Victron Multiplus II 3000 - Unstable Absorption Voltage with 3 Different Units

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ks-vic asked Dec 3, '23

Victron Multiplus II 3000 - Unstable Absorption Voltage with 3 Different Units

Curious whether others are experiencing .10v - . 20v voltage spikes and dips in absorption mode while charging with their Victron Multiplus II 12v 3000 2x120.

The distributor where we purchased our Multiplus has attempted to resolve this problem. They tested 2 other units in their warehouse and they both exhibited this same problem. It's possible this may not be an isolated problem with just my unit. To my knowledge, Victron was notified of the problem in early Oct 2023, and is supposedly working on a fix.

Here's how I replicate the problem quickly. Briefly discharge a fully charged 304ah 4s lifepo4 pack 3-4ah, then turn on the MP charger. It goes into bulk mode for a minute or two, followed by a quick transition to absorption mode a few minutes later. You can see this transition in the far left portion of the Victron shunt trend screenshot below. Current quickly ramps up to 60-65a, then quickly drops while transitioning to absorption mode. Absorption voltage for this particular test was set to 14.30v. No problems up to this point.

Unfortunately, the MP charger only maintains 14.30v for a few min. before it abruptly spikes to 14.48v. This particular time it spikes and dips, above and below 14.30v slowly (~14.40v to ~14.20v), 3 more times before "somewhat" stabilizing at 14.30v approx. 45 min. later. None of these voltage spikes and dips in absorption mode should be occurring.

Important to note, the BMS on our battery pack never goes into high-voltage disconnect (HVD) with the MP absorption voltage set to 14.30v. This same voltage spike occurs even if the pack is charged (carefully) without the BMS. My 10a DC power supply and 75a Powermax converter will both charge this same pack with zero voltage spikes or dips using any voltage from 14.20 - 14.50v.

There were no other loads or connections on the MP's 12vdc terminals except for the 304ah lifepo4 pack during this test. I might add, this problem does not occur the same way every time. Sometimes the .18v spike will happen quickly, sometimes it's delayed 5-10 min. Sometimes there's only the initial .18v spike followed a steady 10-20 min decline in voltage (to as low as 14.20v) before slowly ramping back up to approx. 14.30v 15-45 min. later. There are a number of different ways this voltage spike can present itself. Bottom line: The absorption mode voltage is not stable.

If the MP charger's absorption voltage is set higher than 14.40v, this voltage spike will activate the BMS's HVD. When the HVD activates another more severe voltage spike issue occurs. If there's very little load (<3a) on the MP charger output (with the battery offline due to the HVD), the voltage will pulsate up and down rapidly from ~13.9v to ~14.9v. The voltage will continue to pulsate until the BMS releases the HVD or a load greater than 2-3a is applied.

I've tried setting the Multiplus charger's absorption voltage at different values from 14.20v to 14.50v and even tried different battery chemistries---same results. All tests were accomplished using the latest firmware (v506).

Thoughts? Comments? Suggestions?

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klim8skeptic ♦ commented · Dec 03, 2023 at 08:44 PM

@ks_vic The first thing to look at would be the location of the Multi's voltage sense wires. Ideally they should be located on the batteries + / - terminals. Alternately they can be on the distribution points. The voltage sense cannot be located on the Multi's input terminals.

With the currents expected the cabling and fusing has to be first class. Your using the correct gauge dc cables? Decent (not auto audio stuff) breakers / fuses with minimal voltage drop? Lugs properly crimped? Terminals / lugs clean, flat and properly torqued?

A multimeter is most useful for checking voltage drop along cables and across lugs / terminals /breakers.

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Answer 1 · 2023-12-04T16:32:15Z

Thank you for your reply, @klim8skeptic.

According to the Victron Multiplus II 3000 12v 2x120 manual, "voltage sense" wires are only needed if significant voltage drop is being experienced between the Multiplus DC terminals and the battery when charge current is applied.

When we did our testing only the battery was connected to the Multiplus DC terminals via 2/0 cables approx. 3 ft. long. Voltage drop with a 175a discharge load was less than 1%. When the voltage spikes and dips occur during absorption mode, charge current is typically less than 5a. Since charge current passes through the very same cables and terminations as the discharge current, voltage drop at 5a is nearly zero (unmeasurable).

I confirmed the above by taking voltage measurements at the battery terminals posts and the Multiplus DC terminal posts while the voltage spikes and dips occurred with a voltmeter (mv resolution). The DC voltage readings via the Multiplus bluetooth app (DC output), Victron Shunt bluetooth app (battery neg voltage), and the bluetooth capable BMS app (battery voltage), all read the same voltage when the voltage spikes and dips occurred. This would rule out any voltage drop issues caused by undersized cables or bad terminations.

In any event, we did try connecting 18awg wires between the Multiplus Voltage Senses terminals and the battery. As expected, they had no effect on the voltage spike and dip issue. With no measurable voltage drop there was nothing to correct or compensate for.

Again, this problem has occurred with 3 different Victron Multiplus II 3000 2x120 units. My unit, and two other units the distributor tested. Since all 3 tests were done with a basic test mockup, using only short cables, lifepo4 battery, and the Multiplus, this would tend to confirm the problem lies with the Multiplus itself.

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