All three designs solve the same problem—charging two battery circuits from one source—in three fundamentally different ways. The choice is not made based on price, but on two questions: Do both battery banks have the same charging curve, and does the charging voltage still reach the destination?
The three designs
| Design | Functionality | Advantage | Limitation |
|---|---|---|---|
| Isolation relay | connects both circuits as soon as charging voltage is present | simple, inexpensive, practically lossless | both battery banks see the same voltage |
| Isolation diode | allows current to flow in only one direction | no moving parts | voltage drop across the diode |
| Charge converter | decouples and generates its own charging curve | different technologies possible, losses compensated | more expensive, requires space and dimensioning |
Charge converter, charge booster, and battery-to-battery charger refer to the same design. The name "booster" only describes the most common application: converting a too-low voltage into a sufficiently high one.
When an isolation relay is sufficient
The isolation relay is the right choice when three conditions are met: Both battery banks have the same technology, the cable between them is short and adequately sized, and the charging source provides a charging curve that suits both.
This is the classic case of lead-acid starter plus lead-acid consumer. The relay closes as soon as charging begins and disconnects again when charging ends—this way, the consumer battery bank cannot drain the starter battery. There is practically no power loss because the closed contact is a continuous conductor.
The current rating must be considered: A relay placed between two large battery banks will experience high equalization currents when connecting. The switching capacity must match, and so must the cable.
Why the isolation diode is rarely the answer
A voltage drops across a diode, and this voltage is missing from the battery at the end of the cable. With a charging voltage that is already barely sufficient, this drop determines whether the battery bank will ever be fully charged. This can only be compensated by the charging source regulating correspondingly higher—which in turn requires a regulating system that can do this.
In existing systems, the isolation diode is often the reason why a battery bank chronically remains at 80 percent, without anyone finding a fault.
When a charge converter becomes necessary
The charge converter is the only design that generates its own charging curve. This solves four problems where relays and diodes fail:
- Different technologies. Lead-acid starter and lithium consumer in one system—the converter charges each side with the correct charging curve for it.
- Long cables. Whatever voltage is lost along the way, the converter recovers.
- Regulated alternators. Modern engines reduce charging voltage after a short time and generate peaks during overrun. The converter smooths both into a clean charging curve.
- Different voltage level. Charging a 24-volt battery bank from a 12-volt electrical system without installing a second alternator.
Another, often crucial point: The converter decouples the alternator from the lithium battery bank. If its battery management disconnects under load, the alternator still sees the lead-acid battery on the input side—the load shedding fails.
Sizing the charge converter
The largest variant is rarely the right one. Three limits are decisive, and the smallest one wins.
- What the alternator can continuously supply. A converter that draws 60 amps while the standard alternator supplies 40 will operate it at its limit—with corresponding heating.
- What the cable can tolerate. Input and output cables must be sized for the full current and fused, each close to the battery.
- What the target battery can accept. For lead-acid, charging acceptance is the limit; for LiFePO4, it is usually only the cable.
NoteDevices with adjustable current limiting alleviate the first and second limits: You choose the size based on the target battery and limit the current to what the alternator and cable can provide. A soft start also helps, because the belt is not suddenly loaded.
The decision in four questions
- Do both battery banks have the same technology? If no: Charge converter.
- Is the cable long or barely dimensioned? If yes: Charge converter.
- Does the alternator regulate the voltage down after a short time? If yes: Charge converter.
- Otherwise: Isolation relay, sized to match the current.
Don't forget fusing
Both designs are in the main current path between two batteries. This means: Each cable needs its own fuse, at the end where the battery is located—for a connection between two battery banks, this means at both ends. A fuse in the middle only protects half of the cable.
Typical errors
- Isolation relay with mixed technologies. One of the two battery banks will be permanently incorrectly charged.
- Charge converter chosen too large. It draws more than the alternator and cable can provide.
- Only one end fused. The cable is a connection between two sources—both ends can supply.
- Charging curve not set. A converter with factory settings for lead-acid will not fully charge a lithium battery bank.
- Isolation diode overlooked in existing systems. Explains many cases of chronic partial charging.
