Battery Statistics Check

[awol]

My 36v 10aH li-ion bottle battery has now had aprox 150 charges. I decided to fit the watt meter and see how far I could now get non stop for the 10aH. However after 5aH used I noticed quite a drop in power. I eventually got to 28 miles when the bms shut it off.
The last watt meter readings (from memory) were aprox 6aH, 200wH, 32mV .
Do these figures sound reasonable considering it's (only?) done 150 charges?
How many charges are they generally capable of before loosing capacity?
It's generally not a problem as I can charge every day at 20miles anyway.

 

[Deleted member 4366]

When yoy charge it, what's the voltage? If it's less than 41.3v, it could be out of balance.

 

[jazper53]

Quick question. My battery which is a LiFePO4 9amp after charging I always check the voltage, and it never reads over 40v, but does not change much after 10-15 mile use. The battery is about two years old and have not noticed much of a depreciation except one of the four power lights tends to go out on slight inclines when on full power setting. Is this normal having a lower voltage with LiFePO4 type batteries?

 

[benjy_a]

Lifepo4 cells are lower voltage (3.2v) so packs tend to have more in series to get somewhere near the same voltage as other packs, how many cells in series is your pack? My "48v" lifepo4 is 57v hot off the charger. Settles to 52v but is a 16S pack.

 

[jazper53]

Sorry 36v, not sure how many cells. But I think you have answered my question thx

 

[Deleted member 4366]

Disregard what I said in my last post. I didn't realise that it was LiFePO4. A LiFePO4 charger should charge to around 43.8v. Often, after you disconnect the charger, the battery will gradually fall to anything as low as 40v. To check a LiFePO4 battery, you have to open it and measure the cell voltages.

 

[awol]

No mine is 36v li-ion .
Volts measure 41.95 just after a full charge. I don't know how many aH capacity the battery originally actually held. Do I assume that the 6aH is all that's left in it? or could some surgery identify an internal issue which sorted could get me up to 10aH

 

[Deleted member 4366]

Charging to 41.95v is a good sign. now you have to run it until it cuts off and check the voltage again. Check it immediately after it cuts out and then again later to see if it bounces back.

 

[awol]

Checked again today and got about 6aH again.
There was a flaw in the plan in that when the bms shut it off there was no volts to measure, so I plugged/unplugged the charger which switched the bms back on. Forget the previous fully charge volts, I think the watt meter volts is inaccurate. New readings with my volt meter (which I think is more accurate.)
Fully charged - 41.4v
Dead - 35.7v
1/2 hour after dead - 36.1v
3 hours after dead 36.4v

 

[Deleted member 4366]

Dead at 35.7v is worrying. It should be lower. It probably means that one cell is much lower than the rest. You're going to have to look inside to check the cell voltages.

 

[awol]

Taken a couple hours after empty.
3.8 7.5 11.2 14.9 18.6 22.3 25.8 29.5 33.2 36.9
3.8 - 3.7 - 3.7 - 3.7 - 3.7 - 3.7 - 3.5 - 3.7 - 3.7 - 3.7
All look close to each other.

 

[Deleted member 4366]

It looks like worn out cells. You'd need the meter on it while under load to confirm.

 

[awol]

oohh that's bad news,how would I explain a new battery to my wife already?
Do you mean a meter on the whole battery, or on each of those connectors again?
Would I fully charge first or put a small amount of charge in just to get the battery on again? Cheers d8veh.

 

[Deleted member 4366]

Do you mean a meter on the whole battery, or on each of those connectors again?

Either. You need to find why it's switching off. Either the controller has seen the voltage drop to below 31v or the BMS has seen a single cell below 3.0v.

Charge it up normally.

 

[awol]

or the BMS has seen a single cell below 3.0v.

I am suspecting that one as the bms shuts off on the battery itself, however since the 6aH can get me 28ish miles and I only need 20 between charges I have decided there is no problem as yet. I will monitor the volts under load and I will repeat the above in a month or so to check for any degrading but continue as is until 20 miles becomes too much for it which I hope could be months off yet anyway.
Thanks for the help