# Auxiliary battery



## Colcam1 (Oct 1, 2014)

Hi everyone

I have just joined DIY ECF so I hope this ends up in the right section. I have been wandering for some time what would be the difference using a higher capacity auxiliary battery than the original one. The one on my car is a 40 Ah that is charged directly from the traction batteries through a DC to DC converter. Would it last longer before it needed to draw power from the traction batteries, or would it be drawing more power right from the start.
Hope this doesn't cause too many headaches.


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## Duncan (Dec 8, 2008)

Nah
Both would draw the same amount of power,
The bigger one would give a longer "emergency lights" time if the whole system failed

Most people would go smaller if anything (lighter)


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## dougingraham (Jul 26, 2011)

Duncan has the correct answer. I use an 8 AH LiFePo4 battery as my "Buffer Battery" This is enough to operate the car for about 15 minutes at night if the DC/DC should fail. It would operate the hazard flashers for about 10 hours. My battery only weighs 2.8 lbs.


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## arklan (Dec 10, 2012)

my aux battery is bigger than the original petrol engine battery
it weighs 15kg but it has enough power to run everything long enough for me to get to and from work if the dc converter dies
i had the bigger battery spare and so i chucked it in, the side of the engine bay would have a huge empty space that would look weird if it wasnt taken up by something
also iv heard of dc converters being notoriously unreliable :/

edit to add that the only reason u have a 12v battery is to have some power to turn the dc converter on and to take the spikes out of the dc converter
a bunch of AAs could do the job


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## Colcam1 (Oct 1, 2014)

The reason I asked about using a bigger auxiliary battery was because when I use the lights and especially using the heater in the colder weather the traction batteries start to drain a fair bit. I suspect the auxiliary battery probably needs changing if I can work out how to remove it without dismantling half the car. Only it looks like the aux battery is installed and then the dashboard fitted after.


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## arklan (Dec 10, 2012)

the guys at evworks use one of these in their cars
seems tiny but they also have 2x 35amp dc converters to run everything
i dont think it would be your battery

if your traction pack is draining hard when u use lights then maybe you have a ground fault somewhere?
if u r running power steering, aircon, vacuum pump etc then yeah i would understand

what amp hour and voltage is your pack? and what dc converter do u use?

edit;
when your car is running it will pretty much draw all of its 12v needs through the dc converter including charging your aux battery
if u got a smaller aux battery the dc converter might draw slightly less because its not float charging something as big? need confirmation on that.. either way wouldnt make a noticeable difference


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## Colcam1 (Oct 1, 2014)

Hello Arklan
My car is a small Aixam Mega City. It doesn't have power steering or brakes. There are 12 traction batteries. They are 76ah lead acid in a 4 parallel and 2 series layout, 48 volt supply. I don't think anything is shorting out as I have a small plug in voltmeter in rhe cigarette lighter socket, which I check daily.


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## Colcam1 (Oct 1, 2014)

Has anyone connected super capacitors across their batteries, and has it made any difference to performance or overall capacity of the batteries. I was wandering if it would assist acceleration being as a capacitor can discharge at a faster rate than a battery, or would this affect the controller.


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## arklan (Dec 10, 2012)

it works but many people have said that what your gonna be forking out in caps u may aswell just buy more batteries
youll get more oompf aswell as range 

that said theres plenty of anecdotal evidence to show they work, if u can find a cheap place for them, go for it


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## Colcam1 (Oct 1, 2014)

Hello Arklan
The other thing is would they affect the battery management system when recharging.
​


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## Ziggythewiz (May 16, 2010)

Colcam1 said:


> Has anyone connected super capacitors across their batteries, and has it made any difference to performance or overall capacity of the batteries. I was wandering if it would assist acceleration being as a capacitor can discharge at a faster rate than a battery, or would this affect the controller.


A super capacitor can't increase the capacity of a battery at all...as for improving discharge rate, modern batteries can handle any typical (off track) needs without any problem.


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## Sunking (Aug 10, 2009)

FWIW using an automotive SLI battery is a poor choice for cycle applications as they are not made to do that. SLI batteries are designed for quick short burst of high current and recharged very quickly by the vehicle alternator. If pressed into cycle service are only going to last for 50 to 100 cycles where a good quality true Deep Cycle battery can last a couple thousand of cycles or up to 5 years if given TLC and not abused like over discharged.


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## Colcam1 (Oct 1, 2014)

So would it be best to use a deep cycle for the auxiliary or would any run of the mill SLI batteries be good enough.
Found this article that might be of some interest to others that are not too sure about batteries.

http://www.electric-cars-are-for-girls.com/deep-cycle-batteries.html


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## Colcam1 (Oct 1, 2014)

Ziggy how is the booster pack connected on your VW, is it in parallel or as an alternate set of traction batteries.


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## Sunking (Aug 10, 2009)

Colcam1 said:


> So would it be best to use a deep cycle for the auxiliary or would any run of the mill SLI batteries be good enough.


Lead acid batteries fall into 3 basic categories. 

SLI or Starting, Lighting, and Ignition are constructed using multiple thin spongy plates texture like plate to increase surface area. That in turn make for very low internal resistances so they can deliver very high amounts of current needed to crank an engine. SLI batteries will er much lighter for a given size compared to Deep Cycle and Hybrid batteries . Th eplates in a SLI battery do not fill the jar and have considerable space in the bottom of the jar for the plates to crumble and disintegrate as they corrode. If you press them into cycle service you are are going to get a few cycles before they fail.

A true deep cycle battery has heavy thick plates made to last. The big drawback of Deep Cycle is internal resistance is high and they cannot deliver large discharge rates without excessive voltage sag. Charge and discharge currents need to be limited to C/8 otherwise you will gas then severely when charging, or extreme voltage sag under high current loads. However a good top quality deep cycle if depth of discharge is limited to 20 to 30% can last up to 1500 to 2000 cycles. 

Then there is Hybrid Batteries and hybrids come with fancy marketing names like Golf Cart, Floor Machine, Marine Deep Cycle, RV, and Leisure batteries. Hybrids are a cross between an SLI and Deep Cycle. The plates are heavier and thicker than SLI batteries, bu tnot as heavy as Deep Cycle. As implied they can deliver higher current rates than Deep Cycle, but not as high as SLI batteries. Th elast longer than SLI, but not as long as Deep Cycle. As with all batteries there are trade-offs. 

On especial battery is the Pure Lead AGM batteries. Again they are a hybrid but have very low resistance and can be used for cranking, EV's, and deep cycle batteries. But they have two catches. They are extremely expensive twice that of a high end Deep Cycle, and only have half the cycle life of deep cycles making them very expensive. 

Then there are industrial grade Traction batteries made for forklifts. Fantastic batteries that can deliver very high currents and last a long time. But like Pure Lead AGM are extremely expensive and very heavy because they have a lot of lead in them. 

Like any battery chemistry there are trade-offs, you cannot have it all.


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## Colcam1 (Oct 1, 2014)

How do lithium batteries stack up against lead acid then. Are there different types or do they all have the same construction, just different sizes.


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## dougingraham (Jul 26, 2011)

Colcam1 said:


> How do lithium batteries stack up against lead acid then. Are there different types or do they all have the same construction, just different sizes.


There are three basic types of Lithium batteries. The most common kind used in DIY EV projects are LiFePo4. They are the safest of the three varieties because it is very difficult to have them go into thermal runaway. They seem to be able to handle piercing events like bullets or rebar without doing much more than steaming out some electrolyte. They also are the lowest energy per unit of mass and volume of the three lithium types. At a 20C Lead acid discharge rate they are only about three times better per unit of mass than lead acid. About 95 to 110 wh/kg for LiFePo4 and about 35 to 40 wh/kg for lead. At a 1C rate they are about six times better because of Peukert effect that lead acid suffers from. LiFePo4 appear to have about a 3% capacity loss per year and a cycle life of 3000 cycles to reach 80% of new rated capacity. This means LiFePo4 ends up costing about half of what lead acid costs at the end of the life of the lithium pack.

The LiPo pouch cells are mostly LiMnO2 and are the kinds used in the Leaf. They have higher energy per unit of mass (around 150 wh/kg) but they are quite a bit more fragile and some of this benefit is lost in the required packaging. They would benefit from an active cooling system and they are not too difficult to overheat and are the devils own to put out if they ever catch fire due to the fact that a combustion product is oxygen. These are currently available for a good price as salvage Leaf and Volt battery packs.

The LiCoO2 laptop style cells that Tesla uses are the highest energy density at around 230 wh/kg. They are even easier to make go into thermal runaway so an active cooling system along with really good engineering to contain a runaway event should probably be considered essential to their use.

Any of these can be constructed as power or energy cells.  Power cells can give up their energy more quickly but have less overall energy storage. Energy cells tend to give up their energy more slowly but have more of it per unit of mass. For the most part we are interested in Energy cells. This is because we want long range. Drag racers are most interested in Power cells. This is because they want to use all the energy in just a few seconds.

Hope that answered your questions.


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## Colcam1 (Oct 1, 2014)

Thanks Doug
Would there be any advantage in replacing the auxiliary battery with a Lipo one to save a bit of weight but keep the capacity roughly the same. How could this be charged from the built in charger, they have to be balanced don't they.


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## dougingraham (Jul 26, 2011)

Colcam1 said:


> Thanks Doug
> Would there be any advantage in replacing the auxiliary battery with a Lipo one to save a bit of weight but keep the capacity roughly the same. How could this be charged from the built in charger, they have to be balanced don't they.


You can pretty much drop in a LiFe type 4S battery into a car without any issues if you top balance. The reason for this is the charging system is set to charge a lead acid battery to somewhere between 13.8 and 14.4 and LiFe can tolerate a voltage of up to 3.6 volts for extended periods without issue. There are lots of LiFe batteries being sold for motorcycle use because they don't suffer from having to replace the battery every year which is common unless you ride all year round.

As for having to be balanced, you need to do it once and unless you run it dead it will be fine. I have been using a little 8AH LiFe pack for my buffer battery for a little over 2 years now without issue. I checked the balance at 2 years and it was still less than 0.001 volts difference between the cells at full charge resting voltage. I float this particular pack at 13.6 volts (3.4 vpc) so I can't comment with personal experience on if the higher voltage of a regular car charging system will cause problems long term but because lots of places are selling these 4S packs with no special electronics and they seem to be lasting I suspect it is fine. Was that a run on sentence or what?

Having said all that if your 12 volt battery is in good condition I would just continue to use it. The car already has a place for it and it is tied in properly to the car systems. Your replacement LiFe does not have to be a large battery. With Lead Acid they sized it for cold cranking output. No cold cranking with an EV so it does not need to be nearly that large. My 8AH can drive the car systems at night for about half an hour. But I changed everything out to LED's. It will run the hazard flashers for more than 8 hours. If everything else is off it will drive the car stereo and XM receiver for about 16 hours.

Best Wishes!


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## Colcam1 (Oct 1, 2014)

Hello Doug
I think I would like to match the capacity of the original if I can. I know the auxiliary battery charges at approximately 13.4V, but I don't know if it is charged directly from the onboard charger or through the DC to DC converter. Not too sure how you do a balance charge, unless you have to use a charger designed to charge each cell individualy.


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## dougingraham (Jul 26, 2011)

Colcam1 said:


> Hello Doug
> I think I would like to match the capacity of the original if I can. I know the auxiliary battery charges at approximately 13.4V, but I don't know if it is charged directly from the onboard charger or through the DC to DC converter. Not too sure how you do a balance charge, unless you have to use a charger designed to charge each cell individualy.


If you want to match the capacity of the original Lead Acid car battery you will need to estimate that. A car battery is going to be between 30 and 50 amp hours at a 20 hour rate. They are not rated this way because it is not important in an ICE vehicle. They are rated for cranking amps because that is most important thing in an ICE vehicle. Just buy four LiFe cells that are 40 AH. This will be about a 12 lb battery which should be about 1/3 the weight of the Lead Acid it is replacing. I would top balance them with a balancing charger designed for the Radio Control market. You probably know someone who has such a charger that can be set for LiFe cells. About 10 years ago I designed a balancer for the RC market and LiPo cells called the AstroFlight Blinky. It would take a while but a Blinky would eventually balance even a 40 AH pack. You would only need to do this once or if you ran the battery dead it would be a good idea to do it a second time. The Blinky was able to balance up to a six cell pack to better than 0.001 volt. I no longer have anything to do with AstroFlight. Here is a link to the version of the Blinky with the voltages for LiFe (A123 cells) AstroFlight Blinky

An EV 12 volt car battery is almost always charged from the DC-DC converter. A 13.4 volt DC-DC would give a 3.35 volts per cell float charge on a 4S LiFe pack which is a good place to be. This would keep the pack charged to around 85% which should give a long life to the cells.


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## Colcam1 (Oct 1, 2014)

Thanks for the reply Doug, it has certainly made it a lot clearer how to use the lithium batteries. I will have to look out for some, probably on eBay.


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## Sunking (Aug 10, 2009)

Colcam1 said:


> How do lithium batteries stack up against lead acid then. Are there different types or do they all have the same construction, just different sizes.


If a FLA battery has a CCA rating, is not a deep cycle battery. Cranking or SLI (Starting Lighting Ignition) as it is called in the industry should not be used in cycle applications. They are not made to do that and if pressed into cycle service will fail in short time. Sort of like pressing a Top Fuel Dragster into a Indy 500 race car. Not going to end well.

Having said that if you wish to use a LFP battery to replace a FLA is NOT an AH to AH replacement. which is a really good thing. Battery Rule # 37 says 100 AH FLA = 37 AH LFP. However that rule assumes the FLA battery is discharged at the C/4 rate rather then the specified C/20 rate. If you discharge at a slower rate the formula changes. Generically if you discharge at a slower rate than C/5 use Battery Rule #50 100 AH FLA = 50 AH LFP


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## Colcam1 (Oct 1, 2014)

Hello Sunking
Being as the aux battery is constantly charged from the traction batteries it is more or less kept at a steady state and never really gets any serious discharging. Would this reflect on which battery to use.


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## Sunking (Aug 10, 2009)

Colcam1 said:


> Hello Sunking
> Being as the aux battery is constantly charged from the traction batteries it is more or less kept at a steady state and never really gets any serious discharging. Would this reflect on which battery to use.


Well if that is the case not sure why the Aux battery is there? Sounds like you have a converter?


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## dougingraham (Jul 26, 2011)

Sunking said:


> Well if that is the case not sure why the Aux battery is there? Sounds like you have a converter?


Best practice is to have a buffer battery in case the DC-DC should suffer a failure. This could be an actual failure of the DC-DC converter or a problem with the traction pack. You could continue to drive for a while if the DC-DC fails. And in the case of a traction pack issue your hazard flashers will operate while you call for assistance.

The buffer battery also allows for a tremendous load variation that a DC-DC converter might have trouble with. For example the starting surge of a vacuum pump or the ABS brakes could exceed the ratings of the DC-DC converter which might be adequate in all other situations. The buffer battery will take up the slack.

Unless you want to leave the DC-DC running all the time to keep the clock and radio presets active the buffer battery makes sense.


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## Sunking (Aug 10, 2009)

Thanks Doug for taking the time to explain. Knew I had to be over looking something. Having said that I wouldn't think the battery would not need a lot of capacity?


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## dougingraham (Jul 26, 2011)

Sunking said:


> Thanks Doug for taking the time to explain. Knew I had to be over looking something. Having said that I wouldn't think the battery would not need a lot of capacity?


I am using an 8AH LiFe. I can run about half an hour at night unless I need to run the fan or wipers. The hazard flashers will run more than 8 hours because I switched to LED lamps. I would want to go much less than this.


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## Colcam1 (Oct 1, 2014)

Hello Doug and Sunking, yes there is a DC-DC converter and the reason I wanted to fit a bigger aux battery in the first place was to try and conserve some of the traction battery capacity. There are no power assisted ABS brakes or steering or air-con. These cars are very basic and are classed as quadricycles so 16 year olds can drive them. There are 12 76Ah batteries with a 48V output for a 4.5kW motor. So even though I have changed most of the lights to LEDs it does start draining the main batteries especially when using windscreen wipers and heater for demisting. The defrost setting has a four minute timer built in as standard to cut down some of the drain. I hope my reasons are a bit clearer now.
Thanks for all your help, it's much appreciated.


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## Sunking (Aug 10, 2009)

Colcam1 said:


> Hello Doug and Sunking, yes there is a DC-DC converter and the reason I wanted to fit a bigger aux battery in the first place was to try and conserve some of the traction battery capacity.


Nice idea but in practice does not work that way. The converter is the higher source, and all power will come from it until that condition changes. Like any Float application, the rectifiers or converter supplies all the energy. Your Aux battery sits there fully charged up fat, dumb, and happy and does nothing but Float. Only when and if the Aux battery voltage is higher than the converter will power be drawn.

That is how every battery plant ever made works and exactly how telephone, communications, and electric utilities operate. The batteries set there and Float just waiting to take over in the event commercial power fails. Otherwise the reciters supply all power. All the batteries do is wait for their turn. That is why you do not need a large Aux battery if you are using the main traction batteries with a converter. All it does is act as a buffer, filter, and voltage regulator.


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## dougingraham (Jul 26, 2011)

Colcam1 said:


> I hope my reasons are a bit clearer now.
> Thanks for all your help, it's much appreciated.


Your best bet is to use the smallest aux battery that will work. This will lighten the weight of the vehicle slightly and give you a bit more range. Sun gave the reason why a large aux battery does not help.


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## arklan (Dec 10, 2012)

i have a 40ah lead battery in mine, which is bigger than the car had even when it was petrol
i have the fuel gauge driver which runs off it full time and im about to have an arduino run off it full time aswell so i want to make sure nothing bad happens over the weekend when im not using it.


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## dougingraham (Jul 26, 2011)

arklan said:


> i have a 40ah lead battery in mine, which is bigger than the car had even when it was petrol
> i have the fuel gauge driver which runs off it full time and im about to have an arduino run off it full time aswell so i want to make sure nothing bad happens over the weekend when im not using it.


You should measure the key off load on that battery. If you allow it to discharge very deeply the Lead Acid battery will not last too long. A half an amp load would give you 3 days before depleting the battery too far.


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## Colcam1 (Oct 1, 2014)

OK so the auxiliary battery is just a buffer for the DC-DC converter which is running at a higher PD until there is greater demand on it. The only reason to use a fairly high capacity auxiliary is in case the converter stops working, which will last longer using the lights etc. That seems pretty clear now, I hope it has helped others as well.


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## Roderick (Dec 8, 2013)

Thank you so much for all the explaining. 
Does anyone here have a large lithium battery(100ah) as an AUX and dont use DC-DC? (charge AUX batteries with HV main batteries)

Is this something that's efficient or this wont work?

Rui


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## Duncan (Dec 8, 2008)

Hi Roderick
That's what I do
I only have a small main pack
4P x 40S 16Ah Headway cells and I use 4S 16Ah cells as my auxiliary - just charge the Aux and the main at the same time

When I switch off everything is isolated,
Not had any problems so far - but I just use it for local runs and playing on the track


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## Roderick (Dec 8, 2013)

Duncan said:


> Hi Roderick
> That's what I do
> I only have a small main pack
> 4P x 40S 16Ah Headway cells and I use 4S 16Ah cells as my auxiliary - just charge the Aux and the main at the same time
> ...


Does that mean you do NOT have DC-DC??!
or you have some sort of switch allowing you to control the on/off of DC-DC?

also, 16Ah cant be big enough to keep you going under the rainy evening...given you have power steering, vacuum and A/C.


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## Duncan (Dec 8, 2008)

Hi Roderick
No;
AC, Heater, Wiper, Washer, Power steering, Vacuum
None of that
All I need is 
Current for;
The three contactors - Battery +ve, Battery -ve and forwards/reverse
The controller
Lights (all LED except headlamps)
Horn, Flashers
Normal drain about 4 amps - enough for 4 hours
Main battery will run at 50Kph (50amps) for about 1 hour

http://www.diyelectriccar.com/forum...-dubious-device-44370p8.html?highlight=duncan


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## dougingraham (Jul 26, 2011)

Roderick said:


> Thank you so much for all the explaining.
> Does anyone here have a large lithium battery(100ah) as an AUX and dont use DC-DC? (charge AUX batteries with HV main batteries)
> 
> Is this something that's efficient or this wont work?


Using a 100AH Lithium battery as your 12 v source with no DC-DC converterwill certainly work.

Downsides:

1) You still have to charge it somehow.
2) It is heavy. About 30 lbs which is similar to the 30AH Lead Acid battery you are replacing.
3) It is costly. Around $400 for the cheap 100AH CALB SE cells or as much as $560 for new cells.
4) It is holding energy that cannot be used to move the vehicle.

If you have a 3000 lb car with a small buffer battery (mine weighs about 3 lbs) and the DC-DC converter (about 5 lbs) you have about 8 lbs of stuff you are carrying around all the time. If you decide to go to a 100AH buffer battery and a charger you will be carrying around at least an additional 24 lbs if the charger stays in your garage and an additional 32 lbs if you carry the charger around with you. Assume he DC-DC and charger weigh the same. That additional weight will cost you 3.2 wh per mile. As a percentage a little over a percent. If you have a 100 mile range with the small buffer battery and a DC-DC your range would go down by a mile from the extra weight.

I would not call this efficient. But yes, it will work.


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## Colcam1 (Oct 1, 2014)

Hello folks
I didn't think anyone would have used this thread again. I would like to know if you can buy Calb batteries as 12V units or do you have to add cells together to make them up to 12V.


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## Roderick (Dec 8, 2013)

Duncan> That's perfect explanation, thank you.
Although my car did come with 100Ah lead battery which is extremely heavy to carry!
As I've already got spare 100ah batteries with me, I might use it. However, I will look for a cheap slightly used 20~60Ah cells in future.

Thanks


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## rinnnatajacc14 (Jun 4, 2015)

Hi,
i have no knowledge about the topic.you should contact the
expert for further information.sorry for that.


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