Lithium batteries vs lead-acid batteries: What are the key differences for boat owners?

For over 100 years most rechargeable batteries used lead and sulphuric acid, first invented in 1859 by Gaston Plante. Then in 1970 along came lithium batteries which are 50% lighter, with no memory effect, up to ten times the cycle life, and able to give nearly all their rated capacity at even the highest discharge rates.

lithium-batteries

But even the best lithium batteries do have downsides, writes Emrhys Barrell . The first is cost, at up to four times the price of an equivalent output lead acid battery, and the second is safety. Lithium is a highly reactive metal that reacts violently with water, or even moisture in the atmosphere if the cell is damaged, and if it catches fire, using water to extinguish it only makes the situation worse.

Using compounds of lithium significantly reduces, but does not eliminate the fire hazard. Lithium cobalt oxide (LiCoO2) has high energy density but greater safety risks especially when damaged. Lower energy but safer compounds include lithium manganese oxide (LiMn2O4) and lithium iron phosphate (LiFePO4).

The latter has become the material of choice for moderate performance batteries, and accounts for all the units in our lithium batteries group test.

What is in a Lithium battery?

The actual battery consists of a multiple pack of basic cells, and 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.

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Lithium batteries and BMS (Battery Management Systems)

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 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 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.

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.

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