A Basic Set-Up…

The most basic setup to be found on a narrow boat would be one 12v battery providing both starting and leisure systems with power.

Theoretically this could (and for some does) work, so long as the battery could hold sufficient amps, consumption of those amps was less than the storage and that there is an efficient way of recharging the battery when discharged.

The battery would need to be able to turn over a heavy engine as well as to be able to efficiently deliver domestic power requirements. In reality this is not really a practical long term solution. The big issue would be having enough amps left in the battery to start the engine following a night on the tow path.
Most of us, at some time in our lives, have experienced the inconvenience of a flat car battery and the middle of nowhere is not a good place to be when this happens!

Multiple Battery Banks…

Therefore the vast majority of narrowboats have 2 battery banks. One for dedicated engine starting and the other to provide power storage in order to run domestic systems. Two battery banks may involve more than two battery cases…

Batteries are Individual Cells Grouped Together…

As we have seen in our ‘What is a Battery’ section, lead acid batteries are based around individual cells of 2.1 volts (or thereabouts and for the purpose of simplicity we will refer to cell output as 2V). These 2V cells are commonly grouped together in 6’s within a plastic moulded casing and are therefore recognisable as the 12V batteries we are familiar with. A battery bank may consist of two of more 12V battery cases connected together.

Starter and Leisure Battery Groups…

Starter batteries are 12VDC units built specifically to be able to deliver high amp output in short bursts. Cold Cranking Amps (CA) or Marine Cranking Amps (MCA) are key features of starting batteries. The purpose of a starting battery is to deliver a big burst of energy in one hit.

They differ from dedicated leisure batteries by way of internal construction and are therefore not well suited to handle the many charge/discharge cycles demanded from a purpose designed leisure battery.

Leisure batteries are sold by being rated on Amp Hours. This relates to how many amps can be used from the battery at a certain temperature over a certain time before the battery is discharged down to about 10.8 volts. At this voltage there is not enough oomph left in the battery to power a load and the battery is considered to be flat!

How Many Amp Hours do you Need…?

A common amp hour figure for a marine leisure battery would be around 100 amps. This is rated to indicate that a 5 amp load could be run for 20 hours before the battery would be considered to be flat, theoretically!

In reality and with an eye on getting value for money out of a standard battery bank, the bank should not be discharged lower than 50% of it’s rated capacity.

Therefore your standard 100 amp/hour leisure battery is really only good for 50 amps output before it needs a charge. It will go lower but if you regularly discharge lower than 50% of its rated capacity it will fail sooner. That is a key factor towards understanding how to get the best out of your battery bank.

Therefore it is common practice to wire 2 or more of these standard 12VDC cases together to gain amp hour outputs of up to 400 or 500 amps. This gives extended operation of loads before the need to recharge.

Traction Batteries…

2V cells are also sold individually or grouped together in 3’s as 6 volt batteries. The 2V and 6V battery cases are commonly known as traction batteries because they are used to power electric vehicles such as forklift trucks or golf carts.

The case sizes are similar to the common 12VDC leisure battery and therefore many more plates of a heavier construction can be used internally.

More plates produces more amp output and the heavier construction means they can last longer in age (measured in terms of charge/discharge cycles), being less prone to internal short circuits due to sulphation.

These types of battery can also stand being discharged to a higher percentage than a common 12VDC leisure battery. It is possible to discharge these types of batteries up to 70% without doing significant long term internal damage. A common output figure for a good quality 6VDC battery is 225 amp hours.

Common Voltage…

Narrow boat DC leisure systems have developed as a result of innovation in automotive systems. As a consequence, the commonest system voltage is 12VDC. Most accessories such as pumps, televisions, lighting and accessory sockets are based around 12VDC operation. If we use 2V or 6V cells we must connect together a group of these batteries to produce 12VDC.

Two 225 amp 6 volt battery cases can be wired together in series to produce one battery outputting 225 amps at 12VDC.

Two of these 225amp banks can be wired in parallel to give a battery bank that still operates at 12VDC but now is capable of storing (and releasing) 450 amp hours of energy.

How Big…and why There is a Limit…

It’s now, hopefully, easy to see how we can connect a group of batteries together to give banks of varying voltages and amp hour output. The obvious question is why can we not just go on indefinitely adding batteries to build massive storage capacity (money permitting). One of the big disadvantages of DC electrics is that due to the way current flows down the wire, the more amps we draw, the thicker the wire has to be.

A Bit of History…

In fact Thomas Edison developed the first commercial electrical power grid in Manhattan using 110VDC but the venture was commercial flawed due to the fact DC power cannot be distributed over long distances, principally due to practical restrictions on wire gauge. More amps can be pushed along thinner wires by higher voltages thus AC distribution became the norm by being stepped up to very high voltages using transformers, carried long distances by thin wires and then stepped down again at distribution stations for use by the end user.

You Can’t Take Out More Than You Can Replace…

This is relevant to our narrow boat systems in that a balance has to be struck between wire cost, battery cost, safety and the ability to recharge efficiently and at a reasonable cost. This is why most systems will store 400-500 amp hours with the ability to efficiently recharge when necessary.

And Finally…

Some of you may be aware that 24 volt systems are also used. As we learned above, the higher the voltage, the more amps can be pushed through the wires.
By using 2V cells grouped to provide 24VDC lower wiring costs and greater electrical efficiency can be achieved.

These 24V systems are usually used where high demand accessories such as pumps or fridges are installed in order to be able to safely use thinner wiring. Thinner wires offer less resistance so efficiency over longer cable runs is greater with 24VDC installations. However as the status quo is 12VDC as 24VDC accessories such as pumps, inverters, chargers etc generally tend to be more expensive.