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pforster avatar image
pforster asked

MPPT 75/10 and MPPT 100/30 in Parallel?

On my teardrop trailer which has an MPPT 75/10 and one installed 100W solar panel, I want to add the ability to connect one or more solar panels if needed. So I got an MPPT 100/30 with the intention of replacing the 75/10. However, the 75/10 also has "load" connectors which the 100/30 does not, so I don't know what to do about those (??). Alternatively, I guess I can just ADD the 100/30 for the extra panels, but how would I connect that in parallel? CAN I do that? How should I wire that? Are the 2 MPPTs going to interfere with each other if solar panels are connected to both?

Thanks a lot for any help!

Peter

MPPT Controllers
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5 Answers
wkirby avatar image
wkirby answered ·

You can connect another MPPT, no problem.
Wire the 100/30 to the battery the same as the 75/10 is wired. Connect new panels to the new controller.
If they are Smart MPPT's then you can create a VE.Smart network and synchronise the charging. If they don't have Bluetooth, then they won't synchronise the charge states between themselves, but this is not really a problem at all. Just make sure the charge settings are the same on each MPPT.

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pforster avatar image
pforster answered ·

Thank you very much for this answer! Can I run wires from the battery connectors of the 100/30 to the battery connectors of the 75/10, rather than running long wires all the way to the battery?

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wkirby avatar image wkirby ♦♦ commented ·

I would not recommend that. It's best to run wires from each MPPT to the battery.

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Kevin Windrem avatar image Kevin Windrem commented ·

Combining panels on the input of the charge controller is tricky. You need to match panel voltage and/or current depending on parallel or series connections between panels. It would be much easier to run separate controllers for each solar array. We can advise more accurately if you give us more details about your expansion plans: matching panels to the original or ????

Connecting MPPT outputs together then to the battery really depends on the size of the wire, it's length and how much current you'll be transferring. WKirby is correct, it's better to run separate wires especially if it's a long run. The wire needs to be sized to carry the expected current. If you parallel the two controller outputs you need to add their currents to determine wire size. There are wire ampacity tables on-line to help with the selection. If you runs longer than a few feet, also calculate voltage drops and oversize the wires to minimize power loss if needed. There are on-line voltage drop calculators for this as well.

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wild-thing answered ·

I have a 100/30 on a 12V battery and a 75/15 on a 24V battery in parallel. Both are connected to the same panels.

Runs pretty smooth for 3 years. The 12V system somehow is priorised, probably due to the fact that it drags the panel voltage below 26V. Thus the 75/15 with 24V doesn't even start charging. As soon as the 100/30 goes into float charge, the panel voltage increases and the 75/15 starts to charge.

Sometimes both MPPT algorithms seem to fight with each other. Than I experience a few seconds loading from one charger, then the other wins and takes over etc. This can happen, when the 12V system is in absorbtion charge.

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Trina avatar image Trina commented ·

Wow.

Weird. But we make do! However, technically that isn't parallel even if they are next to each other as they are charging separate battery systems.

I would guess if the 12v is getting "priority" it might be due to the lower voltage to turn on and amps it receives off the PV so unless it is a much bigger ah battery it is reaching charger-on and then float sooner.

Not sure "fight" would be the best term for competing batteries especially if they both get charged up eventually; they get the raw PV output and try and convert to their settings to charge each separate battery. A bit like plugging into a wall outlet that looses power at night!

Interesting.

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Trina avatar image
Trina answered ·

For anyone looking for a convenient voltage calculator this one has worked well for me.

Hopefully it still is on-line!

https://www.calculator.net/voltage-drop-calculator.html

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wild-thing avatar image
wild-thing answered ·

I would say so, if the wire length is not too long and the diameter is big enough

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