Smartsolar MPPT 100/20, battery voltages are below set values

Q&A and troubleshooting
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1

Hi,

I have set a 160W solar panel with a 136Ah Sunwind AGM battery. The recommended absorption voltage for this battery is 14,22V and float voltage 13,60V and I have set these values in the app. However the battery voltages are usually about 0,2V below set charge voltage values. Is this normal operation for the charge controller or is something wrong? Ambient temperature is about 23 - 24 celcius so the reason shouldn’t be the temperature compensation? Could anyone help me with this issue?

Thank you for help in advance!

Screenshot_2026-07-04-14-02-44-50_30b6efbd53acd6f273baafa7ca03da38
Screenshot_2026-07-04-14-02-44-50_30b6efbd53acd6f273baafa7ca03da381240×2772 174 KB

2

For power to flow the voltage has to be higher at the mppt terminals (excluding volt drop over a cable)

It looks like you also have a load output there, are you also using that while the battery is charging?

Sometimes to compensate you can set the mppt 0.2v higher.

3

The 1W/0,1A load is always there. Just took the fuses out of the load and the 1W was still there. First I thought it was the controller using that 1 watts but it shoudln’t be that high right? It should be like 0,02A load for the controller if I remember correctly.

Just checked the battery terminals and the reading was 13,49 volts whilst the controller app reading was 13,42 volts for float voltage. Maybe I should increase the voltages by 0,2 volts then?

4

At 0.07v difference means the mppt is reading well. It is within margin of error. Nothing to worry about there.
But what that means then is its not the mppt that has an issue, but you have voltage drop over your cable. Maybe resistand in lug crimps or smalll gauge etc.
Either change the cable or increase the voltage to compensate.

If you look at your battery spec there should be an allowable 0.2 to 0.3v difference in charge voltages. At least most AGMs all that.

You can always switch the load ouput to always off if its not already set like that

5

I asked the battery manufacturer about the charge voltages. They said that it’s also completely fine to use 14,4V absorption and 13,8V float voltages on a normal solar panel system with a relatively small panel. So it shouldn’t be an issue if the actual charge voltages go near those values.

I checked all the connections and they should all be nice and tight. I have an automatic fuse next to the batterys positive terminal, so the voltage drop could be bacause of that. Or the gauge is simply too small? I have a 6mm2 gauge with a length of approx 1,5 - 2 meters between the battery and controller.

Thanks alot for the help!

6

You must have awesome autocorrect vision. Just re read my post -the spellings were awful- my thumbs were lacking some accuracy there

7

Ps. I changed the load output operation mode to “Always off” and then back to “Battery Life” and now it reads 0W/0A correctly when there is no load.

8

Its possible the resetting fuse is the issue they have pretty high resistance.
6mm² is ok for 20A. Maybe with the length as well it might also be a contributing factor.

Either way your 2 choices are

  1. Redo the DC side to the battery
  2. Compensate the volt drop with settings. The battery will reach the correct voltage at its terminals even though the mppt reads higher.
9

No problem you were totally understandable there. :smiley:

Thanks again for the help.

10

I have compensated the voltage drop (+0,2V) and now it reads about 13,6V for cycle/float voltage and 14,2V for the absorption.

I will possibly investigate the voltage drop later and go with the compensation for now.