LiFePO4 Battery Pack Question

[dilkes]

I purchased a 10ah "48V" LifePO4 pack and BMS (duct tape packaging) last fall from a seller on eBay called lifepo4-power. Of course he's not there anymore. Being in Toronto and suffering thru an interminable winter, I am finally getting a chance to really use/test it.

I consistently find I get 7Ah out of it or about 300 Watt-hours before reaching LVC. Also I find the battery pack (not the BMS) gets warm when used continuously. I see this 7Ah number both from loading it thru an inverter and also measuring while riding using a wattmeter.
My questions:

• Is this what I should normally expect (like 70% of advertised value)?
• If not, any suggestions on what I can do to extract more Ah from it?
• Contacting the vendor is out of the question as he's gone!
• The good thing is I didn't pay much - you get what you pay for??

Any help appreciated.

 

[dnmun]

can you open it up and take pictures so we know what you have?

 

[mateusleo]

Usually the BMS cuts off the power when the battery hits certain voltage to avoid cells damage, this might result in getting lower capacity than nominal...

 

[DAND214]

Are you taking the charger right after it says done? Or reading the watt meter and removing the charger? If so, you need to leave it on for a couple days to see if it comes up. If it sat without being charged once a month with the bms connected you might have it way out if balance.

try charging it a full day and see if it goes up. If not you will need to read the cell voltages to see which one or more are low. You can read the cells thru the bms plug but be careful, you don't want to short a cell it might burn the sense wire up.

Measure each cell and write it down. Then Dnmun will tell you how to correct the problem. He has done it for so many members. If you have questions ask exactly what to do.

Dan

 

[biohazardman]

The rest of your AH is probly the energy you are wasting in heat. A 10AH cheap lifepo4 pack might not be able to supply the amperage needed to power your bike. Possibly you need a 15 or 20AH to get the job without heating your batts excessively. Hard to say unless you supply more info.

 

[dogman dan]

The red flag waving here is the pack getting all hot an bothered. Some warming is normal, but the pack should not be heating itself much past your body temp. If it feels hot, it's not below 110F or so.

This is a sure sign that cells in the pack are getting discharged too fast for their design and or quality of manufacture. Other types may heat up more, and or tolerate more heat. But inexpensive lifepo4 will self destruct if you are discharging it fast enough to make it hot.

15 amps continuous would be the max you should be doing with that size.

At this point, you may also have had some damage to a cell in there. This makes that section of the pack actually only 7 amp hours. That means your max continuous discharge just got even less. 10 amps might be the max now.

 

[dak664]

You measure "about 300" watt-hours but 7 Ah x 48 V is 336 Wh. That difference could be the heat if both numbers are accurate (how did you measure amp-hours?).

Assuming 14 amp discharge over 30 minutes, 36 Wh would be dissipated over 360 milliohms. That could be internal resistance of bad cells but it might simply be a single bad connection. I'd open the pack and look for a point source of heat. Put voltmeter probes directly on the end caps of each cell and measure the voltage sag when you apply a 10 amp load. If those are less than 100 mV (corresponding to internal resistance of 10 milliohm) then look for the high-resistance connection.

 

[dilkes]

Thanks for all the tips. I will:

• leave it on the charger for a few days to see if that changes anything.
• If not, will probe into the individual cells either by disassembling or fussing thru the BMS connections

In one of the earlier tests I did by using an inverter and connecting a load that way, the draw was 15amps continuous and the pack did get warm - not hot, but warm.

 

[dilkes]

You measure "about 300" watt-hours but 7 Ah x 48 V is 336 Wh. That difference could be the heat if both numbers are accurate (how did you measure amp-hours?).

I measure this stuff with a wattmeter inline between the battery and controller. I think the watt-hours it calcs use some algorithm where it calculates watts by the product of (amps) x (whatever the current voltage it is seeing after any sag due to the load). Most of the time this will be less than 48V. Although maybe it is the heat.

 

[wesnewell]

Most ah ratings (and cycle life) for lifepo4 battery packs are based on a <1C load, Charge the battery fully, allowing time for it to balance, which could take days if badly out of balance, then discharge it at .5C (5A) through your watt meter and see what you get.

 

[DAND214]

Did it ever have the 10ah rated?

Most ebay packs are 20 to 30% less than rated.

Dan

 

[999zip999]

What is your motor controller setup ? amp. draw of your controller ? Hills or gunning it from stop signs ? Peddling from a stop then throttle helps a lot. But a lot a ebay 10ah batteries are to be used on a 10 amp controller.

 

[dilkes]

try charging it a full day and see if it goes up. If not you will need to read the cell voltages to see which one or more are low. You can read the cells thru the bms plug but be careful, you don't want to short a cell it might burn the sense wire up.

Measure each cell and write it down. Then Dnmun will tell you how to correct the problem. He has done it for so many members. If you have questions ask exactly what to do.

Dan

I finally got to taking the covers of the BMS and checking cell voltages. The issue I was trying to address was that I was getting 7.5Ah out of an alleged 10Ah LiFePO4 16S battery pack. In any case, after full charge, I measured the cell voltages, then did a full discharge and measured them again. I'm guessing this shows out of balance (or is this normal?). Charged cell voltages range from 3.34 to 3.58 but not sure how to fix it. I have already tried leaving on the charger for a week or so. Here's the data and any suggestions welcome:
Charged/Aft Discharge(no load)
0 3.38/ 3.03
1 3.48/ 3.04
2 3.56/ 3.14
3 3.57/ 3.07
4 3.53/ 3.16
5 3.39/ 3.10
6 3.34/ 3.04
7 3.56/ 3.14
8 3.36/ 2.92
9 3.34/ 2.91
10 3.50/ 3.16
11 3.58/ 3.06
12 3.58/ 3.18
13 3.37/ 3.06
14 3.53/ 3.07
15 3.57/ 3.17

 

[e-beach]

Nice job of checking and posing your battery voltages. Yes, your pack is out of balance. LiFePO4 cells should charge up to 3.65 volts per cell/s.

What is the voltage output of your charger? Is it 54.75 volts or higher?

 

[dilkes]

When I measure output voltage on that charger I used it reads 56.2V. I do have some other chargers and one can output 59.4V. It's a 6 amp charger and I was a little reluctant to charge that 10Ah pack at that rate, but maybe I'll give it a shot? I also have a 3 amp charger that can kick out 58.1V.

 

[e-beach]

When I measure output voltage on that charger I used it reads 56.2V. I do have some other chargers and one can output 59.4V. It's a 6 amp charger and I was a little reluctant to charge that 10Ah pack at that rate, but maybe I'll give it a shot? I also have a 3 amp charger that can kick out 58.1V.

Your 56.2v charger is probably for a 15s 48v pack. You have a 16s 48v pack. Use your 3 amp 58.1v charger and leave it on for at least 12 hours then check your voltages and post them again.

 

[dilkes]

I charged the 10Ah pack leaving the charger on for 24hrs with the 6A Charger that puts out 59.8V. Shortly after taking it off the charger, I checked voltage for each cell. It looks a lot better and more balanced (see below). I then did a load test drawing 6 amps and my wattmeter told me I got 7.836Ah before LVC shut me down...not 10Ah but getting better.
I'm not sure it's worth spending any more time on it. However any thoughts/suggestions are welcome.

Cell
00 3.58
01 3.59
02 3.58
03 3.59
04 3.59
05 3.59
06 3.57
07 3.59
08 3.59
09 3.58
10 3.59
11 3.59
12 3.59
13 3.58
14 3.57
15 3.59

 

[e-beach]

Looks like you are in good shape with the 6A Charger that puts out 59.8V. And the fact that you get almost 80% of your amps from the pack seems about normal also. Using all the amps in your pack would kill the cells anyway so your controller shuts it down before you deplete the pack fully.

Out of curiosity, did you measure the cell voltages at lvc?
Also, is anything getting hot when you use your 6amp charger?

 

[dilkes]

Out of curiosity, did you measure the cell voltages at lvc?
Also, is anything getting hot when you use your 6amp charger?

I got lazy and did not measure the voltages at LVC. I guess that would show if it's also discharging in a balanced fashion?

Nothing got hot during the 6amp charging. That charger was in fact the one the eBay seller sent me with the pack, but I haven't been using it as I thought perhaps it was too much. Maybe it's OK.

Thanks again for the guidance.

 

[wesnewell]

You should have checked the cell voltages after LVC shut it down. That's what will tell you if you have weak cell or weak cell groups. It only takes one cell group hiting lvc to cut it off. There might have been another 2-3 ah in the others.

 

[alsmith]

I learned about to avoid these kind of batteries by going the same route that you are now on.

My 20Ah duct tape pack was only 12Ah so you're looking better than that. And that was also the same after stripping the pack and repairing bad joints etc.
Check Amberwolfs repair threads for what's involved.

In contrast my 15Ah (headway cell) packs from BMSBattery are both still giving 15Ah after a couple of hundred of charge/discharge cycles.