Charging two 12 volt batteries together in serial for 24 volts

[DeanHensler]

I have a 24 volt trolling motor on my boat. I have two 12 volt deep cycle batteries wired in serial to make 24 volts. There is no power hookup anywhere so I need a solar system to charge the batteries. The boat sits out in the sun all day, every day. I only use it about once per week. How many watts do I need to charge the batteries?

[Logan005]

some information is missing, type of boat, does boat already have a gas or diesel engine? but assuming you have two typical 100 or so AH Batteries, I would use an MPPT charge controller with 3 X 100 watt panels wired in series. This would maintain your batteries and even recover batteries that have a small discharge. You will still need a regular stiff charge after any serious use. The best way is to have an onboard 110 to 24v charger and plug it in. Solar will cost a lot and save very little if any at all.

[SunEagle]

Or the other solution would be to charge those batteries at home and then use them when you go out in the boat. Probably the cheapest solution then going with solar to charge your batteries.

[DeanHensler]

It's a pontoon boat with a small 9.9hp outboard engine. I have been taking the batteries out, bringing them home and charging them, then taking them back to the boat for a couple years now and its just too much work. They are big heavy batteries. That's why I was interested in a solar solution to charge them. I didn't think it would take 300 watts because they will get many days of sunlight between uses.

[ButchDeal]

It's a pontoon boat with a small 9.9hp outboard engine. I have been taking the batteries out, bringing them home and charging them, then taking them back to the boat for a couple years now and its just too much work. They are big heavy batteries. That's why I was interested in a solar solution to charge them. I didn't think it would take 300 watts because they will get many days of sunlight between uses.

It will take much more to charge them. That will just maintain and top off slightly. You can't trickle charge the batteries, you will quickly kill them like that.

[jflorey2]

It's a pontoon boat with a small 9.9hp outboard engine. I have been taking the batteries out, bringing them home and charging them, then taking them back to the boat for a couple years now and its just too much work. They are big heavy batteries. That's why I was interested in a solar solution to charge them. I didn't think it would take 300 watts because they will get many days of sunlight between uses.

You still need to get enough power to bring them to a bulk charge voltage. That takes power, not just voltage and time. To just maintain them you can charge them at a C/20 to C/40 rate, but if you really want to charge them after using them you need to get to at least C/10. That means if you have 100 amp hour batteries you need 10 amps of charge power. At 30 volts (a typical voltage to charge 24 volt batteries) that means 300 watts of solar. That would give you a full charge in about 2-3 days of full sun or a week of intermittent sun.

Fortunately solar is cheap nowadays and you can get a single ~300 watt panel like the Kyocera KU340 for about $300. A cheap MPPT charge controller will run you another $170 or so.

As a side note most outboards will give you at least some charging power. The Mercury 9.9hp will give you 6 amps, for example - not enough to charge the battery by itself, but it would definitely help.