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OK, so you’re worried about horror stories where someone has had a BEV for a few years and the battery is down to 50% of it’s capacity due to that awful battery degradation. So, what’s the story and how do you avoid this happening to you?
Modern BEVs come with a “lithium” battery pack, made up of lots of individual cells into groups of cells that with a Battery Management System (BMS) and cooling/heating tubes go together to make that very large battery hanging underneath the car.
The batteries, they’re almost certainly “lithium ion”, and these come in different “flavours”: metal-oxides (e.g. Lithium Manganese Oxide, LiMn2O2), or phosphates (e.g. Lithium Iron Phosphate, LiFePO4). The phosphates are preferable in BEVs as they’re much safer and have relatively long lives.
So, we have the batteries, they’re safe, but they still need looking after, if they run too hot or too cold they won’t work properly, hence the need for heating and cooling, liquid cooled (antifreeze) in some, air cooled in others, and not much at all in others due to battery capacity being increased without regard to increased heat generation (thankyou Nissan). Air cooling has fans and air ducts. Water cooling & possibly heating has pipework and often a heat exchange system (and you thought that heat pump was for your comfort in the cabin?).
That leaves the BMS, the Battery Management System or Battery Mismanagement System in some BEVs. This consists of some electronics and a lot of cables, plus links to the cooling/heating system.
Remember you have lots of smaller battery cells making up modules and modules making up the whole battery. When you charge the BEV it starts off with the BULK CHARGE, this can be delivered at very high rates using a DC Rapid charger, but after the battery reaches a higher state of charge some of those individual battery cells will have more electricity stored away in them than others…. that’s where the BMS comes in, to “balance” the cells. The balancing process involves all those finer cables in the battery pack, they allow the thicker cables to charge the battery whilst the finer cables are used to drop or leak individual cell voltages down to stop them getting damaged whilst others are still charging full rate.
So, how do we get around this slowing down of the charging? Well, we don’t really, we leave the Rapid charger when we have about 80% charge & continue our journey. When we reach our destination we plug into the AC fast charge post or plug it into our wallbox and use these to “balance” the battery cells over a longer time (overnight full charge once a week should do it). If the battery pack doesn’t get that occasional full balance charge then it will degrade much faster.
In addition the BMS can limit the power available whilst driving, this is what you see when the BEV uses “modes”, Eco limits the power available using the BMS, Sport gives the full amount allowed by the BMS, speeds are limited again by the BMS. If you run short of electricty then you’ll see “Turtle Mode” and the BMS progressively limiting the power whilst you get nagged to recharge as soon as possible. Go beyond Turtle Mode and the car will stop & shut down to stop you from draining the battery completely which kills it forever.
So, keep your battery between 10% and 80% whilst using the car if you can, and charge it to 100% once a week just before you use the car. If you’re leaving the car unused for a while, aim for 60-70% charge level, that’s where the lithium batteries are happy, unlike lead acid batteries which degrade unless they’re at 100%.
If a battery is too cold it won’t charge rapidly, it’ll throttle the charge rate. Some cars will heat the battery pack in cold conditions to help, some will just charge very slowly (coldgate).
If a battery is too hot it will be damaged by charging rapidly or by driving (remember high regen rates going down hills), to counter this in warmer weather some cars will cool the battery pack whilst charging and sometimes whilst driving, they will also throttle the rapid charge rates (rapidgate).