How to charge 4 marine batteries with a single solar panel

I’m interested in maintaining charge on 4 12V marine batteries. Can I use a single solar panel like this one and 4 solar charge controllers such as these?

Would I be able to switch the batteries on to boat power without disconnecting the controllers (There are switches that disconnect the batteries, connect them in parallel, or use either battery 1 or battery 2}?

Sure, but you can generally get away with just wiring all 4 batteries in parallel. That is, the “-” on that solar charge controller would go to the 4 negatives on all 4 batteries, and the plus would go to the 4 positives. You generally cannot do this if the batteries are lithium, but marine lead acids usually work. (one flaw is that if one battery fails it will bring down the others)

And if you just look at the solar charge controller, the output - powering the boat - is on it. If you want the alternator of the boat engine to also charge the battery at the same time, if it’s running, you can probably do that as well.

Depends on the goals here, btw. Running a bilge pump, where if that bilge pump fails, the boat will eventually flood and sink? (by a slow trickle of water over weeks)

Then you should go for full redundancy. Have a second solar panel, second battery, and second charge controller on a second bilge pump. Maybe use a different brand of charge controller and solar panel. I just noticed the controller you have linked is not marine rated - it looks like it will allow the salt mist, if your boat is oceanside, directly into it’s housing. Will run for a while, just expect to be replacing it routinely.

I am not an expert in this area, but I have used a 100-watt solar panel to charge a single marine battery for use in a shed. 100 watts is not a lot of power to charge four marine batteries, even under ideal circumstances. Based on my experience, I would stick with a single battery, or two at most.

Here is information on marine charging systems, should be good advice for this situation as well. The author is a SME contributor to a sailing/boating forum.

Marine Solar Charging Systems

Thanks. I’ve inherited a fishing boat that has two banks of 12 V batteries, each redundant and can be switched to parallel with the flip of a switch. The banks can be crossovered if necessary to start the motor if the motor/electronics batteries fail.

I don’t want to run all batteries in parallel as the existing architecture is quite workable. I also heard it was bad to try to charge batteries in parallel unless all batteries are identical. In this case, this is true, but are their inherent problems if some of the batteries are “better” than others?

My primary goal is to trickle charge the batteries to keep them topped off as the boat is sitting in a storage facility without easy access to an electrical outlet. Secondarily, when I’m out on a trip I’d like to have the solar panel supplement my electricity.

The dual bilge pump sound sensible, but I want to change as little as possible and their ain’t much room down in the engine compartment. Would it be worth it to have a manual backup?

Strongly suggest you head to the forums at for these questions. I know we have smart people on this board but the referenced site is helpful and experienced. Despite the name, it is not limited to sail.

Thanks. I’m a member of the Hull Truth, but I’ve never gotten a single reply from that board. I see that sailboat owners has an engines and propulsion subforum that may be useful.

Do batteries charged in parallel have diodes in place to prevent them from charging each other?

Sometimes. It’s quite common to separate banks of batteries used for fire alarm systems in a way that will prevent one failure from affecting the entire set of batteries. Reduced operating time is better than total failure.

Yes, batteries charged in parallel should be as identical to each other as possible.

My earlier comment about 100 watts being a bit light for four batteries was meant to apply if you are trying to charge them after a discharge cycle. Maintaining the batteries after a full charge is a different story. Still, I’d suggest no more than two batteries in parallel for other reasons.

More info would be helpful, please.

I’m too lazy to draw this out or search for a schematic, but I’ll describe the way this is done.

First, most FA control equipment operates at 24 VDC, so two 12 VDC batteries are wired in parallel. For larger systems, there will be two or more of these sets of two batteries, so imagine four batteries arranged in two sets of two.

Second, these batteries are usually arranged in a separate power supply with a dedicated charger. In other words, the power charging connections/terminals are not the same as the load connections. This would be somewhat similar to using a solar panel to charge and then drawing current directly from the batteries for a pump or other equipment.

Since a diode is really just a “one-way valve” for DC current, insert a diode(s) so current can only flow FROM the positive terminal of the charger to the positive terminal of each battery set. Insert a second diode(s) so positive current can only flow FROM the positive terminal of each battery set to the positive terminal of the load.

If one set (pair) of batteries has an internal fault (e.g., a shorted cell), the other set (pair) of batteries will not be affected, since current can’t flow from that set to the faulted set. You can achieve the same result using bridge rectifiers.

There are other considerations, of course. The diodes have to be capable of carrying the current, there will be some slight loss, and you have to consider if the charger will be damaged if it is attempting to charge the batteries while current is being drawn from them. (In a FA system, this is usually not a concern since the failure of commercial power means the charger isn’t charging the batteries anyway.)

It’s possibly unclear that you need FOUR diodes to accomplish the above. Each pair of batteries has TWO diodes connected to the positive terminals (that are in parallel). One FROM the charger and one TO the load.

Do you mean in series?