Mixing panels with different Wattages?

[BilljustBill]

I've been looking for a few extra panels for a 12 volt battery backup system. I found what seems like a good price for 125 watt panels, $330.00 each including shipping.

http://cgi.ebay.com/125-Watt-12-Volt-SOLAR-PANEL-House-RV-Marine_W0QQitemZ280420708090QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item414a606efa

I'll have 2 used panels with about 103 watts each, 2 newer panels at 90 watts, each, one used 60 watt panel, and maybe adding two of the 125 watt panels. My question has to do with mixing those panels. Will it work? If wired in parallel, will I get a sum total of ALL the watts, or an average of all the watts?

Should I place a Schotty type diode on each panel's + side to prevent the stronger panels from backfeeding the smaller wattage panels?

Thanks for you help,
Bill

[Mike90250]

Welcome.

When connecting panels, you can do either of the following:
Increase the Voltage, by connecting panels with the same AMPS, in series
or
Increase the Amps, by connecting panels with the same Vmax in PARALLEL.

example
Series: 18V, 3A & 17V, 3A & 26V, 3A = 61V @ 3A
_

Parallel: 18V, 3A & 18V, 20A & 18V, .6a = 18V @ 23.6A
-end example- Begin truism :

A series string is limited by the lowest amp panel, and the voltages all add together

A parallel string is limited to the lowest voltage panel, and the amps add together.

Schottky diodes are best for direct charging batteries, to keep the battery from backfeeding the array at night, and draining the batteries.

[BilljustBill]

Schottky diodes are best for direct charging batteries, to keep the battery from backfeeding the array at night, and draining the batteries.

Thanks for explaining the Amp gain for a collection of same voltage panels. I understand that a diode prevents drain, but a controller should take care of that and watch the battery charging. I was wondering if those inline panel diodes are a requirement to prevent a higher amp panel from pushing back a lower amp panel? On the back of the used ones from "American Signal", it listed an 8 amp diode to be use..

Thanks again for your help,
Bill

[Mike90250]

Schottky diodes are best for direct charging batteries, to keep the battery from backfeeding the array at night, and draining the batteries.

Direct Charging Batteries is not using a charge controller, just wireing panels directly to the battery. You can do this when you know your array cannot overcharge unattended batteries, like a 5 w trickle charge on a boat with a bilge pump. No need for a controller, just add any needed water monthly.

If you have a charge controller, you generally don't need a blocking diode.

If you keep all your panels in the sun, you won't have backfeed issues.

If you have a lot of parallel strings, you need to attend to fuses for each string, to prevent a bad panel from affecting an entire array.

Each diode installed is power lost. About 1 volt, at your nominal amperage, so if you had a 15A array, you would loose about 15W in a diode. If you instead protect strings with a fuse, you don't have diode losses.

[P.S.]

My idea would by put some smart electronic in loop, that will control that issues.
Is there any kind on the market?