24v solar panel for 12v charging?

[Bene505]

I found a Kyocera solar panel at a good price. It's much bigger than our current solar panel and would be a great addition to the top of our davits.

The only problem is, it's 24 volts instead of 12. Is this a problem?

I already have a Xantrex C60 controller laying around that I think may be MPPT, but I wanted to hear from people who have done this.

Would you buy it, or wait until a good 12v panel comes along?

Regards,
Brad

[mitiempo]

It will work as long as its output is lower than the max voltage the controller can handle. If the controller is MPPT it will give you the most out of the panel, converting to a higher amperage at the required voltage (14.4) and if the controller is PWM it will waste the extra voltage.

It is no different than a multi panel array in series that produces say 48 volts going to a MPPT controller and being converted to the proper charging voltage.
It is well explained in any MPPT manual such as for the Outback controllers.

[Bene505]

Thansk Brian,

I think it comes down to whether I want the extra piece of hardware.

It seems that big 12v panels are getting hard to find. Anyone know a good source?

Regards,
Brad

[btrayfors]

Originally Posted by Bene505

Thansk Brian,
I think it comes down to whether I want the extra piece of hardware.
....

What extra piece of hardware? You need a good controller even with "big 12V panels". These typically have an open-circuit voltage of 17-18VDC....more than enough to fry your batteries if you don't have a controller.

And, might as well spend a few extra bucks and get a good MPPT controller because, as Brian says, they'll give better performance.

I'd go with the "24V panel" and a MPPT controller which can handle the peak amperage and an open circuit voltage of 40VDC or higher (the panel is likely to have an OC voltage on the order of 35.4VDC). Better overall performance and better partial shade performance.

Bill

[Maine Sail]

Originally Posted by Bene505

Thansk Brian,

I think it comes down to whether I want the extra piece of hardware.

It seems that big 12v panels are getting hard to find. Anyone know a good source?

Regards,
Brad

With a 24V panel on a 12V system you will certainly want an MPPT or you're giving up some output current. The Blue Sky 1524 & 1524iX (IIRC) will do 12 & 24V panels and automatically detects whether it is connected to a 12V or 24V system. They basically take excess voltage and turn it into usable current.

Evergreen was one of the few still producing a 12V panel (actually 22.5V +/-) in 150W and up sizes. Most in that size range are 24V.. If I remember the 200W panels were rated at about 22.5V and low 11's for amp output. With an MPPT you could see more than the 11A output.. If you search the net you still may find some of the Evergreen Panels. Last I saw they were as low as $1.60 watt...

[Barquito]

Another related question: Would I still need a controller for a smaller 24 volt panel (5W for a 12 volt house and starter batt)?

[nickmerc]

Yes

[Bene505]

For starting batteries, I'd use a very small 12v panel and no controller. If your panel is small enough, you don't need a controller, just make sure it's small enough.

It's not like you are using your starting battery for any loads between engine starting, so you only need enough to maintain that battery. Years ago I had a panel that was about 4" x 12". I used it to maintain a starting battery.

There's an article by Don Casey on boatus.com that talks about this:

Installing a Solar Panel to Maintain Batteries by Don Casey

...
Wet-cell batteries self-discharge at about 1% per day-more in hot weather, less when it's cold. So to float charge the battery, i.e., maintain a full charge, we need an equivalent charge rate. For a 100-amp-hour battery, that means a solar panel with a daily output of about one amp-hour.

Solar panels, however, are rated in watts. To determine the output current in amps, divide the watt rating by 15-the approximate true output voltage of a typical solar panel. Thus a five-watt panel has a maximum output of about 0.33 amps. But rated output only occurs at high noon; the rest of the time, the panel puts out less. Expect average output to be the equivalent of about five hours of rated output per day. That makes the expected daily output of a five-watt panel about 1.65 amp-hours (5 x 0.33).

Allowing for recharging inefficiencies, this is still about 50% more than necessary to float a 100-amp-hour battery. A good rule of thumb is 3.5 watts per 100 amp-hours of battery capacity. But throw in an occasional sunless day, put blocking diodes in the circuit, and let the bilge pump run once in a while, and five watts will be about right. A small amount of extra capacity won't damage the batteries as long as you maintain the water level in them. ...

I would err on the side of fewer watts for a starting battery, and use a maximum of 2 watts of solar power for each 100AH of battery. It's a simple solution requiring no charge regulator.


Depening on the size of your batteries, your 5w solar panel could possibly be used if you split-up the charging between the house and starting batteries, using diodes. Casey talks about that too.

Regards,
Brad