Ok, the dealer got the repaired ‘brand new’ 24/3000 back from the depot on 3/13/25
Swapped it out. Voltage reads are MUCH closer, reads about 0.07 low now.
Acceptable.
Input frequency STILL indicates 60.3 and it NEVER CHANGES. (Dummy value?)
All of my generators are inverter types, so I’d have to borrow a direct drive type from someone to get a source of AC that I could vary the frequency on. (Applying some pressure to the throttle linkage will obviously swing the frequency more than enough to test this..)
The battery bank was at about 50 aH below its full point of 500 ah. (I’ve added some capacity since the earlier narration.)
Absorb is set to a fixed curve, and for 1 hour. Battery type to lithium.
I set the charger absorb to 28.4 volts, the float to 27.4, and set the Fluke to do min / max /average and let it go at it.
I picked 28.4 because I’ve tested my bank with a small power supply, and I’ve driven the voltage up to about 28.6 or so before individual BMSs will start disconnecting the charge path.
The AC input current to the Multiplus is limited to 15a 120vac, so it can only do about 55 amps DC, which works out to around 10 to 12 amps DC per string.
The meter captured a voltage spike of 29.8 volts!
Whaaat?!!
Unhooked the AC feed to the Multplus and put a 1.2 kW load on the system, (pulls about 45-50 amps at 24v,) and ran it for about 2 minutes.
Turned off that load, and reconnected the AC power and stood there and watched the current and voltage levels. The charger kicked in and ramped up to 50 amps or so, and the voltage quickly started climbing.
The current started dropping as it got closer to the set point of 28.4 but the sound of the charger changed and the voltage started quickly climbing, and went right on past the 28.4 set point. There was another change in sound, and the voltage shot up over 29.6 volts, and then the sound changed again, and the voltage went up over 30 volts! That seemed to trigger some kind of ‘aw crap’ switch in the charger, and it cut off and the voltage dropped down to float. (27.4v)
It finally dawned on me that the individual strings were ‘tripping off’ as the voltage climbed and instead of the charger chopping off the voltage increase, it just kept on going.
To test that, I isolated all but one string of batteries, put a load on that string for a few minutes to get the voltage below 27. Dropped the load, and plugged in the Multiplus.
It synced up and started charging, I let the current climb up to over 35 amps, and popped the breaker open.
POW! Voltage shot up to 31.5 volts and it sat there for a second or two before triggering that ‘aw crap’ circuit again which brought the voltage back down to float.
I tried it several times, all with the same result.
Ok, lets see how it acts with the Absorb set down to 27.9 volts.
Same thing happens. When the breaker opens, voltage slams up to 31 volts before dropping down.
Ok, put the rest of the bank back on line, and discharged it at 45 amps for about 5 minutes.
Pulled the load, power cycled the Multplus, (turned the rocker switch to ‘off,’) plugged the charger back in and set the rocker back to its normal ‘on’ position.
Charger kicks in, voltage climbs back up, and the current starts dropping the closer it got to 27.9v
It hit the 27.9 point while still sourcing about 15 amps, (down from the 55 amps it was running at,) and finally the current dropped to about 1 or 2 amps at 28.2 volts, which is 0.3 volts ABOVE the set point. Finally, at 28.3 volts, the current went to zero, for an overshoot of about 0.4 volts.
Ok, lets try it at the lower set point, (27.9) but with a longer bulk charge cycle.
Ran 50 aH out of the bank, power cycled the Multiplus and plugged it in again. Set meter to H/L capture, and went about my business. Checked about an hour and a half later, and the charger was showing absorb,
Meter showed that the peak was 27.97, so that is much better, less than 0.1 volt overshoot.
To summarize, it appears that a low aH discharge and subsequent recharge (in bulk,) will cause the charger to overshoot by about 0.3 to 0.4 volts, but a longer period in bulk seems to operate correctly. Whereas before, it didn’t seem to make a difference if it ran a longer bulk charge or not, it would overshoot by 0.3 to 0.4 volts so I would have to rate it ‘somewhat’ repaired.
The fact that disconnecting the battery would cause a drastic overshoot of the voltage implies to me that some work needs to be done with the charger circuit to limit that response, ESPECIALLY since the overcharge protection mechanism of a lithium BMS is PRECISELY that behavior.
It was pretty obvious that the charger did NOT respond well to the BMS protection circuits kicking each string off of the bus as the voltage rose.
Ok, Victron, what say you?
Addendum:
Tested the float voltage by clipping a 200 watt 120v light bulb across the neg and positive distribution bus between the battery pack and the Multiplus which is -after- the 500 amp shunt so any current flow could be measured.
Set the float voltage to 27.6 in the Multiplus and ran it once again through the absorb / drop to float cycle.
I am pleased to report that the voltage at the zero current flow point as measured by the shunt showed a nice and steady 27.64 volts. I checked it an hour later, and it was still at 27.64 volts.
Yay!