Help with low voltage cut-off settings Victron MultiPlus II 48V AGM

[MilbankAustralia]

I have a Victron MultiPlus II with 4x190Ah 12V AGMs making a 48V bank with A Victron SmartSolar MPPT with DVCC enabled via VE.Direct.

My little camping fridge has started to use ~70-100W (may be broken thermostat or need a regas, but waiting on a new fridge soon anyway).

So this morning the batteries were down to 46.7V after a day of cloud yesterday and me forgetting to get out the generator. Not good (and generator will be running all morning to boost them back up).

I went to check the low voltage cutoff settings and found the low voltage shutoff is by default set to 37.2V! Way too low to protect the batteries I'd have thought?

I gather that is to stop it shutting off with a voltage drop under load but still, that seems very low. And at the moment we never have more than 300-400W running at a time.

Considering that, I was wondering what the low voltage cutoff should ideally be set at?

I'd really like to use SoC to shut them off but I don't know how to set it up. In VE.Configure I don't have "Enable Battery Monitor" on because I don't know how to enter the settings. I know the battery capacity (190Ah), but do not know the "State of charge when Bulk finished" (which has a default value of 85%) or the "Charge Efficiency" and haven't been able to find out these values for my system either.

I don't have an external battery monitor either so not sure if it is possible, but we don't have any DC loads coming off the batteries at all so it does all come through the inverter.

Can anyone help me with these questions please?

 

[sunshine_eggo]

I have a Victron MultiPlus II with 4x190Ah 12V AGMs making a 48V bank with A Victron SmartSolar MPPT with DVCC enabled via VE.Direct.

MP II to GX via VE.Bus?
MPPT to GX via VE.Direct?

If you do not have an external shunt, there's no point in having DVCC enabled unless you want to use it to restrict MP charge voltage and/or current.

My little camping fridge has started to use ~70-100W (may be broken thermostat or need a regas, but waiting on a new fridge soon anyway).

So this morning the batteries were down to 46.7V after a day of cloud yesterday and me forgetting to get out the generator. Not good (and generator will be running all morning to boost them back up).

11.675V/12V likely isn't too bad under load.

I went to check the low voltage cutoff settings and found the low voltage shutoff is by default set to 37.2V! Way too low to protect the batteries I'd have thought?

I gather that is to stop it shutting off with a voltage drop under load but still, that seems very low. And at the moment we never have more than 300-400W running at a time.

Considering that, I was wondering what the low voltage cutoff should ideally be set at?

I'd set it at 46V.

Ideally, you'd establish an approximate voltage to SoC relationship for your system under load. At that point, you could guess at a reasonable cut-off.

If you very carefully ensure that your batteries are fully charged, you could enable the MPII battery monitor using the 190Ah capacity and the defaults. Turn off MPPT and let your batteries discharge. This could help you establish that loaded voltage to SoC relationship.

I'd really like to use SoC to shut them off but I don't know how to set it up. In VE.Configure I don't have "Enable Battery Monitor" on because I don't know how to enter the settings. I know the battery capacity (190Ah), but do not know the "State of charge when Bulk finished" (which has a default value of 85%) or the "Charge Efficiency" and haven't been able to find out these values for my system either.

Aside from the one-time process above, the MP battery monitor will do you no good. It can't see what's coming in via solar. It will only track AC in and AC out from the battery.

There is no point in having DVCC enabled, since you have no devices that can provide meaningful data to other equipment.

I don't have an external battery monitor either so not sure if it is possible, but we don't have any DC loads coming off the batteries

No, but you have a "negative DC load" that bypasses the inverter, i.e., the MPPT direct charge of the battery that "skips" the inverter.

at all so it does all come through the inverter.

You're underequipped to get what you want.

The additional of a Smartshunt or BMV-702 or 712 WITH the temperature sensor would be a good addition to the system. An advantage of a BMV is the control relay. You could use the control relay of the BMV to signal the inverter to turn off at a given SoC.

EDIT: lastly, when your batteries are at peak absorption voltage, you should check the individual 12V batteries to ensure they are at nearly identical voltages.

 

[MilbankAustralia]

Thanks for the reply!

MP II to GX via VE.Bus?
MPPT to GX via VE.Direct?

If you do not have an external shunt, there's no point in having DVCC enabled unless you want to use it to restrict MP charge voltage and/or current.

I should have specified, it is the MP2GX. SmartSolar to it via VE.Direct. I was using DVCC so they share the temp sensor of the MP2GX mainly, but also to limit the overall charge current when the generator is on and solar is feeding in (this hasn't actually been relevant since setting up the system as I've only used the generator on very cloudy days when the solar is negligible).

The other feature I used it for was to allow the GX to track the solar input, which it does and which I can see through VRM which only the MP2GX is connected to.

11.675V/12V likely isn't too bad under load.

Looking at the battery voltage/current chart in VRM it looks more like 0.4V drop when load goes from -0.7A to -1.7A (which is far more than the 0.013V/amp drop I estimated using the 3.3mOhm resistance from the spec sheet...but that math is not something I am that familiar with the application of either).

When the batteries are fully charged at the end of the solar period, they usually drop down from 50.6V to about 49.4V by the early morning when the solar starts (both values with the 70-90W load on them).

Unloaded, 100% SoC should be ~51.96 from the spec sheet, so does ~51-50.6 seem likely to be ~100% SoC with that ~2A load on? If so at least I know they are usually being returned to ~100% SoC under good solar conditions.

Maybe I will give some more generator top up today and see if I can get the unloaded voltage to 51.96, then turn the inverter off overnight and check the battery voltage early morning before the solar starts as well. Then I can apply a load (inverter and modem) and note the voltage drop under a particular load. I think that is what you are suggesting if I read correctly?

The day they dropped to 46.7V happened overnight after a cloudy day where we used about 4kWh but only generated ~1kWh.

I just saw on the battery spec sheet they recommend a low voltage cutoff to prevent discharge below 1.75Vpc. I think I will be conservative and set it to 1.85Vpc which is close to your suggestion. I think that should accommodate me when using larger loads without risking damaging discharge when it is just the usual loads. Does that seem reasonable?

Aside from the one-time process above, the MP battery monitor will do you no good. It can't see what's coming in via solar. It will only track AC in and AC out from the battery.

So even though the GX communicates with the MPPT and tracks solar input, the battery monitoring feature will not factor this in?

you could enable the MPII battery monitor using the 190Ah capacity and the defaults.

Thanks. I don't understand the default values. Could you please explain how I would find out the "SoC when bulk finished"? I have the SoC to Vpc chart in the spec sheet but don't know how to relate that to the end of the bulk charge stage.
I also don't know what "Charge Efficiency" is, or why the default value is set to 0.8.
Is it more reliable/safer to just use the voltage based cutoffs rather than the SoC anyway? If my typical loads increase and the low voltage cutoff is becoming a problem due to voltage drop under load I guess I repeat the process you described to try and determine the voltage drop under the typical load.

EDIT: lastly, when your batteries are at peak absorption voltage, you should check the individual 12V batteries to ensure they are at nearly identical voltages.

I have an active balancer on the 4 of them and the voltage difference between the 1st and last battery in series is generally <0.0.2V since putting the balancer on, regardless of what stage of charging they are on. Forgot to mention that, it is an unmonitored DC load but hopefully its self consumption isn't that significant.

Sorry for the fairly longwinded response, appreciate your input