question

DVCC is primarily a method to allow a BMS to control the charge and discharge of the battery (usually lithium). Using lead acid battery and without a BMS, then there is not much point in using DVCC. Using smart networking or other would allow the MPPT to read the battery voltage from the bus, rather than its internal measurement, but this would have only a minor effect on the Bulk and Absorption charge phases.

Lead Acid battery charging is simpler than Lithium charging, as when a cell goes over voltage, it burns the extra charge power by gassing off, so allowing the battery to self equalise. This is voltage driven, and the MPPT will read both te voltage and the output current to decide when to switch to float voltage. Adding the load of the fridge (modem is negligible) will mean that the MPPT will switch to Float after the bulk timer expires, rather than on load current. However, as the fridge power cycles (hopefully), then the MPPT will still see the Absorption current tail off to the threshold value while the fridge is idling.

If your batteries are ageing, then cycling the additional load of the fridge through them will heighten this effect. If the battery chemistry will tolerate it, try setting up a monthly equalisation charge.

I have a lead acid system with Cerbo GX, Victron MPPT, BMV700 and Multiplus and I have DVCC set up using SVS (current), STS (temperature) and SCS (current). It does not do everything like for BMS but despite answer above, for lead I believe it should be done. This means that Multiplus and MPPT use the battery voltage from the SmartShunt or BMV and the MPPT uses the correct tail current measure, hopefully you habe MPPT absorption ending on tail current. If you have a battery temperature sensor then that can also be shared on both charging devices. Whilst not as useful as with lithium and BMS, the system is capable so use it. You do not need to use a Smart network as given above, DVCC can do all this through your CCGX.

In short, DVCC is a must with Lead Acid as it will:

• STS : Let the SmartSolar adjust charging voltage using temperature values shared from a sensor connected to the Multiplus II (if you do not a temperature sensor you definitely should; you can also wire it to the CCGX and it supply the readings to both the Multiplus and the Bluesolar all the same).
• SVS & SCS: Let the SmartSolar adjust charging based on values from what effectively is going to the batteries endpoint, using the readings gotten from the SmartShunt and discounting whatever is being directly used).

@freesurfer - What is the Mfr, Model # and total number of batteries? Parallel / Series configuration? Do you cycle 365 days per year? How do you store for winter? How old are the batteries.

Thank you for your responses.

The batteries were old AGM ones and about at the end of their lifetime expectancy. The datasheet states never to run equalisation voltages higher than 14.4V as the batteries cannot be refilled once the electrolyte gasses off, so equalisation was disabled in the MPPT controller.

What probably happened was that one battery in the array turned bad, causing the batteries to become unevenly charged. I do however think that the MPPT not being able to differentiate a tail current from a load, may have accelerated the process after leaving the MPII on, and that it can still be an issue even with healthy batteries which I am hoping DVCC can solve. I have temporarily replaced the old ones with 4 good batteries that have only been in service for about 5-6 years, and still have another 5-6 years of remaining life expectancy.

I did enable DVCC, mainly to limit the charge current to 20A which is the most the current batteries will handle. This seems to work as intended: If my PV has more than 20A available, it will limit the battery charge current at 20A, but still allow excess current over 20A to go directly to the MP rather than limiting the total PV power production (which is one of the highlighted features with DVCC).

Maybe DVCC is mainly intended for lithium BMSes. But one may argue that the smartshunt takes the role as a BMS for the lead acid batteries (just without internal cell balancing like a real BMS would have)

Question is still (and I think pwfarnell is just on point here): Will the system know how to differentiate a tail current from a load when determining when to switch from absorption to float? And if so (considering that there are several devices capable of reading currents and voltages, all of which have their own settings), where do I select which devices to use for these readings? The thing is that the MPPT has no way to know on its own without any info about the loads, and only the SmartShunt can know the true current at the battery terminals.

I use dvcc on my lead acid system so that the multiplus knows the correct SOC of the batteries.

Without it the multiplus doesnt know about the charge current from the mppt controllers.

in fact when my raspberry pi failed the multiplus shut down due to low SOC when in fact the batteries were fully charged.

as long as you have a smart shunt or BMV I'd always recommend enabling DVCC.