Victron MultiPlus C 12/1600/70-16 charges starter from the house bank

DIY
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1

Situation

I have a Victron MultiPlus C 12/1600/70-16 and a Orion XS 12/12-50A in my RV. These are connected to a LFP leisure battery, an AGM starter battery, Smart Shunt 300A and a Cerbo GX for monitoring.

The Orion is connected to charge the leisure battery from the starter battery when the engine is running. The Multiplus is configured to charge the leisure battery of AC when connected and also trickle charge the starter battery.
Additionally there’s a PV panel with MPPT controller feeding the leisure battery.

Observation

I was analyzing the logs via the VRM portal and noticed that there’s a strong correlation between the voltages of the leisure battery and the starter battery. As you can tell from the graphs below:

  • in the morning the leisure battery is almost full at 13.3V and the starter battery is full at 12.95V.
  • Then the solar chager starts charging and the voltage of the leisure battery increases until it’s full at >14V (around 14:30). The solar charger turns off.
  • The voltage slowly decays from the standby consumption (~5W)
  • Then at around 18:30 the AC is briefly turned on and the Multiplus charges both batteries. Voltages increas again.
  • The Voltages of both batteries strongly correlated but are offset by about 0,5V.

Expected vs observed

I would have expected the leisure battery to charge and it’s voltage to increase from the MPPT charger during the day.
I would not have expected the starter battery to increase in voltage as the Multiplus was not being fed AC power. As this is not documented in the manual.

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Hypothesis

Something is feeding power from the leisure battery into the starter battery when AC is disconnected.

Test

I disconnected the trickle charger of the Multiplus and the behaviour stopped.

Hypothesis

The Voltage offset suggests there’s a backfeed diode between the starter battery and the leisure battery (ostensibly to prevent the starter battery from charging the leisure battery or feeding the inverter), but there’s no hard disconnect on the trickle charger when AC is disconnected. Possibly the trickle charger is just a branch of the main charger with a current limiter and a backfeed diode.

Test

Discharge the starter battery and the current and the voltage difference should increase.

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Question

Is this hypothesis correct and does the Multiplus have this behaviour where it will use main/leisure battery power to trickle charge the starter battery?

3

You drain your leisure battery into the starter battery IMO that isn’t useful in most systems.
It was useful 10 years ago when you only had lead batteries.
I recommend to not use the trickle charge in mixed systems and to install a separate charger for the starter battery and that charger is connected to the incoming shore power.

5

The grid connection/“land power” (Landstrom).

6

You could always read the FAQ document which describes the trickle charger. You seem to have independently worked it out.

7

To be fair to all concerned, while your writeup of the specifics is excellent, I’d definitely argue that the “always-on” aspect of the trickle charger should be in the manual itself since many users might never end up here to read the FAQ.

8

Thank you @Ludo @pwfarnell for confirming. @Justin I agree completely. I was pretty sure of my hypothesis, but confused because I couldn’t find any reference in the manual or using a web search. Apparently there was this excellent write up by @pwfarnell, but I hadn’t found it despite some effort.