Hi,
Yes — but only with careful design, the right isolated equipment, and strict safety/charging arrangements.
It’s not a simple plug-and-play setup and has important failure modes (imbalance, poor charging, or dangerous shorts) unless you account for them.
How you’ve imagined it
Battery A (primary): used for 12 V engine start/general loads, charged by the alternator.
Battery B: physically in series with A (B-negative to A-positive), so A-negative → B-positive = 24 V output for 24 V loads.
At the same time, B is used as a “house” 12 V battery, with its negative referenced to A-positive (the midpoint).
You plan to charge B from A via an isolated DC-DC charger (e.g. Victron Orion-Tr Smart isolated).
Why it can work
A galvanically isolated DC-DC charger can regulate B’s charging even though A and B don’t share the same reference.
As long as both batteries are matched and all charging/discharging flows are controlled, you can technically use the pair in series for 24 V while also tapping them individually for 12 V.
The big problems / pitfalls
1. Balancing: In series, both batteries must stay at the same state of charge. If one drifts lower, it can be overstressed. Without active balancing, one may get over- or under-charged.
2. Alternator only charges A: Battery B depends entirely on the isolated DC-DC charger to keep up. That charger must be sized to fully recharge B during normal operation.
3. Short-circuit hazards: The midpoint (A+ / B-) is a “floating” node. Accidental connection to chassis ground could short half the system.
4. Equipment compatibility: Many 12 V devices expect “battery negative = chassis ground.” In your setup, that isn’t true for B’s 12 V loads — making integration tricky or unsafe.
5. Maintenance complexity: A fault in one battery affects the whole 24 V system. Troubleshooting is harder.
What’s required if you try it
Galvanically isolated DC-DC charger (not just “non-isolated”). Victron sells both kinds.
Balancing/BMS or some way to monitor both batteries and prevent drift.
Fuses and disconnects close to each battery terminal and in the series link.
No chassis connection to the series midpoint. Clear labeling is critical.
Identical batteries: same type, age, and capacity.
Careful wiring: keep 12 V and 24 V circuits strictly separate and documented.
Safer alternatives
Separate start and house batteries (both 12 V): Charge the house battery with an isolated DC-DC from the alternator/start battery. If you need 24 V, add a DC-DC step-up converter.
Dedicated 24 V bank: Use two 12 V in series as a permanent 24 V battery bank. For 12 V loads, step down with a 24→12 V DC-DC converter.
Parallel setup with inverter: Keep both at 12 V, run 24 V equipment via a step-up converter or inverter.
Simple sketch (for concept only, not a full wiring diagram)
[BATT A -] ---- chassis/engine 12V ---- [BATT A +] ===series link===> [BATT B -] ---- midpoint (house 12V -) ---- [BATT B +] ---- 24V +
Alternator charges A.
Isolated DC-DC charger (input from A, output to B) handles B’s charging.
Midpoint must remain floating (not tied to chassis).
Recommendation
If you must do this, get a professional installer familiar with mobile power systems to design and test it.
My strong advice: keep start and house batteries separate at 12 V, and use DC-DC conversion for 24 V. It’s safer, simpler, and far less error-prone.
\\Roger