LiFePO4 Battery Replacement - Acura
 

I've had a 4.2 Ah hobby LiFePO4 battery and a 6s Supercapacitor setup in the Acura for quite some time, as the Pb-acid battery had become too weak. I relocated the battery into the glovebox using about 10 feet of 10 gauge wire, with the supercapacitor sitting where the original battery used to be. These are connected in parallel, with the battery keeping the caps topped up, and the caps delivering the majority of the cranking power.

In a rush to get the girls to swim class the other day, the car wouldn't start. Had to pop the hood open and use a portable jump pack to get the car started. I figured the battery was on the way out, but in hindsight I think I just hadn't fully plugged in the trickle charger (designed for use with LiFePO4).

With the inlaws in town this weekend, I decided I needed to complete my battery project, so they have reliable transportation.

The LiFePO4 32700 7Ah cells were purchased for $4 each including shipping from Aliexpress. The price was so low that I was initially skeptical and only bought a few to test ($40 tester also from Aliexpress). Every cell delivered well over the rating (about 7.4Ah), and seemed to have consistent quality.

I struggled to get good spot welds on the nickel tabs, mostly due to bad technique. Then my project sat for ages while I tried to figure out how to solder 10 gauge wire terminals. It sat for so long that one day I accidentally dropped car keys on the battery and sparks went flying everywhere. If you see burn marks on the battery, that's why.

I purchased a Pinecil portable soldering iron that goes up to 90 watts. My USB-C charger could only output 60 watts, but that was still enough to sloppily solder 10 gauge wire to the tabs.

I've got an XT90 connector to attach the battery in the glovebox of the car.

So far I'm only measuring a peak of 60 amps when cranking the car, but it requires about 290 peaks amps, meaning the supercapacitor is doing the bulk of the cranking. Perhaps I'm just not getting an accurate read. 60 seems low to me. I'll throw my voltmeter on there to see how low it sags when starting.

https://ecomodder.com/forum/attachme...1&d=1692400387

https://ecomodder.com/forum/attachme...1&d=1692400387


https://ecomodder.com/forum/attachme...1&d=1692400567

https://ecomodder.com/forum/attachme...1&d=1692410672

https://ecomodder.com/forum/attachme...1&d=1692410672

The balance circuit was $10, also from Aliexpress. These are all pre-inflation prices.

I bought enough to make 2 more batteries, so I'll hone my skills and make a replacement motorcycle battery. The neat thing is I can use the LiFePO4 trickle charger on these while the balance circuit takes care of keeping the cells even.

UPDATE:

I measured 174 cranking amps from the supercapacitors, and 60 from the battery. Battery voltage sagged to 9.64v while cranking.

That distribution of power flow seems about ideal to me. The length of 10 gauge wire must present enough resistance to limit amps from the battery.

I need to replace the 300 amp breaker with a 100 amp one. I'm not even sure a dead short would trip the 300 amp breaker.

Here's the hobby battery I replaced
https://ecomodder.com/forum/attachme...3&d=1602362265

Here's the 350 farad supercaps in a foam box connected where the battery used to live
https://ecomodder.com/forum/attachme...4&d=1602362272

Damn, 7Ah is so low lol, I would've gone for double that minimum. Glovebox is a clever spot though.

I have seen a few YouTube videos of this. Did you notice any improvements to fuel efficiency? Does it start quicker than with a lead acid?

I've been gathering up all my lead acid cores to sell to the local interstate dealer.

Considering something like this but between lacky soldering skills and diesel powerplant, I think I'm going to at least wait out the life of this battery.

Lastly,
Is this comparable to your super cap?
2.85V 700F Super Capacitor 6PCS/1Set, 17V 116F Double Row Farad Capacitor Automotive Super Farad Capacitor with Protective Board https://a.co/d/8eY1clh

How deep of a discharge would be acceptable on a standard lead acid battery? My thinking is you probably wouldn't want to use more than about 30% of the capacity or you start to prematurely wear it out.

30% of a 40 amp hour battery is 12 Ah, which is still more than the 7 Ah I've got in there, but it's not too far off. Double the capacity probably would have been ideal.

Still much better than that dinky 4.2 Ah battery. The car could only sit maybe 6 days before it drained down enough that it wouldn't start. The parasitic drain takes about 0.5 Ah per day.

Car starts about as quickly as with a good battery. The supercaps you linked are twice the capacity as mine, so they would probably start the car faster. One important spec not mentioned there is internal resistance, because that can vary quite a lot and determines if they are capable of supplying the hundreds of amps needed to start a motor.

I don't notice any improvement in efficiency, but it has to be a little more efficient since a lead acid battery wastes about half the energy when it's nearly fully charged, which is what it normally is.

Supercaps are probably the wrong solution for most diesels if it has glow plugs or a grid heater. Those heaters will draw the voltage down before the engine is ready to start.

My diesel does have glow plugs but I live in south Florida so I could most likely delete bypass disable the goalies glow plugs without too much penalty

In my Insight, I was using a very similar setup - a supercap bank and a small LiFePO4 pack.

I now live somewhere that never experiences below freezing temperatures, and have switched to just a LiFePO4 battery. The pack is "PLFP-30L", and I estimate it to be ~7.5Ah. It uses pouches rather than cylindrical cells. It weighs ~2.3kg, or 5.0lbs, and is rated at 440CCA. Putting it on a tester, it measured in around 600CCA. It cranks my MX-5 noticeably faster than the stock Group51 lead acid battery. The reseller offers a 12 month guarantee, even in applications such as starting V8 engines in a marine environment.

I imagine a supercap bank would prolong the life of the battery, but currently I see no need to overcomplicate it if it's working well. Have you done any testing with just a lithium pack?

Ocean/coastal application is ideal as it rarely freezes in the lower 48.

I have to relocate the battery inside since it freezes here. That means I have to find space for it, which the glove box lends itself to. If I want to start off an internally located battery, it has to be large enough to supply the ~300 amps, and I'd have to pass the enormous cables through the firewall. My setup seems less complicated. I'm happy with it so far.

I did try a lithium ion battery as a virtual alternator delete, but I need thicker gauge wire to do that properly, if it can work long term at all.

I got this monster in the mail yesterday.

I may duplicate your setup with the hobby battery. I was initially thinking add this to a deep cycle battery so I can run my inverter for extended periods and still start the engine.

My thought was that the caps would keep the deep cycle from being damaged from the instantaneous drain of starter current.

My car is manual transmission and I usually roll start it.