multiplus frequent rebulk while on shore power

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seank asked Sep 19, '21

multiplus frequent rebulk while on shore power

Hi All,

I recently installed MultiPlus 3000 on our boat managing 4 BattleBorn Lithiums.

As I understand from what I have read so far, is that when the lithium battery type is selected, the rebulk offset is set to .2v. So we are floating at 13.6 and rebulk will trigger at 13.4.

The problem seems to be that when we are on the boat while on shore power (and the charger is in float) and using things that create a DC load like the fresh water pump, diesel furnace, electric heads, this offset gets satisfied and triggers a new charge cycle. So we are re-bulking at least once a day, which on a fixed charge profile sends us through the minimum absorption time specified.

Could someone please help me understand:

Why is this offset so tight?
Is there a minimum dwell time it needs to see the offset voltage for to trigger the rebulk?
Can I/should I change this offset? Can I create a new battery profile?
It just doesn't seem that this is necessary or good to rebulk-reabsorb frequently.

Thanks!

battery charging Lithium Battery

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Answer 1 · 2021-09-19T19:31:32Z

Alexandra answered · Sep 19, 2021 at 07:31 PM

@SeanK

The offset is tight because Lithium has a flat discharge curve. and often the difference of 0.2 volts represents huge AH. Re bulk the voltage would have had to drop for just over a minute to trigger the offset.

Your DC loads are drawing the battery down so the Multi would be charging it? Maybe I am misunderstanding the issue here. Do you have a way to offset the DC loads, for example solar?

You can create your own battery profile as long as you are within the manufacturer recommended specifications. You could float at the lowest voltage allowed- what does the battery manual say the range is?

Since absorption and bulk are the same voltage are you sure the repeated absorption time is not set to do this once a day? The other thing you can do is tweak the settings with lithium unchecked then the re bulk offset is 1.3V. Set a fixed charge curve, then how often the absorption time repeats.

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seank commented · Sep 19, 2021 at 09:42 PM

Thanks Alexandra,

Correct, the DC loads are drawing the battery down, but the MultiPlus should be delivering 13.6 volts onto the bus, so I'm not understanding why it's taking voltage excursions downhill that are triggering rebulk. It's no like we are running electric winches and the inverter is off while at the dock. It seems the MultiPlus should be capable of delivering enough current to hold 13.6v? BattleBorn recommends 13.4 - 13.8 for float in their battery manual and 13.6 or lower in other resources.

I guess at the core of this is understanding why the Multiplus is apparently not able to hold the voltage on the bus steady during float or absorption when there are no significant DC loads.

I watched it through the last absorb cycle and it ranged up and down from 14.1 to 14.4 (the specified absorption voltage). There's onboard battery management systems involved here, which is something I haven't dealt with before. I've been talking to tech support at BattleBorn and they tell me some voltage excursions are normal in the charge cycle in new installations (this one is about 2 weeks old) as the batteries balance themselves internally. I'l continue working with them on this issue.

It sounds like my options for rebulk offset are 1.3 or .2 and nothing in between?

I think I'll try dialing down the float voltage and see if it holds a slightly lower float voltage better. I guess my core concern is why the MultiPlus isn't better at holding the bus voltage steady with reasonable DC loads. We had no issues with an old Xantrex on an AGM bank before.

Thanks!

Sean

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Alexandra ♦ seank commented · Sep 21, 2021 at 04:07 PM

On your gx, is has dc loads checked? Then maybe this will help. Also a way for the system metering on dc like a shunt/bmv

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seank Alexandra ♦ commented · Sep 21, 2021 at 08:23 PM

yes DC loads is checked. We have a Victron smart shunt to monitor.

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Answer 2 · 2021-09-21T15:24:30Z

pmikep answered · Sep 21, 2021 at 03:24 PM

According to this post

https://community.victronenergy.com/questions/55751/are-bulk-rebulk-voltages-different.html

@klim8skeptic says that once you go back to bulk from a rebulk voltage trigger, the Mulitplus should NOT go into Absorption. (Presumably it would jump back to Float.)

I haven't tested this myself. Are you seeing the Mulitplus go to Absorption after being triggered by rebulk?

Update: I wonder how any of the Victron products know the difference between a rebulk trigger as opposed to a normal bulk cycle? In the case of an MPPT, I suppose it could set a flag for to rebulk for the day and, after it shuts off and comes back on the next day, a normal bulk cycle ensues. But the Orion and Mulitplus don't know when it's night / when a day has passed.

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seank commented · Sep 21, 2021 at 08:30 PM

This Multiplus is definitely going to absorption after rebulk. Maybe it's that it is on a fixed charge curve with 2 hours specified at absorb voltage per Dragonfly Energy's advice? Again I don't really understand why the Multiplus can't/won't maintain the bus at 13.6 while in float to prevent tripping the rebulk. The bus drifts down to 13.4 and it rebulks pretty much ever morning at least...

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pmikep seank commented · Sep 21, 2021 at 10:41 PM

Once my SmartSolar enters Float, it stays in Float until the sun starts going down, powering my load (which varies from 280 W to about 500 W (24 V system). It stays in Float as the sun sets until Rebulk voltage, when my MPPT switches back to Bulk in a vain attempt to keep the battery voltage up.

I presume that a Multiplus should also be able to supply a 500 W load in float. But I haven't seen a specification for how much current/power it can supply in Float mode before a Mulitplus has to downshift into Bulk Mode.

In any event, perhaps someone should start a Feature Request thread asking for a"Rebulk Voltage" Setting to be added to the Multiplus settings? (Although I would bet that this Request has already been asked for.)

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Answer 3 · 2021-09-25T14:56:43Z

seank answered · Sep 25, 2021 at 02:56 PM

I did get this figured out. It's always simple once you figure it out. Thanks to Alexandra and pmikep for their input.

The short answer is that the "dynamic current limit" was enabled in the general settings. This is a setting designed to give a generator time to respond to a rapid load increase. So when a load came on the AC bus like the water heater, microwave etc. the Multiplus would interrupt the float (or absorption) charge for a while.

We were floating the batteries at 13.6 per the manufacturers recommendation. If you look at the discharge curve for the batteries it's easy to get from 13.6 to 13.4 and satisfy the .2v offset for a rebulk. (https://battlebornbatteries.com/comparison-one-battle-born-lifepo4-battery-two-6v-gc2-batteries-series/)

It's a little harder at a modest load to get down to 13.2, so I set my float at 13.4 just to make it a little less likely to satisfy the rebulk offset.

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