Squirrel, and the case of the hot batteries!

In the post about our South Island road trip, I described the issue we had with the battery overheating on the last day. In summary, I charged 5 times during that day, 3 times to 95% from a quite low charge, and once to 80%. It was a very hot day, and the last leg of the journey went over a high-ish pass and the drop down the other side was mostly in regeneration braking mode. So I was a bit puzzled as to exactly what caused it, probably just the recharging, but the regen and big hills were a possibility.

I wanted to test this as soon as we got back, but before I could we were locked down in Level 4, and poor old Squirrel started up a home for the local spider population.

A few days ago, we were let out of lockdown, and I thought “Road Trip!”. So to kill two birds with one stone, we decided to go over into the Wairarapa, for a round trip of 180kms or so, with a 500m hill in the way, and to monitor battery temperatures at key points along the way.

Here is a PDF of the relevant data. tempsroadtrip

And here is a text version of just the data:-

Location kms SOC Elev Cell Temp
Home, Whitby 0 78 49 12
Queens St Carpark Charger, Upper Hutt 24 53 47 16
24 80 47 21
Top of Remutaka Hill 45 45 530 25
Featherston Charger 55 46 26 25
55 80 26 28
QE II Park, Masterton Charger 92 41.5 87 27
92 65 87 29
Featherston Charger 128 30 26 29
128 80 26 34
Top of Remutaka Hill 139 56 525 35
Home, Whitby 182 28 49 34

and a graph of Elevation, State of Charge and Cell temperature as the journey progressed.

The green line is the elevation, in metres as shown on the right hand “axis” of the graph, and the blue line the SOC in %, against the left hand axis, and the red line the temp in degrees C, again using the scale on the left hand axis. Distance is shown in kilometres along the bottom axis, starting at zero at home. Click on the graph for a clearer image.

 

So what does this show? Firstly, temperature did not increase during periods of regeneration braking (the downhill bits!). Not at all, in fact the temperature decreased slightly. Even on the steepest, longest downhill, only 1% of state of charge was recovered, nothing at all really.

Secondly, pushing hard up hill did not increase the temperature by a huge amount, just a degree or so. On a long journey, with normal hills, this may not be important. The first temperature increase was starting from a cold morning, where the battery was chilled to 12 degrees C, and it increased to 15 which was the days temperature when not in the shade.

Thirdly, the major cause of temperature was recharging. Small recharges only increased it by small amounts, large recharges caused a big increase.

Fourthly, the temperature never really decreased significantly during the whole 6.5 hours we spent out, despite long breaks to have coffee, walks etc. There is no cooling whatsoever on these batteries, other than contact with the metal case of the battery and air passage over that. Features that certain models of the i-MIEV have, like forcing air conditioned air through the battery while charging, or even just air inlets and outlets to allow air to pass through the battery, are not present on the MIEV minicab.

On this trip, our charges were short and not to maximum charge, but with 4 charges the temperature still got to 35 degrees, and the day was not too warm either.

Conclusion, 4 or 5 good long charges in one (hot) day will take these batteries above their safe limit (somewhere around 45 degrees C ?) and charging will slow down after this point. This limits a days driving to about 300kms, depending on the terrain and how often charges to maximum are done. There is no way of cooling them off, other than leaving for a significant amount of time. 5 hours on the ferry reduced the battery temp from 45 to 35 degrees C. Leaving it overnight reduces them back to the ambient temperature.

A 3  to 4 hour slow charge at 13 amp may also allow the  battery to cool while it charges, I may try this on the next long trip. It will get at least one more 80km leg in, but will shorten the driving day.

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