question

Is the charger rated for the two batteries being at different voltages? One battery will be referenced to ground, the other to +12V. If the charger circuits are not isolated, you have a direct short in it. Even if they are, the voltage differences may prevent it from working.

In your diagram the midpoint and positive connections to the shunt are odd. Guessing the dotted wires are meant to represent them. Might be worth clearing that up.

I have basically the same question. Thanks for asking it, cariboe, and thanks for helping us try to understand, kevgermany.

In my case, I have a Noco Genius Pro charger, which charges each 12v battery separately even though they are connected in series to provide 24v to the Minn Kota trolling motor.

Each 12v battery in the 24v system has a direct + and - connection from the Noco charger. One is connected to Noco charging bank 1, the other to Noco charging bank 2 -- independent of each other.

The Victron 712 manual seems to instruct me to remove the negative charger cables from each 12v battery and connect both of them to the "Chargers and Load" side of the Victron shunt along with the Load (negative) wire to the trolling motor.

My concern is whether this would damage or cause problems with the Noco Genius charger? https://no.co/genpro10x4

Or, can I just leave the negative charger leads attached to the Negative side of each 12v battery in that 24v system, as they were wired before I bought the Victron? I'm guessing that this alternative might remove the noco charger damage risk, but would require me to manually resynchronize to 100% at the start of each trolling motor trip (after I've charged those batteries to full, obviously). This wouldn't be ideal, but would work for my goal, which is to monitor the trolling motor battery level on long trips.

In case this is helpful to anyone else who arrives here after lots of mostly useless Internet search results for how to wire a Noco charger and Victron for 2 12v batteries wired in Series for 24v, I spoke with Noco Customer Support, yesterday, who said "No, the charger negatives can not be connected together" on the Loads and Charger side of the Victron shunt.

Unless there's another way properly and safely to wire the Noco charger and the Victron, this means that the Victron is about 50% functional for me, in that it will require manually synchronizing Victron to 100% charger after each trolling motor excursion and charging session, but should function well as a battery level indicator while on the water after that. This is what matters most in my situation, plus I can set an alarm on the Victron to let me know to stow the Minn Kota Ulterra before the LiFePO4 voltage dramatically drops (which would prevent the Minn Kota from stowing electrically).

I did, however, send a diagram after the call. If they provide a response that would allow full Victron functionality, I'll update and edit this post.

MJ

Sorry, I missed the update from @caryboe

For a shunt to measure current into/out of a battery bank, it must see all of it. When charger(s) bypass the shunt, it's impossible for this to happen. And of course, like @emejotace said, joining the negatives on series connected batteries must not be done. You would create a short circuit, which would make for an explosion as the wires overheated and boiled.

One shunt per battery would do it, but there's no way I know in the Victron environment to achieve this.

As an update for anyone searching ... Noco CS responded to my written question in which I submitted a hand-written diagram showing the 2 12v batteries connected in series and said "That should be fine as long as the charger recognizes the batteries indicated by the mode led and charge leds. If it doesn't recognize the batteries it will remain in standby (power light only)."

I interpreted this to indicate it would be permissible to attempt to connect the two negatives of the charger banks to the "Load and Chargers" pole of the Victron shunt and gave it a try. one of the banks showed normal LED indicators, but the other showed a voltage error.

So ... more information but back to the same conclusion, above: Need to leave the NoCo charger leads attached to both batteries for proper charging, which means the Victron works great to show consumption but won't show recharging. For my application replacing lead acid deep cycles with lithium batteries for a Minn Kota 24v Ulterra trolling motor, it's not difficult to just manually reset the charge level to 100% via the BMV-712's bluetooth interfaced app. Easy to do at the end of the day and easy to double do before leaving at the start of the day, since there's the Noco charger connected all night long.

The reason for adding the Victron is that Lithiums drop dead really, really quickly. Running them to that state is a concern that the Minn Kota won't be able to "Stow" on a dead lithium (but it could on a dead Lead Acid). Manually stowing those things is not fun or easy -- especially on an evening sunset cruise with beverages.