Can I safely charge 2 differing batteries on the same solar charger?

[BobTheNob]

I have a little 30w solar setup, and have a question about charging two batteries at the same time.

One battery is an 80Ah deep-cycle battery that is permanently connected to the system; the other is a 20Ah sealed lead-acid battery I remove for use at night, then want to recharge by day (both are 12v).

Question: Any safety (or other) concerns with just hooking the SLA battery in parallel with the deep-cycle one, given their different chemistries and charge levels?  (I understand there's no point in doing this unless the solar panels are generating, but is there a down side/better way?)

Thanks ...

[TomW]

Question: Any safety (or other) concerns with just hooking the SLA battery in parallel with the deep-cycle one, given their different chemistries and charge levels?  (I understand there's no point in doing this unless the solar panels are generating, but is there a down side/better way?)

Thanks ...

I would be most concerned with directly hooking a discharged small battery to a well charged larger battery. The big battery will try to "instantly" charge the little one and there is a real danger the small one may heat up, melt,  explode or simply start a fire when it outgasses and then the hydrogen finds an ignition source.

It could be done but you do not want to simply directly connect the 2 unless they  are at the same voltage. Charging them together is one of those "will work" things but generally not a good idea.

What you want can be done and is being done but not by myself so no real experience here.

Others will chime in on this I think.

Good luck with it.

Tom

[wpowokal]

What Tom said, with Dc one must balance voltages as close as possible before paralleling (what you are proposing) as opposed to AC when one must adjust both voltage and frequency.

You could have a circuit that limited the current to the smaller battery until the voltages balance, perhaps a control unit intended for duel batteries on a vehicle would give isolation and allow charge from only the solar source. I would still be concerned about the two battery's having different charge requirements.

allan

[ghurd]

Can get them balanced with a 12V light bulb (50W?) between the 2 positive terminals.
In a couple~few minutes, the voltages will be pretty close to the same.
Could hard wire the bulb in the wire, with a parallel switch to bypass the bulb when the voltages match.

I would maybe be concerned with the different type battery requirements, but I kind of doubt a 30W panel will get that much battery too high if they are being used daily.
G-

[zap]

Exactly how ghurd says, a light bulb hooked in series.
I do it with small SLA and NiCd packs to keep the inrush current low.

[Rover]

Could hard wire the bulb in the wire, with a parallel switch to bypass the bulb when the voltages match.

I would maybe be concerned with the different type battery requirements, but I kind of doubt a 30W panel will get that much battery too high if they are being used daily.
G-

Bob , exactly what GHurd and Zap said, notice the little bit about different battery type , although probably not a concern now, but if you increase your solar capabilities by adding more panels...rethink your battery arrangement, not a good idea to mismatch battery type/capacity/age.

[BobTheNob]

Thanks, guys!  It is a small system & it only has to run this way for a week ... I'd heard the light bulb trick somewhere but forgotten about (or didn't really understand) it.  Great tips, thanks!!

- Bob