newcomer question

[MikeB]

Can someone direct a newcomer to info on how to charge from one 12v deep cycle battery (fully charged) to another 12v deep cycle battery.

thank you

[Nando]

No enough information.

How many amps do you need, also the capacity of both batteries.

Normally you need to have a voltage greater than the battery to be able to charge that battery and you have a case where you have two equal voltages batteries.

I have done and requires a small isolated power supply fed by the charging battery and the output voltage added to the charging battery.

At the same time this scheme can be used to feed a lap computer from a 12 volts battery for those PC that require greater than 16 volts supplies.

Nando

[oztules]

Small Boost converter. Look up article http://www.fieldlines.com/story/2006/3/17/185646/194

It's called matching the load   by Flux.

Among other things it explains the function of boost converters. Example circuits are provided also. From the discussion and the circuits, you should be able to configure your own. If not, the possibility exists that this is not for you. in this case, look for commercial boost converters of the voltage you wish to use. for a 12v battery you will require a voltage of at least 13.7v to get good the "to be charged " battery fully float charged from the donor battery.

..............oztules

[drdongle]

Generally you need a DC to DC converter ( 12 in 14-16 out) but as Nando said current is an important consideration and as yet an unknown.

A more efficient approach is to simply charge both from the same source, provided it can supply the required current.

Perhaps if you expained your application we could make specific recommendations.

[nothing to lose]

Well you lose alot of power, but certainly the easiest way is to run an inverter from the full charged battery and a battery charger from the inverter.

I did this a bit to start with when I rented a remote house I kept off grid while there, but had no charging source there.

In my truck was a battery bank, I charged it at my main home from the grid, wind gennies, while driving etc...

 At the remote house was a bank also. It ran the lower power needs durring the day while no one was home. To keep it charged sometimes I would run an inverter from the battery bank in the truck, I think a 400watt inverter would run the charger at 20amps, and over night the house bank was getting a 20amp charge all night. When I needed more charging I ran the charger from a 5K inverter on the truck battery bank and charged at 40 amps. I did alot of things different ways different times, that was one of them.

Of course there was alot of loses involved running an inverter to make 120Vac just to run a charger to make 12Vdc again.

What and Why can be as important as how! Do you just need to keep a 2amp load running steady for a week some remote place and charge once a week? Then inverter and charger may work well and easy.

[MikeB]

Thanks for all the good advice. The situation is that I am setting up a 50w solar panel to charge a couple 115 amp deep cycle marine batteries on a dock. One of the batteries is in the boat, the other on the dock. What I realized is that after a day use of the boat battery, I would like to charge it overnight so I could use it again the next day. (switching batteries is not an option, the one in the boat is very hard to get out) If I will not be using it for a few days, no problem, it should re-charge from the panel. So in essence, the question is how to get stored energy from one battery to the other of the same voltage, I guess. This is my first attempt at solar power and I plan to expand, as there is a small cabin on the lake property, also without grid power.

Thanks again, and I hope I've explained this correctly.

Mike

[thirteen]

could you mount a small solar panel some where on the boat and charge the battery