Separating Starter and Leisure Batteries: Methods Compared

Why separation is needed at all

A house battery has two conflicting tasks. One is to deliver small currents for days and be deeply discharged in the process. The other is to deliver several hundred amperes for a few seconds and then be fully charged again immediately. A battery that does both does both poorly — and in the worst case, it's so empty in the morning that the engine won't start.

That's why the starter battery gets its own circuit. It is only used for starting, otherwise remains full, and is not touched by the boat's electrical system. The house battery takes care of everything else. The only question is how both are charged from the same charging source without discharging each other.

This is exactly the task of the isolating device: allow charging current in both directions, but discharge current only in its own circuit. Three designs solve this with varying degrees of effectiveness.

The three methods

Diode Isolator

A diode only allows current to flow in one direction. Two diodes behind the alternator feed both battery banks without one being able to discharge into the other. The component is passive, has no moving parts, and practically never fails.

The price for this is physical and cannot be designed away: approximately 0.7 volts are lost across each silicon diode. From a 14.4-volt charging voltage, only about 13.7 volts reach the battery — this is sufficient for a float charge, but not for a full charge. A lead-acid battery bank on an uncompensated diode isolator will never be fully charged and will sulfate. This can only be compensated for with a regulator that measures the voltage at the battery itself and allows the alternator to run correspondingly higher.

Voltage Sensitive Relay (VSR)

A voltage sensitive relay (VSR) — also called VSR or ACR depending on the manufacturer — measures the voltage and closes a power contact as soon as charging is detected. If the voltage drops again, it opens. Typical switching thresholds are around 13.2 to 13.7 volts for closing and 12.4 to 13.1 volts for opening, each with a delay of a few seconds to prevent short load peaks from causing it to chatter.

Advantage over the diode: no significant voltage drop, and it works with any simple charger without remote sensing. The decisive disadvantage lies in the functionality itself: As long as the relay is closed, both battery banks are simply in parallel. They see the same voltage and get the same characteristic curve — which only works as long as both have the same technology and a similar state of charge.

NoteA VSR can also get stuck if a charger in the float phase consistently exceeds the closing threshold. In this case, the battery banks are parallel around the clock and the separation only exists on paper. Check during operation whether the relay actually opens when charging is switched off.

Charge Converter

A charge converter — also known as a battery-to-battery charger, B2B, or charge booster — draws current from one circuit and uses it to generate its own regulated charging voltage for the other. This electrically decouples the two battery banks: each receives its own characteristic curve, regardless of what happens on the other side.

Three things follow from this. Firstly, mixed technologies work cleanly — AGM starters and LiFePO4 house batteries are the norm, not the exception. Secondly, the current with which the alternator is loaded can be limited. Thirdly, the alternator never sees the BMS of the lithium house battery: if the BMS disconnects, the alternator continues to work against the starter battery, and the feared load dump is avoided.

The price: more money, an active device with self-consumption and waste heat, and the charging power is limited to the nominal current of the converter.

Switching thresholds and losses are typical ranges; the specifications of the respective device take precedence.

Calculation: How large the charge converter can be

The most common design error is to choose the charge converter based on the desired charging time instead of what the alternator can continuously supply. The calculation is done in three steps.

1. Estimate the continuous output of the alternator. A standard alternator delivers its nominal current only for a short time and when cool. As a continuous value in the engine compartment, about 50 percent of the nominal current is a realistic estimate — so for a 90 A nominal current, about 45 A.
2. Subtract house consumption. What is already running during engine operation — instruments, refrigerator, autopilot — is deducted from this value. With 15 A, 30 A remain.
3. Account for converter losses. A charge converter draws slightly more on the input side than it delivers on the output side. With an efficiency of about 90 percent and slightly higher output voltage, 30 A output current draws about 35 A on the input side.

In this example, a charge converter with 30 A output current is the right size. A 60 A device would continuously operate the alternator at its limit and overheat without the battery bank charging significantly faster — the energy that is not there cannot be transferred faster. If you really want more charging current, you need a larger alternator, not a larger converter.

WarningMany charge converters have an adjustable input current limit. Use it. Without a limit, only the thermal capacity of the alternator determines what happens — and it only reports damage when the regulator or winding has been damaged.

Exceptions and special cases

Mixed technologies. AGM starters and LiFePO4 house batteries require different charging voltages and, crucially, different charging termination points: lead-acid batteries need a float charge, lithium batteries do not. A VSR cannot accommodate this difference because it places the battery banks in parallel during charging. One of the two will then be continuously treated incorrectly — usually the starter battery, because the characteristic curve is set for the house battery.

Lithium as a house battery, regardless of the starter. Here, the argument of load shedding is added: if the BMS disconnects while the alternator is running, the alternator is left without a consumer. A charge converter solves this incidentally, because the alternator always works on the starter battery.

Two identical lead-acid battery banks. The classic case for which the VSR was developed. Here, it is the correct, inexpensive, and robust solution.

Jump start. Some VSRs have a button that manually connects the battery banks to start with the house battery. This makes sense for two lead-acid battery banks. For a lithium house battery, it does not: the cranking current regularly exceeds what the BMS allows, and shutting down in the middle of the starting process is not a good moment.

Typical errors

• Diode isolator without compensation. 0.7 volts less means a permanently partially charged battery bank.
• VSR with mixed technologies. It cannot map the different characteristic curves.
• Charge converter chosen too large. The alternator is the limit, not the converter.
• Input current limit not set. The factory setting rarely matches the existing alternator.
• Isolation device unprotected. Both connection lines need a fuse close to their respective battery, not just a common one.
• Ground connection forgotten. Both battery banks must be on a common negative point, otherwise current will flow through unintended paths.
• Relay never switches off. If it remains permanently closed, there is no longer any separation — and the empty house battery will drain the starter battery with it.

Decision aid

Two identical lead-acid battery banks, simple system, small budget: VSR. Existing system with existing diode isolator and regulator with remote sensing: can remain, otherwise replace. Mixed technologies, lithium involved, or an alternator that needs protection: Charge converter — in these cases, it is not the more convenient, but the only technically correct solution.