Solar charging deep cycles- regulator necessary?

[chris95040]

My house bank is 2x 200ah AGMs. I'm looking for a simple solar solution to keep them topped off during the week when I'm not there.

Assume I use 25% during the weekend (100ah * 12v = 1200 watt hrs, that's a lot of juice for us.)

I suppose on average there's 12 hours of sunlight per day, 5 days, so 60 hours to make up my 1200 watt hours. That implies a (1200 / 60 = ) 20 watt panel.

There are a few things I'm not sure about though:

1. If it's overcast, will solar panels output something like 50% their rated output? More? Less?

2. Does the battery's charge state affect output capability of panel? (Is it harder to charge a nearly full battery, so panel output decreases as charge state increases?)

3. Is there a rule of thumb for what constitutes a 'trickle charge' for a given size and type of battery? In other words, what's the max wattage panel I could safely attach to my 400ah worth of AGMs without adding a regulator/etc?

4. Do you need a diode to stop current from flowing out of the batteries at night, through the circuit created by the solar panel, or is the resistance of that loop pretty high and therefore insignificant?

[deckofficer]

A solar panel's output is given for direct exposure, so it is better to assume at your lat during summer months about 6 hours X panel's rating. So if you need to harvest 1200 whr in 5 days, I would go with at least a 40 watt panel, more if a marine layer of morning fog is normal for your area.

Always use a charge controller, don't wire a panel directly to your house bank.

A 40 watt panel will have diodes.

[chris95040]

Using some sort of charge controller does seem like the safest bet, and likely the route I'll go...

But for curiosity's sake, if a panel's peak output is at or less than what's considered a (maintenance/ trickle / float) charge for your battery set, isn't overcharging impossible?

This of course requires knowledge of what constitutes a float (aka trickle?) for a given battery set, something I'm not certain how to quantify. I believe 2amps (24 watt) is appropriate for a ~ 100ah set, intuition suggests this scales linearly (twice the batteries, twice the safe trickle-current swallowing, in other words my 400 ah set could take 8amps continuously) but I don't know for sure!!

[deckofficer]

Originally Posted by chris95040

Using some sort of charge controller does seem like the safest bet, and likely the route I'll go...

But for curiosity's sake, if a panel's peak output is at or less than what's considered a (maintenance/ trickle / float) charge for your battery set, isn't overcharging impossible?

This of course requires knowledge of what constitutes a float (aka trickle?) for a given battery set, something I'm not certain how to quantify. I believe 2amps (24 watt) is appropriate for a ~ 100ah set, intuition suggests this scales linearly (twice the batteries, twice the safe trickle-current swallowing, in other words my 400 ah set could take 8amps continuously) but I don't know for sure!!

Maine Sail says it better than I can......

Solar Over Charge - 12W Unregulated Panel vs. 220Ah AGM Bank - YouTube

[djmarchand]

First of all your solar panel won't put out any way near the current in your first proposition. If the panel is rated at 100 watts or about 7 amps you will typically get 35 amphours out of it on a full sunny day, much less on cloudy days, sometimes less than 10% if it is a really dark day.

Panel output is influenced by angle of the sun, shading, cloudy days. You can get a bit more output than 35 amphours for a 100 watt panel if you angle it towards the south.

Other answers:

2. Yes, charge acceptance decreases as the battery gets above about 70% full.

3. You can probably get by with a trickle charge solar panel of 20 watts or so. Any more and you need a controller. But it will take forever to replace the 25% that you use over the weekend.

4. A controller will have a built in diode. If you don't use a controller, some panels have blocking diodes, some don't so to be safe add a diode if you aren't using a controller.

So if you want to replace 25% of 400 amphours in the 5 days between weekends- or 100 AH, you probably need at least a 100 watt panel assuming 3 equivalent full sun days in that period.

And FWIW, panels are now cheap and maximum power point (MPPT) controllers are expensive, so I would buy a 135 watt panel (Kyocera makes a good one) and install a 10 amp Morningstar pulse width modulation (PWM) controller. The extra 10% that the MPPT controller isn't worth the cost.

David

[chris95040]

Wow!!!!

Excellent video post, Bob. Settles that one.

David, thanks for the help sizing the panel.


-Chris

[hellosailor]

Chris, what's that phrase about "as reliable as the day is long" ?

Sunlight is funny stuff, you can't just schedule it as needed. So personally I'd say to buy one or two solar panels that are as large as you can, within the restrictions of budget and a good installation place (not shadowed, not in the way, not likely to be hit) and probably a Genasun MPPT controller. Genasun has a number of very reasonable smaller-size controllers and if it is more or better than you "need"...remember that one of these days it will be rainy and overcast all week, and you'll be trying to get in some extra time on the boat, or running the batteries down low without the usual recharge time, or something. Isn't it always something, on a boat?(G)

So this is not one to overthink, but rather, do the best charging you can and one day you will be glad you did. You can't really get wastefully excessive on a small job with one or two panels and a matched controller.

The other path of course leads to much reading an pencil sharpening and a fast learning curve about all things solar.(G)

[mbianka]

Originally Posted by chris95040

Using some sort of charge controller does seem like the safest bet, and likely the route I'll go...

But for curiosity's sake, if a panel's peak output is at or less than what's considered a (maintenance/ trickle / float) charge for your battery set, isn't overcharging impossible?

This of course requires knowledge of what constitutes a float (aka trickle?) for a given battery set, something I'm not certain how to quantify. I believe 2amps (24 watt) is appropriate for a ~ 100ah set, intuition suggests this scales linearly (twice the batteries, twice the safe trickle-current swallowing, in other words my 400 ah set could take 8amps continuously) but I don't know for sure!!

A good solar controller will make sure your batteries will never be over charged and they won't drain at night they are cheap insurance. I've used Morningstar PWM 15 amp controllers on both my 12 volt Gel house bank and 48 volt AGM propulsion battery banks for seven years now without a single problem.

[deckofficer]

Originally Posted by chris95040

Wow!!!!

Excellent video post, Bob. Settles that one.

David, thanks for the help sizing the panel.


-Chris

I can't take credit for the video, it and many others were put together by CF member Maine Sail, aka Compass Marine Inc on YouTube. He has many marine electrical videos on YouTube.