Faulty Lifepo4 pack..can the gurus help?

[benjy_a]

Managed to get to the plug.

 

[benjy_a]

Fully charged, measuring between the pins on the plug I get the following voltages;

1) 3.60
2) 3.62
3) 3.63
4) 3.61
5) 3.61
6) 3.61
7) 3.60
8) 3.62
9) 3.61
10) 3.62
11) 3.60
12) 3.61
13) 3.61
14) 3.62
15) 3.55
16) 3.61

It's quite clear that battery bank 15 is at a slighly lower voltage than the others. I will have to run the pack down over the next couple of days and measure again.

 

[D8ve]

Could the Bristol thing be me D8ve in Bristol but D8veh in Telford

 

[banbury frank]

Hi Just because they show voltage dont mean to say they are fully charged this is where you need to charge each cell pack individual you buy one off the chargers i listed the when the charge light changes to green then they are charged

Frank

 

[benjy_a]

Hi Just because they show voltage dont mean to say they are fully charged this is where you need to charge each cell pack individual you buy one off the chargers i listed the when the charge light changes to green then they are charged

Frank

Thanks for your help frank, problem is that the pack will need to be completely dismantled to do that.

When the battery is cutting out it's the BMS shutting off the output so it must be seeing low voltage on one or more of the packs. This should be obvious shouldn't it when the pack is discharged and the LVC activates?

If I was to take all the wrapping off the cells could I use that charger while still leaving all the packs connected together? Does it matter that they're still all connected in series if I'm attaching the charger to a parallel group of 6 cells?

Thanks.

 

[benjy_a]

Could the Bristol thing be me D8ve in Bristol but D8veh in Telford

Ah D8ve that explains my confusion!

 

[cwah]

You can already start discharging your pack and see how the voltage drop.

So you'll see if the problem is because of your unbalanced cell or not

 

[Deleted member 4366]

There's nothing worrying in those results for LiFePO4. You get a bit more variation in them because the bulk of the charge is between 3.0 and 3.3v; however, after discharge, it might be a different story. The results will speak for themselves.

 

[benjy_a]

Ok, results are in from a flat battery.

All pins on the BMS plug are reading 3.25v apart from one which is reading 2.80v.

The low reading pack is the one that was reading slightly lower at full charge as well.

 

[cwah]

That's when you need a single cell charger.

You need to charge all your cells to max voltage 3.6V and do the test again, so you'll know if it's an unbalanced cell, or a weak cell.

If you're feeling good enough, you can do a single cell charger with any cell phone/usb charger. Just plug the + and - together and track carefully the voltage to not overcharge.

 

[benjy_a]

Ok this is more than just an unbalanced pack. I just took the battery apart and identified the balance cables for the pack reading low. One of the wires looks like it has burned out against a cell.

 

[benjy_a]

I'm guessing that a bump in the road has caused the wire to be crushed against the corner of a cell and shorted it out. Further guesswork is that this has put the one pack out of balance to such a point that the BMS cannot rectify it.

So, I understand now how to re-balance the pack; I have ordered a proper single Lifepo4 charger as listed by Frank (thank you).

My concern is that either the cell or the BMS will have been damaged by that short circuit; any thoughts? The burnt wire is still conducting ok which makes me think the BMS is still ok and hopeful that the actual result of the short circuit is simply an unbalanced pack (everything crossed).

 

[banbury frank]

HI just a word for others Reading this the charger i recommended is for 3.2 volt LifepO4 battery which charge with a 3.6 volt charger It will only part charge Li-ion batteries as the cels are 3.7 volts and need a charger 4.2 volts

Frank

 

[Deleted member 4366]

There's very little difference in charge between 3.55v and 3.61v. Even with a single cell charger, that cell would climb very quickly to 3.61v, so I don't believe it's a problem of balance, but instead I think there's something wrong with that cell. Ideally you need to follow the balance wire to find out which one it is. Maybe you can see something visually, like squished or leaking, so do that first. Another thing to do is charge up the pack, measure the cell voltages, leave it for a day and measure again to see if it's self-discharging, which it probably is if it's faulty.

 

[benjy_a]

There's very little difference in charge between 3.55v and 3.61v. Even with a single cell charger, that cell would climb very quickly to 3.61v, so I don't believe it's a problem of balance, but instead I think there's something wrong with that cell. Ideally you need to follow the balance wire to find out which one it is. Maybe you can see something visually, like squished or leaking, so do that first. Another thing to do is charge up the pack, measure the cell voltages, leave it for a day and measure again to see if it's self-discharging, which it probably is if it's faulty.

OK I have further investigated and identified the 6 cells which are causing the problem. You are right; there appears to be physical damage to two of the cells wheret the sheer weight of the pack has squished the edge of them.

Can I simply cut out the two damaged cells and solder in a couple of replacement new ones? The only other alternative I guess is to remove all 6 and go to 15S 6P.

 

[benjy_a]

 

[cwah]

Buy few spare cells and solder them to your pack.

Removing a string of cell will make your bms incompatible with your pack

 

[benjy_a]

Buy few spare cells and solder them to your pack.

Removing a string of cell will make your bms incompatible with your pack

I know that there is a way that you can wire the BMS so that it can be used with less batteries.....however I would rather repair the pack that's for sure.

I have found some cells on eBay but they are listed as ANR26650M1B whereas mine are ANR26650M1A.

2x A123 Systems ANR26650M1B / 26650 mit U-Lötfahne - Neue Version! | eBay

I'm hoping that the only difference is that these have tabs (which I need to solder them in anyway).

 

[Deleted member 4366]

As a first step, I'd charge up the pack, and keep measuring that string to see whether/how much it self-discharges. It might be Ok and just needs re-insulating.

If it does self discharge below 3.3v, you would ideally need to pull out all the cells in the string and test them individually, but, if it shorted, it would pull the whole string down, so it's likely that they're all damaged, or none of them. It could be that the intermittent shorting only caused it to go out of balance, and no permanent damage is done.

 

[benjy_a]

D8veh thanks for your help...I think I've confused the thread a little so I will attempt to clarify.

Upon examining the pack I have found two separate isuues. The first was the shorted wire from the BMS. The cell that is shorted against is NOT in the pack of 6 causing the issues. That cell and pack appear to be fine, I have repaired the wire and re-insulated the cell.

By complete coincidence, the burnt wire from the BMS is going to another group of 6 cells, two of which are squashed on the edge. It is this group of 6 cells that is reading lower than all the others both at full charge and when the pack is flat.

So, I believe that this point that my problems are being caused by the two squashed cells which is why I was looking to replace them. Would you think this is the right way to go?