CONTINUOUSWAVE

I have mentioned it several times in posts that I have used small solar chargers for 10 years now to keep my batteries topped off. I figured I would show the one I have had the best service from and explain why. I like the NOCO 2.5 watt model and it is also available in a 5 watt version. I paid about $25 for it recently but I have found it for $20. I am on my third one since I have gotten clumsy in my old age and these have glass screens-- so don't drop them. It has suction cups to stick to a windshield, or reversed to stick on the back of my console when I'm underway. At the dock I stick it to my bench seat facing the sun. It has several wiring options with a waterproof quick disconnect. I chose to hard-wire it to my battery so it is charging at all times when the sun is up. It has a diode so it will not discharge at night. It is ignition protected so starting the motor while connected will not harm it. It is fairly waterproof out of the box, but they recommend you seal around the screen edge with silicone for prolonged outdoor use. I did that and so far no problems. My latest model has an LED in the connector to show if it is charging. My battery has always [powered a pump that] pumped out the rainwater and I have never found my battery too drained to start the motor. If you keep your boat at a mooring, the solar charger would be the best way to keep your batteries charged.

I am guessing that a photovoltaic panel rated at 2.5-Watt when connected to a 13.2-Volt DC battery would produce a maximum current of:

P = I × E

I = P / E

I = 2.5 / 13.2

I = 0.19-Ampere

If you an get 8-hours of daylight each sunny day, then the total charge per sunny day would be

Ah = 0.19-Ampere × 8-hours

Ah/Day = 1.5-Ah/1-Day(sunny)

If you had a fairly typical 60-Ah battery, and it was half discharged, you would need to provide it with 30-Ah of charging. If using the above photovoltaic panel as a charging source, you would need

30-Ah × [ 1-Day(sunny) / 1.5-Ah ] = 20-Days(sunny)

Depending on your climate, you could figure how many calendar days it takes to get 1-Day(sunny). In Michigan maybe we'd average about 1.5-Days(calendar) / 1-Day(sunny). On that basis, it might take a month (30-calendar days) for the charger to restore 30-Ah charge to the battery.

Jim I would agree that a solar panel could not possibly do what a regular 10 amp plug in charger can do. I have motors with alternators but depending on how they were used during the day the battery may be left weak. Same for a boat pumping rainwater and not given a chance to recharge. That is why I used the solar charger just to top off the battery while the sun was out. FWIW my last marina had no electric service so it was solar or nothing. I used a 1.5 watt model then and it worked pretty well. A mooring would be a similar situation but I would advise the 2.5 or even 5 watt model just to be safe. Any bigger gets unwieldy. I also use these chargers in cars that don't get used much. They always start right up even when the battery should have been long dead. They are even better in cars that have a lot of parasitic voltage loss like computer and radio memory or ONSTAR which will kill a battery in less than a week.

I suspect that using a photovoltaic charger to maintain the battery in car when it is not being used for a long time only works if you keep the car outdoors in an open area. But thanks for the reminder. I better start my old Suburban. It has been in the garage for the winter, and it hasn't run in about two months.

Also, that photovoltaic panel comes with some nice connectors and cables. It looks like a great deal for the very modest price.

I use this charger for the breakaway battery on my trailer. It works well.
Chuck

Jim--I keep an old Firebird in a closed garage. There is only one window and that faces South West. I simply stuck the charger to the glass and extended the wire so it reached the battery. Even with that less than desirable exposure it did the job. I find that even on cloudy days it charges OK and on super bright days it charges well above what it's rated for. May be I could use my multimeter and get some values if I get ambitious.