# Forums

## Does Max battery charge indicate battery health?

Does the full charge mileage range number indicate the health of the battery? In other words, if the full charge- when the car was new- was 285 and after a year its 275 does that mean there has battery degraded? I thought the car calculates the full range, in part, based on the history of how the car has been driven?

jordanrichard | May 30, 2019

Yes and no. If you constantly start out at 90% and only go down to 75% SOC and then charge back up to 90%, the algorithms can only come up with an estimate based on a mere 15% usage window. You need to think of/treat your battery like a rubber band. If you only stretch it to “x”, you will forget how far it can actually stretch to. So periodically run your battery down to say about 10-15% and then charge back up to either 90 or 100 %. If you do charge to 100%, be sure to start “burning” some of that charge off and get it down to 90%.

The other thing to do is whenever it is that you make note of what a 100% SOC gets you, be aware that ambient temperature can have an affect on the rated miles for a given SOC. Over the years I have noticed that 90% in the winter tends to be 1 sometimes 2 miles lower than it is in the summer.

AERODYNE | May 30, 2019

I have found you can correct the calibration with much smaller ranges, like 70-40%, after storing a few weeks at 55%.

Never been above 93%, nor below 27%.

RM varies between 270 and 266.

72echo | May 31, 2019

interesting. Thank you!

GHammer | May 31, 2019

"I thought the car calculates the full range, in part, based on the history of how the car has been driven?"

No it doesn't, rated miles is based on a usage constant and does not change with usage.

Bill_75D | May 31, 2019

@ hammer - If rated miles are based on a usage constant (which I believe) then wouldn't a decrease in the full rated range indicate a battery losing capacity?

My car had 259 miles rated range when new, and now charges consistently to 235 miles at 100%.

Bighorn | May 31, 2019

It’s based on a rate constant, but it also is not accurate at measuring capacity, so good numbers are probably good, but bad numbers might not be bad. Not worth analyzing.

GHammer | May 31, 2019

"If rated miles are based on a usage constant (which I believe) then wouldn't a decrease in the full rated range indicate a battery losing capacity?"

Perhaps, it all depends on the accuracy what the Battery Management System is reporting as the capacity which as noted above can be variable. For instance, now that the weather is warming up, my reported range is going up. As for rated range being based on a rate constant, that has been confirmed by people examining the code despite what some service center employees have reportedly said.