Batteries, batteries, batteries

The project progresses apace…or at least it was progressing. Now hitting the limits of what I can do without some batteries. But here is where we’re at:

Engine Bay Tetris

With the engine mounted, we could start working out where everything else goes. Through a process of trial and error, and lots of tack welding, we got the inverter and the high voltage junction box mounted in the engine bay, using a combination of 40×40, 40×20, and 20×20 steel profiles. The 40×40 might seem a bit excessives, especially since it is the thick walled stuff we made the adaptor plate from. But this will be the mounting point for the motor mounts that drop down to the rubber bobbins below. These are all now cut and waiting to be welded…

Welding Woes

Our skip-dived welder hasn’t been performing brilliantly for the last week and needed a little TLC. A new earth clamp, some more gas (this time from Hobbyweld), along with the relevant adaptors and some new shrouds have all either been ordered and collected, or are due to arrive in the next day or so. This should allow us to do some much better quality welds when we take the motor back out and finalise it all before painting.

Cooling systems

In what has been an expensive week for little bits, I bought a load of elbows for the 10mm and 19mm pipe that will make up the two cooling loops for the inverter and the motor. I also ordered a very cheap but slightly battered motorbike radiator with 19mm fittings to go into the inverter loop. This has a filling cap so should work fine as a small reservoir. For the oil loop I will use the old power steering reservoir.

Plan is to mount each radiator and pump roughly in front of the things they are cooling. Some little plates and strips welded onto the rest of the mounting cage should hold them nicely.


When I sold the combustion engine, I hung on to the loom, thinking I could re-use lots of it. The plan was to keep the original plugs on it and salvage the sockets from an old ECU. In reality this is looking like less of a good idea. It just leaves me with awkward shaped connectors and a load of wire that doesn’t quite go where I want. So I’ll keep some of the old loom and butcher some of it for the relevant wiring and hook it all up with new connectors.

I still haven’t totally established what the intelligence at the heart of this loom is going to look like. Right now I’m playing with a Teensy 3.2 (for the battery management system), an ESP8266 (for an interface back to my home automation system and perhaps some WiFi-based diagnostics), and a Mega2560 Pro (for managing things like cooling pumps and ancillaries – all of which could be done in analogue fashion and may yet be).


From the start of this project I have had my heart set on the BMW hybrid batteries. Watching the research into these on the OpenInverter forum they looked niccely designed and ideally specified for what I wanted. I did hope the whole pack from the BMW 330e would just drop in to the engine bay for a while but my tape measure soon told me that wouldn’t (quite) work. It might have done if I had gone for the Lexus motor option.

I’ve been tracking lots of different units for sale on eBay but they seem to have gone up since I started looking. Whereas before there were a number for sale for around £8-900, now they’re all up at £1350-1800.

Then, this week Jamie, a fellow forum member who has helped me lots on this project, spotted a pack going for just £500 on Facebook Marketplace. I contacted the seller and thought I’d agreed to purchase it, only to be gazumped when someone else could collect before me.

So, the search continues…

inverter and high voltage junction box in BMW z3>
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