DIY Lifepo4 batteries solar fun

[technerd88]

So I'm working on two 12v packs of lifepo4 cells. They are 20p4s packs with 3300mah 26650's. Im currently testing them out with an inverter and my mini fridge and they are holding up strong! I'm working on these for my box truck conversion and they are exceeding my expectations. 

Here's my thoughts on the process. Having a strong battery bank for the build is important, especially since we will be doing it as we go. I don't want to finish it all and want to figure out what we actually need. Theres a lot of space in the truck but I need to figure out what is practical, but in order to do that, I'm going to need power. I don't have a ton of cash so I built these lifepo4 packs for 230 bucks with materials. 
I tested the fridge, the 32 in tv, my laptop(which is a dell precision)and two usb devices into the inverter and they held up for 7 hours straight before they even got to half charge. Suddenly I can now run my tools and get it all done. 

All I had to do was soldier 20 cells in parallel and then 4 of those packs in series and all of a sudden I was off to the races! The thing about these batteries, is that you can use any standard solar controller with these packs as well and they can be directly charged from the alternator as well. I plan on building 4 more packs so I have enough power for a week of bad solar/sun days.

Have any of you had experience with building your own packs? I'd like to know about your experiences if you have. Mine seems to be working amazing and wasn't too hard to build. 

I was thinking about 400 watts of solar to start and Ill be adding 400 more this summer, so I'll have all the power I need and to have a better absorbtion rate in less light. 400 watts is technically enough, but more pannels=better. I found 400 watts for 460 bucks on ebay, so I ordered em. Seems like a good deal to me

 

[Weight]

You need some type of battery management or you risk undercharging, overcharging or reversed polarity in one or more cells.
It is common practice to spot weld the cell connectors rather than solder. There are devices available to do that. There is less risk of thermal damage to the cell.

 

[jonyjoe303]

I build 2x lifepo4 batteries using the larger 32650 cells (5.5ah) 4s20p also (actually 4s10p connected together in parallel shown in picture) . I highly recommend a tab welder if your going to build more( i have a review on the review section of 12 volt tab welder I use). Tab welder (cost about 100 dollars) make job so much easier. Two tools I never use in my life were the tab welder and a hot gluegun, now that I used them they are my favorite tools for building battery packs.

The batteries have performed good for the past year, only problem was they go out of balance during charging. I had to get active balancers to fix that (also reviewed them). When these lifepo4 go out of balance they trip the BMS early and prevent the battery from getting a full charge.  


To connect to solar I been using an overvoltage relay to disconnect the solar panel when the battery reaches a certain voltage. This prevents the BMS from tripping and causing overvoltage surges which destroy any 12 volt devices connected to battery.


Building your own lifepo4 is the best way to upgrade your system. Even these cheap DIY lifepo4 will outperform the best lead acid systems. And I can't complain about the weight 100ah of usable power for less than 30 pounds of weight. I paid 160 dollars for the 80x32650 lifepo4 5.5ah cells (110ah at 12 volts), so price is not bad when compared to high quality lead acids.

 

[John61CT]

Actually, that is incredibly cheap for LFP.

If brand new and from known-good makers, could y'all post source and spec links? Too many counterfeits and fraudulent products sold out there online, especially direct from China.

I'm not interested in other chemistries, second-hand gear, or cells salvaged from laptops etc

Also, can you recommend forums specializing in the construction techniques, open hardware designs etc?

 

[technerd88]

Well I got my cells from dougsdeals and they were sold as is, but all of em came up to full capacity. Check them out, good prices too.

 

[jonyjoe303]

dougdeals also found on ebay. He don't have the larger 32650 in stock only the smaller fullriver 26650. You can buy 80 for 100 dollars that includes shipping. Those batteries have been in storage for 3 years but still come in there original boxes, some might not be up to full capacity. You can build something in the range of a goalzero (about 40 amps) with those, goal zero is 600 dollar range and only 11.1 volts.

secondlifestorage and endless-sphere are 2 websites that cater to lithium battery builders. Malectric website is the site that sells the 12 volt spot welder, it runs off a 12 volt lead acid battery. With a spot welder there is no limit how large a battery to build, it makes quick work of putting one together. With solder you have to clean the batteries ends, then scuff them up so the solder will stick, I had a hard time soldering these larger cells. With tab welder, you just place the nickel strip on the cell and press the button, go from one cell to the next. 

You join the cells together with a hotglue gun, it drys quickly. 


This is what they look like
View attachment 19505

 

[technerd88]

Half of my pack delivers 100 amps with ease. I tested it with my inverter and maxed it out, and it has only a few mv of voltage drop on the oscope. The whole pack can deliver 360 amps. There are two packs each with 20 parallel cells and 4 packs of those in a series making 14.6v, but the thing is with these batteries, is you only need to charge them to 14.4 and they are full. I didn't use a BMS, because it was shit, and they are staying in balance by themselves anyways. Ill just add a meter and mount it somewhere visible

 

[jonyjoe303]

I would stay on top of the balancing, they sell 3in1 meters for less then 10 dollars. I always monitor the balancing especially if you will not be using a BMS. 

If you do get a balancing problem the active balancers (4s model cost about 90 dollars) work excellent it keeps the battery in perfect balance even when charging at higher amps. I first notice balancing problems when charging the cells, they will always read the same voltage but as they start getting full one of the cells will start to read 3.60 volts and keep climbing while the other cells are at 3.40 volts, I notice that on both of my lifepo4's.

picture of 3in1 meter with active balancers

 

[skyl4rk]

Can you recommend a good source to learn about how to build and use lipo batteries?

 

[frater secessus]

To connect to solar I been using an overvoltage relay to disconnect the solar panel when the battery reaches a certain voltage. This prevents the BMS from tripping and causing overvoltage surges which destroy any 12 volt devices connected to battery.

Stupid question:  Is there such a thing as an electrical soft disconnect?  Like fading out over a few seconds?

 

[John61CT]

Never heard of that, but yes sudden opens can cause damage to both sources and sensitive loads.

A cheap little lead batt can act as an effective buffer.

 

[technerd88]

Stupid question:  Is there such a thing as an electrical soft disconnect?  Like fading out over a few seconds?

Yes, I will build mine with a raspberry pi and small relays to control two big contactors. One for the alternator and one for the solar controller output. it will measure the voltage and when it reaches full voltage, it will cut off the charge and will then monitor the output of the solar controller for voltage drop(load) and will re-connect the batteries when the load is high enough. its pretty easy and will be nicer to the batteries than a BMS is. Those things fail and destroy batteries. its happened in many devices to perfectly good batteries.

 

[skyl4rk]

Can you charge the batteries at temperatures below freezing?

 

[John61CT]

It varies by maker, small changes in chemistry can make a big difference, as with the yttrium doping in Winston / Thundersky "LYP"

But in general no with LFP, they won't get hurt trying to get current *out* below freezing, but charging them then can cause irreversible damage.

 

[frater secessus]

Yes, I will build mine with a raspberry pi and small relays

Following with interest.  Maybe it could integrate a temp sensor, too, to cut charging near freezing?

After I run my present 6v batteries into the ground I'll likely go LFP.  I have a couple extra Pi hanging around begging to be put to use.