How do I charge a small battery off a large battery.

[SteveW]

I would like to maintain a small (12v 7ah) battery off a 12v 100ah battery which is charged both from an alternator and solar.

The 7ah battery is nothing to do with the RV and is for a particular use, but I would like to have it fully charged and handy when needed.

My best guess is to insert a diode preventing current running back from the 7ah to the 100ah whilst, at the same time, allowing for current from the 100ah to keep the 7ah charged.

a) Is this possible?

b) Am I on the right track?

[T1 Terry]

You are on the right track but it must be a Schottky diode as they have a lower voltage drop across them and it needs to be high enough capacity to not over load and fail. The alternative is Zener diode, it will only pass current once the battery is above the Zener's voltage and will not allow back feed. As the second battery is only 7Ah the first idea is probably the easiest, 7Ah pulled out of the house battery isn't going to affect it much. This unit mounted on a heat sink will do the job fine as the 7Ah battery could not accept 50 amps for more than a few seconds before the terminal voltage increased to the point where the transfer would drop to 10 amps or less. To be sure to be sure, you could bridge across both halves of the diode block and that would give you approx. 100 amp continuous capacity and 1,000 amps surge capacity. http://au.rs-online.com/web/p/rectifier ... s/0155513/

T1 Terry

[SteveW]

Ok. There is sufficient reading about Schottky on Google to last me a life time but I have got the general idea. Nothing will be connected to the 7ah battery while it is connected to the 100ah battery - for charging purposes only. Therefore, am I correct in thinking that a small (low amp) schottky diode is all I need?

[T1 Terry]

The difference in voltage between the small battery and the big battery at the time of connection will determine the current flow (amps) If the big battery is up around the 14v and the little battery is down around the 11v or lower as it's full discharged, the amp flow for the first few seconds could be in the hundreds, if it was from a lithium to a lithium, in the thousands, until the cable heated up and started to reduce the current flow. I did a demo quite few yrs back at a Taggerty get together where I used a 5 cell Li battery (15v) to charge a 4 cell Li battery (12v) The 1/0 welder cable was smoking at the brass battery clamps on the heavy duty truck jumper lead set I used, that much current with only a 3v differential, so you need to use a diode large enough to handle that in rush current

T1 Terry

[SteveW]

So the link you sent is the size you recommend. Correct?

[Peter and Sandra]

Could be easier and safer to use a small 12-240 inverter and 240v charger to prevent problems, not as efficient but could be a better alternative.

[BruceS]

I have a small deep cycle battery (7AHr) that is used to run a small 12v water pump for my pure water. (coffee, cooking, drinking) It is a seperate tap on the sink.
When I bought the houseboat it was there. It didn't have any form of charging connected. I intended running a wire to the 12v house batteries (lights, radio, GPS, etc only now) but decided to see how it performed first.
I only need to recharge it twice a year!!!
What I do is connect it to the other big bank BEFORE I charge them. That way both the little one and the big bank come up slowly together.
Seems to work for me. How big of a drain do you get on your small battery?

[T1 Terry]

So the link you sent is the size you recommend. Correct?

It would be big enough to cover the job you have in mind no matter how far you flattened the little battery

[SteveW]

Next to no drain, Bruce. It just powers up a small circuit board while the board is connected to a PC for some firmware alterations. Might only be ten minutes. It's nothing to do with the motorhome. I carry it with me in the Terios - you might recall the Terios has a 100ah deep cycle battery to run the Engel. The 100ah battery is charged by a 200w solar panel mounted on roof racks and which doubles as a car port for the Terios.

[jon_d]

Just stick a ordinary diode and a 10 ohm 10watt resistor in series between the positives.
The battery will always be no more than 0.6 volts lower than the rest voltage of the big battery.

The 0.6 voltage drop will help stop the little battery boiling off when the bigs go to float.

The resistor will naturally limit the current and give a gentle charge. The diode has to be closest to the big battery and the resistor closer to the little battery. This ensure the diode turns on.