Choosing the right Victron Mppt device

[Peter Tattershaw]

I want to upgrade my Vechline PWM to a Victron Mppt. I have a single Vechline Solar panel (120w 18.6v 6.45a) and a Varta AGM 95aH battery.

I am about to put in a new Varta AGM 105aH battery as there is very limited space and can only fit one battery. How do I size the Mppt device making sure that is is big enough so in the future if i was to add an extra panel on the roof the same size as the existing one, that the Mppt can cope.

I have looked at the Victron sizing guide but do not feel confident that i am putting in the correct details so any advice would be much appreciated.

Thanks

 

[Guigsy]

With Victron solar devices, e.g. 75/15, the first number is the maximum panel voltage and the second is the maximum current. For motorhome use, the panel voltage is normally irrelevant. So 15 amps x 12 volts = 180 watts (roughly).

If you're going to double up in the future, then you'll probably need a 100/30.

(Caveats: assuming the second panel is added in parallel as that's normally best, and the second panel is very similar)

Please make sure your wiring will cope with the additional current.

 

[Lenny HB]

If you look at the Victron data sheets they give the nominal panel size in watts.

 

[autorouter]

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[RedFrame]

With Victron solar devices, e.g. 75/15, the first number is the maximum panel voltage and the second is the maximum current. For motorhome use, the panel voltage is normally irrelevant. So 15 amps x 12 volts = 180 watts (roughly).

This isn't quite correct, the second figure is the maximum amps the device will deliver to the Battery.

The first figure, the voltage it can handle from the panels is the limiting factor.

Cheers
Red.

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[Guigsy]

This isn't quite correct, the second figure is the maximum amps the device will deliver to the Battery.

The first figure, the voltage it can handle from the panels is the limiting factor.

Cheers
Red.

Agreed with the first paragraph. I didn't really explain the second number is output current.

For the first number, it's rare that you'll hit the voltage limit on a motorhome roof. You'd need to string 4 panels in series, which is unlikely. Hence I said it's normally irrelevant for motorhomers.

 

[autorouter]

The output current can be calculated because you know the maximum output of the panel, which is 120W. If the output volts is 12V, then the output current will be 120/12 = 10A. Most of the time the voltage will be higher, more like 14V, so the output current will be 120/14 = 8.6A. With two panels in parallel that will be 20A or 17.2A.

If the panels are flat on the roof, ie not tilted towards the sun like house roof panels, then you'll probably only ever get 80% of the maximum output anyway, so that means the output current will be 16A or 13.7A.

So the MPPT 75/15 version will give you up to 15A, and anything over that, on very rare occasions, will be wasted but will not cause any damage or problem. If you really want to harvest every last scrap of sunlight then you'd have to go for a 20A version, maybe the MPPT 100/20. If you're not planning to install more than two panels I'd suggest the 75/15 will be fine.

As Guigsy says, the voltage is not a problem - even even if you decide to connect the panels in series rather than parallel, the voltage will be less than 40V, well within the 75V limit.

 

[Peter Tattershaw]

Many thanks for all your comments it has been really great information. Decided to go with the MPPT 75/15 as there is a chance I might not upgrade the solar panels.