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4.6. Conclusion: how should a battery be charged?

As mentioned earlier, there is no simple recipe that can be applied to all batteries and operating conditions. Also, there is no greater variety of operating conditions and types of batteries than can be found on a yacht.

To get a better idea of how batteries are used and what this means for charging, let us again take the example from section 2.4. Let us assume that the yacht has 3 batteries on board: a house battery, a starter battery and a bow thruster battery.

How are these different batteries used, and how should they be charged?

4.6.1. The house battery

In sect. 2.4 and 2.5.6 three conditions of use were described:

1) Cyclic use, in the partial state of charge mode, when sailing or at anchor. Important here is charging as fast as the battery permits. Temperature compensation is a must to prevent early failure due to overheating and excessive gassing. 2) A mixture between float use and short, shallow discharges when motoring or moored. The risk here is that a 3-step alternator regulator (when motoring) or a charger, (when connected to shore power) is frequently triggered by these shallow discharges to go into bulk and then absorption mode. The result could be that the battery is continually subjected to absorption charging and will be overcharged. Therefore, ideally, the length of the absorption phase should be in accordance with the preceding DoD. See section 5.3.2. for the adaptive charging method, a Victron Energy innovation. Flooded batteries, if being float charged without any discharge occurring, should be switched to the lower 2.17 V per cell level and be regularly topped up with an absorption charge at 2.4 V / cell or more. Again, see section 5.3.2. 3) For long periods of time the battery is left open circuited or float charged, in wintertime for example. As discussed in sect. 4.2.3, most flooded batteries will deteriorate quickly if float charged at 2.3 V per cell for a long time. Ideally charge voltage should be lowered to between 2.15 V and 2.2 V per cell, or left open circuited and recharged regularly. When the average temperature is 20°C or less, at least every 4 months. At higher temperatures they should be recharged more often. From my personal experience and from numerous discussions with boat owners, I do also prefer to leave sealed Exide/Sonnenschein Dryfit A200 batteries or equivalent open circuited or on a lower than recommended float level instead of float charging them at 13.8 V, because, although in theory they can be float charged during long periods of time, only too often the result was damage due to overcharging.

4.6.2. The starter battery

The starter battery is subject to 2 conditions of use:

Shallow discharge due to starting the engine once or twice a day.

- No discharge at all. The best would be no recharge either, apart from an absorption charge once in a while.

In practice however the starter battery will very often be charged in parallel with the house battery, which is acceptable as long as the right type of battery is used and some decrease of service live is accepted (see note, sect. 4.5).

4.6.3. The bow thruster battery

When used, discharge can be deep, and fast recharge will be required. In general the most practical solution is to charge the bow thruster battery in parallel with the house battery. Often spiral-cell batteries are used, because of their very high peak current capability. These same batteries will accept a wide recharge voltage range and are very tolerant to overcharging.