EV BASICS 1O8: How to maintain your EV’s battery health during Covid-19 Lock down | Idle Battery Consumption & Charging

How to ensure health of battery when the EV is not in-use for weeks? This used to be one of the frequently asked questions during holiday season, but is more than relevant for now since most of the electrical vehicles are parked up for over 6 weeks - due to the global Corona virus pandemic.

We see that some EV owners just have left their vehicle unattended for weeks; and some other enthusiastic ones are charging them to 100% every day. Both these cases are wrong, will harm the battery life of your electric vehicle.

Hope this article may help you to understand the battery basics and provide some tips to maintain your EV’s battery healthy.

Should you charge your EV during lock down?

Lithium-ion cells typically have a self-discharge rate of  about 2% per month but an electric vehicle battery tends to discharge faster than this due to the parasitic drain of the vehicle’s electronics.

For example: A Nissan Leaf’s battery may lose about 15 kilometres of range per month when parked up for prolonged periods of time ; but a Tesla Model S that contains a lot more internal electronics may lose up to 15 kilometre of range per day! (assuming the Tesla Model S has been left parked in it's default 'standby mode' rather than fully 'shutdown').

So leaving the EV unattended for weeks is not a good idea! On the other hand, Charging your EV for up to 100% is never a good idea too, as the electrolytes inside the battery will degrade the cathode or positive terminal when the cell is at high voltages. 

The best practice is to keep the battery at between 80% and 50% state of charge (not be confused with the best drive range which is between 80% to 20%).  State of charge of 80% translates to a low enough voltage to prevent the electrolyte in degrading over several weeks; and 50% is high enough to limit the chances of the battery being left for so long that it is over discharged.

How to maintain between 50% and 80% state of charge?

Many of the electric vehicles provide a mobile app that can show the remaining charge in the battery. Some high-end EVs such as Tesla’s also lets you to set the adjustable charging limits, so you can set the charge to automatically terminate between 50% and 80%.

Some other EVs provide an option to easily configure their charging limits via their dashboard console or web portals. But what about cars that don't have these feature? You just have to monitor the battery percentage and manually intervene to stop the charge at the desired state of charge.

You don’t have to stand next to your EV and continuously monitor the battery percentage, just use the below formula to decide when to go and stop charging.

Take an example of a 40 kilowatt hour battery pack where the car's battery is currently a 30% state of charge (i.e: 12 kWh) and you wish to top it up to 80% (i.e: 32kWh). Assume that the EV has a 11 kW onboard charger and you’ve connected to a 11 kW charging station.

The formula for calculating,

Charging time (hours) = Required energy/ Charging Power

i.e = [32 – 12 kWh] / 11 kW = 1.81 (1 hour 48 minutes)

So if you plug-in the 11 KW charger at 30% of battery state and unplug it after 1 Hour 48.6 minutes, the EV’s battery will be ~80% charged. 

Note that the formula mentioned above is “good enough” to give a rough estimate of the charge time requirement. But, keep in mind that it will not guarantee exact 80% when you return, since the charging duration depends on various other factors such as temperature, SoC , SoH, battery’s age, technology…etc.

Is it ok to leave your EV just plugged-in (even after 80% SoC) ?

There has been some debate about the charging strategy for keeping the charger plugged-in for longer duration. I would advise to keep an eye on the SoC of your EV’s battery at least once a week and charge when required (when goes below 50%).

However leaving the car plugged-in continuously should be okay as long as the 80% rule is adhered to which entails that the car has the ability to set the charging limit to 80% in the first place. The battery management system (BMS) of the electric vehicle should regulate the charging pattern to get a gradual top-up rather than continually feeding the car.

It is also important to note the electric vehicle’s 12-volt auxiliary battery, the same as what's found in any petrol or diesel car which powers the headlights, windscreen wipers and infotainment systems, typically only gets charged up when the traction battery is engaged. Meaning, it gets charged either when the EV is fully switched-on and ready to drive or when the EV is charging. On this perspective, it is important to charge the car regularly or turn on the engine.

Conclusion :

Battery state	What should you do?
100% to 80%	Avoid keeping the battery above 80% state to charge to minimize electrolyte degradation and capacity loss.
80% to 50%	Try to keep your EV's battery between 50% and 80% state of charge during these corona lockdown or during long holidays. This is the ideal condition for maintaining battery when the EV is not in-use.
50% to 20%	Consider charging your EV to the recommended state of charge window above.
20% to 0%	If it is below 20% , you must minimize the chance of further discharging; you should consider to plug-in immediately (not at full charging power though!). You just plug-in the charger, the EV’s battery management system will gradually increase the charging current in way that is safe for the battery.