# Thread: Help with my tank circuit.

1. thanks man! You have been most helpful.

2. Would there be a problem of overcharging the gel cell?

I came across this site. It talks about adding a limiting resistor to safely charge the gel cell.

3. You don't need the resistor or the extra diode. That is being anal and isn't really nessessary. plus the "float" voltage the car puts out is well within the range of the Gel cell.

4. I don't know about this setup, but if you have two batteries hooked directly to each other and the alternator (like in my diesel truck) you need two batteries of the same capacity, or else one may get overcharged/fail/worse.

Perhaps having your battery on its own circuit and isolated might be different.

5. Originally posted by DodgeCummins
(like in my diesel truck) you need two batteries of the same capacity, or else one may get overcharged/fail/worse.

How can you overcharge a wetcell battery that charges by having a constant voltage? I can see the argument about having too much inrush current if one is depleted (a smaller battery). If the two batteries are in parallel and they are of different capacities, one runs out faster, but the voltage at the terminals are the same for both, so all the current just comes out of the one with more capacity.

NiCads are trickle charged with a constant current, IE if you hooked a 1.2 volt nicad to a current limited 25 volt source, and set the max current to 50 milliamps, it could charge forever and not get hurt. Where as if you put a wet cell battery in this situation it would kill it after a while because it would try to charge to 25 volts.

So if I'm understanding it correctly, to charge it, you take a wetcell battery (like in a car) and you just hook it directly up to 13.8 volts, if the battery is low, it sucks a lot of current, it is the regulators job to not exceed initial current rating. So if you have two batteries, the regulator cuts out sooner, if they are different capacity, one will take more current, but they will both charge to the same voltage.

Can someone help me understand why you need Isolation? And why this is even a problem?

6. Well I don't know about isolation, but the battery came in today and it solved my problem. Thanks Jeff!

7. Originally posted by ddt
Well I don't know about isolation, but the battery came in today and it solved my problem. Thanks Jeff!
If you don't let it get too discharged, Isolation shouldn't be a problem. If you run the thing for a long period of time with the car off, there is a chance you could exceed that initial current rating..

-Jeff

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