# Replacing 12V batteries with Lithium



## mko (Nov 27, 2009)

Hi

I consider replacing my 12V battery with Lithium (4 CALB cells at 3.4 volts), since weight is on of the key issues in my conversion.

The current (LA) 12V battery is of 60ah. I think 40ah in Lithium would be sufficient, since the battery does not have to maintain a starter motor.

What is your opinion? Is 40ah sufficient, or should I play safe and go for 60ah?
Or is the plan to replace the 12V battery with lithium a bad idea in the first place?

Thanks for your feedback, c.k.


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## MN Driver (Sep 29, 2009)

The 'bad idea' part that I see with replacing the accessory battery in an EV, or even a standard car with lithium is that if you leave the headlights on or a door is cracked open and the dome light is on, your battery discharges and you are out the cost of the 4 cells of 40Ah(or 60Ah) lithium. I'm not sure what the 12 volt needs are for your car but considering the 1.4kg CALB cells(based on the new spec, 1.5kg on the old) or 1.6kg for the TS cells each puts you at about 14 pounds with the copper links and compression hardware(which you might not need).

Have you considered what you are using the lead acid for? Do you really need the 60Ah? If your DC-DC converter can handle the loads pretty well and you just need something to keep the cars electronics active without the DC-DC converter running or to buffer any larger loads, you could consider using a smaller lead-acid battery. Motorcycle batteries are picky in my opinion and sometimes priced too high. A good option, in my opinion is a lawn mower battery, it's a standard U1 size and they are available for $20-25 pretty much everywhere. They are compact and relatively light(especially to 60Ah of lead) and if you leave your headlights on, you aren't out much if it doesn't charge back up, but chances are it will be able to take the abuse a time or a few.

Added: ...I guess the question that I have is, what are you using the reserve capacity for. You might need 60Ah for something but if you do, I don't know what that need is. Usually lead acid is sized up for an internal combustion engine so that way it has enough power to crank over a big engine that has thick oil in -40 temperatures while the battery, being that cold can barely produce enough power to do such a task. Once an ICE engine is running, it normally gets all of its power from the alternator attached to the ICE. In your case the DC-DC converter would handle most loads except for the biggest loads like when you lower the power windows while your blower, run the rear defrost, have the vacuum brake pump, and have the high-beam headlights are all on at the same time, which is an extreme but even a lawn mower 12v battery can handle the peaks just fine and the DC-DC will fill it right back up once the power windows and vacuum brake pump are off. If your DC-DC is strong enough, it might not even need to dip into the 12v battery in which case it might only be there to keep your radio presets and cars computer happy.


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## dimitri (May 16, 2008)

The decision largely depends on whether you plan on having DC-DC converter, how you plan to wire DC-DC converter and what you typical/max load on 12V system.

For example, I wanted smallest possible 12V battery in my EV, so I used lawn mower starting battery and I had my DC-DC switched to the main pack via main contactor, which is controlled by ignition key. Everything was working great until one time my EV sat in a garage for a week without use and 12V battery was drained by small loads I have always on, like BMS, clock radio, EV Display, total less than 0.5 Amp, but enough to drain small battery in a week of no use.

I then changed my wiring to have DC-DC always on, so it keeps 12V battery full at all times. Battery now just serves as a buffer when 12V load exceeds DC-DC capacity ( for example all windows are rolled up or down at once while radio is at full volume, etc), and also serves as a safety in case DC-DC fails while on the road.

With DC-DC always on I didn't want to use Lead Acid 12V battery anymore, so I put together 4 Headway LiFePo4 cells of 10AH and use it as my 12V battery. Cells are essentially always full, so I expect them to outlast the car 

Hope this helps.


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## mko (Nov 27, 2009)

Dimitri, MN Driver

Thank you for your input.

I think I like Dimitri's idea. Since my car won't have any power-windows, -stearing or -brakes (it is 38 years old) there should not be to much load during driving.

Best, c.k.


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## dtbaker (Jan 5, 2008)

I use 'always on' dc-dc convertor without 12v battery at all... no problems. rated capacity is around 400 watts, and I have never seen my draw go over 300 even with headlights, brakes, and vacuum pump running.


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## mko (Nov 27, 2009)

Yes, I considered this as well, especially since here an auxiliary battery is not necessarily needed for registration. However I kind of feel safer having a small 12V battery, just in case the DCDC breaks down in the middle of the night.


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## marcexec (Feb 11, 2009)

dimitri said:


> With DC-DC always on I didn't want to use Lead Acid 12V battery anymore, so I put together 4 Headway LiFePo4 cells of 10AH and use it as my 12V battery. Cells are essentially always full, so I expect them to outlast the car
> 
> Hope this helps.



Dimitri, do you use any kind of BMS or have you measured / balanced the cells in the meantime?


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## dimitri (May 16, 2008)

marcexec said:


> Dimitri, do you use any kind of BMS or have you measured / balanced the cells in the meantime?


Since I am in the BMS business, naturally, I used my own BMS 

However, since 12V battery is hardly used at all and its always charged by DC-DC, BMS on it wasn't really neccessary, just charge all 4 cells individually at first to line them up, then assemble in series.


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## rwaudio (May 22, 2008)

I did basically the same thing as dimitri, but went with a 4s3p headway pack and my dc/dc converter will always be on. I'm a stereo guy so wanted a little more headroom on the peak current draw.



















I am also assembling the dc/dc converter myself so I will probably incorperate it into the new battery "case" since it's quite a bit smaller than the stock battery right now. So I will have a single unit with high and low voltage terminals all in a nice compact package.


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## uigiroux (Oct 31, 2017)

rwaudio said:


> I did basically the same thing as dimitri, but went with a 4s3p headway pack and my dc/dc converter will always be on. I'm a stereo guy so wanted a little more headroom on the peak current draw.
> 
> 
> 
> ...




Hey where did you get those battery blocks? I've been searching online and can't find anything for A123 batteries, just the 18650's...


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## rwaudio (May 22, 2008)

uigiroux said:


> Hey where did you get those battery blocks? I've been searching online and can't find anything for A123 batteries, just the 18650's...


I milled them on my cnc router.
Also, they are headway cells.


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## john61ct (Feb 25, 2017)

40AH LFP is more usable AH than 60AH lead. 

Protect with a simple adjustable LVD cutout sized for max amps. I'd set conservatively at 11.9 or 12.0V, and charge at 13.8V for longevity.


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## Solarsail (Jul 22, 2017)

It has been claimed that Lead Acid batteries are overrated by a factor of two, because the manufacturer's rating assumes the battery will be drained to zero volts, which will destroy the battery. So 60Ah Lead Acid is practically only 30Ah.

If you use a simple 4s balancer-protector, then this board will cut off the power if the voltage on any cell drops to below 2.5V or so, in order to save the cells. These boards cannot drive an engine starting motor, and are limited to about 30A. It also detects overcharging and will cutoff the current. However for LFP, the charger should not deliver more than 4x3.7 = 14.8V under any circumstance.

https://www.ebay.com/itm/4S-30A-12V...807438&hash=item283c676382:g:IIoAAOSwQwBZjHS2

If you will not use such a board, then you can get a low-voltage detector and a disconnect relay such as these:

https://www.ebay.com/itm/12V-Batter...524148&hash=item2cc150d046:g:208AAOSwyWZZP-T4
https://www.ebay.com/itm/12V-24V-30...d=311754827745&_trksid=p2047675.c100005.m1851
https://www.ebay.com/itm/12V-20A-Re...827745?hash=item48960927e1:g:imEAAOSw5cNYRtUk


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