Solar Panel Control Question

[Pete the Cat]

I have a dumb question: I have two banks of golf cart batteries (440 Ah total) and a single 65W solar panel with a controller. The panel does a very nice job of charging one bank and keeping it topped off. I would like it to charge both banks, but I do not want to link the banks through the controller and someone said a diode would rob the meager output of the panel. Any advice??

[cdennyb]

A diode would rob the power? How.?
A diode would only prevent reverse current flow. Are you implying the diode would go between the second set of batteries and the panel? Sounds simple enough. I'd prefer to go thru the charge controller (if it were one designed to charge two seperate banks of batteries. Some only have a solar input, battery output and load output. Others have two sets of terminals for two seperate batterys.

[capt_douglas]

How about a dual battery bank controller?

The other idea is to switch the solar panel to the battery you're using each day. When I had 2 banks I used bank 1 on odd days and 2 on even days. I had a switch that would allow me to connect the output of the wind generator to either bank.

Eventually, I went from a diode isolation bank (which seems to me reduces the voltage by about 0.7V but doesn't affect the current) to a triple bank charger (house banks and engine start). I got better performance from the charger than the diode bank and old charger.

Depending on the maximum voltage from your panel, your controllers characteristics, and the charge rate, the voltage drop may not be a problem (if the controller likes 15V and your panels put out 16V or more, you should be OK). You might check the output voltage/current load curves to see if you can meet the requirements of voltage.

[senormechanico]

If you use a Schottky diode, the drop is a bit less, in the range of 0.3 to 0.5 volts. They're not expensive.
Get one with a maximum current rate larger than the connected panel.
There is a very small amount of power dissipated in any voltage drop, so do whatever you can do to minimize it.

For example if your panel puts out 3 amps, the power lost to a conventional diode would be 3(amps) x 0.7 (volts) =2.7 watts whereas a Schottky diode which drops 0.5 volts would be 3 x 0.5 or a loss of 1.5 watts.

[cdennyb]

10pcs 8 AMP 45 VOLT Schottky BLOCKING DIODES Wind/Solar - eBay (item 180391298403 end time Jul-26-10 05:44:41 PDT)

Here's what I use. Also use them on your panels as blocking diodes.

[hellosailor]

"A diode would rob the power? How.?
A diode would only prevent reverse current flow. "

No, an in-line diode (i.e. isolation diode) will always consume power. In the best case it will cause a 0.2-0.3V forward voltage drop. If the diode is not the right kind or is not matched for the power range, 0.5-0.7V is not uncommon.

If you don't mind throwing away at least 3/10's of a volt (and bear in mind, a nominal 12-volt system only has a range one one full volt from "dead" to "fully charged") then diodes are a wonderful thing. But if you want to squeeze the power all into the battery and not leave any on the floor (so to speak) diodes are best avoided. Except for the very necessary one to prevent reverse flow back into the panel.

[goboatingnow]

if you have a MPPT controller and proper battery sensing then diode drops can be accounted for. diodes cause voltage drop but "consume" very little power.

[btrayfors]

While there are several ways to have the panel charge the second battery bank, I question why you have the four golf carts separated into two banks?

It would seem that you'd be much better off by following current practice of combining all house batteries into a single bank -- full time. Especially since you have a none-too-large total AH capacity (440AH) on a 37' boat.

The advantages of a single bank include:

1. each battery is drawn down less than it would be in a two-bank system, making for longer life;

2. charging can be faster, as four batteries will accept twice the amperage as two batteries; and

3, you don't have to worry about switching in each battery bank separately, or at all. It's automatic.

If you currently have a 1-2-ALL-OFF switch to select between the two banks, you could simply choose to run them full-time in the ALL position. I've done that on my boat for nearly 20 years.

Bill

[Minggat]

Originally Posted by btrayfors

While there are several ways to have the panel charge the second battery bank, I question why you have the four golf carts separated into two banks?

It would seem that you'd be much better off by following current practice of combining all house batteries into a single bank -- full time.

I've done that on my boat for nearly 20 years.

Bill

I was beginning to doubt my setup. Glad you piped in.

[hellosailor]

"if you have a MPPT controller and proper battery sensing then diode drops can be accounted for. diodes cause voltage drop but "consume" very little power. "
It isn't just power consumption in the diode. Let's say your system is putting out 15 volts at 4 amps. The MPPT controller will supply 60 watts of power (less about 4-6% lost in overhead) to the batteries.
Now insert a diode with a 0.5V voltage drop. The MPPT controller sees 14.5V at the same 4 amps, so no matter how it fiddles around--it is only getting 58 watts instead of 60 watts. OK, that's less than another 2% lost, but it is still LOST. The MPPT controller will keep trying to optimize the voltage versus amperage--but the diode's voltage drop is still going to cause losses.

It will be worse without the MPPT, but still...Why throw out power if you don't have to?

As Bill suggests, single bank. Or, charge 'em one at a time.

[Bash]

+3 on single house bank. (assuming here you have a separate start battery.)

[Pete the Cat]

Thanks folks. This is what I needed to know. Appreciate the help. I have my batteries in two banks only because if I ever had a dead cell I wanted to be able to isolate it from my system and still keep going. I use them all the time as one bank for the reasons mentioned. Haven't had a starting battery in 19 years and don't see a need for one.

Good information on options. This is a great resource.

[Bash]

Originally Posted by Pete the Cat

Haven't had a starting battery in 19 years and don't see a need for one.

I wouldn't vote for combining the house bank without a separate start battery. At that point you would be better with a 1/2/all switch that you can combine (all) for charging but keep separate at night so that you never lose the ability to start the engine.