The organisation I work for have a pretty large fleet of vehicles based at 70 odd locations around a city. The vehicles are fitted with computing and other electronic gadgetry and so when vehicles are not on the road they are “supposed” to be parked at their base and connected to a vehicle charger, which is basically a 12v battery charger with a long cable and connector attached.
During an impromptu conversation around the coffee machine, it came to light that many of these charger units probably don’t work for one reason or another and there is no way of quickly testing them. There have also been problems with some of the vehicle charging ports wired incorrectly or simply having blown fuses, and I made a throw-away type comment that why don’t they have a simple gizmo that connects between the charger and the vehicle and can test the charger output voltage and the polarities. I love that glazed look that proceeds the “wow… can you do that”. Of course I can… I’m an engineer. Luckily, the fleet guy happened to have a charger drop cable handy with a connector fitted and from that I managed to find a suitable socket that would fit, and using what I had available in the home workshop designed and constructed a simple tester device prototype for them in an afternoon.
It had to be simple to use as the people using these aren’t mechanically minded. It also had to be relatively cheap and strong as devices like this don’t tend to last long in busy workshops.
The pictures below show the case marked out for drilling, the underside of the assembled and mounted PCB, and the finished prototype device.
If either the vehicle or charger polarity is incorrect, the associated LED lights red. If both polarities are correct the charger voltage is displayed on the LEDs on the left; 13v or higher is “good enough”.
The operator can then press the “Load Test” button which simply connects the charger to the vehicle allowing the operator to see the on-charge voltage, and as long as it stays in the green, everything is ok.
If either polarity is incorrect; as indicated by the polarity LEDs, or either of the polarity LEDs don’t light at all; indicating no voltage present, an interlock disables the Load Test function. This is to prevent shorting the charger out if there is fault somewhere on the vehicle.
If readers are interested I’ll post the schematic and PCB foil but there’s nothing clever in this device. An LM3914 drives the 10 voltage output LEDs and the polarity indication is simply two red/green bi-colour LEDs. The only bit of any real interest was the interlock that prevents the Load Test button from working if there is a fault condition, but this is simple enough and makes use of a transistor and relay.
The one thing I was keen on was to reduce the amount of interconnect wiring to a minimum. This was a fun little project and may have earned me some brownie-points with the management, but it occurred to me they may ask me to create another 70 devices; one for each location and that’s a lot of cases to drill and wiring to solder.
As often happens with prototypes, it was decided to change the voltage measurement scale to start from 11v and not 10v, which now means the Green band starts at 14v - thank heavens for trip-pots. You can see I drilled a little hole in the PCB so I can adjust it even when the PCB is mounted.