jm0278 avatar image
jm0278 asked

New Multiplus-II Cycling Absorption Charge On and Off

Got a new installation: Multiplus-II 12/3000 2x120, Cerbo GX, and SmartShunt 500. There are three Battle Born LiFePO4 batteries connected via a Lynx Distributor. All settings are defaults except for Absorption voltage which is set at 14.4, the midpoint of Battle Born's range.

The Multi goes through Bulk charge okay, but then cycles charging amperage -- not the status light --on and off when in Absorption. The absorption light on the Multi stays on constantly, but the charging amperage goes from 0.8 to none based on the voltage. When the Multi is charging at 0.8 amps, the voltage climbs in a minute or so to 14.1. The charging current then stops despite the Absorption light staying on. After a couple of minutes, the small drain from the Cerbo GX causes the voltage to drop to 14.39 at which point the Multi starts charging again at 0.8 amps.

I watched this behavior for just over an hour and the Multi never entered Float stage. I suspect there's a setting...or combination of settings somewhere that need to be adjusted.




Multiplus-IIbattery chargingcerbo gxSmartShuntinstallation
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3 Answers
snoobler avatar image
snoobler answered ·

This sounds pretty normal.

At 0.8A and 14.4V, your batteries are poked completely full. Continued attempts to charge them @ 14.4V will result in the batteries refusing to draw any more current. These aren't lead-acid where they'll always pull a little current. LFP flat out drops to 0A when it's hit the absorption voltage and is full.

Actually, anything UNDER 15A (5A per parallel battery) @ 14.4V is at 100% SoC.

LFP batteries need very little absorption time - 15-30 minutes depending on your starting current. IIRC, the minimum is 1 hour for a fixed absorption time in VE.Config or VC. You should set absorption to adaptive to help minimize absorption time with LFP.

Is the MP pulling the voltage data from the smartshunt via the GX, or is it relying on its own measurement?

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jm0278 avatar image
jm0278 answered ·

I believe the MP was using its own voltage reading. But I've just been reading about what the Cerbo can do and based on what I've read I think I should change it to use the SmartShunt. I'll add that I did another test yesterday after changing the "fully charged" voltage to 14.2 based on Battle Born's recommendation to set it to 0.1 to 0.2 below the charging voltage. After making that change, the MP charged the same way in absorption as mentioned in the OP but did go into Float mode properly after an hour. (I set one hour based on 20 minutes per battery.) I had been concerned that the MP would never exit absorption because the Cerbo was draining a small amount of power between "on" portions of the charge cycle.

I'm going to enable the GX to be the control point for the system and, after reading up on it a bit more, I'm inclined to enable DVCC as well. I'll be adding a SmartSolar MPPT controller to the system within the next couple of weeks and will add a second sometime next year.

There was/is one troubling aspect of yesterday's test -- right before switching into absorption, the MP boosted the amperage from low single-digits to somewhere in the teens and the voltage shown spiked up to over 14.6. I was seriously concerned the BMS was going to shut off, but in reality, the actual voltage at the batteries was not that high. I'm going to finish the configuration changes mentioned above and will post a new thread if that situation recurs.

Thanks for helping!

1 comment
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Sounds like a good plan.

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dray22 avatar image
dray22 answered ·

I have learned from my own experience using multiple LifePO4 batteries, that they are not exactly the same and one battery can hit fully charged before the others and the one battery causes the voltage to spike. This causes the charging to start and stop over and over. You have to lower absorption voltage until the charging for all batteries stabilize. It helps to review VRM graphs for this voltage limit. Know that when this happens though, the voltage swing happens so fast, it's not caught on 1 minute logging intervals. It can give you a general idea where to start. I had a set in series and used VRM midpoint monitoring to catch the point where the deviation started going up.

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