Battery temperatures and #Rapidgate

Rapid charging any battery heats it up. So can driving the car in anything other than a very gentle style. Some cars can remove that heat with a cooling system; others rely on passive cooling. Whether that heat causes any issues for the user depends on the starting temperature, charging power, software limits and a host of other factors.

The new Leaf 40 is being heavily advertised in the UK right now. It looks great, lots of toys and the quirky Leaf Mk1 looks are history. However the EV-geeks on social media are less than impressed by #Rapidgate. Rapid charging any battery heats it up; the battery has some internal resistance so behaves as a heater. This should be no surprise, 100+ Amps tends to do that.

The Leaf 40 was never going to be the best long-range high-speed car. The aerodynamics aren’t wonderful since this is a facelift rather than a new car, and 50kW charging is last-generation. 50kW charging of a 40kWh battery was always going to take… a while. Indeed the Nissan UK website now acknowledges this, and the fact that charging rate will depend on “charging conditions”. Rapid? Or Quick? You decide!

I’ve been logging my Leaf 24 on and off using LeafSpy, which can record various aspects of the car including the battery temperature. Here’s an example of a rapid charge logged by LeafSpy, in February with an outside temperature of 10°C.

The blue line is the current draw, I’ve reversed the axis (the left one) so that charging (or regen) is “up” and motor draw is “down”.

The green line is battery temperature.

We can see that during the rapid charge, the temperature rises from 13 to 19 degrees. This charge added about 10kWh, or half the battery capacity.

Also notable is that the temperature doesn’t drop during the subsequent cruise at around 60mph, despite the battery being 9°C warmer than the surrounding air.

How does this relate to the Leaf 40? All Leafs to date (2018) have the same battery box, and with good reason – Nissan can now offer a battery exchange between the 24, 30 and 40kWh variants (or should I say 21, 27 and 36 🙂 ). Well, charging a “40” might take 3.5 times as much energy as the charge shown above, giving a temperature rise of 6*3.5=21°C.

Leaf 40 drivers such as LemonTreeLeaf on YouTube report a 24°C temperature rise on a charge. This is similar to my prediction of 21°C.

The battery temperature will depend on the starting temperature, and the way the car is driven. Here you can see the difference between “taking it easy” and “driving a little more enthusiastically”: the battery can be deliberately heated.

The blue line to the left is my commute in the morning. The battery temperature rises by a couple of degrees. It then cools on standing (step in the data). On the home journey I deliberately made use of accelerating and regen in B-mode to warm the battery, resulting in an 8 degree C temperature rise. I found that this increased range, even the next day. Relating this to the Leaf 40; the temperature rise I saw was 8C over 25 miles, using around 6kWh; this is roughly 1.3 degrees rise per kWh used.

So driving in a similar manner using, say, 75% of the 38kWh available in the new car, I’d expect a temperature increase of 37°C. If the battery started at 20°C it could then be at 57°C – certainly not in a fit state for a rapid charge!

So what does this mean for the ’40? The 40 battery has around double the active material compared to the Leaf24. This leads to more heating in the same size box. Users have reported a 49°C limit. At that point the Leaf charges at 22kW, not 50. That means 2-hour charges, which can hardly be called rapid. Nissan are looking after the battery for you. Ultimately this will affect:

-very long distance trips, beyond 2 legs, even in winter;

-Probably limited rapid charging rates even on the first rapid charge in high summer (I have seen a 30°C battery on my Leaf24 in the morning, you do the maths…)

It will not affect normal commutes up to the range of the car; most of my Leaf use has been slow charging overnight and 50 miles a day, something all Leafs are is perfect for. If you need a car to do long-distance (let’s say over 300 miles) and more than one rapid charge in a day, in summer, I suggest you buy one with some battery cooling- or face 2 hour charge times. And take it easy between those charges, because heating due to hard driving can be just as potent as rapid charging!