Why should you not charge your EV right after a ride? Scientists have the answer

Charging an electric scooter or car right after a ride adds heat to a battery that is already warm, and heat is what wears batteries out. IIT Madras's Professor Arvind Pattamatta explains the science and the cooling fixes being tested.

In Short

• Large pouch cells crossed 45 degrees under sustained one-C discharge
• Heat accelerates ageing and higher charge levels worsen thermal runaway risks
• Daily charging between 20 and 80 per cent can extend lifespan

The battery in an electric scooter does not need a heatwave to run a fever. At IIT Madras, researchers placed a large lithium-ion cell, of the kind used in electric vehicles, in the air at 35 degrees Celsius, on an ordinary summer afternoon across much of India.

Run at a steady, hard pace, it crossed 45 degrees, the line engineers would rather a battery never touched.

Then comes what most of us do without a thought. We park, plug in and walk away, handing a warm battery fresh heat.

Professor Arvind Pattamatta of the Department of Mechanical Engineering at IIT Madras has spent five years studying this problem. With the air outside at 40 to 45 degrees Celsius, he tells India Today Digital, "we have seen cases where the cell can catch fire."

WHY DOES AN ELECTRIC SCOOTER BATTERY GET HOT?

Every time a battery gives out or takes in energy, a little of it escapes as heat.

A lithium-ion battery stores energy by shuttling tiny charged particles called lithium ions between two layers, like passengers moving between two platforms. The crossing is never smooth. Resistance inside the cell turns part of the energy into heat, which is why your phone warms up during a long video call.

The harder the battery works, the more heat it produces. Engineers call that effort the C-rate. At 1 degree Celsius, a full battery would drain in an hour. At 2 degrees Celsius, in half an hour.

At IIT Madras, Professor Pattamatta and his team studied infrared images of two flat, soft-packed pouch cells. The results showed that the larger one, rated 60 ampere-hours (a measure of how much charge it holds), passed 45 degrees whenever the air was 35 degrees or warmer and it ran at 1C or faster.

WHY SHOULD YOU NOT CHARGE AN ELECTRIC VEHICLE RIGHT AFTER A RIDE?

Charging makes its own heat, and a battery that is already warm has little room to spare.

Charging is work too. The resistance that warms a battery on the road warms it on the charger, and fast chargers push more current and produce more heat.

Heat also wears batteries out. A review in the journal Energies calls rising temperature the most important accelerator of ageing. It cites tests in which a common type of cell, built with nickel, manganese and cobalt, fell to 85 per cent of its original health six times sooner when stored at 42.5 degrees than at 25 degrees.

A University of Michigan review of battery care advises unplugging a device that gets noticeably hot while charging, and parking electric vehicles in the shade in hot weather. Cars can run their own cooling while plugged in. A scooter, which usually depends on air alone, cannot. Your maker's manual is the best guide to how long to wait.

IS IT BAD TO CHARGE AN ELECTRIC VEHICLE TO 100 PER CENT?

The last stretch of a charge is the hardest on a battery, and the slowest.

Filling a battery is like boarding a Mumbai local. The first passengers glide in. By the end, everyone is squeezing for the last spot, and the charger must slow down.

That strain leaves a mark. A thin skin forms on the battery's negative layer and thickens with age, locking away lithium that can no longer carry charge. The Energies review says it grows more severely above 80 per cent charge, and faster in heat. In the nickel, manganese and cobalt cells it describes, cycling between 20 and 80 per cent instead of 0 and 100 roughly doubled the number of cycles before health fell to 95 per cent. Stored at full charge rather than 60 per cent, a cell reached 90 per cent health seven times faster.

Charge level shapes the worst case. Tests by UL show that thermal runaway depends strongly on charge level. Thermal runaway is a chain reaction in which heat triggers more heat until a cell fails. The Michigan review suggests stopping at 80 per cent where practical. Lithium iron phosphate cells are affected less, and a full charge before a long trip can be worth it.

WHY CAN'T A SCOOTER CARRY A BIGGER COOLING SYSTEM?

Because there is no room, and no weight to spare.

Cars, trucks and electric buses can link battery cooling to the vehicle's air conditioning, Professor Pattamatta says, and liquid cooling is more reliable there. "Air cooling, unfortunately, is not very efficient at high ambient temperatures," he says.

The real test, he says, is a battery that holds its capacity through thousands of hours of use with no risk of fire. "There is no one quick-fix solution for all conditions."

For two-wheelers, he points to extra help such as Peltier cooling, a small chip that pumps heat from one face to the other when current passes through it.

HOW DOES IIT MADRAS FIND A BATTERY'S HOT SPOTS?

A camera that sees heat shows exactly where a battery suffers.

Professor Pattamatta's lab uses a high-speed infrared camera that records a surface's temperature about 1,000 times a second. The camera gives a full map, not a few point readings. "We get a complete temperature field," the professor told India Today Digital in an exclusive interview at the IIT Madras campus. "We know where the hotspots are." The cooling can then be aimed there, and nowhere else.

His group, the Multiscale Thermal Transport Research Laboratory, can also place a battery in a climate chamber and recreate a Rajasthan desert or a humid Chennai afternoon.

In the same study, a T-shaped cold plate, a metal plate with liquid channels pressed against the hottest zone, was tested on a computer model of the larger cell. At 40 degrees outside and a 2C discharge, the authors report it cut the maximum temperature by 41.8 per cent and the gap between the hottest and coolest points by 31.8 per cent compared with natural cooling.

WHAT IS IMMERSION COOLING, AND CAN IT STOP BATTERY FIRES

Submerge the cell in a fluid that boils when the battery runs hot.

Water is out, because it conducts electricity. Dielectric fluids, which do not conduct, can touch a battery safely, but they carry heat away poorly. It is a double problem, Professor Pattamatta says.

His answer is to let the fluid boil. A fluid chosen to boil at about 35 degrees soaks up large amounts of heat as it turns to vapour, the way sweat cools skin as it evaporates. His lab submerges a whole pouch cell in such a fluid, chosen for low global warming potential, and tests it at different loads and outside temperatures.

He says immersion cooling suits very large batteries. Whether it can protect a scooter battery is not yet settled.

WHAT SHOULD YOU DO UNTIL THEN?

Give the battery a pause, and give it a rest from full.

Park in the shade. Let a warm vehicle settle before the charger goes in. For daily rides, charge to about 80 per cent if your range allows, and keep full charges for the days you need them.

The cell that crossed 45 degrees at IIT Madras was not abused. It was only used. Between the end of a ride and the start of a charge sits a gap that you control. It needs no fluid and no camera, and it costs nothing. A little patience is the cheapest cooling system there is.

- Ends