Attaching a Battery Current Probe

This topic is a description of how I attached a clamp-on current probe to the battery cables of my Prius.  Thanks go to Wayne Brown and Ernst Schmidbauer for help in figuring this out. My purpose in doing this was to calibrate a more permanent battery current meter using the sensor built into the system main relay. You could also directly make battery current observations using the probe, although you'd have to leave the car in a partially disassembled state.

The picture at right (click it for a bigger version) shows the attachment of the current probe to the battery cables as they pass under the rear seat on the left side of the car.  You are looking through the left side rear door to where the seat used to be before I took it out.  The first thing, then, is to take out the rear seat bottom.

Removing the Rear Seat Bottom

To remove the rear seat bottom, you just yank it up. There are no screws or bolts, just two clips that release with sufficient force. It is best to pull just above each clip in turn.  The clips would be under the outside thighs of two people sitting normally in the back of the car.  That is, the clip on the left side would be under the left thigh of the person on the left and vice versa for the right.  In the photograph, you can see the sockets the clips fit into at the front edge of the seat support.  The two smaller, round things closer to the middle of the car just hold the carpet down.

Removing the Rear Seat Back

There are three bolts that need to be removed to get the back part of the rear seat out of the car.  If you have the seat bottom out, these are easy to see.  They pass through loops of the heavy steel wire that forms the armature of the seat back and helps the cast foam to hold its shape.  There is one on each side, close to the doors, below the seat back and normally hidden by the seat bottom.  There is also one in the middle.  If the seat bottom is in the car and you don't want to remove it, you can get to the outside two bolts by pressing down and forward on the seat bottom cushion.  You can also get to the middle one this way, but I've found it very hard.  Once you have all three bolts out, the seat back lifts up and hooks on the back, also made from loops of the wire, come out of slots in the car frame.  Thread the seat back sideways out of the seatbelts.

Checking the Probe's Calibration

It seemed a good idea to me to check that my clamp-on current probe was reading accurately.  I didn't have to hand a source of many tens of amps of current, so I devised a way of calibrating it using five amps from a laboratory power supply in constant-current mode.  I simply wound ten turns of wire into a loop and secured it with tape.  Passing five amps throught the wire means that ten times that much, 50 amps, pass through the taped bundle of wires.  I put the probe around this and measaured its output with a digital multimeter.  You can see this set-up at right (click the small picture for a larger one).

When the bundle of wires was centered in the clamp, the output was 498 mV on the 100 amp range, which has a sensitivity if 10 mV per amp.  As I moved the bundle to other areas within the clamp, I saw a small variation in the reading.  In some spots the difference was a little more than 1% from ther centered reading.  The "best" spots for the wire bundle, where the output was exactly 500 mV, were where you see me holding it in the picture and just inside the opening of the jaws.

Now, at the time I was first experimenting with battery current measurement, the weather was hot.  I reasoned that the probe might not be as accurate at the temperature inside the car as in a nice air-conditioned laboratory.  So, I decided to make a quick check on how readings changed with temperature.  Of course, on the day I decided to do this check the weather was not hot at all but undeterred I put the probe in the refridgerator instead.  In the picture at right, you can see it on top of all the cans of soda (click the picture for a larger one).  I found that after a 20 minute soak in the refridgerator, the probe output had increased by about 6%.  Therefore, when cold, the probe over-estimates current.  I concluded that when used in a hot car, I would be under-estimating current.

Last edited September 24, 2002.  All material Copyright   2002 Graham Davies.  No liability accepted.