1
Question of the Week / Re: QotW - 24.05.31 - Why do batteries go flat when we leave them for a while?
« on: Yesterday at 17:39:32 »
The self-discharge rate has a number of factors.
The internal impedance of a cell is polarity-dependent. The designers aim to reduce the forward impedance as required for the intended duty, so that a car battery can supply up to 200 amps for starting, but a low forward value may compromise the reverse impedance, allowing electrons to flow from the negative to the positive pole, so "wet cells" will discharge themselves over time, even if the discharge current is of the order of microamps.
Cells designed for light-current duties (watches, hearing aids...) tend to have possibly nanoamp self-discharge rates but of course much more limited forward current capacity.
Added to all of which is chemical degradation as the electrolyte corrodes the electrodes or even (in the case of NiFe wet cells) absorbs carbon dioxide from the atmosphere. This has the effect of increasing the forward impedance so that the battery is incapable of supplying the design current, but regular charging tends to offset the chemical reactions somewhat.
The internal impedance of a cell is polarity-dependent. The designers aim to reduce the forward impedance as required for the intended duty, so that a car battery can supply up to 200 amps for starting, but a low forward value may compromise the reverse impedance, allowing electrons to flow from the negative to the positive pole, so "wet cells" will discharge themselves over time, even if the discharge current is of the order of microamps.
Cells designed for light-current duties (watches, hearing aids...) tend to have possibly nanoamp self-discharge rates but of course much more limited forward current capacity.
Added to all of which is chemical degradation as the electrolyte corrodes the electrodes or even (in the case of NiFe wet cells) absorbs carbon dioxide from the atmosphere. This has the effect of increasing the forward impedance so that the battery is incapable of supplying the design current, but regular charging tends to offset the chemical reactions somewhat.
The following users thanked this post: paul cotter