Battery Current Loss with Temperature

When a lithium ion battery is charged, a current is driven into the cell an d the state of the battery can be tracked by measuring the integral of the current over time.

When the battery then is cooled, the terminal voltage is reduced, but also, the available current is reduced over time. This link shows a typical dis charge curve for multiple temperatures.

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The variation in voltage can explain a reduced capacity with temperature. But I don't get the reduced mAh at colder temperatures. A battery is a che mical process that releases an electron with each ion that reaches the anod e. Charge and current should be conserved other than internal leakage whic h is typically very small in lithium ion batteries.

So how can the mAh capacity of a cell be reduced by the cold?

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  Rick C. 

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gnuarm.deletethisbit
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On Sunday, March 31, 2019 at 1:44:34 AM UTC+11, snipped-for-privacy@gmail.com wrot e:

and the state of the battery can be tracked by measuring the integral of th e current over time.

o, the available current is reduced over time. This link shows a typical d ischarge curve for multiple temperatures.

But I don't get the reduced mAh at colder temperatures. A battery is a c hemical process that releases an electron with each ion that reaches the an ode. Charge and current should be conserved other than internal leakage wh ich is typically very small in lithium ion batteries.

A plausible hypothesis is that some of the electrode surface becomes inacce ssible at lower temperatures.

If the electrolyte were to freeze solid the electrode behind that frozen bi t wouldn't be accessible.

The bulk electrolyte seems unlikely to freeze. but you might have funny mic rostructure close up against the electrodes which could be more susceptible to freezing solid than the bulk.

It's just a hypothesis and I can't see an obvious way of testing it.

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Bill Sloman, Sydney
Reply to
bill.sloman

  • You answered your own question: reduced capacity with temperature = change in chemistry.
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Reply to
Robert Baer

l and the state of the battery can be tracked by measuring the integral of the current over time.

lso, the available current is reduced over time. This link shows a typical discharge curve for multiple temperatures.

e. But I don't get the reduced mAh at colder temperatures. A battery is a chemical process that releases an electron with each ion that reaches the anode. Charge and current should be conserved other than internal leakage which is typically very small in lithium ion batteries.

Only in the sense that if you don't know what is going on, any explanation that you don't understand is as good as any other.

Rick is right that every lump of charge transferred is an ion being moved, and the number of ions won't change with temperature. Looked at from a chem ical point of view, this is has to be correct.

Some of the ions have to be locked down at low temperatures to explain what the data sheet shows, and that has to be mechanical rather than chemical.

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Bill Sloman, Sydney
Reply to
bill.sloman

I'm confused by that graph's labeling. Why would capacity be zero at the beginning of a discharge curve? The discharge (terminal voltage versus time) at constant current would be a graph with voltage/time that looks like this one, BUT the horizontal axis would be labeled 'seconds' and not 'capacity mAh'.

The most likely reason for poor mAh numbers at low temperature, is inappropriate charger algorithms. Mainly, at low temperature, you expect some conductance diminution.

Reply to
whit3rd

rote:

l and the state of the battery can be tracked by measuring the integral of the current over time.

lso, the available current is reduced over time. This link shows a typical discharge curve for multiple temperatures.

e.

e beginning

constant current

izontal

If the discharge was at constant current, the two would be equivalent. If you really can't understand the graph, I can't help you. But I think you a re being a bit disingenuous and actually understand that they are measuring the amount of charge removed from the battery and the capacity is the amou nt removed at the cut off voltage.

ropriate

ance diminution.

These are discharge curves. They have nothing to do with charging. The vo ltage on the battery varies with temperature. Notice that the voltage vari es much more at lower temperatures than higher. The first two lines are 22

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  Rick C. 

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Reply to
gnuarm.deletethisbit

How do you know? If the charge is terminated when the terminal voltage hits a threshold, and the IR drop when charging is higher (because of series resistance in the cell), then the final as-charged state would be less. If I were testing a battery in an environmental chamber, I'd want to charge it under field conditions, at temperature.

Reply to
whit3rd

te:

nappropriate

ductance diminution.

hits

esistance in

ting a battery

, at temperature.

"Higher" than what? "Less" than what? What are you talking about exactly? The curves presented were generated from discharging batteries for the pu rposes of comparing discharge at different temperatures. Why would you ass ume the batteries were charged at different temperatures?

The measurement was not made to evaluate performance in any particular appl ication. It was made to gather specific information on the battery dischar ge characteristics at different temperatures.

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  Rick C. 

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Reply to
gnuarm.deletethisbit

If a charger terminates charging at 4.2V, and applies a 0.5A charge current, a battery with 2 ohms internal resistance will stop charging at 3.2V + 0.5 * 2 = 4.2V

The same battery, when warm, has 0.5 ohms internal resistance, and stops charging at 3.95V + 0.5 * 0.5 = 4.2V

So, an automated charger will produce different charge states depending on the battery temperature, if it follows such a rule.

Reply to
whit3rd

Ok, great. Thanks

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  Rick C. 

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gnuarm.deletethisbit

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