At 13:57 5/13/2004, you wrote:
tmpm wrote:
<snip><snip>Buy a cheap one that exceeds the wattage you need for every device or cluster them, and remove the little tiny gell-cell and tie it to a truck battery kept on float at about 70-100ma and youre golden.
Do be careful here. There was a time when Lead-Acid battery banks were somewhat common - and so were the explosions from accumulated hydrogen. You see, once a battery is fully charged, all the energy run through the battery (i.e., the trickle charge) just heats the cell and splits water into hydrogen and oxygen, which is a really dangerous combination. (If you don't believe this, note that it's used as rocket fuel.) Gel cells are specially designed to not emit any hydrogen/oxygen and are considered relatively safe when used indoors. Car and truck batteries, since they are designed to be used outside, where the gas can't accumulate, have no such provision. If you decide to use one of them, just make sure that you provide plenty of ventilation to the outside. And, use a fan motor which has no brushes (this would be almost all commercially available blowers and fans).
Also, if you really want to do this, you may find that, depending upon the battery's capacity, 70-100ma may not be enough to exceed the minimum current required to charge the battery - the first bit of current just heats up the cell and produces gas. Generally, lead acid batteries should be charged at a current just great enough to exceed the theoretical cell potential of 2.2V per cell (13.2V for a standard 12V car battery at 25 degrees C). Typically, the optimum charge voltage would be about 13.8V for a 12V battery.
Stephen R. Besch
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