Alternator charging a Lithium bank

Doug Smith

Hello group and apologies for such a very specific question.  This is regarding the ‘stop charging’ cutoff for the 24V Mastervolt alternator with respect to a Lithium house bank.

I have reviewed everything I can find within our internal group email messages on this subject and have found great value in the education I have received. Also scoured the Victron site, Wil Prouse’s videos, PYS and the Marine How To web site for more information.


I have spent the last few weeks mapping out the transition of the equipment I currently have and drawing my schematics for the upgrade to Lithium over the next 12-18 months.

My schematic is attached for planned system. I agree with the idea of one vendor for the system, and have a mostly Victron layout to begin with so will continue that direction. 

Phase 1, installed already or over this winter

Cerbo GX, with Touch GX for display, mounted in battery compartment with the display on the forward bulkhead of the passage berth at eye level.

BMV-712 replacing the Xantrex.

Skylla-I 24|100|1+1 battery charger

Sterling Alternator Protect Device

Currently have a Mastervolt, 24-2500 Mass sine wave inverter, 24/24 inverter, and 24/12 inverters

Phase 2, next winter

We will upgrade to the quattro with the Digital Multi Control panel in the second phase with necessary cabling as required

Solar Arch with Panels, and Smart Solar MPPT controller.

Alpha Pro III, upgrading from the Alpha Pro. Keeping the original Mastervolt 24/110 alternator.

Phase 3, will be to switch to Victron Lithium and BMS when current AGM’s reach end of life cycle and my own full time retirement begins


Scott’s email from SV Tengah described a method of cutting off the Volvo Alternator charging circuit using external relays and using the battery disconnect signal from the VE.BMS to control when the cutoff relay would disconnect the Alpha Pro regulator. This disconnect would be set at a level lower than full voltage, and it appears he set it around 26.5 for float.

Arno, Mark, Bill and others have commented on using Coulomb counters for amount of current out-in to measure when the SOC is nearing 90-100%. 

What I am raising as a question is “why not use the BMV – 712 self-contained relay, as the method to cut off the alternator charging when either or both the voltage hits a threshold or the SOC is near capacity?”  There is an internal relay on the back of the BMV-712, with both normally open and normally closed contacts that is bi-directionally stable, with low current draw, and a built in option to activate the relay using a logic circuit of both voltage and SOC based on a low and a high threshold.  Then use the Alpha Pro “activate and disconnect relays” in the box that are activated when the volvo is running by means of the ignition circuit. When SOC is 90%, it would disconnect the alternator ignition circuit using the relays in the regulator box. Also, does anyone have better information about how those relays are connected to the ignition circuit? I have attached what I have found, but not sure if any of this impacts the circuits running the engine or its system electronics. This would appear to simplify the concerns, but I am worried I am missing something.

A second option is a Cryix-Li-ct 12/24-120 relay.  This would be one more Victron component to add, but appears to be a way to disconnect using the Victron Direct port signal coming from either the Cerbo, BMS or the BMV.

I am pretty deep into the weeds on this so any or all feedback is appreciated,


Doug Smith

S/V Aventura, Amel 54-113

Patuxent River, MD USA

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