I have a 12v solar system with 2 12v batteries (One marine, other a car battery).

My inverter has a minimum voltage input of 11.0v and peak of 15.0v, but when I'm running it under load, I get a voltage of 10.9v - 11.8v, and 12v-14v when not underload.

Is there something wrong with my setup or is this normal for a battery under load?

btw, the panels are 20v and charging the batteries while under load as well as not under load. (Charging current :500mA, solar panel current 7A.)

  • \$\begingroup\$ Doesn't look abnormal (voltage is lower under load, and higher when charging), but to be sure you would have to tell us the current the load pulls and the size and type of the batteries. Having a charging current of 500mA when the solar panel current is 7A sounds stranger to me. Is that under full load? \$\endgroup\$
    – ocrdu
    Commented Jan 19, 2021 at 15:24
  • \$\begingroup\$ Yeah it's weird. The 7A is coming from three 20v solar panels. (two 1.5A and one 3A), which goes to my charge controller. I'm guessing the charge controller keeps it at 500mA while not under load and increases while under load? I haven't measured the current the inverter is pulling. Should I, and where would I plant my contacts to measure inverter current, input or output? \$\endgroup\$ Commented Jan 19, 2021 at 15:27
  • \$\begingroup\$ Are you measuring on the battery terminals or at the inverter? To the extent those readings differ, fattening up the wires will improve matters. \$\endgroup\$
    – user16324
    Commented Jan 19, 2021 at 17:47

2 Answers 2


These readings sound about right. While charging, the voltage will be higher than the nominal 12V, while discharging it will be lower.

This is because of the internal resistance of the battery.

  • \$\begingroup\$ So 10.5v is normal? I had the inverter on all night to see if it would fail due to undervoltage but it never did. The charge controller read 11.5v all night but when I measured my batteries they were at 10.5v (underload) \$\endgroup\$ Commented Jan 19, 2021 at 15:32
  • 1
    \$\begingroup\$ I would consider 10.5 V a bit low with OPEN CLAMPS (no load). That would be deep discharge, something most batteries are not very fond of. But if it is delivering significant power, it think it is fine. Depends on how much power the battery is delivering at the moment. \$\endgroup\$
    – EvertW
    Commented Jan 19, 2021 at 15:46

If you have only 4.5A charging or 12x4.5=66W while operating say a 1500W load , the batteries are taking most of the load and it won’t last long as you can see by the Voltage below 11.5V which is typical <10% SoC ( state of charge) for a car battery and it will start getting sulphated and increase ESR on the plates.

The marine battery can go lower but you need bigger or better batteries and bigger Solar Panels for this load.

The battery voltage drop with the inverter on and load off will tell me something about it’s current. Or the dV/dt voltage drop per minute, which = Ic/C for current and battery capacity in amp-minutes converted to Amp-hours.

An Ammeter would be wise to use or two of them, one for charger, one for output load suitably rated for max current.(either digital or analog)

By computing Battery \$ESR = \Delta V/\Delta I \$ you may correlate this with specific gravity s.g. which are good things to measure health of the battery. ESR must be high, if s.g. Is low in just one cell. This indicates near end of life unless managed better with desulphenator pulsers and proper maintenance and Solar design. Then load R equivalent = V/I into battery ESR ratio determines the voltage drop ratio from no load. I.e. if voltage drops 10% then your battery net ESR must be 10% of the equivalent load resistance (inverter with load ) on the 12V bus.

Cables and terminal resistance will add to this battery ESR (Equivalent Series Resistance)

  • \$\begingroup\$ These are the equations above derived from Ohm’s Law for resistance and Capacitance for charge capacity. \$\endgroup\$ Commented Jan 19, 2021 at 16:10

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