question

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100/30 MPPT solar charger

I have 2 170watts panels on my RV roof, and 2 100 watts panels on the ground. The total watts is 540. Can I use the 100/30 charger if I connect the one's on the ground in series in one string and the one's on the roof in one string. And then parallel the strings together and then into the charger? Or do I need the 100/50 charger?




smart solar set-up help
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If you want to get creative, post photos of your panel's spec labels, and their front cell counts.

Math's is fun.

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sunshine1 answered ·

I would like to clarify my question. I have total 720 watts on my roof. 2 190watts panels on one side and 2 170 watts on the other side all going into a MPPT 100/50. I would like to add 2 more 170 watts panels on the same side as my other 170 watts panels are. I would put those 4 panels of 680 watts into my current MPPT 100/50. I would like to combine my 2 190 watts panels with my 2 portable 100 watts panels with a total of 580 watts. My 190 watts have 24.3v VOC and 9.86a ISC. My 100 watts have a 21.6v VOC and 6.0a ICS. Would that be possible if I put the 190 watts in one string and the 100 watts in one string??



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All panels from a PV array should be identical.

Panels in series (forming a string) must have the same current.

Panels in parallel (or strings in parallel) must have the same voltage.


So better get two smaller MPPT chargers.

Thanks seb71, so what controller should I use for the 2 190 watt panels.totaling 380 watts? Would MPPT 75/20 be a good fit?

Or would the 100/30 be better since I can only get 20 charging amps out of the 75/20 ???

I don't see any 75/20 model.

Have you see the explanation I wrote here?

Ok, I meant to say 100/20. I see that I need the 100/30.

The panels on the RV roof will be installed flat (horizontal) or you will be able to tilt them when camping?

If the panels are horizontal, they won't produce at maximum power, so a 20A charger could be an acceptable match for 2x190W panels, if you can get it significantly cheaper than a 30A charger. In general, you oversize the PV array a little to account for cloudy days and less than ideal orientation relative to the Sun.

I assume the battery is 12V.

Yes they are horizontal, yes they are 12v. I put my speciation on the victron calculator,they came up with 100/30.


Specifications :-)

gro avatar image
gro answered ·

While not best practice according to poster above, I think you will be ok with your proposed strings of two alike panels in series and the two strings then in parallel. My reasoning is based on the following: panel voltage is dependent on the number of silicon cells in series, panels are targeted at particular markets and for charging 12v cells the panels have about 22V as the Max open circuit voltage and Max power voltage is around 18V. Therefore if two panels of different power rating are placed in parallel it will be ok as the voltages will be the same.
All bets are off if they are different cell/voltage panels.


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@Seb71 and you are right. I was operating under the assumption that the panels were the same voltage because that is what my setup is like.

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seb71 answered ·

Don't mix panels with different voltage and current on the same charger.

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zach-lustig answered ·

If I am correct, the max power you can use with that controller is 100V*30A=3000W. The controller can handle the power of your system, but you have to make sure the voltage is below 100V input and the current is below 30A. If you do a circuit analysis with your panel specifications you can figure out which ones should be in parallel and which ones should be in series. For instance, I have a 75/15 controller and two 60V panels. I cannot put them in series because the max voltage would be 120V, exceeding the controller specification. However I can put them in parallel because the voltage would remain the same, however the current would increase. But my panels have 2A max current each, so 4A max is well below the 15A spec for the controller, so it will work.

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The 30A (or 15A for your charger) is the maximum current with which it can charge the battery.

I thought it was both input current and output current limit?

Output is always lower voltage than input, so output current would always be limiting first.

On the input (PV) side, the one that matters is the maximum voltage, to not fry the solar charger.

The PV panels won't "push" current into the solar charger. The solar charger will draw current from the PV panels.

From the maximum battery charging current your solar charger is capable of, you can determine the approximate maximum size of the PV array (in Watt).

Your 75/15 solar charger can charge the battery with maximum 15A.

If you have a 12V system (12V battery), the battery voltage can go up to about 14.4V.

So the maximum power the charger can feed into the battery would be about 14.4Vx15A=216W.

For a 24V system you get twice that maximum power for the same 15A, because the voltage is double (compared with the 12V system) and for a 48V system you get 4 times that maximum power with the same solar charger (and with the corresponding doubling or quadrupling of the PV array wattage).


Simplified, for MPPT chargers you have "power in" (from the panels) = "power out" (to the battery or DC loads). Minus some losses. Nothing is 100% efficient.

Your maximum "power out" is 216W, so your PV array ("power in") can be about 216W. Taking into account losses into the charger and also cloudy days, you can oversize the PV array (total Watt) by some factor.

If you install a 216W PV array, with Vmp=60V, the maximum current into the solar charger (at PV input) would be about 216W/60V = 3.6A.

On the output, the MPPT solar charger will lower the voltage from 60V to the battery voltage (14.4V or whatever), but it will increase the current from 3.6A (current from wires coming from PV array) to 15A in the wires going into the battery.

If your array is smaller than about 216W (or during cloudy days), it won't produce 216W so the charger will not be able to push 15A into the battery (the charger will keep the battery voltage at the required level and will push whatever current it can, up to 15A, based on the available power from the panels at that moment).

I appreciate the explanation, makes sense now. I have been trying the single panel system for proof of concept, now I think I will try two in parallel.

On a related note, is the maximum discharge current through the load output of the controller also 15A? i. e. opposite direction of current as charging the battery?

The dedicated "Load" output from the 75/15 charger is limited to 15A.

For big loads, connect them directly to the battery (but make sure that the battery can handle them).

Good to know, I would connect straight to the battery but it doesn’t record the power consumption that way :/ I’ll find a workaround

The "workaround" is to add a SmartShunt or a BMV (and even a GX device for far more options). But this does not make sense for small systems.