Nothing works on board without electricity. The engine won't start, the fridge won't cool and the cabin will remain dark. If you want to avoid an "electrical disaster", you should look after the on-board batteries in good time and, above all, monitor their charge status. A simple voltmeter, as fitted on most boats, does not help. Even if the pointer of the voltage indicator is in the "green range", the battery may be flat. The voltmeter only shows the current battery voltage, and this can still be above the 12 V mark even if the battery is almost empty.
Only a so-called battery manager can provide reliable information about the state of the boat's power storage. Similar to a water meter, these devices count the amount of electricity drawn from the battery. In the opposite direction, the amount of electricity that is fed back in via the charger or alternator is measured. What is actually in the battery is shown as a numerical value on a display. If 85% of the charge of a 120 ampere hour (Ah) battery is still indicated, there is still 102 Ah in it.
In contrast to this simple percentage calculation, the technology in a battery manager is much more complicated. The measuring procedure for all models is based on the following principle: The current flowing into or out of the battery is measured and the result is totalled. For example, 50 Ah out, 25 Ah in, results in minus 25 Ah. Starting from a fully charged battery (only this state can be precisely determined by measurement), the device also couples the current state of charge. Depending on the model, the display then shows how much power is left in the battery. More convenient devices indicate the remaining charge in Ah or - as mentioned at the beginning - in per cent.
Since the last BOOTE battery manager test in 2004 (issue 9/04), three new devices have come onto the market and another has been completely revised by the manufacturer. As in the previous test, the following test criteria took centre stage in this test series:
- Resolution (which smallest unit can be measured?)
- Own electricity consumption
- Storage maintenance after power underbilling
- Deviation on the voltage side
- Deviation on the current side.
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