Lithium 12v Battery Pack Replacement, $300 or less (LiIon/LiFePO4)

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ee241

Member
Joined
Oct 5, 2017
Messages
7
One of things that troubled me the most when I bought my 2016 Spark EV, was the 12v system was still using a big heavy lead acid battery. There has been a couple other threads that discussed this, and the possibility of replacing it, but I have not seen anyone actually post their success, so I decided to roll the dice and try it myself.

My first attempt was using 18650 3.7v Cells, 4 in series, which equates to a maximum charging voltage of 16.8, and a "dead" voltage of 10. I had read that the 12v charging system charges at around 15.1v, so I overbuilt the battery pack a little bit expecting that it would never be fully charged...

I installed the pack after fully charging it to 16.8v, and installed it with a fire extinguisher on-hand just in case ;) (see picture below) The Spark accepted the higher voltage without any issues or complaints, or error lights. I proceeded to drive the car for about 10 miles with the seat heater and headlights on, no issues at all. Checking the voltage of the pack when I got back, it was still around 16.5v

A week passed without any problems. I periodically checked the voltage of the pack, and after about the 5th day, the voltage had dropped down to about 13 volts. I kept checking voltage frequently for the next few days which seemed to stay around 13v, until I got a special LiIon Balance Charger in the mail.. I then hooked it up to the pack, and attached a balance lead, which showed the cells were all around 3.1v... Now, for those who aren't fammiliar with 18650 cells, 3.1v is pretty much just about dead. While they can be discharged down to 2.5v, they have pretty much lost all their capacity before then and the voltage will start to drop very, very quickly after around 3v.

So, while I did not encounter any issues, I decided to remove the 18650 LiIon pack, and write that off as a failure, since if the vehicle was left without being used it might die faster than the lead acid would.




My next attempt I will be using 26650 LiFePO4 batteries, which are commonly used for 12v packs. These have a nominal voltage of 3.2v each, and a maximum voltage of 3.7v. Using 4 cells in series, you get a 12.8v battery, with a maximum charging voltage of 14.8. This looks to be a lot closer to what the Spark keeps the battery at, since at no point after discharging the pack did it push the voltage back up above 15 volts. Also, after some additional research, LiFePO4 cells can take 4.1v without being badly damaged, although it is not advised, so if the pack was charged to 15.1v it would not pose a safety issue. LiFePO4 cells are among the safest types of Lithium batteries in general vs LiIon. The downside however is the price, and energy density.

The 50AH AGM Battery in the Spark has a theoretical capacity of 600Wh (12v * 50aH = Wh). I have not weighed it yet, but it's pretty darn heavy, requiring 2 hands for most people I'd say. The 18650 pack I made shown below, contains 52 cells, each with a tested capacity of about 8.5 Wh, which would give us just under 450Wh, but that's only if charged to the full 16.8v which we saw is never ever going to happen... Now, a LiFePO4 battery pack that's about the same physical size and weight as the 18650 LiIon pack, would only hold about 250Wh.

But, since the Spark does not need the 12v battery to ignite engine cylinders, or really do anything except power the infotainment system, lighting and seat heaters, I think we can get away quite easily with a pack this size. The 450Wh LiIon pack lasted about 5 days of use before the car might have been doing some charging. which is pretty impressive to think about.



I currently have 20x 10.2Wh LiFePO4 Cells on order, this is a smaller physical size and weight than the pack shown below, but will be the correct voltage. I certainly will post back here with my findings. My goal is to design a battery pack that performs as well as the 50AH AGM would in our cars, that weighs a fraction that it does (hopefully might see an increase in KwH/mi), lasts longer (calendar years), and costs $300 or less..

https://imgur.com/piTPaSc -I tried to embed it, but it didn't seem to be working, sorry.
 
I've been using this $130 20 Ah LiFePO4 in a Prius for the past 3 years:

https://www.batteryspace.com/lifepo...-256wh-10c-rate-24-0---un38-3-passed-dgr.aspx

There's usually coupons for 10% off if you search the net, which essentially gets you free shipping.

For a few bucks more you can get one with LED balancing, or for even more one with balance plus over/under voltage protection. I don't bother and just get the cheap one.

6404.png


I also use one in my 2-stroke stand up jetski
 
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