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rtthorn avatar image
rtthorn asked

MPPT 150/70 stops charging

I have an MPPT 150/70, I have had the system running for 6 months, no problem. Its a 48V system, with a LiFeP04 battery and Aims inverter. The frequently I have noticed that it starts to charge in the morning as usual, but will cut out to zero amps of charging current after a couple hours when the battery is partway charged. The system is pulling around 10A from the battery and noting from the MPPT. If I turn on my well...a large load, the charge controller will shoot up to full power, usually around 45A charge current. (Smaller loads will not trigger it to start charging again, not even my 20A furnace). When I turn off the well, it will either go back to zero right away or after a short time. Sometimes it will just keep charging after cycling the well on and off.

I am frustrated trying to determine why the charger would shut off like this. I did take a look at the battery, it is made up of two 24V batteries in series. They are balanced. I also tried hooking up a 24v battery charger to them (while hooked up in parallel and they charge just fine using the external charger.

The charge controller and battery are in my basement, located next to each other and its about 50F-60F in the room.

Anyone have any ideas what could be causing this, or what I might do to troubleshoot some more?



battery charging
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5 Answers
snoobler avatar image
snoobler answered ·

That screams high voltage disconnect (HVD) by one or both battery BMS due to a single cell going over-voltage.


Can you get individual cell deviation?


You said you charged both in parallel. That wouldn't show any difference. Have you tried charging them individually to see if there's any change in their behavior?

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rtthorn avatar image rtthorn commented ·
I checked the cell voltages right after the MPPT cut out. They were all at 3.299, except one was at 3.291.
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snoobler avatar image snoobler rtthorn commented ·

If the system is under load , they could be pulled down very quickly; however, if you know you're at a lower SoC, then that's probably not the case.


What brand batteries?


What temperatures are you dealing with?


Are you using ESS?

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rtthorn avatar image rtthorn snoobler commented ·

This system is installed in my basement next to a 1000 gallon water tank, the temperature in the room is very stable in the mid 50's. This is a "Big Battery" Grila 48V battery that is made up of two 24V batteries hooked up in series.

I'm not sure what Ess is.

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snoobler avatar image snoobler rtthorn commented ·

MANY folks have issues with Big Battery units going out of balance. they're not particularly good units made from commodity cells sourced from secondary supply chains in many cases.


  1. Did you try charging them individually?
  2. When the system is at max voltage, are the two 24V batteries the same voltage?
  3. Do you have the absorption and float at the same voltage like many Big Battery specs suggest?


Recommend you try 55.2V Absorption and 54.4V float.


The 55.2V may require 5+ hour absorption period to ensure you can get 98%+ charged. This will allow the BMS more time to balance the cells.

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rtthorn avatar image
rtthorn answered ·

Thanks for the response! I will try charging them individually and see what the results are. I forgot to mention that when the batteries get low, the inverter switches to on grid mode and charges the batteries. When this happens, i get a full charge with no problems. It seems that the issue only happens with the charge controller. Is the High voltage disconnect a function of the charge controller or of a typical BMS? I have had to return both 24V batteries under warranty due to failed BMS.

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kevgermany avatar image
kevgermany answered ·

What are the MPPT settings and battery charge requirements from the data sheet?

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rtthorn avatar image rtthorn commented ·
I have the standard setting set on the MPPT:

Max charge current 70A

Absorption Voltage 56.8v

Float voltage 54.00V


for the Battery:

Max charge current 100A,

Charge range 55.6v to 58.0v

max charge voltage 58.8v


I have tried lowering the charge current, that has no effect.


Today I charged each of the 24v Batteries with an external charger, no issues. It seems that only the MPPT doesn't like to charge the batteries past 50% or so.


I will measure the cell voltages next.

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kevgermany avatar image
kevgermany answered ·

It's worth increasing the voltages in the MPPT. Set absorption up to 58V.

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rtthorn avatar image rtthorn commented ·

I bumped absorption voltage up to 58.0V, it did not make a difference, the charger cut out at 48% state of charge as usual.

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kevgermany avatar image
kevgermany answered ·

Does sound like the MPPT. Couple of other things to check. Resistance between the MPPT and batteries. Voltage at MPPT and batteries when charging. Any absorption time limit?

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