Just digging up an old thread about rechargeable batteries:
"For long term storage it is recommended to store them flat actually, since the very slow self discharge can also induce some crystal formation."
That's true for NiCd batteries, blr. But it's not recommended at all to store NiMH batteries flat, even for long-term storage. Instead, for long-term storage NiMH batteries should be stored with about 40 percent charge still left in them.
"...NiCd and NiMH batteries should not be stored FULL??"
Not if you're going to store batteries for more than a few days to a week, Joe Bloggs.
"Generally for such low drain devices (mause remote clock etc) rechargeables are not recommended. You wont save much money and the low drain plus self discharge is not so good for the cells eighter."
Very true, blr.
"About the slow overnight charger, it is a farly safe device, meaning that it won't destroy your cells quickly, but it is not the best solution. First of all, these are usually "dumb"chargers, having only a timer to terminate the charging. No matter how deep your cells are discharged (see below) the charger always keeps the current on for say 14 hours, means partially discharged cells get overcharged. Second, rechargeables (except SLA) generally respond better to quick charging. The slower the charging (and discharging) the larger the particles that form on the electrodes and particularly on the Ni one. I won't go deep into this, but NiCd and NiMH generally charge better at currents between C/3 and C. The problem with quick charging is that you have to have reliable charge termination method. Quick charger with only timer is BAD. You need a charger that will properly sense the battery state of charge and terminate once the battery has reached full charge. -dV technique is fairly good and modern chargers can apply it even for NiMH batteries, which have lower voltage drop at the end of charge compared to NiCd."
One drawback of -dV termination is that such termination only works properly with chargers that deliver at least C/3 in charging current to a given battery. (Slow chargers can't use -dV termination, because that feature won't work correctly or at all at such low charge rates. DUH.) IMO, the best charger is a fast charger that uses a sophisticated means of charge termination and negative-pulse charging. For NiMH batteries, slow chargers are almost as bad for such batteries as fast chargers with only a timer for charge termination, because NiMH batteries have a low tolerance for overcharging, and the slow chargers can be overcharging NiMH batteries even though the cells are cool to the touch!
"IMO, the best plug in charger on the market today... Charges 1-4 AA or AAA cells while separately monitors each cell. You can charge different cells together. Works with both NiCd and NiMH chemistry... It ain't exactly cheap but it's worth every cent."
I agree with the recommendation of the type of charger mentioned in the above quote, and any similar high-quality charger on the market. This "independent-charging-circuits-for-each-cell" setup reduces the risk of having one bad cell spoil the entire set of batteries. Chargers that require cells to be charged in sets of more than one and chargers that have only one charging circuit that's commonly shared between two or more cells will get the entire charge cycle wrong: With this setup, either the stronger cells become undercharged, or the weaker cells become overcharged. Either situation will eventually kill the entire set of batteries prematurely.
"Deep cycling any type of battery only shortens its life. You don't need to deep cycle your NiMh cells every time. If you don't overcharge them severely, an exercise cycle is needed once in a while (say once every 20-30 cycles)."
I agree here.
"The problem with most PCDP and other portable gear is that the in-build charging circuits are usually dumb, i.e. timer controlled. Thus you face the choice of eighter overcharge a partially discharged battery or drain it flat and charge it. In both cases the lifetime of the cells is reduced. That's why people get so short lifetimes of say their gumsticks. Not many of these expensive cells survive much more than 100 cycles, whereas if cycled properly the cell will be good for at least 200-300 cycles to nearly full capacity."
Actually, recent built-in chargers in PCDPs and other portable gear use a simple heat sensor that terminates the charge, which is better than a timer alone, but nowhere near as effective as true -dV termination. And thermistors used in such circuits have so wide of a tolerance that none that I've seen actually gets the charging job right: They either undercharge (Panasonic) or overcharge (Sony, Sharp et al) the batteries that had been designed for those portable audio devices! Furthermore, the built-in charging circuits lack reverse-pulse charging technology, increasing the likelihood of the "memory effect" building up in the cells. In other words, the built-in chargers in PCDPs and other portable audio gear are "smart" chargers - but in the most rudimentary sense.
Finally, I've read something about NiMH batteries being less than ideal for longetivity. NiMH batteries, as that article stated, won't last much more than about 300 deep charge/discharge cycles - while properly charged NiCds will last at least 1000 deep charge/discharge cycles.
