MLucas said:
Joe - I would say 1.) Because we can, this car begs for modifications. 2.) More seriously, the factory 12v battery is a bit of a pig taking up a lot of space and needs maintenance. If you haven't checked your battery lately - probably be a good idea to do so soon. 3.) With the low requirements our 12v system needs something smaller, lighter and more compact would have several benefits.
I agree that reducing the existing i-MiEV 12v battery volume would free up a bit of room. I consider the space under the front hood as a not-very-elegant packaging job, anyway - and, what's worse, everything under that hood gets wet when I wash the car.
The photo below shows my Headway LiPO4 12Ah four-pack that I use for hobby tinkering. It has three connectors: JST-XH used for the CellLog8S for cell monitoring and low voltage alarm, the JST-PA to interface with the PowerLab8 cell-level charger/balancer, and the Anderson PowerPole for the I/O through a 30A fuse.
My i-MiEV puts out 14.26v at around 75degF, measured at the accessories outlet. This is pretty darn close to the Headways' 100%SoC of 4x3.65 = 14.60v and, although safe, I wouldn't like keeping it at this voltage all the time as a decrease in one cell would drive the others into overvoltage if not controlled by a BMS.
Inasmuch as I don't have a distributed BMS (which I would put in if I were to install this Headway pack into the i-MiEV for any length of time) I presently top-balance this pack using the PowerLab8, and then monitor it using the CellLog 8M or 8S which has a cell-level low-voltage alarm. The problem with keeping the CellLog permanently attached is that it draws a minute amount of current and over time it could unbalance the pack.
If this pack were to get partially depleted in the i-MiEV, it could easily draw the full dc-dc converter output current (60A?) as its internal impedance is significantly less than the OEM FLA battery so I'd have to use a larger fuse. Dunno if I'd want that.
My preference for the i-MiEV battery replacement is a nice small (~15Ah?) AGM VRLA sealed unit. Since my i-MiEV's 12v battery is still going strong and shows no corrosion, this project is on the back burner... I hope it dies so I can stick in the AGM as I hate flooded lead-acid. I kept an EV conversion with 24 golf-car batteries in my garage over a winter a few years ago, and the outgassing fumes put a layer of rust on all my tools and equipment.
Back on-topic, on my presently-unused ICE vehicles outside (under covers), I use small 15W solar panels feeding the 12v batteries through regulators as that's less work than pulling out their batteries. A couple of those batteries are 20 years old and still provide sufficient starting juice!