- Diode battery isolators cause additional voltage drop: 0.4 to 0.8 V for silicon diodes and 0.1 to 0.4 V for FET transistors used as diodes.
- The alternator in the engine compartment registers an ambient temperature of 40°C or even higher while the house battery, lower down in the boat, is much colder e.g. 20°C. This results in an additional under-voltage of approx. 0.6 V or even 1.2 V for 12 V or 24 V systems respectively. - The house battery will usually be deeply discharged and should really be charged with a high (absorption) voltage. This is particularly the case when the alternator on the main engine is the only source of power and runs briefly every day to charge the batteries.
- In contrast, the starter battery and often also the bow thruster battery are practically always fully charged and do not need any absorption charging.
- Often different battery types are used for starting, for the bow thruster and for house service. These different batteries all have their own charging recipe.
5.2.3. A wide range of solutions
It would be exaggerating to say that there are as many solutions as boats, but there are certainly many ways to, more or less, overcome the above-mentioned problems. Several, but certainly not all, will be discussed hereafter:
5.2.3.1 Keeping it simple and low cost: the microprocessor controlled battery combiner
Let the alternator charge the starter battery, and connect the service battery to the starter battery with a battery combiner (for ex. a Cyrix battery combiner from Victron Energy). When one of the 2 batteries is being charged (the starter battery by the alternator or the service battery by a battery charger), the Cyrix will sense the increasing voltage and connect both batteries in parallel. As soon as the voltage decreases the Cyrix will disconnect the batteries from each other. The advantage is simplicity and cost: the alternator does not have to be modified or replaced. The drawback is a somewhat longer recharge time of the house battery because bulk charge will stop at approximately 30 % DoD (or worse in case of important voltage drop in cabling or a low alternator voltage due to high temperature) and then be followed by float charge. This means that the battery will be cycled between 30 % and 70 % DoD. The solution is to oversize the house battery by 20 % to 50 % and do a 100 % recharge when shore power is available. Most alternators with built-in regulators can be modified so as to deliver a higher voltage. Adding a diode in series with the voltage sense input of the regulator increases output voltage by approx. 0.6 V. This is a job for the specialist. We will not dwell on it here, but it is a low cost improvement that, together with 5.2.3.1, will charge batteries quite fast. Severe overcharging is a risk only in case of very intensive motoring every day, and even that problem can be solved by temporarily switching off the alternator (but never disconnect the main output of the alternator from the battery with the engine running, because the resulting voltage spike might damage the rectifier diodes in the alternator).
5.2.3.2 Increase alternator voltage
5.2.3.3 A multi-step regulator with temperature and voltage compensation
When choosing a multi-step regulator (bulk-absorption-float, see chapter 4), I would suggest to go for the best and choose a model with:
- Voltage sensing. This requires additional voltage sensing wires to measure and regulate voltage directly on the terminal posts of the house battery or on the DC bus. Voltage-drop in cabling and isolators is then automatically compensated.
- Temperature compensation. This requires a temperature sensor to be mounted on the house battery.
This solution is often used when an additional high output alternator is fitted.
5.2.3.4 The starter battery.
The solutions as suggested in 5.2.3.2 or 5.2.3.3 will improve charging of the house battery, but what about the starter battery?
Let us assume that when the main engine is running, the batteries are charged in parallel by using battery combining relays, or a diode or FET isolator. Nearly all of the charging current will then flow
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