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torrmundi avatar image
torrmundi asked

Low charge rate from MPPT 75/15

I have four 140W panels and four Blue Solar MPPT 75/15 regulators. No loads are connected to any regulator. I have three 210 AH AGM batteries.

I recharged batteries with my engines to 88% SOC (according to my Magnum battery monitor). On a sunny day, I disconnected all loads from my battery bank.

Three of the MPPTs are solid green, solid yellow (fully charged)
One of the MPPTs is solid green, fast blinking yellow (bulk charge)


Battery voltage is 12.88 Vdc, and the monitor reads +1.6Adc.
I understand the issue with multiple regulators and the tail charge (one regulator dominates charging, the others go to float stage due to <1A charge current).


There should be at least one panel providing +10.85Adc (140w/12.9v). Why only 1.6Adc?


battery charging
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klim8skeptic avatar image klim8skeptic ♦ commented ·

What is the PV voltage?

What model/specs of your panels?

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torrmundi answered ·

Four of Kyocera KD140GX-LFU, Pmax 140watts, Voc=22.1vdc, Isc=8.7A, Vpm=17.7v, Ipm=7.9A.

At the time I took the other numbers, the 4 regulators had:

Batt1: 13.5v, PV1: 19.0v

Batt2: 13.3v, PV2: 20.0v

Batt3: 13.9v, PV3: 17.5v

Batt4: 13.3v, PV4: 17.5v

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Hi Torrmundi,

You have 1 battery bank of 3 paralleled batteries, not 4 individual batteries?

Are the battery voltages are from the mppt terminals?

Given the voltages, it looks like mppt 1 & 2 have gone to float, and mppt 3 & 4 are still in bulk or absorb.

Slight differences in the wiring will give different voltage drop for each controller. This will cause mppt's to not switch bulk>absorb>float at the same time.

Have a good look at the difference in battery voltage, and each mppt battery terminal and report back.

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torrmundi answered ·

I should add that the firmware on the MPPTs was v1.08; I have now updated to v1.46 and am hopeful of better numbers.

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torrmundi answered ·

Much better today, with two of four MPPTs bulk charging. However, with aggregate of -9A (freezer on), I should have all 4 MPPTs charging. I see my MPPT batt is +13.67V, but my battery monitor is showin +12.63V. So it seems I have about a 1V rise on the wires connecting the MPPTs to the bus bar.

Can I raise my absorbtion and float thresholds to temporarily allow bulk? I think the real answer is to have feedback from the BMV to the MPPT. That's new hardware, as I have a Magnum shunt.

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torrmundi answered ·

I've added Smart Bluetooth Dongles to each MPPT 75/15 and added Smart Sensor to the battery bank. I'm now sharing the battery voltage to all MPPT controllers via VE.Smart network. This addresses the line voltage rise due to charging current.

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