Multiplus 12/3000/120 reads DC voltage incorrectly (at least 1.2 volts low)

Q&A and troubleshooting
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My RV electrical setup has been working well for the past 1.5 years (since I installed it), I haven’t made any recent changes, but the Multiplus just started to read and display the DC voltage incorrectly (and function incorrectly because of it).

Here are the major components: Multiplus 12/3000/120 with DMC and an Android tablet as a dedicated VictronConnect display screen connected through USB-MK3, 12V 400Ah Renogy smart lithium battery bank with built-in BMS, Renogy battery monitor that communicates directly with the BMS (no shunt), and other components that are probably not relevant to the present issue (e.g. Renogy 40 amp MPPT, 600 watts Renogy solar panels, Renogy 40 amp DC-DC charger). A Voltage Sense wire connects directly from Multiplus to battery bank. The Multiplus firmware had been V510, but today I updated to V556, allowing a reset of all configuration settings to factory defaults. This firmware update and configuration reset had no effect on the problem. My charging configuration had been consistent with the Renogy battery parameters, e.g. absorption 14.40V, float 13.50V, current 80A, lithium batteries, fixed charge curve.

Here’s the problem: The battery bank BMS reports 13.2 volts (and around 390Ah) via the Renogy battery monitor, which is confirmed by a house voltage display and a multimeter reading taken directly on the battery bank and directly on the main DC lugs of the Multiplus. But VictronConnect shows the battery voltage as 11.98 volts in inverting operation (both with and without input AC connected, and both with and without moderate output AC load; and disconnecting the Voltage Sense wire merely changes the battery voltage to 11.97 apparently due to very small voltage drop along the main DC cables). Multimeter reading directly on the DC lugs of the Multiplus confirms 13.2 volts.

Yesterday while charging the battery bank from around 280 to 400Ah, the BMS was reporting e.g. 13.7 volts (confirmed by multimeter readings), but VictronConnect was reporting 12.33 volts and 65 amps charging current (limited based on an AC input current limit setting of 10 amps at around 115 volts and some house power draw). As the battery neared full charge, the voltage shown by VictronConnect fluctuated rather wildly between 12.04 volts and 14.89 volts while remaining in Bulk charging mode, and the voltage measured on the battery bank with a multimeter fluctuated between around 13.5 and 16.5 volts. The BMS cut off charging (0 amps) and displayed a voltage of 13.8 volts, along with a battery or cell overvoltage error code. Even though the BMS had cut off charging, the Multiplus stayed in Bulk charging mode (never got to Absorption), presumably because it had an incorrect “understanding” of the actual voltage.

Further background: A few days ago, after three weeks of unmonitored storage of the RV, I was surprised to find the battery bank “dead”, with the BMS shut-off with an undervoltage error code. Battery bank terminals showed zero voltage (due to shut-off by BMS). The Multiplus would not charge the “dead” battery bank because of the zero voltage, but I was able to reactivate with a portable charger followed by solar MPPT charging. After reaching around 280Ah, I charged further with the Multiplus, whereupon I discovered the above described issue.

All wiring and connections seem to be in good order. The battery bank now seems to be behaving normally. It seems that there may be an internal problem in the Mulitplus regarding voltage measurement.

Do you all have any idea of what is going on? What further diagnostics should I carry out? I can’t understand why the voltage reported by VictronConnect is well below the actual voltage measured at the DC lugs of the Multiplus, and the Multiplus seems to be operating based on that incorrect understanding of the DC voltage. Thank you for considering this issue!

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There should be 2 voltage sense wires, both positive and negative….

Whilst this may not fix this issue, the Multiplus is a bit notorious for mis-reading the dc voltage, but usually by a few 100 of mV (typ <0.4V error).

I would note that if using Lithium batteries with a BMS, it is ESSENTIAL for the BMS to communicate with the charger to prevent over charging errors.

When you describe the Multiplus voltage as fluctuating wildly, this was probable because the BMS was initiating a disconnect, due to high cell voltage. A temporary solution to this problem would be to scale back the charge voltage settings in the multiplus.

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Thanks MikeD for your reply.

Sorry my description was bad: yes the voltage sense cable includes 2 voltage sense conductors (positive and negative).

Unfortunately, I’m not aware of a way to make the Renogy battery BMS and the Victron MP charger communicate with each other. Up until a few days ago, the MP charger and the Renogy batteries were “playing nicely together” even without direct communication between them. The charging behavior had been perfect, simply based on the charge parameters set in the MP.

Yes, that seems to be what happened. But the MP charger stayed in Bulk mode and kept trying to push current to the BMS-shut-down battery bank, even though the charger was (fluctuatingly) exceeding its absorption voltage setting of 14.4 volts. The actual voltage as measured by a multimeter fluctuated as high as 16.5 volts, and the voltage as reported by the MP VictronConnect fluctuated as high as 14.89. So even based on the MP’s “understanding” of what the voltage was, it should have switched to Absorption. In any event, I’ll try to adjust the MP absorption voltage setting to e.g. 13.1 (and similarly reduce other voltage settings including low limits) and see if the actual charging voltage goes up to around the specified parameter of 14.4 and then hopefully switching from Bulk to Absorption.

Does anyone know how/where the MP actually measures the voltage, and might there be some circuit component in the MP that is fried/dead/malfunctioning?

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Hi MikeD,

As you suggested, I applied a voltage offset to all voltage parameter settings of the Multiplus, and with the new settings it seems to operate properly. So, basically, it’s just a matter of letting the Multiplus work with its incorrect understanding of the voltages.:laughing: I do still wonder though, what suddenly caused this significant voltage error (>/= 1.2 volts), after it had been working well for 1.5 years. I’ve seen many comments in this forum about the typical 0.4 volts error of the MP. For example, I noticed that you also replied and gave advice to user Brownian_Motion in his post dated 1/1/2025 regarding the voltage error of his 24/3000 MP.

Do you have any further thoughts/advice on my situation? I’ve still been poking around with further diagnostics, and reading more related posts/comments online.

Is there any way to get input from a Victron engineer?

Thanks again.

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P.S.

by the way, regarding communication of the Multiplus with the BMS, I read in this forum that there actually is some software that has been developed by outside users and available (eg on Github) to make the data from the Renogy BMS available to the Victron universe, but such programming is beyond my skill, understanding and hardware. I was happy with the way my simple system worked (with Digital Multi Control and Android tablet displaying Victron Connect via USB/MK3).