I'm doing a PV project with WiFi-enabled statistics.

I'd like to monitor the power coming in and going out, the battery charge level, and the estimated battery charge and discharge time.

I'm designing a PCB to have everything neat, and to not have to connect any wires.


  • I chose ACS Hall-effect sensors instead of shunts, ACS712 5 A / 20 A modules (or going to 758/770 for PV) and ACS758 or ACS770 50 A depending on which I will be able to get my hands on. I don't believe that two pins of an SOT8 package can handle 30 A.

  • An MPPT charge controller with built-in inverter for PV applications. It has AC backup when the battery voltage goes below 30% threshold.

The datasheet of the PV controller states that the charging voltage for the battery is 13.8 V ± 0.5 V, so I'm unsure if it will gradually bump the voltage up to charge the battery or just flat line it.

Here is the problem with the battery charge monitor. I'm not sure how to detect the voltage of the battery itself when it's being charged. If the inverter charges the battery at a constant 13.8 V there is no "standard" voltage divider way, if I understand correctly.

Also for the power-out calculation I need to know:

  • AC power going out

Power in:

  • AC power coming in
  • PV power coming in

and for the battery charge time:

  • current battery voltage level (without load and without charging)
  • power coming into the battery
  • power going out of the battery


  • \$\begingroup\$ What is your actual question? Can you show a block diagram of how everything is connected? \$\endgroup\$
    – winny
    Oct 12, 2018 at 9:46

2 Answers 2


The output voltage can't be higher than battery voltage + wire voltage drop. The charger can't charge the battery constantly at 13.8V, rather this is a float voltage once the battery is full.


To calculate how much energy is being stored on the battery it's simple I*V*t which gives a result in Watt hour (Wh).

The problem is that it's quite difficult to know how much energy is in the battery itself as it will depend of many factors like temperature, internal serial resistance of the battery and so forth, which changes over time as well.

None the less, you can calculate how much energy is provided to it, and how much energy is drawn from it, with that data in hand, you can run some computing to analyse and estimate the state of life and the energy it has.

You should measure the battery voltage when it charges directly on the battery leads, to avoid any connection serial resistance that would create a voltage drop. Unfortunately there is no better way, as you will always have the internal resistance of the battery.


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