Lithium: another league

 

Size and Weight

High energy density of Lithium means lighter batteries.

Lead acid batteries have a low gravimetric energy density between 30-40 Wh/Kg, lithium has a density of 110-175Wh/Kg meaning simply that more energy can be stored in a smaller space and thus lighter batteries.

Cycle life

Lithium does not suffer sulphation!

Lead acid batteries do not handle deep discharge well primarily due to sulphation. To 100% depth of discharge (DOD) they will last only 200-400 cycles suffering 20% capacity loss reaching their end of life (EOL). Sulphation is not a factor in LFP and internal resistance is hardly affected by cycling. Rather eventual fatigue from mechanical ageing is the cause of EOL, even so at a very low rate of 5% capacity loss over 1000 cycles with end of life reached only after 2000.

Capacity

Lithium provides constant output over a greater charge range!

Again sulphation is the culprit, compounded by high discharge depths. To prevent damage to lead, discharge is rated to a 50% state of charge (SOC) this with a charge acceptance current limit around 80% leaves only 30% of usable capacity. LFP has a very flat discharge curve between 10-90% SOC either side of which voltage rapidly drops or increases, Battery Management Systems are installed to cut off charge/discharge preventing damage at these ends allowing a constant voltage supply over 80% of the total capacity.

Charge time

High charge acceptance of Lithium means less charge time!

With lead, resistance to charge increases during charging, to overcome this, voltage is elevated while current is kept constant, bringing the battery to approximately 80% SOC - ‘Bulk phase’. At this point terminal voltage is reached and shouldn’t be exceeded, resultant increasing resistance lowers the amount of current accepted to achieve the last 20% - ‘Absorption’ and ‘float’ phase. These last two phases are necessary to prevent sulphation, damage and rising temperature but greatly increase charge time. LFP internal chemistry allows for the acceptance of higher constant currents over a larger range of SOC. Higher charge rates have been found to increase the mobility of lithium ions between electrodes and lower internal resistance allowing for a much faster charge.

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