Battery bank design is a fairly deep subject as the boat can be yacht, powerboat, fishing vessel, RIB or many others. Numerous questions arise as to 'how long the boat is unattended' or 'does it have a bow thruster', so the subject is fairly complex. I will try and break it down in easy chunks that can be selected at will.

O.K. - I know some smart individual will tell me that his or her boat is too small and only has room for one battery. All things are a compromise so please bear in mind that these designs are flexible and may need adapting to your own needs.


1. Boats that have an engine need a dedicated engine start battery. The battery can be one of two or more and the option to switch from one to the other can be an advantage. One rule is paramount and that is that electronics should not be running from the engine battery when the engine is started as the surge generated can severely damage electronic circuits. This damage can sometimes not show for several years, before the damaged equipment fails. The same rule applies to mains or A.C. battery chargers - they should not be switched on when the engine is started or expensive damage is likely to ocurr to the battery charger. If the engine has a dedicated battery (a 'bank' is more than one battery as in 24 volts) that is NOT used to supply the electronics then one of these problems is solved. If an automatic bilge pump is left on because the boat is on a mooring, then this should NOT be connected to the engine battery in an ideal situation.

2. Electronics need a separate supply from the engine so we now have a separate ship's supply battery, so what happens if the float switch jams and that battery goes flat? Ideally we guard against this by having a second ship's supply battery. We now have THREE batteries - one for the engine that is dedicated and two which can be interchanged for the electronics and other on-board supplies. In the case of a sailing yacht, this configuration guarantees that if navigation lights fully discharge one battery, the second is still available for the GPS and Radar, while the engine supply is left untouched! In the case of a small RIB we compromise by having only two batteries which can be switched for ship's supply, but ideally we run the engine controls from one only - calling that battery No.2 while the ship's supply comes from number one. Optionally in the RIB the easy route is to have an 'either or' situation with both electronics and engine from the same supply, but then we ensure that we do not start the engine while electronics are switched on if we can help it.

I told you it could get complicated, so what about charging. Ideally, we arrange to charge all three batteries independantly so that one cannot discharge the other. More of that later!

3. SWITCHGEAR now becomes important. In the three battery design, the normal situation is to have a single engine isolator, a 1- BOTH -2-OFF selection switch for the ship's supply (call it 'domestic') and a by-pass switch (can be 1-BOTH-2-OFF) that can switch the engine battery into the same circuit as either or both of the domestic supplies. (See diagrams when I have drawn them!) We now have a true redundancy situation in which neither the engine or domestic requirements should ever leave you in serious trouble without power.

The diagram below illustrates one method of installing the three batteries and charging each via a blocking diode assembly. These diode assemblies are available for 2 or more batteries and also with the option of two alternator inputs.

Should the engine battery fail for any reason the emergency engine start bypass switch can be used to select either 1, 2 or both of the 'domestic batteries'. Needless to say they should all be correctly marked!

Pitfalls and problems/solutions with the above design!

As previously stated - everything is a compromise on boats, so the following problems might arise with the above design and installers need to be aware of them.

1. Blocking diodes loose approximately 0.4 - 0.5 volt across them. Given that most marine alternators throw out in excess of 14.00 volts and that a fully charged conventional lead acid battery reaches full acharge at 13.8 volts, then it is quite possible that the battery will only receive 13.5 - 13.6 volts on charge. Is this a problem? The quick answer is NO! Most of the extra volt or so measured on a battery under charge is float voltage which disappears rapidly when the charger is switched off and the voltage settles back to around 12.7 volts or so, providing the battery is in good order. In my own experience, providing that the system is working correctly and all connections are clean and tight, there will be no significant difference to a system that does not use blocking diodes. It is possible to use relays in place of blocking diodes and for the relays to be energised during charge only. This eliminates any voltage drop, but has the disadvantage of allowing a bad battery to steal charge from a good one. This paragraph is given by way of technical explanation as all semi-conductor diodes experience a voltage drop, which is their forward conduction voltage. If you have a good quality digital meter (e.g. Fluke 77 or similar) you can use the mode with the little diode symbol and measure this volt drop, providing you have the meter leads the right way - red to anode and black to cathode. If you are lucky enough to have a high quality battery sensing type of alternator fitted, there will be an extra sense cable from a terminal on the alternator to the engine battery. This type of alternator charges until the battery reaches absolute full charge and diode voltage drops are no longer a consideration. However, in the marine leisure industry (as in the automotive industry) machine sensing alternators are frequently fitted, where the battery voltage is measured by an internal connection in the alternator - these types of alternator cannot compensate for extra voltage drops in the system and it is these that most d.i.y. installers will be experiencing. End of explanation!

2. Some marine alternators (usually some of the machine sensing types) fitted by leisure engine manufacturers require a positive voltage on their main output terminal in order to power their internal charge control electronics. If this scenario occurs, the symptoms will be that all wiring is correct, but because ALL charge connections are through the blocking diode, no charge voltage can be measured - i.e. the battery voltages will not increase when the engine is running! (Diodes act like one way valves) The solution is simple - place a link across the input to the diode assembly and the output to the engine battery. This will have the effect of maintaining the required activation voltage on the alternator, while still controlling the other batteries charge through the blocking diode assembly. Under these circumstances, if you have a three battery system, you can use a twin blocking diode unit (as opposed to a three output) and take the input into the blocking diode from the main alternator + output terminal, while leaving the existing connection to starter motor/engine battery as existing. I have only experienced this scenario on a couple of occasions and it can be easily proven by fitting a temprary link as described, when the system will start to charge normally. However, remember the golden rule - NEVER CONNECT OR DISCONNECT an alternator while the engine is running so make your modifications when the engine is not running.


Once there were lead acid batteries that required topping up with distilled water. Now we have lead acid, lead calcium, nickel cadmium, gel type, sealed, maintenance free and a host of variations. Which do we use and what difference can they make to the installation (and your pocket!)? What are the differences?

Lead Acid batteries have been around for decades. They rely on internal electrodes made from a lead paste held in a grid and immersed in an electolyte of dilute sulphuric acid. When the battery is fully charged, the specific gravity of the acid is around 1.25+ and the acid is at the stongest. When the battery is discharged, the sulphate component of the acid is attacking the lead and causing sulphation, which very quickly damages the battery permanently. Lead acid batteries self discharge around 40% of their capacity over two months and give off hydrogen in large quantities when being charged. Hydrogen is lighter than air, so don't do what one multi-million pound uacht manufacturer does and fit a drain at the bottom of the battery compartment to let hydrogen out - it will already have left into the cabin space! The good point about lead acid batteries is that theyare good working battries with the capability to accept high charge and to discharge verey high current for engine starting or traction. Design life is around 2 years (surprised?) but they often last 4 or 5 if kept charged.

Lead Calcium batteries are modified lead acid types. They have the advantage of far longer life, lower self-discharge (12 months+ for 40%) and have less hydrogen gassing during charge. They are frequently marked at 'gel' batteries that need no topping up. Some manufacturers fit them with little green eyes which show if they are charged or not. I prefer to use them in boats and vbehciles and have some on my own boat that are now over 8 years old. They do have a slightly different terminal voltage and some marine chargers can be set for either conventional lead-acid or gel batteries. Many batterie can be lead-calcium, gel and maintenance free all rolled into one and marketed as 'leisure' batteries. If in doubt, before decidingon a particular battery, ask for the technical specification sheet from the supplier, or visit the manufacturer's web-site.

Nickel-Cadmium batteries are no longer used in the marine field. They require special constant current charging facilities, are initially very expensive and can be corrosive.

Lithium-Ion and other forms of Lithium batteries are becoming larger in capacity every day. The small ones are used in mobile phones and larger one up to 100 A/hr are used in missiles and will probably be available for marine use in the not too distant future. They have an extremely high energy storage capacity, virtually no self-discharge, but the larger they get, the more expensive they become to produce. There are also some disadvantages with instability and heat.

ALL batteries contain valuable and toxic metals and chemicals. They need to be disposed of through licensed disposal sites such as your local council, to comply with the Environmental Protection Act and the Waste Disposal Regulations. If they are just dropped over the side of the boat, the first thing they will do is to kill the local wild-life. Most marinas have disposal arrangements for batteries.

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