Mixing leisure solar panels

By leisure I mean ones designed for caravan/motorhome etc.

I have a 55W panel and our new caravan already has one on the roof, I estimate 100W or so. Having 4 people and their devices (yeah don’t moralise) uses up a surprising amount of power so I was thinking of adding my smaller panel to the roof as well. Thing is they are different brands with different VOC – the larger one I measured at 20.9V and the smaller at 21.9V under the same conditions (next to each other at the same time). I’m not exactly sure the model of the larger one but having Googled the brand it seems the nominal VOCs are similar.

Is there any reason not to just wire them up in parallel? It seems that efficiency might be slightly reduced if I do that – but I could end up with more power generated overall. Fact is I already own the panel and the roof mounting kit.

I have an AGM battery and the PWM controller I bought for my panel has a 5A output and can be configured for AGM batteries. The one that came with the van has a 10A output but has no such configuration so I’m guessing it’s using a slightly lower voltage. I think if I wire both charge controllers together they will be confused – possibly the one that is expecting a flooded battery might not charge at all if the AGM compatible one is using a higher voltage. So I will probably get an MPPT controller – I might be better off with two since the panel voltages are different.

Or should I just sell the smaller panel and get an MPPT controller for the existing one? In terms of consumption, we managed ok this summer with a mix of overcast and sunny days but I didn’t realise there was a panel on the roof and I clipped the smaller panel and its controller directly to the battery. I don’t know what they did and how they interacted.

Thing is they are different brands with different VOC – the larger one I measured at 20.9V and the smaller at 21.9V under the same conditions

That’s near enough that you could connect them in parallel to one controller.

100W @ 21V is ~4.8A. You’ll get slightly more current when the PWM controller drags it down to battery voltage, but not much, so only ~65W @ 13V via the PWM controller.

If you add another 55W @ 21V, that’s another ~2.6A.I would be wary of connecting two PWM controllers with different settings to the same battery. Your new PWM controller isn’t big enough to take the combined output, so would suggest connecting this to your existing controller.

You’re throwing away quite a bit of power by not using an MPPT controller though: If you fed the 100W panel through an MPPT controller, you’d get pretty much the full 100W into the battery – say 7.5A – a 50% increase.

If you added the second panel in parallel with the first, through an MPPT controller you should get around 11.5A into the battery.

Victron make very good MPPT controllers – something like the 75/15 would cope with both panels for ~£55 (you could probably get away with the 75/10 if you wanted to save a couple of quid). You also get various / customisable battery settings and loads of bluetooth graphs and stats to nerd over on your phone :).

https://www.12voltplanet.co.uk/victron-smart-solar-7515-mppt-charge-controller.html