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tom-morton asked

Inaccurate battery voltage reading mppt 100/30

On my boat I have 2 victron mppt 100/30 controllers supporting 2x 160w solar panels each and charging the same AGM battery bank. I also have a 100/30 controller supporting a single 160w panel and charging the same bank. The outputs from all three controllers are wired to a common busbar which is tied to the battery. One of the 100/30s and the 100/15 show battery voltage at the controller that matches the voltage meter on the boats electric panel. The voltage on the bus bar is also consistent with these readings.

But the other 100/30 consistently show higher peak and lower low battery voltages at the controller terminal than the rest of the system, and consequently is not charging like it should. Both the 100/30s have the same settings, same software version (1.37) and are mounted next to each other The errant controller is closest to the bus bar so it’s weird that the voltage reading changes on either end of the short (6”) supply wires.

Any advice or insight appreciated. Thanks

MPPT Controllers
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1 Answer
Mark avatar image
Mark answered ·

Have you tried to manually measure the voltage DIRECTLY at the terminals of the MPPT's (with a good/accurate multimeter) & see how that reading correlates with the MPPT internal readings?

If it does correlate well/better, then it could have something to do with voltage drop in your wiring, fuses &/or connections. Is all this identical for each controller?

Another posibility is the that the voltage drop could be different between MPPTs based on large differences with the instantaneous current output level.

For example, if 2x MPPT are providing a high current output but the 1 has its solar panels shaded (so it's current output is low), then the 2x MPPT's doing all the work will be showing a higher battery voltage reading - as they would have a higher voltage drop within their wiring/fuses/connections... (so their output would actually be ABOVE the true battery voltage)

You could try to swap 2 MPPT controllers around if you have 2 of the same size & suspect one to be off.

Or you could think about adding a Bluetooth 'shared battery sense' device that you can mount directly at the batteries & provide common/accurate battery voltage readings to all 3x MPPT's via Bluetooth.

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tom-morton avatar image tom-morton commented ·

Thanks for your reply. I did measure the voltages at the most terminals with my multi meter and they matched the the internal readings as described. Interestingly the moot that is showing the higher battery reading is the one that is connected to the panels that have more shading. The Mppt that is giving the accurate reading is connected to panels that are farther away from the batteries but are less shaded, I ran on size higher wires for those to minimize the voltage drop.

Today the 15a and “good”. 30 a controller both showed a max battery voltage of 13.05, but the errant 30 controller showed 14.29A. I will try swapping the input wires on the 2 30 A which should tell me if it is the controllers or the wiring that is causing the issue. Thanks again

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Mark avatar image Mark ♦♦ commented ·

No worries.

Good to know that the real voltage as measured directly at the MPPT terminals match the MPPT internal readings.

If your getting >1v difference then I would say that you have more than just a typical voltage drop issue. I think you have a very bad connection/high resistace somewhere in the wiring or fuse for the unit that's reading high battery voltage.

Have you checked your fuses or are you using circuit breakers? If it's a cheap circuit breaker it may have a poor internal contact & high resistance.

Maybe measure the resistance (with your multimerer) between the MPPT output terminals & the common bus bar. I think that will pinpoint one cable with high resistance - then you can find your issue. You can also do the same resistance measurement directly on the terminals of your fuse or circuit breaker.

It's best to disconnect the opposite cable (pos or neg) when doing these resistance checks.

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tom-morton avatar image tom-morton Mark ♦♦ commented ·

Thanks, this morning the errant mppt controller is reading about .7v higher than the others as well as my battery voltage meter. My panels are all wired directly to a terminal block, then for the 30amps controllers 2 panels are combined at the block and wired to the controllers inputs. The outputs for all three controllers are wired to a single common bus bar. The only fuse on the system is between the bus bar and the battery, so if that connection was bad it I would think it would be affecting all the controllers. I’ll will check resistance on my wires and also swap the wires to the two controllers and will report on what I find. Appreciate your help!

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JohnC avatar image JohnC ♦ tom-morton commented ·

Also, check the pv polarity to the errant mppt. Pull the wires to be sure..

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tom-morton avatar image tom-morton commented ·

Just to follow up I swapped battery supply wires from the good to the errant controller and it worked properly.

I noticed that the pos wire on the errant controller was warm and the terminal connection didn’t seem great. Cut an restripped a new end before hooking it back up and made sure all terminals were tight and everything is working well now. So just a bad connection. Thanks for the input.

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wkirby avatar image wkirby ♦♦ tom-morton commented ·

Glad it's fixed. Thanks for coming back to let us know what the fault was.

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Mark avatar image Mark ♦♦ tom-morton commented ·

Nice, good to hear - lucky it was fixed before you melted a terminal!

Please also note that the individual wire strand size can also have a reasonable effect on these low voltage/high current conections.

Smaller stands provide more contact surface area to the terminal & accordingly lower resistance.

You should be using a cable with a maximum individual strand diameter of <0.4mm. But 0.32mm or 0.25mm is preferable & commonly available.

Extract from MPPT manual;

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