>
>I'm considering using 8 rechargable _ALKALINE_ D cells instead of the
[SNIP SNIP]
>I think the D cells should give me 12 amp-hours, which is about what many
>small lead-acid batteries are good for. I suspect that this might be adequate
>for my needs, particularly since the corrector plate is relatively small and
>should not require nearly as much heat as an 8" or 10" SCT corrector to keep
>it dew-free. I arrived at the 12 amp-hour figure as follows:
> 1. I measured the discharge curve of a conventional alkaline AA penlight
>cell, and found that it had put out 1.5 amp-hours by the time the cell voltage
>had dropped from 1.5 volts to 1.15 volts. It actually put out nearly 2 amp-
>hours by the time the discharge curve took a nose dive and the cell was really
>dead.
> 2. I assume that amp-hour capacity is directly proportional to cell
>volume, for two cells of the SAME DESIGN, but different sizes. I measured a D
>cell and a AA cell, and found that the volume ratio was roughly 12/1.5, so a
>_conventional_ alkaline D cell should be good for 12 amp-hours.
> 3. Further assuming that a _rechargable_ alk. D cell has the same
>capacity as a _conventional_ alk. D cell, so it should also be good for 12
>amp-hours.
>Thanks in advance for any info. -- Gerry
Gerry,
Unfortunately, the 'typical' Ni-Cd D-cell it only rated at 1.25 volts (not 1.5) and about 1.5 to 2 amp-hrs. A 'heavy-duty' Ni-Cd D-cell might be rated for about 4 or 5 amp-hrs at 1.25 volts. These 'heavy-duty' cell usually cost as much as eight to 10 dollars EACH and your dew heaters alone will probably suck up that much energy in a short time.
For the same price, albeit with less portability, you can get a 12 volt 20 or 30 amp-hr deep cycle gel cell someplace like Sears. They make one designed for wheelchairs that works great. I epoxied two wooden blocks to each side of one of these and attached two nylon ropes to each wooden block to use as a handle. It's kinda heavy, about 15 or 20 lbs, but I can put it in the trunk without worry of it spilling if it tips over. I can run a telescope drive, three dew heaters, and a small fan all night.
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