Battery Maintenance on narrow boats

After my son bought a narrow boat I started to look into battery management. Unlike a caravan where the battery may have a few weeks float charge for a week on the road the with only the lights as a load. The narrow boat may spend weeks away from the marina with shore charging availability, as a result have a very short time with mains charging.


With lights, pumps, and central heating the over-night draw on the batteries is heavy. Fitting a large alternator will not in its self cure the problem as on a float charge there is a limit to how quickly the power can be replenished. Also solar cells tend to be powered at the same time as the engine so again not much help as it is the ability of the battery to accept charge which is more important then the availability of power.


Only by increasing the voltage above float charge voltage can the batteries be replenished within the time available.

Fork lifts and the like have a two stage charge and the voltage on a 12 volt battery could well reach 16 volts the volts at a fixed current and is matched to 80% charge at which the charge rate is then dropped to the equalising charge current for a fixed time and the batteries are slightly over charged to ensure all cells reach the fully charged state. But in this case the battery is not used while being charged.

The car and van is a compromise with slightly over float voltage for a 12 volt battery between 13.8 and 14.2 volts selected by the manufactures to be likely to ensure the best battery life for a fixed voltage charge.


To raise the volts over 14.2 although the increased voltage can help with a discharged battery it can damage a charged battery. So to pump more power into a battery in a short charge time the three stage charge cycle is used. At first fixed current but once a preset voltage of 14.2 to 14.8 according to battery type is reached the charge rate is reduced this reduces the chance of over charging. While in this stage normally the current is measured. Once this drops to 1% of the amp hour ratting the battery is considered as charged and the voltage is reduced to the float voltage. However any draw on the battery while being charged can delay this change in charge rate so a maximum time is also included. At the charge rates delivered by on shore power supplies this is OK as the maximum current is relatively low maybe 25 amp. Also the charger often has a split output and the engine battery is floated at 13.8 even when the domestic is being supplied with 14.8 volt.


To charge while under way there are a number of options. Two alternators with different voltages is one method again the engine start battery having a lower voltage to the domestic. But the regulators required to step charge a domestic battery are not normally fitted as standard to the alternator so the alternator has to be specially altered. The prolonged high rate of charge combined with limited engine compartment ventilation can also cause overheating problems and alternator failure which because it has been altered is often not covered by warranty. 

Using either one or two unaltered alternators to charge the battery through blocking diodes or split charge relays would result in either domestic battery being under charged or engine battery being over charged. Also with blocking diodes the volt drop across the diodes needs to be compensated for.


The best method of charging the domestic battery at a high rate while still float charging the engine battery will involve some switched mode conversion. There are a number of DC - DC converters and these basically take the voltage and current supplied and transform it to a higher voltage to charge the domestic battery. The simple ones take the power from the engine battery and once the voltage rises showing it is on charge the power is taken from this battery and feed to a domestic battery using the step charge process to quickly charge the second battery. These using are mainly designed to allow the domestic battery to be remote from engine battery for example mounted in bows and the maximum current is normally quite low around 15A. However with high amp hour batteries near the engine compartment the energy transfer is likely to be lower than that which would be transferred using a simple split charging relay.


There is however one unit which works on much higher currents and also uses a different method to monitor when each stage should be applied so reducing the possibility of the battery being over charged due to domestic drain foxing the monitoring system.     


However as we start looking at the best charging system we also are looking at a lot of money to buy and install the system.


So looking at cheap method what is required. Main thing is a method to treat both batteries different. If the alternator is machine sensed I can't produce a voltage over the voltage set by the regulator. So if it supplies the domestic we can stop the engine battery over charging by using a diode but it is unlikely to do too much harm to engine battery anyway.  

Where a battery sensed alternator is used we can fox the alternator to over charge by inserting a diode in the sensing lead by charging the domestic direct and from the domestic charging the engine battery through a blocking diode we can charge a two different voltages. However every time we engaged the starter it would likely blow diode so also a split charge relay would be required to ensure the domestic batteries are not able to supply starting current.


Two blocking diodes could be used but this would mean lifting up the alternator voltage to 1.2 volt above the designed voltage.


As one starts to add the extras required to allow the duel voltage control one quickly reaches the stage where it is just as cheap to buy the ready made switch mode unit.


So one has to work out is it worth it to extend battery life. Traction batteries are expected to last around 10 years when correctly maintained. However in narrow boats this can be more like two to four years. For the hire fleet which are sold on when only a few years old then unlikely to be worth while fitting the expensive unit. For the holiday boat which spends most of it life at the marina again unlikely to be worth it. With boats not based in a marina with shore power and lived in 24/7 then cutting down power requirements by using solid fuel cookers etc. will help far more than increasing the power from engine charging. And do you want to run the main engine anyway? Few people can afford to be moving every day so we need to look at some charging method other than main engine.