Testing a DIY LiFePo4 mini battery
 

I've been wanting to build one of these forever, and recently some cells became available on the ebay market that really motivated me to try.

https://www.ebay.com/itm/173071152948

Unfortunately these sold really quickly but i'll keep my eyes open if more become available and ill update this thread.

This isn't really meant to be a how-to more of just I'm trying to do this and maybe it will fail maybe it won't. I wouldn't want to give out instructions on how to do something unless I knew it wasn't going to end in a fire....literally. That being said the LiFePo4 flavor of Lithium battery is to my understanding the safest, most stable form. These cells in particular are beasts. They can actually supply 200 amps continuous, that's not just some marketing wank. They also are rated at 8amp hours and that seems to be more of an under rating. So they make a viable replacement for a car battery especially in the 2p configuration I have them in, which is good for 16-18 amp hours. Right now im just in the viability stage of this project. As in will this thing do what I need it to do, to be worth continuing. So the setup I have them in is somewhat crude, but temporary. So far they have worked very well. They started the car no problem, then again, and again, and again, all one right after another with a cold engine. Then I let it warm up and idle, monitored voltages and temperatures, and everything looked really good. I've driven it for around 100 miles in all the conditions of a city dweller. There is no BMS on these cells, so i have been very closely monitoring the pack voltage, as well as the voltages of the individual cells to make sure they aren't drifting. As I advance with this ill be adding some kind of Battery voltage monitoring, some kind of an enclosure /w a better mounting solution. Right now im letting the car just sit and i'm tracking the voltage on the pack, so i can determine how long it can sit before getting over discharge territory. Updates to come, questions/feedback welcome.


https://cimg7.ibsrv.net/gimg/www.evo...ccd35825f9.jpg


P.S. I don't have a weight, its very light compared to lead acid. It's going to have more stuff added to it though, if it lasts, so I'm waiting till then to get a final weight. According to the ebay listing they weigh 380 grams each and there are 8 cells (6.7lbs) to get you in the ballpark.

Battery holders here:

http://www.batteryspace.com/batteryh...38120cell.aspx

Copper bar for bus here:

https://www.mcmaster.com/#catalog/124/3872/=1bec9s7

Once I know this thing works well and is safe I'll do a how to.

So I let the car sit for a little over 96 hours, unfortunately i left my openport 2.0 switched on so it was drawing more current then it normally would have, but it fired right up no problem. After my commute though I noticed the cells had drifted a little bit. not by much, and nothing alarming, so Im going to try and rebalance them at the top and see where they land after some more driving.

I've been using a Ballistic Evo2 12 cell LiFePO4 battery for over a year now, however my car is not a daily. It is hooked up to a charger when the car is not in use. So far no issues at all. Just curious how much did your battery setup cost? These types of batteries are a little on the spendy side.

I'll check out that battery and see how it compares. I really want to be able to daily this thing, but i also want it to be able to sit for a week or more if i need it to and start up fine, so fingers crossed. I would estimate I'm into this battery for ~ $90 dollars. I bought enough supplies / cells to do a second one for my Evo X if this one works out well. If i could do it over again i would get thinner copper, i went with 1/4" which is overkill and I could have gone thinner and saved some money.

Not bad at all. If you are looking for a LiFePO4 charger that supposedly conditions the cells try the Optimate Lithium 4s. So far it's been working for me. We'll see though.

are you really only $90 bucks into it? Can you post a link to the where you bought the cells?

I used have a 4S diy battery with 20Ah A123 prismatic pouches on my Lexus IS-F and work great for over a year until I have to get some collision repair and the idiots probably left a door open for a while and the pouches expanded.

The car work great and I never had any issues even after living and inside light on over night. Leaving the car park over a week was fine and the car was my daily driver. I checked the balance of the cells periodically but it was always perfect even after a year. It seems to that there is not really a need for a BMS or even balance charging if you drive the car constantly.

I was looking at this ones:

https://www.ebay.com/itm/TENERGY-LiF...EAAOSwgmJX1INY
http://www.batteryspace.com/lifepo43...iABEgIjQPD_BwE

Yah that cost is accurate. I got the cells off ebay and I linked the listing in my first post. Unfortunately the seller sold out. I follow him because he is a battery recycler and he gets some really good stuff in and sells it very reasonably. I expect he will be getting more of these in the future. Thats usually what happens anyway. They are headway like what you linked except a higher current lower capacity version 8amp hours and 200amp continuous. I got them for $33 dollars per 4 and im using 8 per battery so thats $66 for the cells $20 for the copper bus bar and 8 bucks for the battery holders. I did buy some misc. tools for cutting and machining the bus bars, but nothing expensive. Just a new saw blade for my hack saw and some new drill bits. I do plan on spending some more money on a nice mounting solution once i know they are gonna work out. So far so good though, they are doing great. That sucks about your old battery. I too have had many bad experiences with body shops, you gotta babysit the hell out of them or **** goes off the rails fast. I literally had an optima red top just disappear out of one of my cars and get replaced with some random battery from the wrong car while one of my older cars was in the body shop for some fender work.

So far i have been using a combination of buck and boost converters, along with a modified pc power supply to handle my charging / balancing, but i will probably need to get something more convenient so thanks for the tip.

Found this they seem really reasonable:

http://www.evassemble.com/index.php?...roducts_id=175

what do you think?

Those look really good, and with a capacity of 15AH you could probably get away with a single string (4S1P). I'm running 4S2P to get approx 16AH.

Yeah that's what I am thinking

There are more cells available if anyone is looking:

https://www.ebay.com/itm/4-HEADWAY-3...4AAOSwUYNaS8aC

just a quick update, everything is still working great. I have probably done at least 100 starts so far, many below 40F (about as cold as it got where I am). Somewhere around 2500 miles driven. I have some ideas I want to try out for a management system. The parts are on order. I'll post more details on that soon.

Side note, I had a cell die on my bench while i was running a capacity test. It died open circuit, which is really good. It means, at least in this instance, that it didn't fail to a short and theoretically had this happened in a pack it wouldn't have started a fire. I need to replace that cell now though so I'm gonna try out one of those china retailers and see how that goes. The shipping is horrible, but i really don't see any better alternatives right now.

This is a rough sketch of what I have in mind. If anyone notices any flaws please let me know. Otherwise I'm going to build this up and test it out. Its only a charging circuit meant to prevent in-balance and overcharging. While I have ideas for undervoltage protection that's not part of this yet. https://cimg8.ibsrv.net/gimg/www.evo...77c1b17062.jpg