Need an opinion

[DougT]

I'm trying to disconnect a water tank control panel from the grid. In 15 years it has never registered a single kilowatt of use but we still pay the $24.00 a month meter charge. I'm going to hook a 24 volt solar panel to the control panel to power the system. My problem is that the RF antenna is 12 volts and the water tank level transducer is 24 volts.Each of these use virtually no power. I'm going to use two 12 volt 7amp hour batteries wired in series to get my 24v volts. My question is: Can I also pull my 12 volts from one of the batteries in the series. My common sense says that this would not be a good idea if you were pulling a heavy 12volt load from one battery and a light 24volt load from both batteries, but since both loads are negligible and the solar panel will easily keep the batteries fully charged I think It would work with out a problem. Any opinions.

[ghurd]

I wouldn't pull power from just one.

An LM7812 would supply 12V from 24V.  Not very efficient, but better than ruining batteries.

G-

[ghurd]

"A 24V panel" to a pair of 7ah?

Meaning 2 tiny 12V panels?

Might be able to run a panel to a battery, each controlled by a PB137ACV.  Like 2, mini 12V systems.

Then series the batteries for 24V.

I was just thinking 24V controllers are not cheap or small.

G-

[commanda]

If I was doing this, I'd make the battery/solar panel 12 volts, and use one of those little 12-24 volt switchmode inverters. You'd need to know precisely how much current the 24 volt powered controller required, so you could size it properly. The ones I mean are the little pcb mount ones. The smallest are like an over-sized IC.

Amanda.

[richhagen]

I would not consistantly pull more power from one battery than the other, as you will likely also consistantly overcharge the underused battery in the series string, resulting in premature failure.  I have seen some relatively inexpensive 12V DC to 24V DC dc/dc converters on Ebay for small power draws, similar to what Commanda explained, which seem like a better long term solution.  Rich

[paborralho]

When you say negligible, are you saying something like 50mA? 100mA? In that case, I, (always voting for the KISS party) would connect it to one of the batteries, as you suggest. At most, to keep balance you can put a similar load to the other battery, or even a switch to change periodically from one to another. Inverters, step up an down, are not that eficient and you would be consuming more than needed and would have more parts that can malfunction some day.

Have fun