Managing lithium batteries
Q
Lithium (Li-ion) batteries may become ever more popular on boats as their price:performance ratio is dropping. However, I’m concerned about whether they are actually suited to the UK boating environment.
My Torqeedo electric outboard manual says to avoid allowing its lithium battery pack to be exposed to frost, presumably because it could damage or shorten the battery’s life. It’s little trouble to take my Torqeedo battery home for the winter but it would not be practical to take my boat batteries.
My other worries concern the lithium battery charging requirements. Sterling Power’s Alternator-toBattery charger allows the user to select the correct charge regime from any one of 10 different battery types, including lithium, but most boat battery charging devices – including 240V mains chargers, wind/water turbines and solar panels – do not.
So are lithium batteries suited to UK winters and would they require all new chargers and regulators? Chris Mardon, by email
DUNCAN KENT REPLIES: All batteries lose performance and capacity in low ambient temperatures. However, Li-ion batteries perform better than lead-acid in very cold conditions, continuing to function right down to -20°C.
Li-ion batteries have a higher cell voltage than lead-acid and much stricter charging tolerances are required to properly protect them. Although a Li-ion battery charger is a voltage-limiting device similar to those for lead-acid batteries, it utilises a different regime. The first, ‘bulk’ stage can be faster as it can take a higher current, but once around 70% full the second ‘absorption’ stage can take longer. Unlike lead-acid, Li-ion batteries don’t need to be fully charged, nor indeed should they be as doing so can overstress the cells and make them unstable. For this reason, rather than have a third, ‘float’ level as you do for lead-acid cells, the charge parameters on a multi-stage Li-ion charger are usually preset to switch off at the end of the absorption stage and only cut back in when the voltage has dropped to a predetermined level.
It’s also important, whatever the charging source, to ensure the cells in a Li-ion battery receive a balanced charge by regulating the charge to each cell. This, then, requires a lithium Battery Management System (BMS) to ensure the procedure is carried out correctly and safely.