Snargleblarg
Well-known member
- Joined
- Jul 27, 2021
- Messages
- 46
Well, my math above was off by 10x anyway. 10mA at 400V is only a measly 4W. And 6 parallel shunts would only get you to 24W. Leftie says the pack warmer in the original Leaf uses ~300 watts (https://www.mynissanleaf.com/viewtopic.php?f=17&p=615851#p615851) so nowhere near that.
Where did the 10mA number come from? Is it known to be accurate? For which packs? I checked datasheets for a few BMS chips and they support up to 150mA or 300mA balance current per cell using internal balancing switches. But then there's gonna be a lot of power dissipated in the IC - combined current from multiple cells. So perhaps they used external discharge switches and heat-sinked them to the cells. This configuration is recommended in the datasheets for higher power balancing. It's possible. No reason to believe the 62kWh pack has the same cell balancing circuit as the smaller packs.
I think Nissan just increased the cells from 4 parallel to 6 parallel to increase the capacity 50% (which makes me wonder if the 40kWh pack is really 41.3kWh). The missing dedicated pack heater seems like a compromise made to find space for the extra cells. I'm not suggesting this cell-balancing method would keep up with the dedicated warmer. I'm just saying, what else do you expect a "software pack warmer" to do? Running the microprocessor full speed could generate some heat, but it would be highly localized. My guess is that the pack designers were high-fiving each other just to fit 50% more cells and the software pack warmer solution was just a band-aid to show some effort. A design compromise and perhaps they're mostly relying on the larger pack thermal mass to reduce the liklihood of a low temperature event that lasts long enough to freeze the cells.
I'm in the Dallas area and my Leaf is garaged at night. So, similar to you, my car won't naturally see these conditions. My reaction to that is ... I don't care what the pack warmer does. Why is this a curiosity for you? Are you planning a move to a colder climate? I'm always wary of the damage that could be done with dry ice. Skin contact, fast thermal movement or thermal variations across a rigid structure ... risks I want to avoid.
Where did the 10mA number come from? Is it known to be accurate? For which packs? I checked datasheets for a few BMS chips and they support up to 150mA or 300mA balance current per cell using internal balancing switches. But then there's gonna be a lot of power dissipated in the IC - combined current from multiple cells. So perhaps they used external discharge switches and heat-sinked them to the cells. This configuration is recommended in the datasheets for higher power balancing. It's possible. No reason to believe the 62kWh pack has the same cell balancing circuit as the smaller packs.
I think Nissan just increased the cells from 4 parallel to 6 parallel to increase the capacity 50% (which makes me wonder if the 40kWh pack is really 41.3kWh). The missing dedicated pack heater seems like a compromise made to find space for the extra cells. I'm not suggesting this cell-balancing method would keep up with the dedicated warmer. I'm just saying, what else do you expect a "software pack warmer" to do? Running the microprocessor full speed could generate some heat, but it would be highly localized. My guess is that the pack designers were high-fiving each other just to fit 50% more cells and the software pack warmer solution was just a band-aid to show some effort. A design compromise and perhaps they're mostly relying on the larger pack thermal mass to reduce the liklihood of a low temperature event that lasts long enough to freeze the cells.
I'm in the Dallas area and my Leaf is garaged at night. So, similar to you, my car won't naturally see these conditions. My reaction to that is ... I don't care what the pack warmer does. Why is this a curiosity for you? Are you planning a move to a colder climate? I'm always wary of the damage that could be done with dry ice. Skin contact, fast thermal movement or thermal variations across a rigid structure ... risks I want to avoid.