A fixed-current alternator-to-battery charger either cooks the alternator or leaves output on the table, because the safe ceiling is never constant — higher at cruise on a cool morning, lower at idle in an engine room at 65 °C. Alt2Bat measures the alternator case temperature and runs your Victron Orion XS chargers so the alternator sits at its maximum safe output, whatever the conditions.
A probe on the alternator case feeds a control loop aimed at a setpoint you choose — 105 °C by default, the accepted marine practice for case temperature. Below the setpoint the current limit climbs; near it, it eases. The loop settles where heat in balances cooling out: the true sustainable output for the present RPM, airflow and ambient.
Drives up to two Orion XS 12/12-50 units over VE.Direct using Victron's own External Control mode: a fresh current limit every second, while each Orion keeps its bulk, absorption and float algorithm and its battery preset — LiFePO4 included. The combined target is split between the units by their individual limits.
Reads engine RPM and coolant temperature from NMEA 2000 — any engine gateway or ECU that publishes them. At idle the belt is left alone; with cold coolant the ceiling is reduced; with no RPM at all charging is held at 0 A. Boats without engine data run on temperature alone.
Alternator temperature, commanded and measured current and battery voltage, sampled every ten seconds and drawn against clock time on the phone — so you can see the loop find the ceiling on a real motoring leg.
Each unit's stored settings — absorption and float voltage, engine-shutdown detection, input lockout, current limit — read and edited from the web page. The box captures each unit's configuration as a profile and re-applies it if a unit is ever replaced; one button syncs unit B from unit A.
Three saved networks and an open setup access point when none is in range. Firmware updates install themselves only while the engine is off and nothing is charging, and a version that fails to settle rolls back on its own.
The windings of an alternator run 30–50 °C hotter than its case. Holding the case near 105 °C keeps class F and H insulation well inside its rating — the same principle as a dedicated smart regulator, but using the DC-DC chargers you already have and leaving the alternator's own internal regulator untouched.
Below the setpoint the combined limit rises at a rate you set; inside the deadband it holds; above it, it backs off. Proportional and integral gains, deadband and ramp rates are all yours to tune from the phone — with sensible defaults that suit a small marine diesel.
The combined target is shared between the two Orions in proportion to the current limit you give each one, and clamped to it. If one unit drops out, the other takes what it safely can.
A 120 A Valeo alternator on a Yanmar 3YM30AE, in a 65 °C engine room. Driven flat out by a fixed-current charger it overheated and tripped, delivering around 48 A effective. Under Alt2Bat it sustains around 70 A at a 105 °C case — hour after hour, with the temperature trace flat.
The heavy cables are the standard Orion XS alternator-to-house layout: both chargers take their input from the starter battery, deliver into the house bank, and are fused at both ends. Alt2Bat adds only light-gauge connections — a VE.Direct cable to each Orion, the probe on the alternator case, the thermal switch, and one NMEA 2000 drop cable that also powers the controller.
Per Orion: one cable from the starter battery to IN+, one from OUT+ to the house bank, each with a 60–70 A fuse at the battery end; 16 mm² under 5 m. Negatives to the common negative bus. The alternator-to-starter-battery cable is never touched.
One standard VE.Direct cable per Orion, plugged straight into the controller — nothing to strip or solder. The probe bolts to the alternator's rear housing and plugs into the controller. One standard drop cable puts the box on the NMEA 2000 backbone and powers it — no separate supply. A third socket, CERBO, takes an optional VE.Direct cable to the Cerbo GX — the Cerbo then sees the two chargers as one Orion XS.
A normally-closed 130 °C thermal switch on the alternator replaces the wire bridge on Orion B's remote terminals. Above its trip point unit B shuts down by itself, with no software in the loop; Alt2Bat re-normalises onto unit A.
Alt2Bat only ever sends a volatile current limit. If the box stops talking for a minute — power lost, crashed, unplugged — each charger reverts by itself to the fixed limit stored in its memory. A dead controller can never leave the alternator uncontrolled.
An open, shorted or implausible probe reading cuts the combined limit to 0 A immediately, and stays there until the reading is good again. The chargers stay awake and connected throughout.
Every cut-back acts through the Orion's current limit or enable line. The alternator output is never open-circuited under load — no load-dump spike, ever.
A normally-closed 130 °C thermal switch on the alternator, wired to unit B's remote terminals, trips above the software setpoint. It works with no software in the loop at all.
Charge voltages, chemistry preset and the engine-shutdown detection live in the Orion, not in the box. Alt2Bat only caps the current; nothing it does can overcharge a bank.
Alt2Bat claims its own address on the bus and publishes alternator voltage, current, temperature and charger state as the standard charger PGNs (127508, 127506, 127507, 127750). A Cerbo GX or a chartplotter lists the alternator as a charger — no VE.Direct cable to the Cerbo required.
Over-temperature, a probe fault and a charger error are raised as standard NMEA 2000 alerts (126983 with 126985 text). A test button on the Setup tab lets you check how your own displays react before you need them to.
CAN error counters by type, the address it claimed, and the state of your engine gateway — how old the last RPM is, whether the gateway is silent or sending "not available", how many fast packets were lost. "No engine data" tells you why.
Status, Graph and Setup tabs at altcharger.local on the boat WiFi — no app. Live alternator temperature and target, each Orion's state, input and output, error code and off-reason; the engine card; and every control parameter editable in place.
Temperature against setpoint, combined target and what each Orion is actually delivering, battery voltage, engine RPM and coolant, WiFi and bus health — refreshed every two seconds.
The last two hours of temperature, current and voltage on a clock-time axis. Watch warm-up, the approach to the setpoint and the hold.
One button records the raw traffic on both Orion ports and uploads it for analysis — the fastest way to show us a unit that misbehaves. It can also be requested remotely, so the boat records on its next check-in.
Alt2Bat sets the current limit of Victron Orion XS chargers; it is not a charger itself. Battery protection and the engine-shutdown detection stay configured in the Orion and keep working with the box disconnected.
Not included: the Victron Orion XS chargers (one or two) and their VE.Direct cables. Powered from the NMEA 2000 bus: one standard drop cable supplies the controller, brings engine data and puts the charger on the bus.
Contact for pricing and availability. Tell us your alternator, engine and which Orion XS units you have or plan to fit.