As suggested on this thread and in other EV forums, I was convinced to swap the 12v SLA battery in my EV for a 12v Lifepo4.
The advantages are clear: less weight and flat voltage curve should help my EV be more efficient.
I thought of doing this as a second stage of my ongoing project, but as I found a fairly cheap 12v automotive Lifepo4, I decided to go along with it in parallel to my efforts of the buffer battery setup.
So about a month ago I ordered this one:
Smarter Shopping, Better Living! Aliexpress.com
www.aliexpress.com
Costing about $235 shipping included.
It's a 12v 45Ah Lifepo4. In theory it has the ability to directly replace a car's 12v SLA starter battery. I say in theory because although it's rated for hundreds of CCAs (cold cranking amps) which is needed to start an internal combustion engine, I wouldn't count on a high lifetime in this scenario. I've seen teardowns on YouTube of better known Lifepo4's that claim to be suitable as a starter battery (such as the LiLead 110). They really aren't much different than your solar oriented deep cycle Lifepo4's.
The only difference maybe in the BMS programming, which will tolerate a very high current surge for a few seconds.
The LiLead for example uses standard EVE prismatic cells. These are rated at standard 1-2C rates discharge. There are some more professional models that use cylindrical Lifepo4 cells, or even better high C rated headway cells. But these are majorly expensive (around $1k).
Anyways...
An EV 12v battery doesn't need alot of cranking amps since there is no cranking going on. It's just needed to power the contactors of the HV battery and from then on the onboard DC-DC converter supplies all the 12v power (in addition to charging up the 12v).
I got this specific model and not a standard deep cycle Lifepo4 (which costs roughly the same) because this one has a wider temperature range, which I thought would be better suited for a battery under the hood of the car (and not indoors). The seller claims that they use Lifepo4 pouch cells for these types of "starter" batteries (which personally I'm not too fond of), instead of prismatic which they use in their energy storage models.
So... I finally received it!
Of course the first thing I did was to charge it up and do a proper capacity test. Just out of curiosity and to test how it works before installing in my EV.
Here was a disappointment: It tested out only to 20Ah instead of 45Ah!
I did this charge-discharge test twice @0.2C rate but got similar results both times:
View attachment 74844
The second time I charged it up, I connected a shunt to see how many Ah went in up to 14.6v (as recommended) and sure enough it was only 20Ah.
Capacity is not so critical in an EV because it's use is very little as mentioned. But I was hoping that I would get closer to the advertised capacity, which would allow me to run some auxiliary loads straight from the 12v system for a while before draining too much.
In any case I contacted the seller and opened a dispute on Aliexpress.
The seller claims this is OK because starter batteries are "rated differently". I personally think it's B.S. he scammed me with false advertising stating its a 45Ah battery.
We will see how this dispute goes (I requested a refund of $100).
Capacity aside this little battery is actually pretty cool! It's got a nice look to it, with rubbery casing (doesn't look/feel like cheap ABS plastic) and properly sized terminals for car batteries (positive larger than negative so that you won't mix them up).
It's also so lightweight! Only 3.4kg !
I don't know if 3.4kg should have hinted to me something about the accuracy of the 45Ah claimed capacity or not. Does this sound logical in terms of Lifepo4 energy density?
I went ahead and removed the 12v SLA. Darn that battery is heavy!
It's a 60Ah SLA, just like you get in a fuel burning ICE vehicle. It's amazing how manufacturers try to reduce costs by putting off-the shelf ICE batteries in EVs even when they aren't really suitable for an EVs needs.
View attachment 74847View attachment 74848View attachment 74850
I used a suitcase style hand held scale and it came in at 16.5kg !
That's a weight saving of 13kg in favor of the Lifepo4!
It was a pretty simple swap once I found the correct bolts to open in the battery tray.
The Lifepo4 is a bit smaller than the SLA so it has a bit of room to move in the tray (sorry I didn't take pictures of it installed as I was in a hurry). Not too much though, but I will address this soon using some foam padding to get a snug fit.
I was actually surprised to see that the SLAs voltage was at 12.25v after removing (which was after a short drive, so it should have been fairly charged up).
I don't think the battery is close to end-of-life, because it's less than a year old of my ownership as the EV was bought new.
Maybe it's just a sign if how inefficient SLAs are...
I tested the Lifepo4 in the car before driving away and all was good.
I did about 150km of driving with the Lifepo4 and it seems to work perfectly!
I had the 12v SLA in the trunk (with tools) just in case something would go wrong.
But now I'm more confident with it, so a did a little drive without the SLA in the trunk.
I don't know if it's only psychological or not, but the front end of the car seems to feel lighter and a bit more responsive.
I have a battery monitor connected permanently, and the Lifepo4 was holding nicely around 13v when the car was in "auxiliary mode" (HV battery disconnected and therefore DC-DC converter off). I turned on the headlights, radio and even A/C fan and it held up well.
Sorry for this post being so long, but I wanted to document my experience and maybe it will help others too.
I hope the Lifepo4 will hold up well in all seasons, but only time will tell.