Posted by David Sherman on Thursday, January 19, 2006 at 7:02PM :
In Reply to: Re: 24V to 12V inverter posted by doryboy57 on Thursday, January 19, 2006 at 12:15PM :
Your system will work okay if you have a small 12V load you use only occasionally, like perhaps a radio. If you draw any significant current for any significant amount of time (fans or lights), the unbalanced current drain between the two batteries will cause the low (grounded) battery to be undercharged while the high one will get overcharged. To some extent you can tolerate this if you swap them periodically or at least check the water in the high one frequently, but in the long run it's asking for trouble. The M211 brush rig I bought from the FD here supposedly had some mysterious electrical problem that caused it to use up batteries frequently and be unable to hold a charge. When I got into it, it turned out they'd mickey-moused all the new accessories (2-way radio, rotating hazard light, and turn signals) by simply tapping into the mid-point of the two batteries. I disconnected all that junk and it can sit for 6 months and then start right up and the batteries stay evenly charged.
APB's device is a very good solution at a reasonable price. Another option, for higher current drains, is a "charge equalizer", which is basically a fairly low current converter that produces an output voltage equal to exactly half the input voltage. When a heavy 12 volt load draws more than the equalizer can supply, the load draws the excess current from the low battery, but when that load is turned off, the equalizer charges the low battery back to what it should be without overcharging the high battery.