It is springtime in Missouri. March rather carried a bit of late winter and so we’ve had a late bloom but a welcome one.
This week we talk a bit more about the VW Thing progress. This vehicle has turned into a pleasure to work with largely because everything has been so accessible. Rerant on the importance of component placement for maintenance purposes. But some vehicles just make this easier than others.
IN 1973 I was graduating high school and had pretty much decided to move on from my 1954 Dodge Coronet feeling I had gotten my money’s worth on the $60 purchase price. The VW SuperBeetle was kind of the craze and I recall spending some time at the local Wieser Motors looking at the shiny new SuperBeetle’s with what looked like enormous tail lights. I wanted one pretty badly. I recall they were about $3200 loaded. I rather purchased a brand new Ford Pinto, in all ways a better value at $2185 cash money hard come by. But not so much a better car.
That year VW introduced the Type 181 THING in the United States, Trekker Elsewhere and Safari in Mexico. It really was NOT a Type 82kubelwagen. The Kubelwagen was based on the original beetle pan and both the THING and the Karman Ghia shared a much larger pan. But at $3150 it was just a little too wierd for a little too much. Although 140,000 were manufactured between 1968 and 1980, only 25,000 entered the U.S. and in 1975 it failed to meet the new safety standards for windshield intrusion.
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With this build we are not necessarily striving for the best electric vehicle. We are using the THING as a platform to try a number of new things. Most notably of course is our AZD windfall of Siemens 1PV5135 liquid cooled AC induction motors and the AZD DMOC645 inverter. But the high voltage desired let’s us do more experimentation with the CA60FI cell from China Aviation Lithium Battery Company. We have never done a build with this SMALL a cell, either physically or with regards to AmpHour Capacity.
We get kind of a constant stream of inquiries about sizing battery packs and components. And it is always a wag without notice. But the process is pretty simple. First you start with the car. What kind of car do you want to convert? And it goes almost immediately to what kind of performance you want to achieve.
The car is mostly about weight. How heavy is it. In removing the motor and exhaust and tank etc. and replacing it with a controller and electric motor, you can expect to actually lighten the vehicle by 100 lbs or so. But you lose that gain when you add in the batteries and boxes. And the result is a car that is typically 400-500 lbs heavier than stock. So if you start with a 2600 lb car, you will normally be looking at 3100 or so.
We use a rule of 10:1 though it is admittedly very rough with regards to energy requirements. A 3100 lb car will nominally require 310 Watt-hours to move it a mile.
From there you go to range and performance. Now that you know you need 310 Wh per mile, if you want a max range of 100 miles that’s a 31 kWh pack. And at 80% DOD, you will get about 80 miles. So this is a consideration.
More difficult to define is performance. Acceleration. Not only is it difficult to calculate and define, but it is even difficult to compare in discussions. I have been astounded at the lack of performance of many builds that the owner found perfectly acceptable. Cars that wouldn’t get out of their own way were declared quite responsive in description. All things being a matter of feel.
I usually start with what is. The original thing had a 46 horsepower motor and accelerated from zero to 60 in a dizzying heart pounding 23 seconds. At a younger age, I needed two shaves in that kind of time. 46 horsepower is 46 x 746 watts very very roughly so a motor that peaks at 34kw would be sufficient. But it would not feel good.
The Siemens with DMOC645 peaks at about 118 kW. That’s a bit of an overkill. I don’t really need to hurtle down the ramp with a zero to 60 of 5 seconds in a THING. We chose the motor because we HAVE the motor and needed a vehicle to really test and improve Collin Kidder/Ed Clausen’s work on teh Generalized Electric Vehicle Control Unit.
But yes, that takes you to drive train selection. The other end of it is HPEVS claim that the AC-50 is good up to 3500 lbs. We are BIG fans of this Curtis 1238 HPEVS AC-50 drive train – up to about 2500 lbs. At that point our frenzied admiration begins to wane a bit. There are no hard and fast lines. It kind of goes to expectation. With enough gearing, certainly an AC-50 will drive a 3500 lb car and we would have no real concerns about longevity of the motor or overheating. But it might be a much more sedate driving experience than we like to enjoy.
One of our viewers contacted me this week about a conversion he was doing for a customer with a VW Vanagon Pritsche. This vehicle looks like a VW bus with a pickup truck bed and a crew cab and would actually make a GREAT electric vehicle. The conversion shop was trying to make the case to the customer that the AC-50 would be suboptimal in a vehicle that with battteries was just destined to breech the 4000 lb mark despite best efforts. I told him he was precisely correct and that I could not imagine delight and joy with an AC-50 in train. He bought one of our AC-75’s instead and his client is lucky to have him on the four function calculator. The AC-50 does about 110 ft-lbs of torque in the best case while the AC-75 is 180 ft lbs. Identical product with an extra 70% of torque and kind of focused on the low towing and tugging end of the rpm spectrum perfect for such a truck with a VW transmission.
So sizing a motor to a car is a bit of art, but you can’t go TOO wrong here. The AC/DC selection os always problematical. But your motor/drivetrain pretty much narrows your controller options rather sharply. And now we are getting down to cases.
A SOliton 1 has a hard limit at 350vdc as an upper limit as to pack size. If you pick the Soliton1, it can do up to 1000 amps battery or motor. So our pack MUST be less than 350vdc AND if we want to take full advantage of the motor and controller, we really need to be able to squirt 1000 amps without horrendous sag in our pack voltage.
In the case of the Siemens/DMOC645, the eTransit Connect is about 335 volts – almost exactly 100 LiFePo4 cells. And the charts we have show an upper range for this motor of 300-400 amps even for a few seconds. So we don’t need cabling or batteries beyond really 300 amps here.
100 cells is a LOT of cells. But 300 amps is really pretty tame with regards to current. THat points us to a smaller cell size, and a lot of them.
The CA60FI cells we have tested to 12C and are very comfortable that 10C regularly would not be at all damaging to the cell cycle life. Particularly for short durations. Hopefully less than 23 seconds in this case.
So our cells would do 600 Amps and indeed we could go to CA40FI cells and still run this vehicle.
320v x 40 AH is just over 12 kWh. We’re guessing at this point 2600 lbs and so 260 Wh per mile. That’s less than 50 miles. The CA60FI will give us 19kWh. 74 miles max range and probably a little under 60 miles at 80%. And we are at 5C to do the 300 amps. I like CA60FI for this application.
I share this so you can see the wag and figure tradeoffs as we actually do them. You start with car. Go to drive train. Then go to battery pack. Weight, performance, range.
So I get to play with 60 Ah cells. Unfortunately, we had already designed and purchased battery boxes for CA180FI cells which are MUCH larger. We planned 36 of these with an HPEVS system. But as you go down in cell size, they become more granular and easier to fit into spaces. So as it so happened, we were able to fit the CA60FI cells into the same boxes with very little excess space and get the full 100 in as well.
There are those who will tell you that you MUST constrain the cells to contain swelling otherwise your cells will go bad. Hogwash actually. Swelling means your cells HAVE gone bad. You need to constrain them to the degree they don’t fall out into your lap in the case of a rollover. ANd you don’t want them shifting around as you drive. A byte of foam rubber or styrofoam is actually quite useful to take up excess space.
We do reinstroduce the inertia switch. In an internal combustion engine vehicle, this unit shuts off the fuel pump in the event of a crash. Really any minor collision will trip it. This is actually a very key safety element, but also a bit problematical. If you are in a collision and incapacitated, you may wind up with a heavy foot on the accelerator but curiously unaware of it as you are UNCONSCIOUS. This could lead to the poor prospect that you are INJURED in the crash and KILLED in the aftermath as the vehicle tears itself up trying to accelerate to full speed AFTER the crash, and indeed maybe upside down. This little device has a magnetic ball in a cage that is for the most part stuck there, and the switch is normall closed applying either power or a ground to the electric fuel pump. A stiff whack and the ball breaks loose, triping a little mousetrap type device opening the switch. This cuts off the fuel to the engine. Accelerator position is largely moot when there is no fuel. The engine is cutoff.
The only problem with this is if you smack a shopping cart in the grocery store lot, or even hit a curb hard, you can trip it as well. And indeed, many parking lot fender benders you could drive home except the fuel pump is cutoff. If you KNOW about this, and know where the switch is located, it’s a simple matter to push the red button on top and reset it. If you do NOT know about it, you have to tow an otherwise perfectly operational car.
All that pretty much applies to electric cars as well. Perhaps more so. Electric motors spun up very high in RPM often explode in Claymore mine fashion – shrapnel akimbo. In any event, you don’t want your car jumping about after a collision. So we started including this switch in our builds with the Cadillac Escalade.
In this case, the DMOC likes to have power applied all the time. So we will put together a circuit with a Tyko Klovac Czonka EV200 contactor mid pack. We’ll run 12v through a slap switch on the dash to the contactor to keep it energized all the time. The inertia switch we connect between the contactor negative coil terminal and frame ground. If the inertia switch trips, the contactor looses ground and opens, disconnecting the pack circuit. Alternately, if you hit the slap switch, the 12v is removed from the contactor coil with identical effect.
At this point, I would recommend this setup for any build.
It does have a downside unless you run the contactor only with the key on. The coil is always drawing about 300 ma of current. Interesting aside here. Richard took the original speedster for a drive a month or so ago and failed to put the MAINTENANCE switch to off for storage when he got back. I didn’t notice. Yesterday, we had a visitor we were going to take for a spin in the car. It was strangely unresponsive.
The original Spededster has a stereo that hasn’t worked in a year. As I don’t really need music much in the Speedster, we haven’t done much about it. But it does draw current 24×7. We discovered our 57 cell pack of CALB SE180AH cells had incredibly drained to about 6.3v total. That’s a TOTALLY dead pack as best as I can tell and should have cost me 57 cells.
Young Hauber of course had almost pathologically balanced the cells at 2.75 volts. Our bottom balance trick. As I left the shop yesterday we appeared to be at about 183v with all cells very tightly grouped in voltage. I think we’ll get away with this with ZERO loss of cells, though undoubtedly we almost HAD to suffer some loss in capacity. But none evident at the moment.
Tesla actually ran into this problem with the Roadster. A couple of customers “bricked ” their Teslas this way. We did too. Without 12v, we couldn’t even plug in to J1772 to recharge it and indeed our charger would not charge the pack as it couldn’t detect its PRESENCE at that low a voltage. We connected two 12v batteries in series with the pack using a manual connection scheme. We quickly charged it up to about 40v and then removed the two 12v batteries and continued charging.
Meters, stereos, etc. We’ve encountered this before. No matter how SMALL the current drain is, or that it is only from the 12v side, doesn’t matter if you are parked for months. Eventually you will drain it.
It also points up the importance of bottom balancing. In truth, bottom balancing doesn’t really do much in normal operation. It is only important when you overdischarge your cells. The message is, sooner or later, you are GOING to overdischarge your cells. BMS as a solution? How do you power the BMS? It is more likely the CAUSE of this problem. You need a pack disconnect, and you need to disconnect it if the car is going to be parked more than a few days. And a few days can all too easily turn into a few months. WE are just not accustomed to doing regular checks on a car that hasn’t been used or moved in 17 weeks.
So there it is. Fearless leader and battery expert has hosed up ANOTHER electric car with a stupid mistake. My curious ability to fall on my own sword in public remains my most endearing feature. I guess I just don’t have a good “embarrassment” gene although this one does bring a little tingle.
The curious thing about the THING is that you have to have 12v from somewhere to START the pack so that the DC-DC converter can get power to make the 12v the first time. I haven’t worked out that part just yet. But probably a 12v battery you switch on and off.
I detest 12v batteries.
49 thoughts on “The Thing about THINGS”
If you want an off the shelf 12V Li battery, http://www.shoraipower.com/ is making nice little 12V motorcycle batteries. I picked one up at my local motorcycle shop who carries them.
With you on the inertia switch. In the GT40 I built some years back I had two: one ran a monster relay that disconnected the battery. The other disconnected the alternator field. This stemmed from the observation that the main source of fire in a bump is sparks from shorted electrics. Same effect as cutting the pump on the 302 – runs a bit poorly with no spark.
Continuing the safety overkill theme (my first job was in road accident research) I have a 12 volt contactor inside each battery box that is only closed with the key on or the charger connected. The 12 volt supply to the coils goes through a loop of heat detecting cable and an inertia switch in each pack so in a bump both packs should be finger safe. And an overheat in either box on charge or discharge will disconnect the pack. I need a 12 volt battery to make this work, but I reckon they are cheaper than a set of CALBs and a new garage.
A main pack low voltage alarm/disconnect would be nice. I’ve always wondered about that 260GBP cost option solar panel on the Nissan Leaf for the 12v battery.. It might actually be a life saver(?) even if its output is puny.
If anyone is interested in Jacks solid state ac unit. DX.com flog peltier devices, All sizes and outputs, including small water jackets. Sorry if the links show the wrong currency, I’m in deepest, darkest Lithuania awaiting instructions from Ms. Moneypenny.
http://dx.com/p/f40550-high-temperature-power-generation-thermoelectric-cooling-module-white-172103 ($195.70, 10 of)
I like the idea of an Impact Switch, I’ll have to add one of those…
I have been thinking about the 12V bus for a while and how I want it to operate. Here is my plan:
I will install a momentary push button in the dash. It will momentarily apply pack power to my 24VD DC-DC convert. This will energize a 24vdc latching relay to hold the pack power on the 24VDC converter. So, you only have to hold the ‘Start” button down long enough to energize the 24v bus and latch a 24vdc relay.
The PLC is powered from this 24vdc bus and boots in a couple of seconds. It will then monitor the position of ignition switch. If it is switched on it will turn on an output that will power another 24vdc relay to apply pack power to the 12 DC-DC converter. It will also power up the 4″ Color display in the dash.
When you turn the ignition switch off, it will start a timer in the PLC. After the timer expires it will turn off the output that powers the 12vdc converter and then turn on a relay that breaks the latch on the 24 vdc converter. This will power doen the 24vdc bus and remove any load from the pack.
The time delay will be long enough for someone to get out of the car and plug it into a charging station. (Thinking 5-10 minutes)
However, if you plug the car into a power source for charging, it will hold both the 12v and 24v bus on as long as the pack is charing. Once charging is complete, it will disconnect the pack.
I will also keep the display in the dash on as long as it is charging so you can see the progress.
The only draw back I can see is when you want start the charger after the 24v latch has timed out. In that case, you will have to push the start button once to get it to charge.
I like this approach because it does not rely on my absent minded memory to turn a switch off when I get out of the car. It basically disconnect the pack automatically after a short delay…
Any thought or comments are welcome….
Jeff, what about using the start position in the car’s ignition switch for your momentary contact for the latching relay?
I do not want to put pack voltage throught the ignition switch. There won’t be any 12vdc until the contractor is pulled in…. This is the chicken befor the egg issue….
The Thang sure is progressing! I have to absolutely agree in endorsing the “POWERLAB 8” as a magic machine for batteries. I normally use the unit I got from you attached to my PC so the settings can be easily modified and graphed in real time. And bottom balancing saved my wallet mucho denario. As you are aware one of my cells was discharged to below .078v and it did swell somewhat. The battery was brought back using the Powerlab 8 and it even holds a full charge. I just hope that it will last.
My Sevcon controllers have “drivability” switch options for handbrake, seat switch, etc. The inertia switch could be tied to one of those and leave everything else functioning, it just wouldn’t go anywhere.
Also is there any plans for cruise control in the gecu program?
Don, yes, there will be cruise control. From a technical standpoint this can be problematic because all the GEVCU will likely know is what the motor controller is reporting for motor shaft RPM. This is all well and good but doesn’t take into account people who are using multi-gear drive trains. What if you have a 5 speed stick, set the cruise for 2000RPM in 4th gear,and then shift to 5th? If you don’t touch the brake the VCU won’t know any better and your speed will increase accordingly. This same problem can happen with automatic transmissions. Going up a hill the trans might downshift which would lower your speed. True cruise control requires a wheel speed sensor. Really good cruise control and traction control requires 2 to 4 wheel speed sensors. Will people put wheel speed sensors on their DIY cars? I don’t know. I could easily support wheel speed sensors but not everyone is going to have them.
Could you tap into the speedo sensor? For people with transmissions that still have mechanical drive speedo’s, something like this, http://howellefi.com/vehicle-speed-sensor-1990-up-tpi.html
Yeah, a speedo output would work too since it is tied to the speed of the vehicle. The bottom line is that cruise just needs some way of knowing how fast the vehicle is really going. For some reason I forgot about the speedometer but most any car is going to have something like that. It is true that older cars will almost certainly have mechanical gauges but the item you linked to or some other electronic retrofit could work.
The CC could also watch the current and not go over a set limit. This way it would do a bit of automatic Driving with Load type energy conservation when facing an uphill. Obviously it could also control regen on downhill to slow down the vehicle.
Some of the older GM’s (80’s models) have a VSS pluse coming out of the speedometer. Mechanical drive cable to the spedometer, the spedometer displays speed and outputs vss pluse back to ECU. These were really primative car computers, the example I am most familier with are the G-bodys which are Monte Carlo, Grand prix, Regal, etc. Any one of these would have cable that would interface to almost any standard GM trasmission. The spedometer wouldn’t even have to be visible.
The inertia switch on a normal car could of course be an input to the ECU. Rather more directly it is a failsafe wired to the fuel pump. That’s to cut off the fuel.
We’re going the same route. Skip all the other things that may or may not be working after the crash. If this contactor loses its ground, this pack loses its circuit.
There are currently plans FOR a GEVCU program. That’s the plan. Because it is then a software program, you can add anything you like.
I kind of hAVE cruise control on the Escalade. It works. But in an electric car used mostly locally, it has limited utility.
We’ll focuse first on forward/reverse throttle and brake. And from there, almost anything can be added. If you check the forums, there is not only a discussion, but a full blown specification of what we will do INIITIALLY.
But that’s rather the point of an open source project. There are essentially open ended. If you desire it and can code it, it is yours. I think the side step to Arduino Due and a CANbus/wifi shield is good work to ensure sufficient hardware to go a LONG way toward fulfilling a LOT of desires for featuritus.
Good point Jack, if the control fails internally in a crash for whatever reason it may not be functioning properly so interupting the contactor with the inertia switch is a better way to go.
If you’re looking for a bargain on inertia switches look in pick and pull junk yards. These switches are used in many Fords and can be had for a few bucks. Find the switch extract it and as much wire as you can get attached to the mating plug. It is a hell of a lot cheaper than buying a new one for $55 or more from the EV parts dealers. Heck if you’re converting the right Ford the switch is free. All you get for the $55 is the same Ford switch with a rubber weather resistant cap.
Yes Randy. What I really want is a used inertia switch from a junked out Ford for my electric car. Maybe a set of four baldy tires too at $5 each as long as they have a little tread left in em. And one of those stretchy car seat covers in a bag that you can put over the front bench and make it just like new.
Hit er with a little red primer from a spray can on the holy places and we’re ready to cruze….
Not to worry too much. I have had to use the pull your own salvage yards for years to keep my little beasties alive. It is a treasure trove of excellent parts for your car. After all your car is nothing buy a pile of used parts anyway. Yes, there are balding tires and tattered seats and just plain junk but you and I know to leave that behind. You know, you don’t go hunting in the yard for junk. You hunt for good usable parts. We do know how one feels about digging for quarters but in fact repurposing and recycling good used parts is a good thing. It’s not all about money. That hundred dollars I save can buy me one more cell for my pack. There is a method to the madness. Yes its fine to shop and hunt for good deals. Not everyone has to do one stop shopping. Happy Hunting. Ebay also has plenty of good deals for new switches.
Pay no attention to criticisms about your idea of using a used inertia switch from a wrecking yard. Why not? Its not like they have been cycled a million times (most likely cycled once), AND they have to remain durable for many years due to the rampant lawsuit pressure if they fail. AND, you can get a fairly young one if you are so inclined. If you get one off a late model car, it is virtually new. The quality of those OEM inertia switches MUST be high, versus an aftermarket device.
Jack, will you be posting Friday’s episode onto You Tube? I generally watch the show from You Tube, a lot of times with my Android App. I know you may not have a lot of viewers on You Tube and may not be practical to continue to use You Tube. I was just curious as to wait or go ahead and download it from your site. Thanks Jack.
I also prefer to watch from YouTube, because it works on my Mac, Android phone and at home on the LG TV directly. Not to mention immediately in HD. Everything Flash gives me a headache so I end up downloading the episode it’s not in YouTube and watching with VLC on the Mac, but don’t want to download the huge HD version so I end up watching the SD one. Besides YouTube can be set to notify immediately when an episode is available whereas website needs manual polling. I’m grateful for the show by any delivery method obviously, but YouTube is the most usable way. And yes, you can use YouTube without Flash, in case you were wondering.
There is also a podcast available for EVTV videos. Podcast URL is http://feeds.feedburner.com/EvtvMotorVerks
You can use it on smartphone or smart TV.
More info at http://evtv.projectooc.com/podcast/
Tnx for the links!
Nice Catch-22 you have there – you can’t get 12v until you have 12v. If your set-up is like mine, the 12v DC-DC converter is controlled through a pack-rated relay controlled by 12v. Ironically, the power source for the relay coil is the DC-DC converter through the ignition switch. So…I added a pack voltage rated momentary switch that by-passed the relay (my boot-strap switch) which powered up the converter, and turned on the ignition switch which then powered the relay. Releasing the boot-strap switch then left everything controlled through the relay and ignition switch. I may forget sometimes to hit the boot-strap switch first, but thankfully turning off the ignition switch turns EVERYTHING off so I don’t have to worry about an inadvertent dead pack. Downside is my clock’s always wrong since it needs constant 12v to keep time. Oh well….
You might get a 4 cells 14.8V/4.2AH pack for some 80 dollars. They are lithium. They are less than a pound and they might wake up your electronics. Looks like a big key for your car, but hey it is electric after all.
7.4V/450mAH weight 28 grams, you need two of them for about 15 dollars. Are they big enough?
Have a look at model cars and batteries to get more ideas.
You only need them for jump starting. It should take less than a few seconds to get the inverter running and take the stress of the poor little batteries. But beware of anything else that might leach on the 12 volts.
Is Jack and EVTV about to washed away by the floods?
When I was there in December the Mississippi water level was so low that they were short loading barges. Whats it like now? You have a good location for electric river runabouts.
It’s very high here now.
It would be a good location, but oddly we have no dock, no marina no real place to tie up boats here.
Hope you are well above flood level.
hi Jack, you said on the show last week that your capacity on that marked cell you tested would end up being 165ah . any estimates on the capacity for the other cells in the pack. (unmarked cells)
They’ll still be up in the 180-185 range Michel.
Rod Adamowotsit accepted your $5k is a contract and broke it. You are fully entitled to (and expect to) have your money back, plus costs if he attempts to cream you in a court of law.
On my ev I fitted a tracker not because the car is expensive but the parts I’ve fitted are. With the tracker you have a n/o help button which I removed and wired up to my voltmeter which when a pre programmed voltage is reached I receive a text saying help. This will also be set for over charge, I know this is pretty basic stuff but it sure helps you sleep, also the car alarm sounds when the help text is sent.
Check out these connectors. Could be some parts for a better charge connector. Great show. http://www.sabritec.com/Catalogs%20in%20PDF/Specials/High_Power.pdf
My car has a lead acid 12V battery that I use to close the main relay. It also runs the rest of the 12 volt system when the car is shut down. Any parasitic loads will just drain the 12 Volt battery. If I want to park it for long periods I just disconnect it or put a trickle charger on the lead battery. The DC to DC converter is disconnected from the pack when the key switch is closed. It’s amazing how quickly it drains the battery. Especially the JLD404 meter. It only takes 24 hours to drain a 12 volt battery.
That has been our experience with 12v batteries. They are heavy. They online last a year at best. And they are drained immediately by trivial loads.
We use DC-DC converters that are on ALLL the time. Then if we park the car for any extended period, we hit the maintenance switch to disconnect the main pack. That turns off the DC-DC converter. And nothing gets drained. Later, switch on the maintenance switch and all comes to life.
Problem is, you have to remember to hit the disconnect. Here’s what happene when you DON’T.
OK Jack, I’ll add my $0.02. The topic of the 12V aux battery is one that you and I have different opinions on. I’ve listened to your reasons for not having one and I agree with all of them. You and I simply place different weight’s on the pro’s and con’s for having an aux battery. That doesn’t make either one of us right or wrong, it just gives us a difference of opinion. I’ll add my take on the Pro’s and Con’s to having a 12V lead aux battery. Jack please add more pro’s and con’s as well. I think it will help the viewers to have a fuller understanding so they can make their own educated decision when converting their vehicles.
Pro’s and Con’s for having a 12V SLI aux battery:
They are heavy and take up additional space
The SLI batteries only seem to last a couple years and die just like they do in an ICE car.
It doesn’t take much of a parasitic load on the 12V system to drain it in a couple days.
Adds a level of redundancy to the 12V system, if either the DC-DC or battery fails, the vehicle doesn’t immediately shut down.
Allows for the use of HV DC battery contactors to completely remove all HV power from exiting the battery pack, whenever the ignition is turned off.
If a 12V load is left on for extended periods, you destroy a cheap SLI battery and not your expensive HV pack.
Adds a level of “stiffening” to the 12V system to allow for a smaller sized DC-DC so that the DC-DC can be sized for the average load instead of he peak load.
The battery provides short circuit current for fuse clearing.
The last item in the Pro column I’ll elaborate on. If you have for example 60A DC-DC and no aux battery (or even smaller), you risk the entire vehicle powering down in the event of a short on the 12V system. I’ve recently witnessed this issue on a client’s vehicle (with a 110A DC-DC) who was adamant about not having an aux battery. The largest 12V loads in the vehicle are the most likely culprits because they have the largest fuse sizing. For example the air conditioner condenser fan, or the heater blower fan. Each may take 10-15A running continuous. They might have a 20 to 30A fuse protecting that circuit because the fans take 2 to 4X rated current on startup. The problem occurs when the fan or wiring on that circuit fails shorted. Non engineers would expect the fuse to blow but it doesn’t. What happens is that the DC-DC converter immediately goes into current limit or shutdown before the fuse has enough time to blow. Looking at a Littelfuse 20A ATO fuse curve it would take 100ms at 90A for the fuse to clear. 100ms is plenty long enough for the DC-DC to current limit and shutdown without blowing the fuse. The fuse supports even higher currents for less time, like 200A for 20ms. If you had a 12V battery it would provide the necessary current to clear the fuse without shutting down the 12V system. A headlight circuit would also be a prime culprit because they are most often protected by self resetting circuit breakers which are even slower acting.
An interesting discussion. Extending Peter Dambier’s suggestion, if 12 volt cycle life and weight is an issue, how about trying four Headway 38120S cells as an aux battery? Extending Jack’s analysis of the 787, adjust the bus voltage to 13.5 volts – 4 x 3.38 volts? Total weight would be about 1.5 kg and cost about £60. I’m happier with LiFePO4 than LiCoO2
In favor of an aux 12v battery one thing that is not being discussed is emergency flashers in a side of the road incident. As the last shows battery draindown mishap demonstrates, a properly bottom balanced pack or lifepo4 aux battery need not die if extremely discharged. So instead of having a 12v lead acid battery let’s turn the discussion to what is needed in the way of an aux battery of the chemistry we all know and love. It may take a relay that disconnect it once it reaches 10 volts or it may take nothing at all, just a well balanced group of 4 cells.
For those using an auxillary battery you may want to look into this simple device about
$60 and easy to install. It will disconnect your aux battery from its loads when battery drops below preset voltage, saving the battery. Unit comes with a backup circuit which allows you to reconnect the load to energize your dc/dc convertor.
The secondary purpose for an auxillary battery is the buffering it provides in reducing voltage fluctuation in an electrical system with voltage sensitive microprocesors.
For those with no auxillary battery dont overlook sizing your dc/dc convertor properly to accomodate the 60+ amp load put on the system when the ABS pump is activated. An undersized dc/dc convertor may prevent your ABS system from functioning.
Yes, everyone has a ready answer to this easy little problem.
Now if I might, one question. How do you power the Low Voltage Disconnect circuit?
I know we have a ton of suggestions here but just wanted to add my tried and tested method.
I use a 400w chennic DCDC with 4* 60AH Sinopoly cells.
The cells are TOP balanced for 14.0v (3.5v per cell).
When the car is not running or charging, the DCDC is disconnected from the pack (HV side) and from the accessory battery (12v side). This is important as the chennics seem to become a drain when there is no HV attached!
The 60AH accessory battery soaks up all of the drains quite nicely. I have been parked up for about 6 weeks now and I am still over 13v
I’m a bit confused by what you mean. Myself, I’d use the main pack to complain itself (passive circuit) when the conditions of “ignition off” and pack voltage low is met.
For light 12V backup duty any Lead battery is good for at least seven years, surely!
Andyj you cannot put Jack and a lead acid together into the same car. It is too heavy and lead acids do have an allergic reaction and they would only last one year and a half in the first place. That is why I proposed Lithium and John Hardy even determined the exact species.
Cars are very much centered around 12 volts. 24 volts and 36 volts never made it, not even in boats. You always need a separate 12 volts.
I dont know how airplanes made it. Their stuff is centered around 28 volts even on the space station. Maybe we should look for 24 volts stuff for our cars from the airplane industry.
There is only one lead acid battery in Karin’s i-MiEV and the lead acid and me do sometimes disagree. There is a good reason why model airplanes and hamradio turned away from lead acid and I know from people who successfully expelled lead acid from their cars but I also know an airplane company who successfully burnt down a factory using a single column of lithium cells but a BMS greatly helped them or they would not have made it.
We dont want a BMS in our car. There is no space for it. It tends to get hot. It is bleeding it is a mess, no way. There is no BMS for the 12 volts in an ICE car, is there? Also it is lead acid what BMSes were made for. No BMS, no lead acid. 4 or 5 cells 3.2 volts …
12.8 volts too low. 16 volts a bit high. That is the problem.
Do you mean lead acids are allergic to Jack? Otherwise I agree with you in entirety.
Actually the only reason we need a live battery in any automobile, except for emergency blinkers, is because those who make electronic products use dynamic memory devices. To be honest I have not understood this, since static memory is now in common use. So the question is why, even in a common automobile radio, or any other device with such attributes, still uses such devices.
Anyone who has ever had to change their automotive battery, or had it run down for some reason, and gets back in their car after doing so, and turning on the radio, has experienced the pita of having to reset their favorite stations, not to mention the clock.
Seems the first car I owned, a 1954 Ford Victory, didn’t have that problem with its five button preset.
Makes one ask..
“why do they do that”.
I meant to write 1954 “Victoria”
We went from mechanical to electrical
Boats often have two keys, the second being a simple plastic ‘massaschakelaar’ (mass switch) which physically -as in powered by your muscles- switches off all 12V power..basically a maintenance switch for your 12V but, as it is usually attached to your keychain, not one you can forget.
cheap and Reliable, a Dutchmans dream! but yeah, anything that does not retain memory when powered down gets a memory wipe.. Our JLDs last pretty long though, so have not had any problem with this setup.