Configuration of odd panels

CALLD · February 2015

I'm upgrading my array in a few week's time and have been scratching my head a bit over how best to configure it. Due to the rolling blackouts in our country getting the equipment you're looking for can be hard as all the solar suppliers are swamped. I've managed to secure 2x305w modules and a Victron 150/70 MPPT charge controller. I wish to add these modules to my existing array of 2x300w + 2x140w modules. My existing array works by having 3 balanced strings in parallel which ensures that all the series cells share an equal load current. However with the new MPPT charge controller it would be better to increase the array voltage rather than have 5 parallel strings and a very high current. Having the 5 parallel strings though is the only way to ensure a balanced load current on the modules. The proposed array consists of the following modules:
2x305w, 36.6vmp, 44voc, 8.3Imp, 8.8isc.
2x300w, 36.6vmp, 44voc, 8.2imp, 8.7isc.
2x140w, 18.2vmp, 22voc, 7.7imp, 8.3isc.
I previously had the 2x140w modules wired in series to act as a 280w module with similar voltage characterics to the 300w modules so I could parallel them all. I could obviously keep this configuration and just add the 2x305w modules in series, but the MPPT charge controller would like the array voltage to be a bit higher to be optimized for a 24v battery bank. So here is the question:
Can I parallel each 305w module with a 300w module, then hook the two units up in series to give a unit with a rating of 73.2vmp and approximately 16.5imp, then take the 140w modules, parallel them to give a unit of 18.2vmp, 15.4imp and 16.6isc. I would then connect it in series with the first unit.
Conventional wisdom would tell me not to connect unequal cells in series, but because they are really closely matched I feel it may just work especially with an MPPT charge controller. Or are the higher current rated modules going to reverse-bias the lower current rates modules?

What I hope is that the MPPT charge controller will find the optimum voltage and current to allow the whole array to work in a more or less optimum way. I imagine the larger panels will run at a slightly higher voltage than vmp while the smaller panels will run at a slightly higher current than imp but still less than isc.

Any other advice please?

Regards,
David.

BB. · February 2015

Those panels will work fine in parallel (and the 2x "12 volt" panels in series, then in parallel with the others). Vmp for each panel (or pair of 12 volt panels) is virtually the same. (10% difference from high to low Vmp is about maximum--Within 5% is perfectly fine).

You should have a series protection fuse (or breaker) in each solar panel string. Probably around 15 amp fuse/breaker--Check the solar panels specification sheet.

With a 24 volt battery bank, this array should be fine (and array cables relatively heavy and short to keep voltage drop low).

If you had a longer wire run, then with the "right" MPPT controller you could put two panels in series and use smaller diameter wire for the longer distances (but you have "three sets of Vmp~36 volt panels--You would need another "Vmp-36 volt" panel for 2x2 array.

-Bill

CALLD · February 2015

Thanks Bill,

Yes I'd feel a lot better know the module cells are balanced as far as possible. Having the big modules all in parallel and the small modules hooked in series to act as a 36v module and then paralleled with the rest is the best configuration in my mind. I've ordered 40m of 16mm2 solar cable to do my cable run from the array to battery, hopefully it will handle the current without too much loss. The main concern is that the array voltage will be high enough to enable the MPPT controller work at it's best. The Victron controller says it needs at least 7v over the battery voltage to start-up and at least 2v over the battery voltage to run. My AGM batteries absorb voltage needs to rise up to 29v in summer and 30v winter. With a 36v panel that leaves me with 6v to spare under optimum conditions. I'm hoping the cable drop won't be more than 2v.

Configuration of odd panels

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