|Re: Re-engineered AA Woodstocks|
Message #13 Posted by sylvandb on 14 Apr 2012, 11:45 p.m.,
in response to message #12 by BobVA
I do like the low-discharge cells. Costco usually has a nice price on multi-packs of the eneloop. I've seen the similar duracell NiMH there as well, but not recently.
I wouldn't charge NiMH in a Woodstock (or any LED HP) since their charging circuits were designed for NiCd and may damage either the cells or the calculator.
That is the safer approach, if you don't break the calculator constantly taking the battery in and out.
However the chemistry in NiMH and NiCd cells is so similar, that the simple low-rate charger that HP provides is perfectly fine for NiMH, and that is a lower risk than constantly messing about in the battery compartment. (IMHO)
Since NiMH has larger capacity than the old 300mAh cells provided by HP, the charge rate will be lower when charging NiMH. Most likely it will result in either a very slow or even a trickle charge rate. Luckily NiMH is pretty efficient so will usually charge even at the very low rates provided by the HP.
The slow charge for both chemistries is specified as C/10 (C is the amp-hour capacity of the cell) and usually needs 140-160% or 14-16hours to full charge. (Even tho C/10 is typically considered the "forever" charge rate I wouldn't leave it on charge for more than a total of 2x the capacity or 20 hours, especially with low self-discharge cells.)
In general faster charge rates are safe for both chemistries as long as the cells don't get hot. Obviously the faster the rate, the more risk of overcharging / overheating the cell if you don't stop charging when full. Even C/5 is usually OK if you don't forget them for too long. If you are babysitting the cells then up to C or 2C is OK but will likely reduce the lifespan of the cells. Any faster than that you'd better have a smart charger and a shield over the cells just in case a cell violently vents.
And the key difference between NiMH and NiCd? When doing a fast charge (C/3 or more), the "full now" voltage does not dip as much in NiMH as it does in NiCd, so fast chargers don't know when to stop unless designed for NiMH. You won't have that problem with the HP charger. It is nowhere near fast.
PowerStream has some good info: