brick
10-11-2009, 05:38 PM
You have probably heard the term “afterboil” in the context of engine overheating. The story usually involves an older car (with a marginally designed or age-compromised cooling system) driving just fine on a hot day. The owner parks with the temperature needle in its normal range only to come back a short while later and find it firmly in the red…temperatures dangerously high.
What you may or may not have considered is that the same thing can happen with your hybrid battery. What one has to understand is that all of that current rushing in and out of the cells generates heat internally, while the temperature sensors are on the outside of the battery. The designers have done some things to help, such as placing the sensors in little “wells” that project somewhat into the cell. But there is still some time delay between the center of the battery heating up and the computers knowing about it.
This doesn’t play much of a role at low current levels, such as those encountered on a long highway drive. The internal heating of the battery is slow enough that there is relatively little temperature difference between the interior of the battery and the location of the sensors. (Note that as long as there is heating there will always be some difference by definition. Heat transfer only occurs when there is a temperature difference, and there will always be some heat generated inside of a working hybrid battery!) As current levels increase, so does the rate at which heat is generated inside of your battery cells. By the same principles, this means that there will be a larger temperature difference between the center of the battery and the outside (where the sensors are). It could be some time after that event before that energy reaches the surface. This should be clear to anyone who has used a ScanGauge to track the slow rise and fall in reported battery temperature, even under severe use.
Now back to the whole “afterboiling” thing (if you will forgive the use of an imprecise but familiar term). It is important to note that reported rise in temperature doesn’t stop just because the hybrid system has been shut down. NiMH batteries are heavy beasts, and have a lot of thermal mass. Meaning, they can store a lot of heat! The interior of the battery may be quite hot when you shut down, and the temperature that you see on the ScanGauge may not really be the peak temperature for the day. I’ll give some anecdotal evidence to demonstrate how significant this effect may be.
Earlier today I took a drive out to the local state park for a little R&R. The road into the parking area involves about a 100ft climb and then a 200ft descent over a very short distance. I tried to mitigate the influx of current using “B-mode” but I still maxed-out the battery on the way down at 78% SoC. When I reached the bottom and parked the temperature read a very safe 29C. Curious, I checked on the temperature periodically over the course of about an hour and a half. By the end of that period the reported temperature had risen to 36C, or a 7C delta. That’s still a very safe temperature, but consider what would have happened (and has happened to me in the past) if that descent had been started with a reported temp closer to 40C. That is fairly normal for a hot summer day after a bit of driving around town. The ending temperature would be uncomfortably high..well above the 45C that is considered the highest “comfortable” temp for this battery chemistry. Also note that the maximum reported temperature is still lower than the max actual internal temperature. By the time the maximum reading is read, the temperature gradient has lessened with the distribution of heat to the outside.
I expect that the engineers who designed our systems took this into account when setting temperature thresholds at which current levels are limited to prevent overheating. But I think that it is still prudent to take our ScanGauge readings with a grain of salt and limit current throughput whenever possible. And yes, I do dread that hill. ;)
What you may or may not have considered is that the same thing can happen with your hybrid battery. What one has to understand is that all of that current rushing in and out of the cells generates heat internally, while the temperature sensors are on the outside of the battery. The designers have done some things to help, such as placing the sensors in little “wells” that project somewhat into the cell. But there is still some time delay between the center of the battery heating up and the computers knowing about it.
This doesn’t play much of a role at low current levels, such as those encountered on a long highway drive. The internal heating of the battery is slow enough that there is relatively little temperature difference between the interior of the battery and the location of the sensors. (Note that as long as there is heating there will always be some difference by definition. Heat transfer only occurs when there is a temperature difference, and there will always be some heat generated inside of a working hybrid battery!) As current levels increase, so does the rate at which heat is generated inside of your battery cells. By the same principles, this means that there will be a larger temperature difference between the center of the battery and the outside (where the sensors are). It could be some time after that event before that energy reaches the surface. This should be clear to anyone who has used a ScanGauge to track the slow rise and fall in reported battery temperature, even under severe use.
Now back to the whole “afterboiling” thing (if you will forgive the use of an imprecise but familiar term). It is important to note that reported rise in temperature doesn’t stop just because the hybrid system has been shut down. NiMH batteries are heavy beasts, and have a lot of thermal mass. Meaning, they can store a lot of heat! The interior of the battery may be quite hot when you shut down, and the temperature that you see on the ScanGauge may not really be the peak temperature for the day. I’ll give some anecdotal evidence to demonstrate how significant this effect may be.
Earlier today I took a drive out to the local state park for a little R&R. The road into the parking area involves about a 100ft climb and then a 200ft descent over a very short distance. I tried to mitigate the influx of current using “B-mode” but I still maxed-out the battery on the way down at 78% SoC. When I reached the bottom and parked the temperature read a very safe 29C. Curious, I checked on the temperature periodically over the course of about an hour and a half. By the end of that period the reported temperature had risen to 36C, or a 7C delta. That’s still a very safe temperature, but consider what would have happened (and has happened to me in the past) if that descent had been started with a reported temp closer to 40C. That is fairly normal for a hot summer day after a bit of driving around town. The ending temperature would be uncomfortably high..well above the 45C that is considered the highest “comfortable” temp for this battery chemistry. Also note that the maximum reported temperature is still lower than the max actual internal temperature. By the time the maximum reading is read, the temperature gradient has lessened with the distribution of heat to the outside.
I expect that the engineers who designed our systems took this into account when setting temperature thresholds at which current levels are limited to prevent overheating. But I think that it is still prudent to take our ScanGauge readings with a grain of salt and limit current throughput whenever possible. And yes, I do dread that hill. ;)