to the house battery because this battery has the greatest capacity, the lowest internal resistance, and is partially or fully discharged. This means that the voltage drop across the isolator and wiring from alternator to house battery will be higher than from alternator to starter battery. It might very well be that to achieve an absorption voltage of, say, 14.4 V on the house battery, the output voltage of the alternator has to increase to 15.4 V (i. e. a voltage drop of 1 V from the alternator to the house battery). With 15.4 V on the alternator output the voltage on the starter battery could very well be 15 V (!) because only a small percentage of the current flows to the starter battery. The result is that the starter battery, already fully charged, is “forced” to 15 V although it should be floated at, say, 13.8 V. a) Improve the situation by reducing voltage loss as much as possible and leave it at that. The starter battery might need early replacement, depending on how frequently the conditions referred to above occur and which type of starter battery is used. Gel batteries or flat plate AGM batteries are not recommended here, because they are relatively sensitive to overcharging (they will start venting and dry out). A wet battery (low cost) will survive if topped up with water when needed, and an Optima AGM spiral cell battery is also a good option because of its wide charge voltage range and its tolerance to overcharging. See sect. 4.5 for an estimate of battery service live when overcharged. b) Add 1 or 2 diodes in the wiring to the starter battery to reduce voltage. Now the risk becomes undercharging, if the service battery is only occasionally charged sufficiently to reach the absorption voltage level (think of a sailing yacht on a long trip). What to do?
c) Insert a series regulator in the wiring to the starter battery, like the “eliminator” from Ample Power.
d) Charge the starter battery with a separate dedicated alternator.
5.2.3.5 The bow thruster battery
Optima is the ideal battery for this application. It can deliver extremely high currents and also withstands high recharge currents as well as a wide recharge voltage range. So alternative a) of 5.2.3.4 would be advisable.
5.3. Battery chargers. From AC current to DC current
5.3.1. Introduction
In chapter 3 and 4 we have discussed how batteries should be charged, and how batteries will fail if not properly charged. In section 5.2 it became apparent that charging batteries with the alternator on the main engine is a question of compromising. With battery chargers it’s somewhat less complicated, because most high output chargers have temperature and voltage sensing facilities. Some also have 2 or 3 outputs. And nearly all have 3-step charging. There is a great variety of chargers to choose from and it is also much easier to install dedicated chargers for the different batteries on board than adding additional alternators on the main engine.
5.3.2. Optimised charging
I hope it became clear from the previous chapters that charging batteries requires careful consideration, especially when conditions of use do change over time.
Victron Energy has incorporated in its latest battery chargers the knowledge that resulted from practical experience, discussions with battery manufacturers, and numerous lab tests on a wide range of batteries.
The innovation of the charger is in its microprocessor controlled ‘adaptive’ battery management system:
- The user can make his choice between 5 different charging recipes depending on which battery type has to be charged. All recipes can be modified to fit a particular battery type and brand.
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