Help - How best to save my 36v 20 Amp Hour LiFePO4

[sierraipa]

I left my e-bike not plugged in to a trickle charger for about a year. Now, it has no juice in it. I plugged the charger in to get it charged back up, but I think that the voltage has dropped so much that the charger isn't able to bring it back up to the proper charge. Is there a way to save this battery? I have read about RC Flyer folks using a NiMH charger to save a Lithium chemistry battery by charging for a few minutes on the NiMH charger until the voltage was high enough to allow the LiFePO4 charger to do its job.

But, they are talking milli-amp hours. This is a whole lot more voltage, so I want to make sure I don't make any mistakes. Has anyone been successful with this?

Thanks!
Kevin

[sierraipa]

Here are a few more pieces of information. The battery is showing a charge of about 15 volts.

The battery is a Li Ping battery with the following specs:

Voltage: 36 Volts
Capacity: 20 Amp Hours
Dimension: 300x105x150 mm / 11.8x4.1x5.9 inches
Weight: 7.5 kg / 16.5 lbs
Charging Voltage: 45-46 Volts
Charging Current: <5 Amps
Rated Discharging Amperage: 20 Amps
Max Continuous Discharging Amperage: 40 Amps
Maximum Discharging Current: 60 Amps
Discharging Cut-off Protection: 40-50 Amps

[Icewrench]

When left sitting around the bms on ping batteries will drain a cell bank or 2.
I ruined a ping battery by not charging it for a few months and 2 cell banks drained to a useless level.
A cell phone charger direcly connected to a low cell group might help bring that group up, but not likely to solve the problem.

[999zip999]

Where do you like ?

[dogman]

Likely you have had the bms drain at least one cell group to zero. The best fix will be to identify the ruined cells and replace them.

Does the charger even turn on if you try to use it? If so, nothing to lose by trying charging it. Though unlikely, you may as well try to bring the bad groups up to full charge with a cell phone or other 5v wall wart. Not like you can screw it up worse at this point.

Start by checking the voltage at each group with a voltmeter. Likely you will have to replace cells, so nothing wrong with opening up the top of the battery, exposing each cell groups solder.

[sierraipa]

Thanks, Guys. The charger seems to be trying to charge, but it doesn't appear to have any effect. Unfortunately, the Li Ping batteries are wrapped with duct tape. I guess I will be unwrapping everything to get to the cells and then check each one. You would think that the BMS would prevent the very exact issue that is being suggested - that one or two of the cells goes to 0 volts. I am really disappointed if that is the case as that was a pretty expensive battery!

Thanks for your input. Hopefully, I can get to the root of the problem.

[999zip999]

find the bms plug and test voltage of each cell post voltage in each cell just voltage per cell. 2.5v-3.8v ect. Voltage of pack.

[sierraipa]

So, I got up the courage to open the battery (ie. remove all the duct tape). I tested the voltage for each group of cells. It looks like the way Ping does the batteries is that there are small cells grouped together to make about 6 volt groups. And there are 6 of them (series - 36v). I am surmising this because that is what the voltage across the soldered joints were throughout the battery. So, I was surprised to see that the voltages all looked pretty good. And the positive lead and the negative lead going to the BMS had about 33 volts potential difference. So, Now I am thinking maybe it was a bad BMS?

The negative (discharging) lead out of the BMS had erratic readings for the potential difference compared to the positive lead. So, I cut the BMS out of the loop and the bike seems to be functioning okay. I am now charging directly to the battery with a 1.5 amp LiFePO4 charger. I know that the BMS is useful for balancing the cells, but is this something that I really need if I am diligent about charging properly and not discharging the battery too far. Or, is it more about getting the extra bit of performance out of your system?

Thanks!
Kevin

[dnmun]

post up a picture of your battery. is this the v1 or v2.5 pack? leaving it sitting for long periods drains all the charge from the first 3-4 cells. do not charge that pack without the BMS or you will damage the other cells too. do not discharge the pack either until it is balanced and the BMS is restored. using it now will result in total loss of the pack.

measure the voltage on each cell, list them. take pictures of the pack close up on the tabs of the first few cells, they may be all swollen up now.

[fechter]

Yes, getting individual cell measurements would be very helpful.
After charging with the BMS bypassed for a while, you might try with it again with the BMS hooked up.
The BMS is supposed to prevent any cell from going too low or too high. Without it you risk damaging cells.
A bad connection from any of the cell taps to the board could cause your problem. This is one of the most common failures. Carefully check all the wires and connectors.

Draining a lithium battery to zero very slowly does not automatically destroy them. I've had many that recoverd with proper charging. It's best to charge them at a very low rate until they reach the normal 'healty' voltage range then increase the charge current.

Running them to zero during a high rate discharge can reverse the cells and that will kill them for sure.

[dogman]

33v sounds like a pretty low number to me, even if all 12 cell groups are good. 3.65v per cell group is the target.

Keep charging till you find voltage closer to 42v fully charged.

Figure out how to get 12 voltage readings. The way the cells are soldered up, one big blob of solder connects the + tabs of one group to the - tabs of the next. As you go along reading voltages, you will be putting your - lead of the voltmeter on the same blob of solder as you just had the + lead on just seconds ago.

So figure it out, how to get 12 readings. Once you have all 12 cells charged, if you get to there, try the bms again. One set of low cells will trip the bms. At 33v, it's not charged enough to have the bms turn on discharge. So your bms may not be stacked after all.

If you identify a low cell group, like 0v, try putting a cell phone or other 5v power supply on it for awhile. You'll need it at at least 3.5v or thereabouts before the bms can take over balancing the pack.

In the process, you may have to manualy discharge a cell group before it gets to 4v. You can do that with a light bulb, like a headlight of turn signal bulb.

[sierraipa]

Dogman,

That was very helpful. I am looking at photos that I took of the battery after removing the "lid," and I can see the points where I would get 12 different voltage readings. I think that this will be a weekend project. Hopefully, I can put the BMS back into the loop and won't have too much harm done.

Thanks,
Kevin

[sierraipa]

All,

I want to thank you all for your help and to let you know the final results. That was my first foray into looking at the innards of a battery pack, so your patience is much appreciated. Well 10 out of the 12 cells were at 3.4 - 3.5 volts. There were 2 cells that were at 1.0 v though. I used a couple of phone chargers to get them into the 3.1 v region before I ran out of time (had to send the bike back with a friend that was visiting as I was giving it away to him). Well, I hooked the charger up, and it just took off like it is supposed to do. You all were correct that it was the BMS that was keeping the charger from charging and the pack from discharging since there were those 2 low cells.

The next morning, the battery was showing a full charge, and I instructed my friend to leave the charger one for several more days to ensure the BMS had time to do its balancing. All was looking good, but then we found that the charge just wouldn't hold. One of you mentioned that there might be swelling, and indeed that is the case. I have since learned that a prolonged period of discharge (especially when the battery has previously been over-charged) will result in swollen cells. When I took off the duct tape, it was quite apparent that swelling had occurred. I also learned that this is a bad, bad thing I took the logical next step and ordered a new battery.

Chalk it up to an expensive lesson learned, but I will now be far more vigilant when it comes to caring for my batteries!

Thanks again for all your help. It was a fun project to try and ressurect the battery pack, and I learned a lot in the process.

Thanks,
Kevin

[dogman]

Well, it was worth a try. Somebody might want to buy the remaining good cells, to repair thier ping they did the same thing to. Or make a 12v for lights, or something.