Building a balance cable for a 10-cell LiFePO battery...

[g1nko]

I bought a LiFePO battery off eBay. The cells pretty far out of whack. I measured the cells and got these readings:

4.23 4.07 4.07
4.25 4.05 4.03
4.24 4.05 3.97 3.99

I have a Thunder AC6 battery charger/balancer. It can balance up to 6s. I was hoping I could make a balance cable that would balance 5 cells at a time on the theory if it worked, I could hook 5 up to one charger and 5 up to another.

Last night I soldered up balance cable onto the 5 cells, a total of 6 cables, as shown on the left side of this diagram:

.

I put a 6-pin JSH connector on the end and hooked it up to the converter for the Thunder AC6. On the far end of the converter I measured 4v off the brown/red, orange/yellow, green/blue. I got 20v off the brown/blue. It seemed to me from that, all the connections were solid so I plugged it in to the Thunder AC6 which promptly told me I had a connection break. I couldn't find any issue with my connections and checked the voltages across all the pins and everything was consistent.

Perhaps I just don't understand setting up balancing as clearly as I thought I did. I'm going on the theory that the presence of the other five cells connected is causing the issue. Is what I'm trying to do not going to work or is splitting the cells across two chargers like this fairly commonplace? Any ideas on where to look for problems if it is?

 

[dnmun]

if it is charged to those voltages when you received them then it is not lifepo4.

 

[g1nko]

There was a thread I started in General Discussion about why the batteries seemed to be dying before they were discharged. The voltages not being LiFePo4 was mentioned in that thread, also. That discussion ran it's course so I started this one in the battery section because it related specifically to the balance cable. It bears repeating, however, since nobody every answered.

LiPo would be in soft metallic bags. I have some LiPo RC batteries for a helicopter, so I know what they look like. These are in rigid aluminum cases. I'm assuming aluminum, because they are not magnetic. They look like this:



I know the voltages are high. They don't look like LiPos I've seen, and they were marketed as LiFePo4. Is it possible to guess chemistry by appearance?

 

[agniusm]

What is the capacity and weight of one cell? That should give indication if it is lifepo4 or not.

 

[dnmun]

there is a BMS on it now. what is the charger voltage?

if the charger voltage is about 42V then i suspect it is a 10S limn2o4 cell and the 4.2V would make sense for balancing it.

 

[arkmundi]

Two things: 1> repeat that with those voltages, it is not LiFePO4 and 2> I never did get my Thunder balance charger to work with my battery pack and don't know why. Just gave up as it was becoming a sink hole of wasted time.

 

[wesnewell]

Your pack already has a balance cable on it. The 11 wire plug that plugs into the bms. Lipo comes in many shapes and chemistries. Your pack probably isn't lifepo4. It could be LCO, NMC, LMO, NCA, or something else with a nominal voltage of 3.6-3.7V. My guess would be either lco or nmc.
http://batteryuniversity.com/learn/article/types_of_lithium_ion
To answer your post, balance wiring is the same for any chemistry.
http://scriptasylum.com/rc_speed/lipo.html

 

[g1nko]

I know it already has a BMS on it. I was going on the theory that with the voltages all over the place like that, there was some issue with the BMS. I wanted to bypass the built in BMS and use a balancer that provided more control over the charge levels.

What I really wanted to know was

• is it possible to split the charging of the 10 cells into 2 groups of 5 or is the battery series connection between cell5 and cell6 going to throw that off

• what battery chemistry I have since it really doesn't seem to be LiFePo4 as advertised

Since nobody has said "no" to the first, I'm going with it's possible. Since arkmundi had the same issue I seem to be having with the Thunder, that may be the issue and I'm not going to spin my wheels on that solution any further.

A quick google search for limn2o4 batteries shows a huge number of sellers advertising batteries which have almost the same exact physical appearance as mine. Given the charger is, indeed, around 42v, I think the original seller advertised the battery incorrectly as LiFePo4 and the 4.2v readings I got are not as worrisome.

 

[dnmun]

why would you bypass the BMS if it has the 42V charger already. none of this makes any sense and now you have attracted abuncha nonsense comments so i doubt if this is gonna turn out well since you don't know fact from fiction.

 

[wesnewell]

They make up to 14S balance chargers. You have a 10s pack, so you don't need to split the pack to charge it unless you just want to. If you want to make 2 5s balance plugs and charge them separately , you can do that too. Or you can charge each cell individually. It's all up to you. Batteries are simple.