Electric A/C in a 1969 Lotus Elan +2: A Costly Experiment in Voltage Sag

At the end of last summer, I began an epic installation of electrically driven air conditioning in my fragile little 1969 Lotus Elan +2. I wrote in great depth about how using an electric compressor isn’t the slam-dunk it’s made out to be by those who sell them, why I decided to do it anyway due to space constraints and the unavailability of a compressor bracket and two-groove pulley for my Lotus-Ford engine, the installation of a high-output alternator needed to drive the electric compressor, the wiring modifications performed so all this current coursing through the car wouldn’t light it on fire, and how I bought an 80-amp power supply to allow me to test the electric A/C system in the garage without running the engine and fuming up the house.

It was already October by the time I wrote this piece saying that the system was functioning but not performing as well as I’d hoped, and it was approaching Halloween when I determined that the system needed a Gilmer Drive toothed pulley set to keep the belt from slipping, added up the costs, and basically said that the entire endeavor was perilously close to a fool’s errand and I really should’ve just paid a machine shop to fabricate a compressor bracket and a two-groove pulley and used a conventional belt-driven compressor. I was disappointed and frustrated with the whole thing. Both the car and I needed some space apart from each other, so I didn’t really touch it over the winter.

The electric compressor, the inverter feeding it, and the rest of the components were installed in the nose of the Elan +2 last fall.Rob Siegel

About a month ago, we had an unseasonably warm day, so I made my first stab at reassessing the project. I reconnected the Samlex power supply to the Lotus’ battery using the pair of cut-off jumper cables I’d configured for the task, stuck a thermometer into the vents, and turned on the system. The temperature began to drop, but just as it did last fall, the setup seemed to run out of steam, though I guess that “steam” is exactly the wrong metaphor to use for an A/C system.

The Samlex 1280UL power supply that can deliver the 80-ish amps drawn by the compressor and fans.Rob Siegel

I put a voltmeter on the relay that goes to the inverter that feeds the compressor, and saw that it was receiving voltage, but the level was dropping. When it fell a bit below 12 volts, the compressor was reading this as an under-voltage condition and was shutting down. I went to the back of the car and verified that the power supply itself was indeed pumping 14 volts into the battery. So the culprit was “voltage sag,” something many of us have experienced while trying to jump-start a car in the cold with jumper cables that aren’t getting a good bite onto the battery connectors.