Wiring in solar panels with different voltages

[BlueStringPudding]

Currently I have a 56v solar panel wired into the domestic battery bank via a MPPT controller. I've got an opportunity to add further panels to the setup, but these will be putting out just 16-18v when charging.

 

I want to add these new panels as cost-effectively as possible by minimising the amount of new cabling and kit I need to buy for them. Being a very different voltage I'm assuming I need to buy another controller for them. But hopefully I can link the controllers at the battery end, which slightly reduces the amount of new cable I need to buy (as long as the present cable is fat enough to cope with the maximum combined amps passing through it). I've drawn a diagram of the likely setup here:

 

 

Is this way of wiring them up going to be the cheapest solution for me? (Please don't suggest I get different solar panels as that's not going to happen )

Edited July 7, 2013 by BlueStringPudding

[smileypete]

Diagram looks good needs a fuse in the positive near the batt end to protect the cabling tho.

 

For the 16V-18V panel you could also use a PWM controller, this'll do up to a panel current of 10 amps:

 

http://www.ebay.co.uk/itm/EP-Solar-Landstar-10A-12-24V-Solar-Charge-Controller-/271232347402

 

Theres a SLIM possibility the 18V panel could be put in series with the 56V one through the MPPT, but would need to know the make and full spec of both panels and MPPT to be sure.

 

cheers, Pete.

~smpt~

Edited July 7, 2013 by smileypete

[BlueStringPudding]

Oh yes, there is already a fuse there, I forgot to draw it in.

 

Theres a SLIM possibility the 18V panel could be put in series with the 56V one, but would need to know the make and full spec of both panels and MPPT to be sure.

 

cheers, Pete.

~smpt~

Is there? I thought a variation of more than 5% or so meant different panels couldn't be wired directly together.

[smileypete]

Oh yes, there is already a fuse there, I forgot to draw it in.

 

Is there? I thought a variation of more than 5% or so meant different panels couldn't be wired directly together.

If they go in series they need to have the same current output, and probably be the same type of cell too ie mono, poly.

 

Also the MPPT needs to be able to handle the additional voltage and power from the combined panels.

 

cheers, Pete.

~smpt~

[BlueStringPudding]

If they go in series they need to have the same current output, and probably be the same type of cell too ie mono, poly.

 

Also the MPPT needs to be able to handle the additional voltage and power from the combined panels.

 

cheers, Pete.

~smpt~

Okay... Idiot electrics brain working here:

 

(Thinks out loud...) In parallel circuits current add up, and voltage is limited to the lowest voltage in the circuit. Which is why I rigged my five 10w panels (that produce the same voltage) in parrallel to charge the starter battery from a cheap and cheerful little controller.

But in series voltage adds, and current is limited to the lowest value in the series. So if the different panels in the domestic solar circuit were producing different currents, I'd essentially lose the higher one and end up with the lower one, so will have lost some power. That makes sense. In which case I'd rather buy another controller and some more cable so the new panels (200w) will be giving me as as much power as poss, because I've no idea what current my present panel produces.

What difference does the mono vs poly make?

Edited July 7, 2013 by BlueStringPudding

[smileypete]

Mono and poly have a different voltage/current characteristics and won't play so well together.

 

If the 18V one is 200W, then the other one which I suspect is 36V not 56V, would need to be around 400W.

 

Given most panels are well below this, it seems they're very unlikely to match up in this way, so an additional controller will be in order.

 

cheers, Pete.

~smpt~

Edited July 7, 2013 by smileypete

[blackrose]

Will you be wiring in these panels while reversing through a bridgehole or going into a lock with a strong weir stream near the entrance?

[BlueStringPudding]

Mono and poly have a different voltage/current characteristics and won't play so well together.

 

If the 18V one is 200W, then the other one which I suspect is 36V not 56V, would need to be around 400W.

 

Given most panels are well below this, it seems they're very unlikely to match up in this way, so an additional controller will be in order.

 

cheers, Pete.

~smpt~

Thanks for the advice, Smiley. It definitely puts out 56v, I've measured it on more than one occasion. Doesn't mean it's rated at that, could be rated higher. But the glass is crazy paved so its likely to produce a lot less than its rated at.

 

Will you be wiring in these panels while reversing through a bridgehole or going into a lock with a strong weir stream near the entrance?

Eh? What have I missed?

Edited July 7, 2013 by BlueStringPudding

[smileypete]

Thanks for the advice, Smiley. It definitely puts out 56v, I've measured it on more than one occasion. Doesn't mean it's rated at that, could be rated higher. But the glass is crazy paved so its likely to produce a lot less than its rated at.

Ah, even less likely to work then. Crazy paving is a polycrystalline cell.

 

Another way to get an idea of the voltage is count how many solar cells are in the grid pattern on the panel, though it's not always that obvious what are the 'bus bar' strips or the gaps between cells. Each cell makes 0.5V at optimum output current, so 72 cells means a 36V panel.

 

Still there's a couple of ways to measure voltage, one with no load connected (Voc, Voltage at open circuit) and one with an optimum load connected (Vmp, voltage at maximum power). The latter can be done by connecting to an MPPT in turn connected to a heavily loaded batt bank. The MPPT will work out the optimum load for the panel and the Vmp measured while it's doing so.

 

Confused yet?

 

cheers, Pete.

~smpt~

[BlueStringPudding]

Not yet, Pete. But it still means the the two lots of domestic panels need their own cabling and own controller. Which is what I wanted to check.

[Arducius]

Eh? What have I missed?

 

You've posted this in the "boat handling" section. I'm, sorry but the only solution here is for the forum to form a lynch mob and come after you with pitch forks and flaming torches.

 

 

... or ask a friendly moderator if they could move the thread.

[Paul C]

I'm not 100% convinced the two MPPT controllers will work alongside each other happily, by that I mean that you'll get full/optimal power from each bank of solar cells while there are 2 controllers. For example, by solar installation can achieve say 150W (11.5A) (as reported by its built in display). But if I turn the engine on, the alternator charges the battery (typically at a higher voltage) and the MPPT controller 'turns itself down' and only puts in say 20-30W or so (a couple of amps). I can only think this is because the MPPT controller, in trying to gauge how much charge to put into the batteries (its a 3 stage charger) uses battery voltage to sense the approximate state of charge. So if there were 2 MPPT controllers, one would sense the other's voltage input into the same batteries and possibly turn itself down.

 

I think some MPPT controllers can be 'linked' so they can operate in parallel together.

 

If not, then its a case of upgrading the existing MPPT controller and getting panels which match, or nearly match, the existing ones. Then you may possibly need to connect them in 2 strings or some other wiring variation, to not exceed the input voltage of the MPPT controller.

[Justme]

In bulk they will both give out 100% of what they can.

 

After that then you are limited by what the battery can accept.

 

That will happen with one, two or ten controllers.

[Paul C]

As I say, its something I'm not sure on - not having 2x MPPT controllers myself, to try it out with. I would ask, if they do give out 100%, then why do the more sophisticated controllers have a network port to connect multiple controllers together, for them to communicate with each other?

[Justme]

As I say, its something I'm not sure on - not having 2x MPPT controllers myself, to try it out with. I would ask, if they do give out 100%, then why do the more sophisticated controllers have a network port to connect multiple controllers together, for them to communicate with each other?

 

 

So when they are approaching the end of a charge phase they can better manage the charge. They can also sort out the absorb phase & any EQ charges. Also you could have the main array in shade yet the secondary one is still capable of producing power.

 

Just like paralleled inverters (or generators), one will be the master yet the slave could be doing the inverting.

[BlueStringPudding]

You've posted this in the "boat handling" section. I'm, sorry but the only solution here is for the forum to form a lynch mob and come after you with pitch forks and flaming torches. ... or ask a friendly moderator if they could move the thread.

Oh yeah! It was supposed to be the Boat Maintenance and Tinkering Shenanigans section. Damned iPad and poorly coordinated fingers.

 

Mod! Oh, moooood! Please can you shift this thread for me? Thanks.

I'm not 100% convinced the two MPPT controllers will work alongside each other happily, by that I mean that you'll get full/optimal power from each bank of solar cells while there are 2 controllers. For example, by solar installation can achieve say 150W (11.5A) (as reported by its built in display). But if I turn the engine on, the alternator charges the battery (typically at a higher voltage) and the MPPT controller 'turns itself down' and only puts in say 20-30W or so (a couple of amps). I can only think this is because the MPPT controller, in trying to gauge how much charge to put into the batteries (its a 3 stage charger) uses battery voltage to sense the approximate state of charge. So if there were 2 MPPT controllers, one would sense the other's voltage input into the same batteries and possibly turn itself down. I think some MPPT controllers can be 'linked' so they can operate in parallel together. If not, then its a case of upgrading the existing MPPT controller and getting panels which match, or nearly match, the existing ones. Then you may possibly need to connect them in 2 strings or some other wiring variation, to not exceed the input voltage of the MPPT controller.

Thanks, Paul, although as mentioned in my original post I am not going to get matching panels. I'll be making the best out of the ones I am getting instead.