Adding a Battery Current Meter to the Prius

This topic is a description of how I added a battery current meter to my Prius.  Thanks go to Wayne Brown and others for help in figuring this out. My purpose in doing this was to learn more about how the car operates and adjust my style of driving to make best use of it.  For example, I found out how to brake (when safety permits) so as to maximize the amount of energy recaptured as battery charge by regenerative braking (see also Findings).

The Honda hybrid cars (Insight and Civic) have displays of battery current in the instrument cluster, so my desire to know this is not without precedent.  Their displays are "moving bar" type, so you can read them at a glance but they are not very precise and don't tell you the current in amps.  My display is digital, like the speedometer, and is as precise as I could make it.  Both types of display have advantages and disadvantages and, in the end, the availability of parts had a large effect on my choice.

The picture at right (click it for a bigger version) shows my battery current meter as seen by the driver.  I have mounted it so as to be visible when glancing in the direction of the speedometer and/or MultiFunction Display.  It would have been easy to mount it behind the steering wheel, but I didn't want to create yet another place for the driver to look.  It has no illumination and so can't be seen at night.  I'm happy with this and don't plan to use it at night since I want to pay maximum attention to the road.

I have divided my description of the battery current meter into three parts - the Display, Accessing the Sensor and Wiring Up.  Each is relatively independent and I include variations that you might like to make in each part.

The Display

The first order of business is to arrange to display the battery current to the driver.  Since the sensor (described below) outputs a voltage signal, I started out using a digital multimeter as a display.  From there, it was an obvious step to a digital panel meter.  These are widely available with both LCD and LED displays.  I chose an LCD display because it uses very little power and therefore requires only a small battery.  An LED display would be visible at night, but then you have the problem of supplying enough power to run it without changing the batteries too often.  I decided to build the display as a self contained unit, including the battery, and so to run only two wires to it, carrying the signal from the sensor.  The kit of parts for the display follows:




Part Number



LCD Panel Meter
200 mV sensitivity



You-Do-It Electronics


Cat# PM-200

All Electronics Corp.


Item # 1764296754

evaluecomp on eBay


Type N Battery Holder


Philmore No. BH511

You-Do-It Electronics


Radio Shack Cat# 27-401

Radio Shack


12V "keyless entry device" Battery


Duracell MN21

Home Depot and others


Energizer A23



Miniature Slide Switch


Philmore No. 30-9184

You-Do-It Electronics

You may need to use your ingenuity if you can't find this stuff at local electronics hobby stores such as Radio Shack as I haven't found on-line sources for everything.  You will also need some epoxy adhesive, hookup wire and some wire to run to the sensor.  How the display is fixed to the car, I leave up to you.  I made an aluminum bracket that slips in between the top and the face of the dash.  This allows the display to be removed without leaving a mark on the car.  An easier alternative would be Velcro™ tape, but for reasons I can't explain I didn't want to stick anything on the dash.

How you need to assemble the parts should be fairly obvious.  At the right are front and back pictures of my finished display unit (click for bigger versions).  The strange angle of the bracket matches the slope of the dash where I want to mount the display (see picture, above).  The battery holder is also at this angle and rests on the dash, supporting the assembly from the rear.  The bracket and battery holder are fixed to the bezel of the panel meter with epoxy adhesive.

Before fixing the battery holder in place, you may have to modify it to be sure it contacts the negative end of the battery.  Unlike the N cells for which the holder is designed, the 12V battery is recessed at the negative end.  If the spring doesn't poke into the recess and touch the metal contact, use needle-nosed pliers to tighten the spiral of the spring for the last turn.  (Click to read about how I came to choose this battery.)

The last component is the switch.  I was able to find at You-Do-It Electronics a miniature slide switch that was so tiny that it fits on the edge of the bezel of the panel meter.  Its dimensions are 0.23" by 0.75", with 5/64" mounting screws on 19/32" centers.  I drilled a couple of holes and filed them out with a square file to make the slot for the slider.  Instead of fixing the switch with the screws, I bent the tabs that the screws go into so that they stuck forward a bit.  I put the switch in place, making sure the slider was correctly positioned in the slot, and then heated the tabs with a soldering iron so that they sunk into the plastic bezel of the meter from the back.  This would probably hold the switch, but I covered the tabs with blobs of epoxy to make sure.

The battery is wired to the panel meter though the switch in the obvious way (see also schematic, below) using Radio Shack 22 AWG solid core hookup wire, but just about any fine wire will do.  It doesn't matter which side of the battery is switched.  However, it has been discovered that if you connect the battery the wrong way round, the panel meter is damaged and no longer works.  So, don't do this.  Look carefully at the polarity of the meter and the battery and make sure you get it right.  The meter supply polarity should be marked on the printed circuit board, although you may need a magnifying glass to see it clearly.  The positive terminal of the battery protrudes slightly; the negative terminal is recessed.

The wire from the sensor to the display hangs over the dash, so I was looking for something fairly unobtrusive.  I found a two-conductor telephone cord that was quite thin and was a similar color to the car's trim.  I cut off the telephone plug from one end and used that.  Any kind of two core wire will do.  The picture at right (click it for a bigger version) shows the current meter from the outside of the car.  The exposed back of the panel meter, battery, etc. and wiring give it a stylish, high-tech look.  This is exactly what I intended.  No way was I too lazy to make an enclosure for the meter.  Who said that?

Accessing the Sensor

The Prius has a current sensor built into the System Main Relay (SMR).  This allows the car itself to know what's going on with battery current, but we can also tap into it to tell the driver.  The sensor output is passed as a voltage to the Battery ECU, which encodes it as digital data and passes it to the Hybrid Vehicle ECU a the serial data link.  It would be nice to intercept the data on the serial link, but this would involve decoding the protocol in use and would need some fancy electronics.  Intercepting the sensor output signal is easier and a simple digital panel meter can be used for the display as described above.

The battery current sensor signal passes from the SMR to the Battery ECU over a short solid yellow wire inside the trunk.  The picture at right (click for a bigger version) shows the paraphernalia at the left side of the high voltage battery.  To see it, you need to detach and lift up the felt covering in this area, which you can see being held aside in the top left of the picture.  In the center is the orange service plug.  To the right are the two connectors to the Battery ECU, which is inside the battery housing.  To the left and somewhat further forward in the car is the single connector to the SMR.

At the Battery ECU end, our yellow wire appears on the upper connector and is the topmost wire in the right hand column.  The two positions above it are blank.  It can be seen quite clearly in the larger picture.  However, be careful, because below it and to the left are yellow wires with red and black stripes, respectively, that are the positive and negative supply voltage wires to the current sensor in the SMR from the Battery ECU.  Make sure your yellow wire has no stripe and enters the upper connector in the right hand column (seen from the rear of the car) and is in the third position down with two blank positions above it.

At the SMR end, our yellow wire appears at bottom left.  The connector has only seven occupied positions, two in the upper row (one on each side) and five in the bottom row.  The battery current signal is in the bottom row at the very left.  Again, the supply voltage wires are right next to it and must not be confused with the signal.  Above it is the negative wire with its black stripe and next along to the right is the positive wire with its red stripe.  Make sure you get the yellow wire with no stripe.  If you get a supply wire instead, you'll have plus or minus 12 volts, which will read as over-range on the meter and may damage it.

How you make a connection to this wire is up to you.  If you don't mind a miniscule modification to the car, I think the best way is to use an insulation displacement splice.  This clips over the existing wire and your new wire and punches through the insulation.  You don't have to cut the existing wire.  Or, you can splice in some other way.  I happen to be unreasonably paranoid about modifying the car.  I took a heavy duty paper staple, straightened it out and soldered it onto the end of my sensor wire.  I put two layers of heat-shrink sleeving over the soldered joint to reinforce it and act as a sort of handle.  Then, I poked the staple into the back of the SMR connector so that it slid in between the plastic housing and the metal contacts of the appropriate pin.  If you pick the right staple or piece of stiff wire, this forms a decent connection but can be removed without a trace.

The reference for the current sensor signal wire is the chassis of the car.  I find this surprising, but it's true.  I wasted a lot of time trying to find a better reference at the ground wire of the SMR, only to discover that the relay currents flow back to chassis through this wire and cause it to drop about 75 mV.  I think using a shielded wire is a good idea, but you could decide on any kind of wire with two conductors to carry the signal and reference from the trunk into the cab of the car.  The picture at right (click it for a bigger version) shows my wire from the SMR connector.  You can see the shield in the forground running to a mounting screw of the service plug.  At the back, a blue wire runs to the staple poked into the connector at the pin where the solid yellow wire enters.  You can see the solid yellow wire quite clearly in the larger picture and you can also see the yellow wire with the black stripe above it that carries the negative side of the high voltage and must be avoided at all costs.  The yellow wire with the red stripe cannot be seen but you must find it and also avoid connecting to it.  The white wire in the middle is my attempt to find a better reference, which I mentioned above.  I no longer use this wire.  Maybe I'll find a use for it some day.

Running the sensor wire from the trunk into the cab of the car isn't very hard if you're prepared to take out the rear seat.  I had the seat out to get at the battery cables to calibrate the sensor, so I can't really help if you're not up for this.  It isn't difficult to take out the seat - removing three bolts and a bit of heaving will do it.  A bunch of wires runs from the trunk into the cab just above the SMR where we've been poking about.  You can easily thread your sensor wire through alongside them.  Once the wire is in the cab, you can run it along the left side of the rear seat, under the cushion, down to the floor and push it up under the plastic trim over the sill of the rear door.  This will get you under the driver's seat without exposing the wire to damage or tripping.  Here, you can wire up to the display as described in the next section.

Wiring Up

Now you have a wire from the sensor and a wire to the display, it remains only to connect them up.  This is where I put the small amount of circuitry to get the display to read in amps.  That is, there are a few components in the connection between these wires which I hide under the driver's seat.  My main reason for doing things this way is so that I can replace this little bit of electronics with something else whenever I want to.  For example, I have a LabJack data acquisition system with which I can record samples of the battery current in my computer, so I'll need to connect the sensor to that.  Also, I plan to build a slightly more complex electronic circuit to present changes to battery charge, rather than current, on the display.  If you plan only to display battery current, you might want to move the circuit I describe here to the panel meter or the trunk and have just one wire running straight through.  Here is a schematic of the whole battery current meter:

On the right is the display wired to the battery via a switch in the obvious way.  On the left is the connection to the battery current sensor in the SMR and the car chassis.  In the middle are some resistors that arrange for each 0.027 volts that the sensor outputs to register as one count (1 mV) on the panel meter.  This makes the panel meter read in amps, with a resolution of 0.1 amps.  (Click to read about how I figured out the value 0.027.)  The capacitor, which I won't mention again, is intended to prevent the pick up of any electrical noise on the wire to the display.  Before I added it, I occasionally got some odd readings.  By using a potentiometer in this circuit, you can adjust the display to read correctly so you don't have to worry about the tolerance (error) of the resistor values.  Just connect a known voltage of two or three volts instead of the sensor input and adjust the potentiometer until the display reads this voltage divided by 0.027.  For example, if you apply 3.000 volts, adjust for a reading of 111.1.  Actually, I needed this adjustability because at first I had only a rough idea what the division factor would turn out to be.  You can dispense with this component if you use a 3.9 M resistor in place of the 3 M and a 150 k in place of the 100 k (because 150 / (3900 + 150) = 1 / 27).  In this case, wire all three points where the 50 k potentiometer used to be together.  Either use resistors with small tolerance, such as 1% or 2%, or perform a calibration as above using the potentiometer on the back of the panel meter.  Of course, if you're not to worried about accuracy, you can use 5% resistors and take what you get, but in this case you should be cautious when exchanging data with others.

When this is wired up, either as I've shown and the calibration performed or with fixed resistors, you're ready to turn on the display and try it out.  If you drive off soon after turning the car on, you'll get some serious current flowing out of the battery because the car runs on battery power until the engine warms up a bit.  On my display, this registers as a negative value.  When you brake, you get current into the battery and I register this as postive (no minus sign).  This makes sense to me, but if you want it the other way round you only have to reverse the wires at some point.


These are some things I've found out using my battery current meter.

Maximum Current During Braking

During heavy braking from speed, I have seen peak battery charge currents of a little over 60 amps.  I think that after the peak, the current settles out at around 55 amps (I'm a little vague here because I glance rather than stare at the display).  So, to judge the amount of brake pressure that does not engage the friction brakes (safety permitting), I keep the charge current at 50 amps or below.  Once I got the feel of this, I found that I could avoid the friction brakes entirely (unless something unexpected happens) without falling out of the general flow of traffic.  I think I'm getting better fuel economy (55 m.p.g. on the display today!), but I could be kidding myself.

Maximum Battery-Only (a.k.a. "Stealth") Mode Current

The maximum current you can draw from the battery while the ICE is off depends on the battery state of charge.  So, to use the meter to stay in battery-only driving mode, you need to develop an awareness of this that is somewhat more accurate than the Energy Monitor icon.  I can usually pull 25 amps when the icon is at 3/4, only 15 or 20 when it drops to 1/2.  After a long run with the ICE on, the battery reaches its nominal charge level and I can pull 30 and sometimes even a few more amps.  I know when the battery reaches nominal charge because the current meter begins showing very small values, many readings falling below an amp.  Anyway, over the days it gets easier to be aware of the probable battery charge level at any point in my commute and I can hold the car in battery-only mode much better than without the meter.  Consequently, traffic permitting, I can chose when to stay in battery-only and when to run the ICE based on where I am and what's coming up.  When I want to hold battery-only, I'm effectively driving by the current display and, within reason, letting the speed of the car drift.  Pulse driving over hilly local roads is much easier and boosts my fuel economy.

Last edited October 10, 2002.  All material Copyright ©  2002 Graham Davies.  No liability accepted.