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John Dale (Battery Direct) looks at the architecture of battery bank wiring and gives some advice on making the right battery selection for your boat. 

I think we would all agree having a healthy battery bank on-board is crucial for safe enjoyable boating.

My 40 odd years as a marine electrician has seen some terrible unsafe birds nests, not only in general boat wiring but also at the heart of it all, the battery banks.

These birds nests are responsible for over-heated wiring, radio interference and low consumer voltages resulting in poor appliance performance and excessive power usage

When it is necessary to have multiple positive or negative battery terminations, please take one cable only from the positive and one from the negative terminal to an organised busbar. (not buzz bar. It is a bus as it has multiple seats) Connect the different circuits to the busbar be it positive and/or negative. Keep it simple – keep it clean.

Parallel and Series wiring architecture for battery banks.

Connecting batteries to increase voltage, capacity or both is generally seen in boats, motor homes and off-grid power banks. Be aware, there is a correct configuration that is not always evident.

Firstly, all cable used when wiring the battery bank should be of the same gauge. Each connecting cable should be as short as possible and preferably the same length. Cable has resistance and keeping batteries in balance will prevent early failure.

Each battery needs to supply the same amount of power. It is also important for each battery to be charged equally. Cable resistance, no matter how little, will effect the current.

Parallel connections will add the capacity of each battery while maintaining the voltage of each individual battery. Two 100AH 12volt batteries in parallel will result in a bank of 200AH 12volt.

Note, contrary to the images, it is recommended parallel configurations should be limited to three batteries only so to reduce battery to battery power transfer while not on charge. It is also clear all batteries be the same model and age.

Illustrated are two common methods of parallel wiring. However there is a big difference when current draw is calculated form each contributing battery. The correct example on the right is balanced and all batteries contribute equally. Note the negative termination.

The example on the left is a ‘no no’ as it is unbalanced as each battery contributes different current.

Due to the extra length of cable from the bottom to the top, the top battery has additional resistance resulting in voltage drop and therefore supplies less current when compared with the bottom battery. In fact, the bottom battery will contribute almost twice the current of the top battery, with the obvious variation of the centre two..

This result can be shown mathematically which I will not go into here.

Series Connection

Used when the battery voltage needs to be increased. The example below will add the voltages of the batteries (in series) but the capacity will remain that of one of the batteries only. If this bank has 220AH 6volt batteries, the bank will result 220AH at 12volts.

It is possible to connect as many batteries as necessary to achieve the required bank voltage. However, it is also necessary to ensure they are all are the same model and age and state of charge.

Since batteries in series are links in a chain, the current through the string will be restricted by the battery with the highest impedance.

Note: It is advisable to charge all batteries in parallel prior to the series link up to ensure equal state of charge.

Why in series? The maximum Amp Hour capacity of a battery is governed by the battery wattage. The largest 12v made is 270AH and if you need more capacity – the 6v battery can go to 420AH – 2v battery 3000AH.

(This restriction is due to the ability the surface area of the battery has to dissipate heat.)

Series / Parallel connections will increase voltage and capacity. If these batteries are 6v 220AH, this bank will produce 440AH at 12V. The example is a common bank in a cruiser with a few home comforts. Note the take off of the negative cable to balance the parallel connection as noted above..

The cabling requirements for the series and series/ parallel connections remain as mentioned above. There may be several ways to achieve the bank you need. To consider the best option for you may be; cost, size or weight of each battery or consideration of redundancy. ie. Loose a 6 volt battery in a series configuration, you are left with an unusable 6 volt system. Loose a 12volt battery in a parallel configuration, you still have 12v all be it at half the capacity.

Main sub head as splits the feature into two parts

Battery Selection

The difference between start battery and cyclic (House, Domestic) battery is perhaps obvious but we still see start batteries fitted to take up the duty of the house supply.

The modern battery is designed to produce higher amperage, the main difference is plate design. A start battery has thin porous plates to optimise surface area producing high current on demand. However, cycling produces heat and the thin start battery plates will buckle and shed material in short order promoting early failure.

The Deep Cycle battery has dense thick plates which also defends against distortion making the battery heavier. Since batteries are priced by lead weight, they will cost more. The weight and density of these plates also determines the AmpHour capacity or run time. It is prudent to compare the weight of the battery to optimise selection.

Since the weight of lead determines capacity, the bigger the battery, the more lead the more capacity.

It should be clear that it is possible to have a high CCA from a small battery but not capacity. For instance a battery which may produce 680 amps and only 20AH opens a sensible question – how much power on demand and for how long ?


Flooded (Wet) batteries

Flooded batteries are being used less frequently due to a few disadvantages, The suphuric acid (39%) is very corrosive destroying many a tee shirt. Further, the electrolyte will gas readily under a heavy charging algorithm. The gassing, due to high internal impedance, high charging voltage or high ambient temperatures, expels the excess energy and splits the water molecule. The resulting Hydrogen gas is flammable and the accompanying sulphuric mist is carcinogenicity. These batteries are no longer being fitted under bunks or indeed, in any human occupied enclosure.

Maintenance Free (Calcium)

To solve many of the wet battery problems, the Calcium Maintenance Free (MF) option was introduced.

The gassing problem was solved by alloying calcium into the lead plates but unfortunately at the same time, inhibited charging.

Charge voltage was then increased to 14.8 ~ 15.3. This meant that a discharged MF battery may not fully charge using the alternator alone. The alternator will maintain a partially discharged battery after start up only but is considered a poor battery charger.

 However, when heavily discharged, the 14.8 ~ 15.3 volt calcium algorithm is necessary.  And in fact, since the battery electrolyte is sitting quiet and stratified, 16 volts is then needed to de-stratify the acid water mix after a full charge. If not the battery may slowly loose charge once more.

Sealed AGM start battery

Total convenience comes in the form of a sealed battery – most problems solved!

Other than the obvious advantages, the charge voltage requirement generally is 14.2 ~ 14.7. Most alternators can manage these parameters.

A down side for some, the battery may dry out in high ambient temperatures. Temperatures in the tropics and engine rooms exceeding 30oC. will promote electrolyte lose through the pressure relief valves. We cannot replace it. The battery looses both high current capability and capacity.

Choose the start battery type that suits your environment.


Deep Cycle batteries are available as flooded, MF, Sealed AGM, Gel and Lithium.

With all types, the maximum capacity of a battery is determined by its ability to dissipate heat through the surface area of the box. To increase the required capacity you may need to go down in voltage.

Excluding lithium, the AH capacity of a battery is always qualified by a C rating.

C20 (the standard) meaning the discharge amperage is such as to discharge the battery over 20 hours.

For instance:

Sample battery, nominal capacity 107AH @ C20

C20 (5.36A to 10.50 volts)  107.2AH

C10 (10.0A to 10.50 volts)  100AH

C5 (17.5A to 10.20 volts)  87.5AH

You can see how the battery changes capacity by the rate of discharged

Note: Lithium battery capacity is not affected by the discharge rate.

The deep cyclic battery is often used in 6v and 12v units. The choice may be determined by the system voltage – 12v or 24v and / or the total capacity required. The decision may be influenced by your available foot print and height restrictions. You may be able to achieve the same end result with either 6v or 12v units.

Flooded Deep Cycle battery

As with the start batteries, the flooded option is often used in high temperature environments such as the tropics. It may also have a price advantage.

The cyclic life is generally approximately 800 at 50% depth of discharge and a shelf life of 5 years. However, the batteries life experience will determine the shelf life. Look after it, ie keep it fully charged, and it may last longer. Neglect it and it definitely will be shorter.

Maintenance Free Deep Cycle battery

Just as the MF start battery, this battery requires the correct charging algorithm. It has a cyclic life of 200 ~ 280. This may sound low, but is perfect for the runabout going out every couple of weeks and last some years. It costs less than the flooded accessible and sealed options. Otherwise all other rules above apply.

Sealed AGM

To seal a battery, the electrolyte must be immobilised and in this case it is achieved by soaking the acid water mix into glass matting between the plates. AGM is considered the preferred method of immobilising the electrolyte. It has low resistance to optimise discharge and charge rates.

To ensure good plate to electrolyte conductivity, the plates are compressed together at assembly. Further, to discourage gassing, the chemistry is held in a positive pressure which raises the gassing point.

You will be familiar with the pressure cap on your radiator – to raise boiling point.

Cyclic life generally sits around 850 cycles and a shelf life around 5 years similar to the flooded options providing it does not experience high temperatures – unlike the flooded options.

Some have a CCA rating suitable to start the engine but primarily is a cyclic battery.

Lead Carbon

Lead carbon AGM batteries have carbon alloyed into the plates to enhance performance. The reduction of lead by percentage reduces the onset of lead sulphate mainly on the negative plate. Sulphation is the main cause of premature failure in lead acid batteries. Remco says their Lead Carbon battery in contrast to the Std AGM has a cycle life of 1400 cycles @ 50% DoD

Lead Silicon

The Sulphuric electrolyte has been replaced with a silicon based electrolyte sitting in the battery as a damp powder. It must be periodically charged with a current equal to 20~ 30% (model specific) of the battery capacity. If not, the high cyclic life promised will revert to a standard lead acid AGM specification.

Gel battery

Again, the electrolyte is to be immobilised and in this case , a silica gel is added converting the electrolyte into a jelly substance. Otherwise, there is little difference in construction. Generally, the Gel chemistry has a high cyclic life due to robust deep cycle technology.

The gel battery is often physically larger for the same capacity than the AGM option and is not capable of producing high current on demand for starting.


Lithium batteries – LiFePO4 (Lithium Ferris Phosphate) have many superior attributes over the lead acid battery. Lack of weight, high cyclic life, fast recharge and constant voltage throughout the majority of discharge process. Capital outlay is reduced when one considers the cost of ownership. Once you calculate the cost per cycle, the savings are obvious.

The lithium battery should not be totally discharged and to guard against it, the BMS (Battery Management System) is designed to cut power at 90% discharged. If you have no warning of this loss of power, it could be dangerous when an auto pilot or navigation display is being relied on.

Even though this may not concern you, it may be an insurance concern.

The regulation can be found at: AUS/NZS 3004.2:2014 stating ‘Lithium Batteries Require’

1/ an audible / visual alarm.      2/ Stop charge.     3/ Have a BMS.

There may be only one or two battery brands that will fully comply.

Choosing your battery is a matter of considering your environment.

  • Firstly it must be capable of fulfilling the chosen task be it start or cyclic.
  • It must be able to cope with temperature variations and charging systems you have on board.
  • The Chemistry must be safe for the crew.
  • It must fit your boat and your budget  

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