Ideas for using a 24V 40Ah LiFePO4 battery with damaged cell to power a 12V 500W inverter?

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I acquired the battery in subject from one of the local recycle groups. It consists of nominally sixteen 3.2V 20Ah LiFePO4 pouch cells arranged in 8s2p configuration (pouches in pairs soldered to a PCB that puts them in series). One of the pairs developed a fault and was desoldered and removed by the previous owner. So my pack only has fourteen cells.

I'm brainstorming ways to use this to power my 12V inverter during power cuts. I have a charsoon antimatter hobby charger (that can balance charge up to 6 lithium cells, but will charge anything up to 30V), a 3D printer and a 30W soldering iron.

So far I've come up with the following ideas:
A. Solder a positive wire after the 4 top cells and use that half of the battery as a 12V 40Ah pack. Disadvantage is all the dead weight from the unused cells.

B. As above, but split the PCB to eliminate the bottom half of the cell. Disadvantages are that it's hard to cut a PCB at home (I was thinking of scribing and snapping so as not to make any dust), and the risk of mishaps in the process.

C. Remove all cells from PCB and re-make as a 4s3p pack (12V60Ah). This leaves only two unused cells. Disadvantages are that my low-power soldering equipment may not be up to the job, might overheat and destroy the cells, and I have no idea how to assemble the foil tabs into a pack (parts/tools/technique)

D. Put a shunt across the damaged cell position so the remaining cells becom a 7s pack, reconfigure the BMS wires for 7s, use the battery to drive a 500W 24V to 12V converter (they accept input voltages of 18-30V so 7s will still work). Advantages are that it makes the best use of the cells left in the pack. Disadvantages are that it requires a €25 voltage converter and a separate voltmeter to estimate battery SOC (since inverter only sees regulated 12V output), and the converter is only 90-95% efficient so we're looking at 50W of waste heat at 500W. Plus I don't know whether a DC-DC converter will drive an inverter input?

E. Use the top 4 cells directly as a 12V pack. Use the bottom 3 cells through a DC-DC boost converter to reach the inverter's minimum 10.5V input. Advantages are efficiency for the top cells while still using the whole battery. Disadvantages are having to switch inputs when the top cells are depleted.

Any feedback on the above ideas, or any new ideas will be most welcome :)
 

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andrewmm

Joined Feb 25, 2011
574
how experienced are you at charging / discharging cells , and the protection circuits required ?

I raise this as these cells can explode if treated wrong, with no or little warning and spew liquid metal across the room,
at the least it burns down the house, at the worst, it kills you.

They are a LOT safer than Lion ones, but still have a lot of stored energy,


1599306783933.png
 

BobTPH

Joined Jun 5, 2013
2,543
@andrewmm

The LiFePO4 batteries are the safest type of Lithium batteries as they will not overheat, and even if punctured they will not catch on fire. The cathode material in LiFePO4 batteries is not hazardous, and so poses no negative health hazards or environmental hazards.
From here: https://www.batterystuff.com/kb/articles/battery-articles/lithium-battery-overview.html#:~:text=The LiFePO4 batteries are the,health hazards or environmental hazards.&text=LiFePO4 life expectancy is approximately 5-7 years.

Bob
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I've operated li-po batteries in R/C cars and helicopters for years so I'm familiar with the care and feeding of lithium batteries . Never LiFePO4 though, I've had NiCd, NiMH, flooded Pb, AGM Pb, Li-Po and Li-ion, this is my first foray into the chemistry. So far seems very similar to Li-ion, just at lower voltages.

So, quick update - I soldered a shunt across the terminals of the removed cell.

I also reconfigured the BMS plug by removing the wire for the dead cell and shifting the wires on the BMS side connector one position towards the negative so the BMS now "sees" a 7s battery. I noticed empty solder pads near the positive side of the BMS plug (where the SMD components would go if the BMS was to handle 2 more cells) so I assumed "optional" cells go on that side. Hence I shifted the cell wires so as to use the first 7 cell positions starting from the negative side, leaving the empty slot on the positive side. Tested with a multimeter and the voltages are consistent with a 7s pack at BMS side. Pics attached.

So, presently I'm using the BMS that came with the battery. From my testing I found that it discharges at 0.06A when cell voltage exceeds 3.7V (tested with a DC/DC converter, not an actual cell of course).

The output goes open circuit if I pull the BMS plug while it's charging, so it seems under-voltage protection is operational too despite the missing 8th cell. No idea what the over-current protection limit is but so far it took 20A just fine.

I'm seriously considering the last two options on my list (especially the one with 24V -> 12V converter). Any thoughts?
 

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andrewmm

Joined Feb 25, 2011
574
@LMF5000

that's great , sounds like you know what your doing,
I have a worry with the forums, that lead to the post.

The worry is the experience I have seen over the decades on many forums, of people seeing something, and trying it out without your level of knowledge,

So i just wanted to put a warning into the thread,

@BobTPH
Thank you for highlighting what I had said "They are a LOT safer than Lion ones "
BUT , they are NOT INTRINSICALY SAFE, miss treat them and your in trouble.
 
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