Lithium: what you need to know continued...

these can vary considerably in size, and shape. They can be flat in format, known as prismatic, in which case the battery may only have four in total. Or they can be cylindrical, in a ‘swiss-roll’ format, with strips of anode and cathode rolled together with a separator in between. These cylindrical cells can be remarkably small, around the size of an AA battery for the 18650 size, with the result that an 85kW pack, as used in the Tesla car, has no less than 7,104 of these cells!

The nominal voltage of the lithium-ion cell is 3.2V, which means that multiples of four of these cells give you a battery with a nominal voltage of 12.8V, which closely compares to the lead acid battery, which has six cells of 2.1V and a voltage of 12.6V. This allows you to make a straight swap of a lithium battery for lead-acid. Battery charge management

In order to preserve the expected life of a lithium battery, and to maintain safety, it is essential that it is not discharged below a certain voltage, at a maximum current, and the cells are kept at equal voltages. Also the charge current and voltages should not exceed certain values. For large battery packs in cars this has required a separate and sophisticated electronic battery management system as well as a purpose-designed battery charger.

The breakthrough with the current new crop of marine lithium batteries is that they all include integral battery management systems, either in the case or as a simple plug-in device, and as a result of this they can be charged with existing chargers.

The only stipulation is that the charger must deliver a maximum voltage of 14.4V, typically a gel lead-acid setting.

However some of the suppliers indicated that if you are going to get the maximum capacity into their batteries, you should use a charger with a specific lithium setting, and some supplied us with one.

Or if the battery is in a boat or motorhome, with an engine-driven alternator, they recommend you fit a

‘The 85kWh battery pack in a Tesla car is made up of 7,104 AA-sized cells’

battery-to-battery charger between the engine start battery and the lithium service battery, to give maximum charge rate, while protecting both the alternator and battery from excessive currents. The BMS

The Battery Management System (BMS) is the key component of all lithium batteries and battery packs. If the voltage drops below a certain preset level, usually somewhere between 9.0V and 10.0V, or the current goes too high, the BMS switches off an internal relay and the current stops.

If this happens, some manufacturers leave the battery permanently switched off until a charge voltage is applied to it, whereas others allow the battery to reset after a few seconds once the load has been disconnected. Lithium batteries will require a suitable smart charger

The BMS will also balance the individual cells in the battery – that is bring all the voltages to the same level. This is important for long life.

It will also shut the voltage across the terminals down for transportation. Engine starting

None of the lithium batteries we tested claim to be suitable for engine starting. For this you need a very high current for short bursts – in the order of 200-400A, and they are just not designed to produce this regularly. Warranties

The warranty figures quoted for lithium batteries should be treated with caution. Most are based on figures provided by the makers of the cells, and these can often be hedged with caveats, such as tapering replacement values, rather than full replacement, and stipulations about how the battery has been charged and discharged, which could be difficult to prove if you have a claim.

For this reason you should read the small print carefully, and not assume that a unit with twice the warranty period is necessarily twice as good.