On Sun, Feb 22, 2009 at 10:42:09AM +0000, Wayne Stallwood wrote:
On Sun, 2009-02-22 at 09:44 +0000, Ted.Harding@manchester.ac.uk wrote:
It is, of course, known that it's good for battery life to submit it to a discharge/recharge cycle every so often; but I've not heard of this being built-in to laptops' power monitoring. (But then there's a lot of stuff I haven't heard of).
This is only true for specific battery chemistries. Specifically NiCd and NiMH.
Lithium Polymer and Lithium Iron don't need a full charge-discharge cycle because they don't suffer "memory effects". The only care these cells need is that you avoid storing them below 30% charge and avoid heavy charge/discharge cycles in freezing temperatures .
The 'memory effect' in NiCd batteries (I don't think it occurs in NiMh batteries at all) is very specialised and very specific. The chances of it occurring in normal usage are vanishingly small.
It was originally diagnosed in some batteries on a satellite which had extremely regular charge discharge cycles due to the satellite going round the earth and thus in and out of sunlight at exactly the same time each day. The effect *only* occurs with these extremely regular charge/discharge cycles.
All the wonderful "our charger discharges your battery first" extras are completely pointless. The real guaranteed way to make your NiCd batteries useless is to leave them charging indefinitely which is what a *lot* of older/cruder chargers do, especially the ones that come with cordless tools.
Using good modern delta-V chargers (usually called 'automatic') there's no need to discharge batteries before charging.
Oh, the other *big* killer of NiCd cells is charging/discharging more than one cell in series. Discharging multi-cell batteries fully is a real no-no as the weaker cells get 'reverse charged' before the stronger cells are fully discharged. Similarly fast charging the cells in series results in the weaker cells reaching full charge first and overheating before the stronger cells are fully charged.
Some systems *have* to charge batteries in series, if you do this then the batteries need to be well matched to minimise weak/strong problems.
I use a lot of AA and AAA cells and I *only* use chargers which charge all the cells individually, none of the chargers discharge cells first, I'm still using cells bought many years ago which must have been through a *lot* of cycles.
All of which is, of course, completely irrelevant to the pressent discussion! :-)
(Oh by the way a *careful* Google search for "NiCd memory effect" will find the full details of the satellite I relate above)
However the battery monitor stuff that tells you percent discharge, runtime etc does require calibration every so often and I know that some Thinkpads at least will eventually perform an automated cycle to recalibrate if left on Mains for a log period of time.