charge controller data display incorrect?

I have just installed a 90watt panel on the roof of my truck camper and connected it to a HQRP controller with remote display, and the controller to the battery, which is group 24 84 amp hour battery. All polarities are correct. Controller says charging at various voltages between 13.8 and 15.8, and seems to be working well. However, I do not trust the readouts--when it says the battery is at 100%, a momentary load or shade will drop the SOC reading to about 50%! I know the battery can't lose half its charge in 5 seconds of shade or with just a LED interior light on for a few moments, so what would cause that? When I turn the light off or the sun hits the panel again, it immediately goes back to reading 100% SOC. Also, the green "charging' light is always on during the day even when it says 100%, isn't it supposed to go off when the battery is charged? I guess it is charging the battery, but I don't trust the SOC reading. Once the sun goes down, shouldn't the SOC stay close to 100% if there is no load? Any ideas?


  • BB.BB. Super Moderators, Administrators Posts: 29,634 admin
    Re: charge controller data display incorrect?

    Generally, it is very difficult for a simple solar charge controller to accuratly estimate the state of charge of a battery bank.

    A real battery monitor will install a current shunt to measure the current+time at the battery terminal... sort of like a fuel totalizer on the line to a gas tank.

    10 amps * 1 hour out of a 100 AH battery bank means the battery is at 90% state of charge. 12.5 amps * 1 hour in means the battery is nearly full charged again.

    Regarding the voltages you are reading... I would double check them with a known good volt meter. 15.8 volts is pretty high for charging (typically around 14.5 to 14.8 volts maximum, unless equalizing at ~15-15.5 volts) at ~room temperature.

    Not a lot of 12 volt dc appliances like >15 volts (may cause damage).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: charge controller data display incorrect?

    Unless that charge controller has a built-in battery monitor (and I've yet to see one that has) there is no way it knows the SOC of the battery. In all likelihood it is simply reporting an arbitrary SOC based on Voltage level, which is not accurate.

    It is perfectly normal for a battery's Voltage to drop back from charge set point to a lower value with no load on. It is not normal to charge a 12 Volt battery to 15.8 Volts. Let's hope the meter reads inaccurately. Otherwise too much of that will destroy your battery.

    You might want to have a good read through the battery FAQ's to get some idea what should be going on:
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