Energy Unlimited by Victron

5.Charging batteries with an alternator or a battery charger

5.1. The alternator

The main engine of a boat is normally fitted with a standard automotive alternator. Standard automotive alternators have a built-in regulator with temperature compensation. The temperature is measured in the regulator itself. This is a suitable arrangement for cars, where the battery temperature will be roughly the same as the temperature of the regulator. Moreover, in cars the battery will virtually always be fully charged. The battery will only be discharged to a small extent during engine starting. After that the alternator delivers sufficient power, even with the engine idling, to supply all consumers and to recharge the battery. Because the battery is actually never deeply discharged, and in general plenty of charging time is available, the absorption phase discussed in chapter 4 is superfluous. The alternator charges with a current dependent on engine rpm until the pre-set float voltage is reached. Then the alternator transfers to constant voltage. Generally the voltage is pre-set at 2.33 V / cell at 20°C, i.e. 14 V for 12 V systems and 28 V for 24 V systems. This charging system works perfectly given the following conditions: - the battery is a flat-plate automotive battery - the battery is nearly always fully charged - the temperature difference between the regulator on the alternator and the battery is limited - the voltage drop along the cable between battery and alternator is negligible (i.e. less than 0.1 V, including switches, isolators, etc.).

Problems occur as soon as one of the above conditions is no longer fulfilled.

The following sections shortly discuss the practice of charging batteries with an alternator.

For an exhaustive discussion of alternators, alternator regulators, isolators and other related equipment, I recommend reading Nigel Calder’s standard work “Boatowners Mechanical and Electrical Manual” as well as a visit to the websites of Ample Power (amplepower.com), Balmar (balmar.net) and Heart Interface (xantrex.com).

5.2. When the alternator has to charge more than one battery

5.2.1. Introduction

The bare minimum on a boat is two batteries: one to start the main engine and a house (or accessory or service) battery. To make sure that the engine can always be started, all accessories (navigation equipment, lighting, autopilot, refrigerator, etc.) are supplied by the house battery.

The starter battery (sometimes 2, for 2 engines) should have no other load than the starter motor of the main engine and must never be allowed to discharge, otherwise the engine cannot be started.

Often there is a third battery on board, the bow thruster battery, and there may be even a fourth, the electronics (navigation) battery.

The batteries are separated from one another by relays, diode isolators, or other devices that will be briefly discussed in the next sections. In larger systems the starter battery often has its own dedicated alternator. Battery voltages may also be different, some 12 V (starting and electronics) others 24 V (house and bow thruster)

5.2.2. The problem

When using a standard automotive alternator-regulator to charge several batteries simultaneously, the following problems arise:

- In a boat, cable runs are often much longer than in cars so that there is more voltage drop between alternator and battery (example: the voltage drop along a 5 metre long, 10 mm 2 cross-section cable is 0.5 V at a current of 50 A).