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Solar charging, cheap

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ebsail

.
Nov 28, 2010
241
O day 25 Nyack. New York
Bought a 30 watt solar panel ($58) and controller ($18) and just wired it directly to one of the two group 24 batteries in the boat. We leave the master switch in the "both" position so both batteries charge. No permanent mounting. We just leave the panel face up on the seat in the cockpit and presto, fully charged batteries in a few days. Theoretically puts out almost 2 amps per hour in full sun. The boat is on a mooring, so no more trips to a dock to charge up. I just put the panel face down in the lazerrette when we're sailing. Should have done this years ago. Ernie
 
Mar 2, 2011
489
Compac 14 Charleston, SC
It's amazing how much cheaper solar panels are these days. I used a cheap harbor freight brand 30 watt panel and controller to recharge a group 24 battery. Using a 50 lbs thrust trolling motor to get in and out of the marina, I'd drain the battery down only 20-30% each trip and it would charge back up in a few days. I figured I had about 2 hours of trolling motor run time at a medium setting which would push the Hunter 23 we had at the time to about 4 mph.
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
Try not to confuse voltage rise with a capacity increase. In order to get current flowing into a battery you need to increase the voltage. A voltage increase while charging is not representative of state of charge until you can hit and hold an absorption voltage for a while. If you are at absorption voltage (roughly 14.4V) you can safely guess that you're at close to 80% state of charge or higher. This is with healthy batteries. Batteries suffering from sulfation will come up to 14.4V well before they are actually at 80% SOC and this is then misleading. If you have an inexpensive shunting controller then the last 20% of charge will take a loooooong, loooong time regardless of sun.

A 30W panel will pump out about 1.8A for 3 - 4.5 hours per day, roughly. This is also with no shading. So if we figure 1.8 X 4 hours this = 7.2Ah per day of charging.

With a 100Ah battery at 50% state of charge 50 Ah's, plus about 15% charge inefficiency, would need to be replaced or 50Ah X 1.15 = 57.5Ah.

57.5Ah / 7.2Ah per day = 8 days to get a single 100Ah battery from half full to full, in perfect conditions with a 30W panel.

A 200 Ah bank would take approx 16 days to reach full, from half full...... These are averages and you may do slightly better and you very well can do considerably worse. A 30W panel simply can not take a group 24 battery from 20% state of charge to full in a couple of days..

It your OB motor or alternator can get the bank back to 80% state of charge regularly then base these figures on 20% of the capacity.

A 100Ah battery at 80% SOC - 20 Ah X 1.15 = 23 Ah / 7.2 = 3.2 days to go from 80% to 100% in perfect conditions.

Shunting controllers will get you back to 80% state of charge and do just fine for that but then they really slow down, due to their ON/OFF nature, and can drastically impede the time it takes to get from 80% - 100% state of charge. These controllers can add multiple days to the "topping up" portion of charging...
 
Oct 2, 2008
1,424
Island Packet 31 Brunswick, Ga
Maine Sail said:
Try not to confuse voltage rise with a capacity increase. In order to get current flowing into a battery you need to increase the voltage. A voltage increase while charging is not representative of state of charge until you can hit and hold an absorption voltage for a while. If you are at absorption voltage (roughly 14.4V) you can safely guess that you're at close to 80% state of charge or higher. This is with healthy batteries. Batteries suffering from sulfation will come up to 14.4V well before they are actually at 80% SOC and this is then misleading. If you have an inexpensive shunting controller then the last 20% of charge will take a loooooong, loooong time regardless of sun.

A 30W panel will pump out about 1.8A for 3 - 4.5 hours per day, roughly. This is also with no shading. So if we figure 1.8 X 4 hours this = 7.2Ah per day of charging.

With a 100Ah battery at 50% state of charge 50 Ah's, plus about 15% charge inefficiency, would need to be replaced or 50Ah X 1.15 = 57.5Ah.

57.5Ah / 7.2Ah per day = 8 days to get a single 100Ah battery from half full to full, in perfect conditions with a 30W panel.

A 200 Ah bank would take approx 16 days to reach full, from half full...... These are averages and you may do slightly better and you very well can do considerably worse. A 30W panel simply can not take a group 24 battery from 20% state of charge to full in a couple of days..

It your OB motor or alternator can get the bank back to 80% state of charge regularly then base these figures on 20% of the capacity.

A 100Ah battery at 80% SOC - 20 Ah X 1.15 = 23 Ah / 7.2 = 3.2 days to go from 80% to 100% in perfect conditions.

Shunting controllers will get you back to 80% state of charge and do just fine for that but then they really slow down, due to their ON/OFF nature, and can drastically impede the time it takes to get from 80% - 100% state of charge. These controllers can add multiple days to the "topping up" portion of charging...
Great info Maine, thanks.

I have found a great reference and memory refresher to be Don Casey's "Sailboat Electrics".

Keith
 
Jun 11, 2004
1,203
Oday 31 Redondo Beach
is a controller necessary?

If you have an inexpensive shunting controller then the last 20% of charge will take a loooooong, loooong time regardless of sun.

A 30W panel will pump out about 1.8A for 3 - 4.5 hours per day, roughly. This is also with no shading. So if we figure 1.8 X 4 hours this = 7.2Ah per day of charging.


Shunting controllers will get you back to 80% state of charge and do just fine for that but then they really slow down, due to their ON/OFF nature, and can drastically impede the time it takes to get from 80% - 100% state of charge. These controllers can add multiple days to the "topping up" portion of charging...

If the panel is putting out 1.8A into a 200Ah bank, less than 1% of capacity, do you need a controller? At what point is a controller necessary?

I assume small panels, like a Ganz 12 watt I have hooked to 100Ah battery, don't need one (and don't have one built in) so wouldn't have that problem. Am I right?

Thanks.
 

ebsail

.
Nov 28, 2010
241
O day 25 Nyack. New York
I agree with everything Maine said but our electrical consumption is so low that the 30 watt panel does the job. It only has to make up for an ocassional evening when we're using the running lights. When I checked the boat today, the (cheap) controller had shut down charging and both batteries were over 13 volts, so it works well enough for our needs. Ernie
 
Mar 2, 2011
489
Compac 14 Charleston, SC
Maine Sail said:
Try not to confuse voltage rise with a capacity increase. In order to get current flowing into a battery you need to increase the voltage. A voltage increase while charging is not representative of state of charge until you can hit and hold an absorption voltage for a while. If you are at absorption voltage (roughly 14.4V) you can safely guess that you're at close to 80% state of charge or higher. This is with healthy batteries. Batteries suffering from sulfation will come up to 14.4V well before they are actually at 80% SOC and this is then misleading. If you have an inexpensive shunting controller then the last 20% of charge will take a loooooong, loooong time regardless of sun.

A 30W panel will pump out about 1.8A for 3 - 4.5 hours per day, roughly. This is also with no shading. So if we figure 1.8 X 4 hours this = 7.2Ah per day of charging.

With a 100Ah battery at 50% state of charge 50 Ah's, plus about 15% charge inefficiency, would need to be replaced or 50Ah X 1.15 = 57.5Ah.

57.5Ah / 7.2Ah per day = 8 days to get a single 100Ah battery from half full to full, in perfect conditions with a 30W panel.

A 200 Ah bank would take approx 16 days to reach full, from half full...... These are averages and you may do slightly better and you very well can do considerably worse. A 30W panel simply can not take a group 24 battery from 20% state of charge to full in a couple of days..

It your OB motor or alternator can get the bank back to 80% state of charge regularly then base these figures on 20% of the capacity.

A 100Ah battery at 80% SOC - 20 Ah X 1.15 = 23 Ah / 7.2 = 3.2 days to go from 80% to 100% in perfect conditions.

Shunting controllers will get you back to 80% state of charge and do just fine for that but then they really slow down, due to their ON/OFF nature, and can drastically impede the time it takes to get from 80% - 100% state of charge. These controllers can add multiple days to the "topping up" portion of charging...
I agree with your math and performance of the "cheap" shunt controller. For my use (like the original post) which is occasional and never more than 2-3 days in a row and 7-14 days apart, the shunt controller works fine! I never discharge more than 20-30% of full and recharging is usually 3-5 days on average at that level of discharge.

Once mppt controllers and high dollar solar panels drop enough in price, I'll certainly buy them. I simply can't justify spending 5 times as much for something I don't need. Someday when I retire and do some cruising for weeks on end, I'll invest enough to run all my boat gadgets and small flat screen TV!
 
Jul 1, 2012
155
Catalina C22 Georgetown
Wow, where'd you find a 30 watt panel for $58? I just bought a 15 watt from Harbor Freight for $50 (and got a free 2 year warrant with a coupon), and a 7 amp controller for $25, and I thought those were pretty cheap. It does pretty well when my boat is stored, 4 hours of full sunlight and about another 4 hours of 50% shade, and it does as good a job as my trickle charger.
 

ebsail

.
Nov 28, 2010
241
O day 25 Nyack. New York
Got it on e bay from an outfit in Calif. Can't find the receipt now but I post it when I find it. This is a stock item for them.
 
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