Multiplus 12/3000/120 slow to respond to DC load during absorption

DIY
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1

Hey all this has been an ongoing issue for me. I’ve posted a few times in the past.

I have a LFP battery (12v360 amps) integrated bms, multiplus, lynx shut. The system has been rock solid except for the absorption phase.

I typically float at 13.5v, and bulk to 14.1, with 1h absorption interval. During absorption I notice high voltage spikes (up to ~15.2v). This was brief of then <1 second with almost no detectable amps.

To me this was a dead ringer for sell imbalance, but I no longer think that and am looking for other advice.

  1. I narrowed it down by making sure all dc load was consistent during absorption. No spikes in voltage happened. Absorption shows 0 amps of current ~10 min in the cycle and is basically holding the batteries there for 1h (minimum I can set).

  2. I can induce a spike by putting a load on the system. The fridge is great at this, but water pumps have been shown to do this sometimes. At first I thought it was flyback from the colapse of the induction field, but solved for this by flipping the breaker during operation (no path for fly back) and spike still occurred.

  3. Finally I just watched the output of the multiplus and shunt. I see a load Start during absorption and the voltage drops below target. Multiplus increases current to restore voltage to 14.1. Load drops (fridge cycles off) but the multiplus is still supplying the additional 7amps or so of current. Voltage spikes 5 seconds or so later. At which point the multiplus cuts way back. Voltage drops again below target, multiplus gradually brings it back inline. Normalizes at 14.1v with almost 0 current detected.

Basically, it feels like the multiplus isn’t sampling the voltage fast enough to respond. I’d hate to see what happens when a larger load is dropped than 120watts.

Is there’s something I’m missing that could be an easy answer. Any sanity check would be much appreciated!

I’ve tried all sorts of things. DVCC on, off. Long slower charge, wire voltage sense on multiplus direct to the battery. Probably more I haven’t thought of.

Oh, I don’t experience this issue with the Solar charger mppt 100/50 when there’s enough sun to keep the battery charged.

Almost feels like maybe I should charge at a higher rate so battery actually has some capacity left when in hits its desired absorption. Is that crazy?

Should shared current sense be enabled?

Thanks for any insight. I’m at my wits end.

2

Did some tests this evening with dvcc SCS, STS, SVS, the works. Increased bulk to 72amps. I hit absorption v sooner and absorption continued supplying current longer than a longer slower charge. No voltage spikes occurred while there was measurable current supplied.

However voltage spikes did occur again with cycling of pumps only after current was held at 0amps with current threshold set to 0.01amps.

I think, basically, my batteries are super full and the multiplus is unable to exit absorption phase unless an hour passes.

That also explains why I don’t have this issue with solar as it can exit on tail current.

Are my only options a more agressive charge with a relatively low absorption voltage so there’s headroom longer or manually exiting?

Why is there no exit on tail current for the multiplus?