I have a wireless phone and recently i had to replace it's lithium battery.

The previous battery was totally dead and would work for like 30 minutes or 1 hour before it showed the "low battery" indicator and a single 3 min phone call would totally kill it and shut down the device.

I left the new battery charge for a good 20hs and after about 15 hours of light use (mostly standby while not being in the base) it shows a low battery icon, the manual says it's supposed to last for 7 days in standby with a 300mha battery and the new one i got is 750mha, even if the battery is really crappy i guess it should last at least the same.

Any chance this is related to how that phone knows how much battery is left? I read that there is no reliable way to know the charge left on a lithium battery just by measuring it's voltage so maybe the phone just tries to "remember" the previous cycle lowest voltage to try to estimate how much is left? if this is the case then the new battery would need to run a full cycle before the phone can reliably tell the charge? (kind of like laptops).

How does it work?

  • \$\begingroup\$ I think you can get the charge state by measuring the voltage under load. Then using some math(s) you can calculate form the voltage at that current the amount of energy left. \$\endgroup\$ – Joren Vaes Jun 2 '17 at 13:56
  • \$\begingroup\$ They use a model of the discharge process; in some cases it apparently has some stored information, and phone forums have discussions about resetting this. \$\endgroup\$ – Chris Stratton Jun 2 '17 at 14:14
  • \$\begingroup\$ Light use probably still consumes 100x the power of standby \$\endgroup\$ – Andy aka Jun 2 '17 at 14:16
  • \$\begingroup\$ @Andyaka assuming it actually goes into standby. Software / state problems can prevent that, and screen off does not mean the processor is suspended. \$\endgroup\$ – Chris Stratton Jun 2 '17 at 14:20
  • \$\begingroup\$ @ChrisStratton I'm not sure why I should assume that given what I said previously? \$\endgroup\$ – Andy aka Jun 2 '17 at 14:23

You are correct that the charge in a li-ion battery can't be correctly measured by voltage alone. The discharge curve and temperature dependency of the voltage make any estimates meaningless.

However Li-Ions have a very low self discharge rate and low in cell losses when charging and so a coulomb counter is normally used to measure the charge state. Basically they count the electrons into the battery and then they count them back out again.

These counters do however need to be calibrated, they need to know where the full and empty points are. This is normally done by fully charging the battery and then running the battery until the system dies due to low voltage. Once this cycle is done the system should correctly know the battery capacity and be able to correctly assess the charge state.

Since this is a wireless phone signal strength may also play a part, if the signal is weak the power usage may go up to compensate. With a cell phone being in an area with poor coverage can drain the battery very rapidly, home phones this is less of an issue but could still happen. Also things like display lights on the phone could make a difference if that is a user controllable option.

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  • \$\begingroup\$ this is pretty much what i was expecting, thanks for the detailed explaination, i will wait for the phone to die and see how long it takes and if in the next cycle it get's more accurate \$\endgroup\$ – Joaquin Brandan Jun 2 '17 at 14:35

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