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bertie-bezuidenhout avatar image
bertie-bezuidenhout asked

Calculating Battery Capacity Percentage

How is the percentage battery capacity calculated on the Victron Connect and how often is it being updated? I have a new installation with a Victron Multiplus and Cerbo CX with a MicroCare MPPT (South African product) and, although the voltage is shown correctly on the Victron Connect, the percentages are, as far as I am concerned, completely wrong. This causes the system to function incorrectly, as far as I am concerned. With the batteries (Lithium Phosphate) almost fully charged, the percentage charge is shown as 20% causing all the power consumption to be fed from the grid.

The MicroCare MPPT has a RS485 output and if I could find out how to incorporate that into the Cerbo CX, it may help.

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2 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

Victron uses a "charge accumulation" mechanism to calculate state of charge. Simplistically, once calibrated to 100% for a battery known to be fully charged, any current that flows out of the battery will deduct from the SOC and any current the flows into the battery will add to the SOC. Charge/discharge efficiencies, temperature and other factors contribute to the actual calculation.

I assume you are getting the SOC number from the Multi. Because you have other charging sources and may also have DC loads, the Multi's SOC calculation WILL be completely wrong. That's because the Multi only knows what's happening on it's DC connection, not what's actually going on at the battery terminals.

The only way the Multi's SOC is accurate is if it is the ONLY charging source and the ONLY load on the battery. That is, no other DC connections. A battery monitor that connects to the battery terminals is required so it can accurately monitor ALL current in and out of the battery. This is true even if all Victron components are involved.

Victron BMV battery monitors include a local display if needed. Otherwise the Victron Smart Shunt can be used. It has no display but does have a Bluetooth interface built in.

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bertie-bezuidenhout avatar image bertie-bezuidenhout commented ·

Thank you, Kevin Windrem, for explaining how Victron uses a “charge accumulation” mechanism to calculate state of charge.

Before I get to a follow-up question, I want to say that it is a pity that I did not know this before I bought the new Victron Multiplus Inverter and Cerbo GX. I often wonder if people who sell renewable energy products know what they are selling and exactly how it works and will fit into the overall picture. I bought these components because my other 48-volt 5-kW Inverter faulted and stopped working, and I did not want to replace everything. So, I kept the MPPT, and my intention to only replace the Inverter was communicated to everyone concerned before purchasing the Victron Multiplus. So, it is a bit disappointing to learn what I know now.

The second question is: how will the SOC be calculated if I replace the MPPT with a Victron MPPT to charge the batteries from the PV panels and, at the same time, the Inverter is busy charging the batteries? You mention a “charge accumulation” mechanism to calculate state of charge. Can you (or anyone else) perhaps explain that in a bit more detail.

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rslifkin avatar image rslifkin bertie-bezuidenhout commented ·

Adding a SmartShunt would be the cheaper option compared to replacing your existing MPPT. It won't be able to identify that the power is coming from solar, but the shunt will let it keep track of all power going in and out of the batteries, even from non-Victron devices. That way it'll correctly calculate battery.

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marekp avatar image marekp bertie-bezuidenhout commented ·

If you need to replace your MPPT charger and chose to do it with MPPT made by Victron, the Victron MPPT will be connected to Cerbo with VE.direct cable.

That connection will "inform" Victron system of any energy produced by MPPT and this energy will be included in SOC calculations.

But the safest way is to install the Victron shunt as per @rslifkin suggestion.

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Kevin Windrem avatar image
Kevin Windrem answered ·

Right now, Cerbo selects the Multi's SOC calculation since it's the only one in the system. Adding a Victron MPPT solar controller won't change that, so the SOC calculation will still not incorporate the solar contribution and will therefore be incorrect.

You must add a Smart Shunt or other Victron battery monitor for an accurate SOC calculation that includes the solar contribution. You must turn on the "Has DC System" in Cerbo's settings for it to properly calculate the energy from your existing solar controller. Cerbo will then report that contribution in the "DC System" tile.

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