Measuring capacity of Lithium Ion tool batteries

[JimS]

Have a battery powered string trimmer (Ryobi 40V) with 3Ah battery.  Recently noticed the battery didn't last very long so charged the battery and put it on a constant current load device at work.  1A load.  It ran about 2.5 hours before the internal circuit detected it was at the low limit (about 30v) and cut off the output (the battery has a circuit board with some electronics on it that others have detailed on the web).  Anyway - so fairly close to the 3Ah rating.  In actual use it runs out of charge in about 30 minutes.  I wonder if the internal resistance is going up so at higher load the losses are greater and thus the short run time.  I don't have any quantitative numbers for capacity or run time from when it was new (about 1.5 years ago) but just observed it was short lately.
 
For comparison I just got a blower with the same type battery but 4Ah.  Running the same trimmer it ran for about 1.5 hours and still had about 1 bar of charge (1/4?) remaining.  Planning to do a similar load test on it tomorrow.
 
Going to try to get a warranty replacement as it still has a few months left on the warranty but wondering if they will reject it because it still does charge and have partial capacity.  See lots of reports on line of other issues such as the charger refusing to charge the battery giving a red light that definitely shows a bad battery and mine is not giving that obvious of indication.
 
Also wondering what is the appropriate discharge rate for testing.  For some batteries it is something like 1/10 or 1/20 of Ah (for example for 4 Ah battery use 0.4 A load which should last 10 hours) but for these I have found higher loads typically used.

 

[RAL]

Battery capacity specs usually are C/20 for SLA batteries.  I haven't seen an official convention for Li-ion batteries, but several manufacturers seem to use C/1, which makes sense for use in a power tool which places a heavier load on the battery.  Over about 250 discharge cycles, I'd expect to see about a 15 to 25% loss of capacity.  If you're only getting 30 minutes of use out of the battery, I'd agree that it's not as good as it should be, 
 
Your comparison to the 4Ah battery is a good way for estimating what the 3Ah battery should be able to do. Seems like it should give you a bit over an hour of use. 

 

[JimS]

They said decrease in run time is normal and the run time has to fall under 15 minutes for warranty replacement.  Guess I need to keep running it, which is a real pain, and hope the run time falls further.  Might do a little more testing at C/1.  That's getting right to the power limit of my electronic load (165 W) for the 4Ah battery but doable.  Would be interesting to find a graph of capacity vs. discharge cycles to get some idea of what further deterioration will be and how fast.

 

[RAL]

The web site below has a chart that shows capacity vs charging cycles.  From other things I have read, capacity changes can vary significantly between battery types (eg a cellphone battery vs a 18650 cell) and also from one manufacturer to another. 

 
https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
 
Warranty coverage only when the battery falls below 15 minutes isn't much of a warranty.  Makes me wonder how you prove it to them.  Do they test the battery if you return it to them for replacement? 

 

[JimS]

Yeah, 25% capacity is bad.  The charts you link to don't even go that low. 

 

[LarrylLix]

Way to destroy a LioN battery? Run it as low as the protective circuits will allow, try it again a while later, and let it sit discharged.

Better yet, add a temporary load and drain it into the dirt. These batteries cannot take full discharge.

After it sits dead for a while test again.

Sent from my SM-G781W using Tapatalk

 

[JimS]

Maybe so but the batteries have a PC board inside with some circuitry.  At about 30V it cuts off and won't let you discharge it further.  Are you suggesting letting it sit at 30V for a while will make it loose capacity significantly?

 

[ano]

Maybe so but the batteries have a PC board inside with some circuitry.  At about 30V it cuts off and won't let you discharge it further.  Are you suggesting letting it sit at 30V for a while will make it loose capacity significantly?

Yup, and so will charging it to 100% and leaving it there. Li-ion batteries like to live at charges of 20% to 80%.  This is the levels many electric cars work at, and they will typically retain 90% of their capacity after 10 years.  

 

[JimS]

I saw some information that reducing the max charge voltage a little gave about 90% capacity and doubled the discharge cycles.  I suppose something could be added to cut the charger off a little early but not an easy mod.

 

[ano]

I saw some information that reducing the max charge voltage a little gave about 90% capacity and doubled the discharge cycles.  I suppose something could be added to cut the charger off a little early but not an easy mod.

It's not that hard. Many electric cars "hide" the top 10% and bottom 10% of the battery from users. So you car says its 100% charged but it's really 90%.  I do this with an eBike and eScooter I have.  Say a full charge for a cell is 4.2V, if you charge to 4.1V instead, that is about 90%, and now your battery lasts twice as long. If you charge to 4.05V, about 80%, you have now tripled its life.  Charge to 4.00V (70%) and now the battery lasts 4 times as long.
 
Now you really don't need to do this. Instead don't charge a battery until JUST before you use it.  So charge to 100% THEN use the battery right away.  It will be down to 90% or 80% soon. Just don't leave it at 5% over time. If your battery is dead, charge it to 50% until you want to use it again. Then charge to 100% just before you use it. Just don't charge to 100% then leave it like that.
 
You may not know it but your iPhone only charges until 80% all night, then when it thinks you will wake, it charges the remaining 20% to 100%. This extends the life of the batteries.