How to test battery capacity?

[LOLgas]

I've owned my 2015 i3 for almost a year now, I know I have a few more years/miles on the battery warranty, but was wondering: How do you check the battery capacity to see if it has dropped below 70%?

Additionally, would y'all suggest I wait until the "last minute" before the warranty expires to get a little more life/value? Not sure if I would keep or sell after battery fix/replacement/whatever. TIA

 

[alohart]

A rough indication of usable capacity can be found in the hidden service menu (search if you are unfamiliar). The Batt. Kapa. max value is only an approximation of actual usable capacity. In truth, a battery cell's capacity depends on the charging or discharging current with lower currents resulting in higher capacity. A BMW dealer would charge and discharge the battery pack slowly (i.e., low current) to measure the usable capacity. Those who have paid for a capacity check almost always find that the usable capacity increases to above the 70% of new capacity limit as a result of the dealer's capacity test, so if the capacity isn't considerably lower than the 70% limit, it's likely that a capacity warranty claim would be denied.

Maybe a better way for owners to measure capacity would be to charge to full, reset the trip meter, drive until as close to empty as possible, and then check the driving efficiency in iDrive (e.g., miles/kWh) and divide that into the distance driven which would result in the energy used in kWh. Then correct that by the percentage of the battery capacity that was used during the drive to result in a reasonable estimate of the usable capacity of the battery pack. Of course, this would depend somewhat on the ambient temperature. The capacity at near room temperature would probably be close to the conditions that a BMW dealer would use.

 

[jadnashuanh]

If you can afford to wait, and you have something like a Kilowatt meter, you could drive until nearly empty, then hook it up with the OUC plugged into the Kilowatt meter, and measure how much it takes to reach full. Now, that's not an absolute result since there's some energy lost in the conversion from 120vac to the dcv needed to charge the vehicle, and the heater and cooler may run, but if that's less than the 70%, it would end up being much less than the battery minimum.

When I installed my EVSE, I bought a panel mounted voltage/power/amperage meter and hooked it to the input power to it. I can see that the EVSE in just standby mode draws about 3W, and can see the input voltage, current, and it calculates the power (in watts) supplied. The panel from Amazon was only about $20. In the junction box, I added an extender and cut a blank cover plate to snap the meter in. I added a fuse for the meter and a switch to disconnect it if desired. All in all, less than $30.