Measuring LiPo cell charge level

Den tirsdag den 2. februar 2016 kl. 21.10.06 UTC+1 skrev Tim Wescott:

-Lasse

Huh, I measure alkalines with my 10 meg DMM.

(sorry I don't mean to redirect your thread... I don't know about LiPo's, don't you use them in your RC hobby?)

George H.

Thanks. It sounds like the answer is "no". That doesn't make me happy, but it sure saves me some time.

My recent experience is the voltage unloaded represents the state of charge reasonably accurately.

Is that always at room temperature? (I have no idea what the voltage temp. co. of a battery is.)

George H.

Gibbs Free Energy should give it to you, if you can find the thermodynamic parameters.

might help.

gives some example data. You'd think that the manufacturers data sheets would cover this, but nothing seems to come up.

My experience is that the state of charge is irrelevant. Doesn't matter how many electrons are in there if your application can't get 'em out. My experience is that the dominant failure mode of Lithium cells is high internal resistance. Take most any cellphone or laptop battery with a short operational life and measure the watt hours you can get out at low current. It's often near the stated capacity, but it still won't run your phone or laptop.

Remote Control hobbyists love the LiPo batteries. I have many of them.

My good charger will tell me the charge state of the battery as soon as I start the charging process. It is just an estimate based on battery voltage, but it is always well within the expected value, based on how long I have been flying and the data from the telemetry unit.

You cannot accurately determine the battery charge state by measuring voltage, but you can certainly an estimate that, depending on application, can be quite valuable.

Yep. That's what I've seen, which includes destructively testing cell phone batteries to full discharge. Although they may show less than

This might help: I have a CBA II and can reproduce the plots if you bribe or inspire me. Well, maybe not. I don't have a load capable of handling a 40C discharge.

Define "low". If you are trying to keep the batteries alive and don't want to destroy it, you don't want to discharge it to less than about

Collective wisdom and magic: Volts SoC 3.99v = 80% 3.85-3.86v = 60% 3.76-3.78v = 40% 3.69-3.71v = 20%

and for a 3S battery: Volts SoC 11.98v = 80% 11.55-11.59 = 60% 11.29-11.34 = 40% 11.08-11.14 = 20%

Any chance you're dealing with a low voltage cutoff which might be producing a "low" voltage? Those are sometimes used on LiPo batteries to prevent trying to charge a dead battery:

I'm using a LiPo battery in a gizmo (some experimental stuff for control- line flying, actually). It shouldn't need to be charged after every flying session, and my prototype super-styling and compact on-off switch was leaky. It would be nice to have a quick in the shop way of checking the state of charge.

Maybe I'll just make a load tester...

The battery voltage should be sufficient. However, watch out for temperature effects. The battery will be internally warm after a flight and might produce erroneous results.

Landing with power left in the battery is somewhat of a disgrace. The fun way is running the battery down to where the controller decides its time to use the GPS to find its way back home.

No load needed. A DVM plus a chart or graph should be sufficient. The open circuit voltage won't be very accurate or useful between 90-100% SoC, but elsewhere the slope is somewhat constant and the drop off below about 10% is quite rapid and obvious. If you want more accuracy, use one of the various battery fuel gauge IC's.

Battery Fuel Gauge: Factual or Fallacy?

Maxim Battery Fuel Gauges:

Digikey on battery fuel gauges:

Ti on battery fuel gauges:

Battery failures: