12v Multiplus Compact 1600 - Chronic Undercharging. Bulk phase last seconds low battery SOC

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adf888 asked Jan 12, '22

12v Multiplus Compact 1600 - Chronic Undercharging. Bulk phase last seconds low battery SOC

I have a bank of 4x 100Ah AGM batteries connected to my Multiplus. Even if the battery voltage is down to 12v at the point I start charging, the Bulk phase lasts seconds. If I use any charge curve apart from fixed I also end up with silly scenarios e.g. the Multiplus going to float with a charge rate in excess of 20A!

I have connected a VE.BUS smart dongle with both temperature and voltage monitoring. I did this to rule out any issues relating to voltage drop.

I have also noted that if I enable the Multiplus battery monitor, SOC is ALWAYS 100% - no matter the battery voltage, charging status or load on the battery etc. This seems very wrong to me. I’ve tried charging with the Multiplus battery monitor setting disabled and see a marginal improvement but not what I’d expect in terms of charging behavior.

I have a Smartsolar MPPT which in the summer months keeps the batteries charged nicely. However, in the winter months, I rely on the Multiplus (Fed AC by a generator) and it really isn’t behaving anywhere near as well.

Have I got the Multiplus configuration incorrect somehow or does this permanent 100% SOC indicate some sort of fault?

MultiPlus Quattro Inverter Charger

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Answer 1 · 2022-01-12T15:43:09Z

wkirby answered · Jan 12, 2022 at 03:43 PM

Do you have a screenshot of the actual Voltages that are recorded by the MultiPlus during the (short) absorption phase?
If the MultiPlus is seeing the 14.4V (no matter how soon) then it is going to go into absorption and float soon after.
If there is a bad connection between the battery and the MultiPlus then it will see the 14.$V at its terminals with 20A even if the actual battery Voltage is lower. Battery link connections should be checked too. In colder weather the battery internal resistance will be higher causing the absorb Voltage to be reached more easily.

The battery monitor does not calculate SOC by battery Voltage. It counts energy flowing into or out of the MultiPlus to produce a SOC reading.

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adf888 commented · Jan 12, 2022 at 04:33 PM

Thanks, that is helpful.

No screenshots during the short bulk phase as of yet but I can confirm that it hits this pretty much immediately.

I checked with a multimeter and under load from the charger, the difference between the battery terminals and the charger output is about 0.2v. Near enough zero under no load. All connections have been checked repeatedly (Physically, visually, via multimeter and temperature). Isolator, inline fuse and battery busbars are each adding a small drop rather than it being a single poor connection.

This and the cold weather was what led me to introduce the "on battery" voltage & temperature monitoring to try and compensate for this.

Don't know if it makes a difference but the batteries are pure lead carbons - only mentioning as you mentioned internal resistance.

Not sure I get your explanation of how the Multiplus calculates SOC? Whilst using energy flow makes sense (Assuming losses are configured correctly to minimise accuracy drift) it still needs at least a starting reference point for the charge state of the battery and I'm unable to give it one (Or reset its view of the world which is currently widely out - 12.0v rested open circuit is not 100%!)

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