How measure capacity of LI-ION pack?
- Thread starter boraicho
- Start date
dustNbone
10 kW
Just get something like this
https://www.ebay.ca/itm/DC-100A-Met...bo-Monitor-for-Car-Battery-Solar/361386546620
It will tell you everything you need to know.
https://www.ebay.ca/itm/DC-100A-Met...bo-Monitor-for-Car-Battery-Solar/361386546620
It will tell you everything you need to know.
dogman dan
1 PW
https://www.ebay.com/itm/Digital-Monitor-LCD-Watt-Meter-60V-100A-DC-Ammeter-RC-Battery-Power-Amp-Analyzer/222929217724?epid=77498103
Seriously, just spend 10-15 bucks on one of these, and then run the thing till the bms shuts off, while measuring the watts directly.
Sure, you can do it with an amp meter, but to do that, you need it to discharge at a constant rate the entire time, then do the math with the average voltage, meaning the volts at mid discharge. ( about 3.8v per cell usually)
That will get you a guesstimate, but what good is a guess, when ten or twelve bucks gets you a pretty darn exact measurement in watt hours.
Heres my guess, you have 19 amp hours usable.
Seriously, just spend 10-15 bucks on one of these, and then run the thing till the bms shuts off, while measuring the watts directly.
Sure, you can do it with an amp meter, but to do that, you need it to discharge at a constant rate the entire time, then do the math with the average voltage, meaning the volts at mid discharge. ( about 3.8v per cell usually)
That will get you a guesstimate, but what good is a guess, when ten or twelve bucks gets you a pretty darn exact measurement in watt hours.
Heres my guess, you have 19 amp hours usable.
jonyjoe303
100 W
- Joined
- Aug 31, 2017
- Messages
- 233
you need a drok coulombmeter, I use these on my lifepo4 but works on li-ion. You program your battery capacity and it counts how many amps it used until depleted. Its bidirectional, it counts amps to charge/discharge. You can find on amazon for less then 30 dollars.
On your 21 ah pack, it starts counting down as you use it, when you down to 0 amps on the lcd and your pack is still producing power then its probably 21 amps, if it quits with 5 amps still on the lcd, then you not getting the full 21 ah. This can handle 8 to 80 volts up to 75 amps.
On your 21 ah pack, it starts counting down as you use it, when you down to 0 amps on the lcd and your pack is still producing power then its probably 21 amps, if it quits with 5 amps still on the lcd, then you not getting the full 21 ah. This can handle 8 to 80 volts up to 75 amps.
BVH
1 kW
- Joined
- Mar 26, 2009
- Messages
- 411
With all these power meters, at what rate should a LiFeP04 pack be discharged to obtain true capacity? I know with Lead Acid, Peukert factor is in play and hense I think the standardized .1C discharge rate? Can a .25C or .5C rate be used and will it still be accurate?
With Lithium packs, the Peukert effect is negligible.
But even if it wasn't, you would need to discharge it at the rate you will be using it at in order to determine the actual usable capacity.
If you test at a lower rate and it gives a higher capacity, that capacity is irrelevant, because you could never use it.
The only thing to worry about is powering it externally so that when th ebattery's BMS shuts off the batteyr at LVC, it does not lose the data.
But even if it wasn't, you would need to discharge it at the rate you will be using it at in order to determine the actual usable capacity.
If you test at a lower rate and it gives a higher capacity, that capacity is irrelevant, because you could never use it.
?? The linked meter in the post you replied to also shows Wh on it's screen, so it will monitor capacity.boraicho said:
The only thing to worry about is powering it externally so that when th ebattery's BMS shuts off the batteyr at LVC, it does not lose the data.
dogman dan
1 PW
Yes, the capacity it has would be measured at the rate it will be used. Typically, easiest to just do that on the road, returning to close to home near the end of the discharge. Then ride around the block ten times if that's what it takes to be close to home when you hit 0% usable capacity.
Your capacity WILL vary by factors that affect peukerts. Rate of discharge, age of cell, and particularly temperature. Anything that changes the internal resistance of the cell, will cause a change in the peukerts.
But yes, compared to lead, the variation with lithium batteries is much less, but its still there. The closer to the end of the discharge you get, the more your IR increases, so your peukerts changes as you discharge every cycle. To maximize usable discharge, lower the rate once you get below 75% discharged.
Your capacity WILL vary by factors that affect peukerts. Rate of discharge, age of cell, and particularly temperature. Anything that changes the internal resistance of the cell, will cause a change in the peukerts.
But yes, compared to lead, the variation with lithium batteries is much less, but its still there. The closer to the end of the discharge you get, the more your IR increases, so your peukerts changes as you discharge every cycle. To maximize usable discharge, lower the rate once you get below 75% discharged.
BVH
1 kW
- Joined
- Mar 26, 2009
- Messages
- 411
I'm fortunate to have both a TDI Systems 1500 Watt electronic DC load and a 4000 Watt DC Eload. I find that I typically ride at between 250 and 500 Watts most of the ride so I'll put one of these power measuring devices in-Series with the load, dial in about 350 Watts (in Constant Current Mode) and take it down to about 2.9 Volts per cell, knowing I could go further but that there is very little capacity left below 3.0 VPC. Thanks for the info.
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