Solar Panels - How big to maintain batteries

[dthiesmeyer]

I have two AGM batteries that I would like to keep fully charged between uses. I go out once or twice a week for 3-4 hours and have found that my batteries are not being fully charged by 15 to 20 minutes of running the motor going back to the marina. I typically use my Lowrance GPS, radios, and occasionally my Raymarine autopilot. The autopilot seems to drag down the batteries within a couple hours. I have researched solar panels and controllers on this site and would like confirmation that a 50w to a 100w panel with a 10w controller would suit my current needs.

Thank you in advance for your responses.

Dave

[NiceAft]

Dave,

I too am interested in such a set up. My knowledge in this area is practically non existent. Let's hope (for my sake at least) the sure to come information is quoted from “solar set up for dummies”.

Ray

[Dragos Filimon]

Practically you need a setup to cover what you use out of batteries. You have low consumers, probable max 3 amps hr( AKA 36 watts in an hour) for 3-4 hours means about 150 W. You have about 2 days between the rides. About 50 hours available to put back the 150W. Soooo, you have to charge some 10 W per hour (8 hr sunlight per day)

Conclusion (I hope correct) any panel bigger than 10 W will do.

[Dragos Filimon]

The autopilot seems to drag down the batteries within a couple hours.

OOOps I did not see that seems to be higher draw.

How big are the batteries?
I have about 400 amphr of batteries and my 100W solar panel takes about 1 week to load them up to max from totally drained.

[grady]

The autopilot seems to drag down the batteries within a couple hours.

OOOps I did not see that seems to be higher draw.

How big are the batteries?
I have about 400 amphr of batteries and my 100W solar panel takes about 1 week to load them up to max from totally drained.

If the auto pilot kills the batteries in a couple of hours I would guess your batteries are in need of replacement. Autopilot draw should average out to around 2A. 2A times 2 hours = 4 amp hours. Or possibly you have really small batteries.

[kevinnem]

you have to think of everything in "cycles" ,

in your case, you have a 3-4 hour draw maybe 2 times a week then 5 days of "rest". A small panel could certainly make a big difference. For ease of math, I will assume you use it every 3 days, ... so use it day 1, charge day 2+3

I am going to make some assumptions, please tell me what ones are good, and bad so we can make the estimates better.

Lets assume you have 1 large battery for running this (you might have a second starter battery, I am talking deep cycle that you use to run these systems though.)
Lets say this battery has a capacity of 100 amp - hours
Lets say your a good boy and you only discharge it 50%/trip. That is 50 amp hours.
so we have to rechage it at a rate of 50 amp hours every 2 days, or 25 amp hours per day. if we assume you will have to do this in 10 hours of daylight, you need to average 2.5 amps of charging at 14 ish volts. This is an *AVERAGE* of 35 watts but you will need to get much more then this at peak sun time to even out the low sun time. I would look for a solar panel at least 50 watts, and like more in to the 70+watt class.

I have likely over estimated the amount of draw the AP makes. but I called this "worst" case.

ALso, you should know not all charging time is the same,. as the battery fills up , it charges slower.

Kevin.

[dthiesmeyer]

I have two Optma sc34dm 750 cca batteries that were new last year. I believe that they are still in good condition. When my auto pilot ran it down, the batteries may have been already low. I currently have both batteries on a dual charger, so they should be fully charged the next time I go out. I will test the auto pilot again with fully charged batteries. I am leaning toward getting a 100w flexible panel that I can mount on the sliding hatch. Still not sure what controller to get. Any recommendations on a controller?

[sailboatmike]

Whilst the hatch is a great mounting point the issue becomes that solar panels lose a disproportionate amount of power generation when shaded partially.

[March]

dthiesmeyer wrote:
I have two Optma sc34dm 750 cca batteries that were new last year. I believe that they are still in good condition. When my auto pilot ran it down, the batteries may have been already low. I currently have both batteries on a dual charger, so they should be fully charged the next time I go out. I will test the auto pilot again with fully charged batteries. I am leaning toward getting a 100w flexible panel that I can mount on the sliding hatch. Still not sure what controller to get. Any recommendations on a controller?

Blue sky controller/booster. This is what we've been using for 15 years. They claim it "boosts" the electrical charge from the pannels by 15%, if need be. It has been working well, with one intriguing exception: about 12 years ago, boat just out of storage, it refused to charge, Called the company and they told me to change some jumpers in the back. That was really weird because the controller was mounted in the wall; how could the setttings of jumpers be altered over the winter? No matter, I followed their directions, and it started working again, like it used to.

The Optima batteries were a big disappointment. 5 years ago I invested into a bank of three. One broke down by the next summer (warranty was only 6 months ) the second one, the following year. I decided to replace the whole set with regular, cheap-o, wallmart batteries. Even if their life span is about 5 years, we still come ahead, especially if stored in the carage, with trickle charge, over the winter.

Aside from the AP, depthfinder, GPS, radio, etc., the hungriest consumer is a Waeco fridge. By trial-and-error, mostly, we decided that two solar pannel (160 W) are more than enough. Been toying with the idea of adding an extra flexible solar pannel on top of the bimini, or on the sliding hatch, (about $150) but I don't think it's necessary. Plenty of juice from a three-bank battery. Even if the level of the house batteries goes down to 60% overnight, three hours of sunshine are enough to get us back in the green

[kadet]

Sorry Optma sc34dm are not true deep cycle batteries they are cranking batteries with a nominal rating of 55ah any battery that quotes it's CCA is not suitable for deep cycle work. After a year of use running an auto pilot without continual charging they would be near depleted of electrolytes and need replacing.

Given shadow, non optimal sun angle, charging loss and your high latitude etc. expect to only get about 1/2 the rated power out of your solar panel for about 4-5 hours a day. So if you get a 100watts it will produce about 3A for 5 hours so you can put back about 15ah maybe a bit more or less depending on your regulator choice.

BTW AH is a very poor way of calculating things, get yourself a watt meter and then work out how many watt hours you need.

I run a fridge, autopilot, stereo, chart plotter and laptop plus the odd light etc.. I am sub tropical so get a bit more light and need 180watts to go for a few days to week between mains charging with a 40amp mains charger, according to my watt meter. I currently only have 80watts but am going to install another 100watts on the top of the bimini and a second battery. The current setup requires the generator to be run for an hour or two ever second day.

Also do not trickle charge AGMs they hate it and only run them down to 50% then charge them and charge them fast hence the 40amp charger.

[dthiesmeyer]

I ended up buying a Kingsolar 100W Semi Flexible Solar Panel and a Blue Sky Solar Boost 3000i MPPT Controller. This combination does exactly what I needed it to do. Since I installed this system, I have always had fully charged batteries when I went out to the boat. The last time I went out, I sailed over 13 miles, most of that with the auto pilot, and never had the batteries go below 12.5 volts. I mounted the solar panel on the main hatch and it has not caused any problems with handling of the main sail. I am very happy with this installation and would recommend it for anyone interested in adding solar panels.

Thank you all for your input on this project.

Dave