Sat Jan 13, 2018 10:07 am
mmmike: Like I said; in the 2011-12 Leafs, the charger is essentially already in the car trunk, under a plastic cover. Undo a couple M6 bolts and you have access to the HV bus (charger side), CAN and a bunch of other signals. Seems like a very convenient place to work compared to having to put the vehicle on a bridge and working underneath. All this said: I'm working off of videos and trying to work my way through the service manual, I have not touched any of the covers in the car or worked on it. I like to be prepared.
EVDRIVER: Totally agree. However, I don't think it is fundamentally possible to make the design 100% safe. In fact, the Leaf's battery pack is demonstrably unsafe in some cases; there have been at least two documented total vehicle burnouts, including the passenger compartment, due to faulty charging and consequently pouch cells bloating and catching fire, taking the rest of the car with it. The steel container did very little to stop this from happening. This is also not the purpose of the container.
What I believe the designers to have done, and this is of course speculation, is that the container is designed to detach and move independently from the car in the case of a collision, reducing the weight that the local chassis needs to decelerate in case of front and rear collisions in particular. That way they could design the rest of the car like any other car and did not have to take the battery pack's crash characteristics into account in the rest of the spaceframe. The battery compartment could then individually be designed so as not to cause immediate danger when it bumps into something. The shape of the container as well as the bolt position and orientation seem to indicate this kind of a design strategy.
Design for safety is not something you can do in isolation, and certainly not without proper simulations or real-world crash tests. In order to actually design a foolproof and well-designed pack, you would have to be somebody at Nissan with intimate knowledge of the car's frame and mechanics. This is obviously not a possibility. All we can do is try our best, with the best crowdsourced and engineering background information possible, to make something that could be construed as safe and reliable.
So when designing a container for an extender pack for safety, you have a fairly large design space to work with. It will have to cope with:
- Charging, discharging safety. Undervoltage and overvoltage checks, balancing, temperature checks, cooling. This is the BMS
- Service safety; presenting a safe state of the vehicle when the vehicle is off and not charging; these are the contactors and interface connector
- User-facing safety: when using the vehicle normally, the battery pack should not raise additional concerns in the use of the passenger compartment and trunk, nor should the user have to take special precautions when loading the vehicle. No loose packs on the rear seats, no walls with explosives behind them that get triggered when you callously throw a wood axe in the trunk.
- Crash safety: high physical load tolerance, crush safety and should not present a fire hazard to first responders
From what I understand, there is a decent amount of void space under the trunk; this would be a prime location for the extender pack. Out of the way for users and very easy to package. This takes care of most user-facing safety. Disconnecting the pack and installing a first responder's loop should take care of service safety. It being in the rear of the car, away from the passenger cage (would it be?), in the rear crumple zone (very rare to face impact) also makes it relatively impact safe. There is no way to avoid a massive fire when the pack is impacted, but the immediate danger can be avoided with a fire break. Like the firewall in an ICE car, this only has to be one or two inches of (supported) stone wool to be effective enough. The only thing I would package very carefully are the contactors; I'd put those in semi crush-proof containers, just so they fail open.
Now, don't take this as disparagement to anyone, not in the least because I'm just as much of an idiot with safety sometimes, but you are totally right that most conversions are very slap-dash. Leaf XPack is probably the best example of somebody seemingly not even being aware of the safety issues and doing some exceedingly dangerous things (e.g. the parallel/serial relays on a dodgy switch). I don't condone that kind of work, but on the other hand he does provide very valuable and pioneering information to this community.
