How can residual charge in a battery be measured?

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Jean-Paul

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Jean-Paul asked the Naked Scientists:
   
Hello,

My question is, with mobile phone batteries is there a way to tell how much life is left in this rechargable type ?

What do you think?

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Offline Vern

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How can residual charge in a battery be measured?
« Reply #1 on: 06/02/2009 14:41:19 »
I have used several cell phones until the batteries became impossible to charge. It seems just from my own experience that before they became unusable each successive recharging lasted a shorter time before needing charged again.

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lyner

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How can residual charge in a battery be measured?
« Reply #2 on: 07/02/2009 00:44:52 »
If a battery is newish, then it is possible to tell the state of charge by looking at the voltage. However, for a useful rechargeable design, the voltage may not vary much over the majority of the charge range. It can be done, though -with a certain degree of accuracy.
For an old  battery, the actual capacity will get less and the only way you can tell its health is to test it by charging and discharging it, recording the volts all the time. An 'intelligent' system could make an informed guess about how much charge you have left during a few subsequent charge / discharge cycles.
But it's a bit like testing matches by striking them to see if they would work.

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Offline techmind

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How can residual charge in a battery be measured?
« Reply #3 on: 07/02/2009 18:23:40 »
My question is, with mobile phone batteries is there a way to tell how much life is left in this rechargable type ?

Your question could be interpreted two ways, depending on what you meant by "life". You could mean "how much charge is remaining before it's flat and needs charging again", or perhaps you meant "how many weeks/months before it won't hold a useful amount of charge after charging, and will only be fit for the bin".

Depending on the battery type, the former can be gauged by measuring the open-circuit battery voltage. This method doesn't work well for NiCd or NiMH because their battery voltage doesn't change much as you discharge the cell. For Li-Ion, the open-circuit cell voltage lies typically between 3.6V "empty" and 4.2V "full", with a fairly linear relation between voltage and charge (so for example 3.9V, halfway between 3.6 and 4.2 correlates to half-full). Unfortunately as a battery gets older (after many charge/discharge cycles) the battery internal resistance increases, which means that the useful energy available (particularly for heavier loads) decreases, and the battery may have insufficient voltage when loaded to be useful even when the open-circuit voltage is okay.

Sophisticated "battery gas-gauge" silicon chips in some appliances literally measure the charge flowing in and out of the battery to gauge the remaining energy, but even this isn't foolproof as batteries self-discharge to some extent, and they go flat much quicker when loaded at low-temperatures, for example.


For the second possibility - when is it time to dispose of a rechargeable battery - then when you charge it and it doesn't last any "useful" or "reasonable" time then that's it.
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"