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First Lithium build. Could use some advice

896 Views 4 Replies 2 Participants Last post by  floydr, 
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I was recently given three Renogy SMART LiFePO4 12v 100Ah batteries that would no longer take a charge. After opening them up, I found moisture inside and corrosion on the BMS boards. The batteries were in a portable light tower and I assume we're exposed to rain. Upon testing I determined that the Lithium pouches were all still in great shape so I decided to proceed with building a 36v battery out of the pouch cells. According to Renogy, the batteries in their factory configuration cannot be run in series, so they were limited to 12v applications; I imagine that's simply a function of the BMS.

This is my first Lithium build, so I was hoping for a little guidance from people who have more experience. Ultimately I want to use the finished product to power my 36v electric golf cart. Here's what I am thinking right now:

Currently the pouches are arranged in 4 groups of 4 pouches with a 4s BMS managing the cell groups. They're already tied together well with a PCB board with balancing leads on each parallel group and 4 heavy Guage wires on the terminal leads. My current thought on building this battery is to get a 12s 36v BMS unit and connect the existing pouch blocks in series, each group of 4 pouches will have a balancing lead. Here's where I am in need of some direction: the cart has a 500a controller, doubtful it ever comes close to delivering that amperage, but all the BMS units in my price range are certainly much lower amperage ratings than 500a. My current line of thought is to get a Bluetooth BMS unit at say 100a and use the BMS primarily for charging, monitoring, and balancing, and connect my controller leads directly to the battery outputs either directly or through a continuous-duty high-amperage contactor or low-voltage cutoff device. This should bypass the high-amperage limit of the BMS, yet still be able to protect the cells from over-discharge.

I have top-charged the cells and they all charged up to within .01 volts of one another. The pouches are all already nicely arranged and are wired together through their printed circuit boards; they also have well-made metal casings for each group of cells. My main concern is proper wiring for the application. Is my current plan the right way to build this 36v battery?

As a side note, I have posed this question in some other groups (mostly solar based forums) and the only responses I've gotten thus far are along the lines of "Why are you only building 36v?" To get that out of the way, I already have a 36v series cart, I have a 36v charger capable of charging LiFePO4 batteries, and I have cells to build a 36v 100Ah battery; basic economics, I don't want to spend a fortune on new motor, charger, and motor controller, and this setup would allow me to get my feet wet without breaking the bank.

Any insight you can give me is most appreciated! I am looking forward to gaining a new skillset and gleaning knowledge from those of you who've gone before me.

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if the cells are all at the same voltage 2,5 or above it may just be as simole as connecting bms to the cells. let me wake up. there should be 4 groups of cells in each 12v battery. I would go with a smart bms Ant or JBD 12S minimum Ant Bms's usually cover a range 10-20s, 8-24s etc.
The cells are probably all ready in 4 groups of cells in each battery, if possible check each individual cell capacity. Not just voltage. Charge up each 4s to 14.6 after sitting for a few hours you may find the voltage most likely be around 13.36v-13.8v, 3.34v-3.45v per cell this is normal. Connect all three 4s together in parallel charge to 14.6v. Let sit untouched except for voltage checks for at least two days check voltages of individual cell groups and 4s sections. connect each 4ssection in series with the next section. Add The Bms following the instructions included with the BMS.
Charge your new 12s battery to 40.8v-42v. 90% of the LiFePO4 battery Capacity is between 40.8v-36v. (3.4v-3v) Your resting voltage of your battery may be higher than my 12s LiFePO4 battery.
later floyd
ps check my Calculations as I haven't had my first cup of coffee yet.
 
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Discussion Starter · #4 ·
Thank you. Yes there are 4 groups of 4 cells in each battery. I did charge them all then wired them parallel to balance them. They have been sitting now for 24 hours and are all right at 13.8v with almost no deviation between individual cell voltages.

I did order a 12s JBD today, supposed to be here Friday, so I will have a little time to do more cell testing.
 
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