Development of an EV Monitoring System

by Ian Hooper, 22nd January 2009

 

Introduction

My latest R&D project has been development of (what I call) an Electric Vehicle Monitoring System, or EVMS for short. So what does this EVMS thing do? At present, its a voltmeter, ammeter, power meter, battery gauge, fuse, precharger, leakage detector, temperature sensor and charging interlock, all rolled into one! (And all for about $100 worth of parts.) I've long believed there was plenty of scope for integrating the many subsystems in an EV, so here's a start at least - plenty of room for improvement yet.

Tech specs in brief: the brains of the operation is an ATMega8 microcontroller (centre), with a 2x16 character LCD display for feedback (not pictured - attached to rainbow wire on right). There is an ADM699A external watchdog timer (top right) to make sure the unit shuts down gracefully in the unlikely event of a microprocessor failure. Current is being measured with a 600A Tamura hall effect current sensor (not pictured, attached to cable at bottom). Contactor switching is done via BD681 darlington transistors (on the left) - it may need better ones for switching multiple contactors in future. The white cylinder is a piezo buzzer for beeps and alarms. The black rectangle at bottom right is an isolated 12-12V converter for creating a -12V potential (needed for op amps etc). Click on the image to the right for a larger view (electronics gurus can probably deciper most of the circuit from the picture!)


Circuit as of 22nd Jan 2009

Battery Gauge

All EVs need a means of monitoring the state of their battery to make sure they're not damaged via overdischarging etc. It is also useful to see instantaneous power for quantitative measures of vehicle efficiency during operation. All the existing products I've seen on the market are surprisingly expensive for what seems to me like a fairly trivial job, so I thought it'd be useful functionality to include.

At present it provides realtime feedback of voltage, current, power, and amp-hours, plus a state-of-charge indication. Pack voltages outside the programmed max/min range will give a warning with alarm. Current above maximum will raise an error (see Digital Fuse below).

Integration with a battery management system would be a very useful addition such that the EVMS can warn the driver if any single cell goes below its rated minimum voltage. One for the To Do list.


Realtime V/W/A/Ah feedback


Battery State-of-Charge meter

Digital Fuse

Most people will be familiar with how expensive the big semiconductor fuses and high power DC circuit breakers are. As well as this, they have very soft trip curves - i.e they can carry well above their rated load for a short while without tripping.

The EVMS is monitoring the current in realtime as part of the battery gauge, so can also quickly detect when current is outside an acceptable range. And since it has control of the contactor(s), it can immediately isolate all battery banks in the case of an over-current condition. (Combined voltage rating of contactors should necessarily be higher than your pack voltage, i.e always able to break the circuit in an emergency!)

Of course ideally you'd still have at least one fuse/breaker as well - redundancy is the name of the game with safety - but this digital fuse could certainly replace one (e.g with a split pack, one conventional fuse to protect rear cells, digital fuse for front cells).

Contactor Control and Automatic Precharging

The EVMS currently manages two contactor (groups) plus a precharger on one contactor. Ideally EVs should have contactor(s) for isolating every group of batteries around the car and/or electrically breaking up sections of their pack for safety whenever the key is off - let's call these the "auxilliary" contactors. You should also have a "main" contactor between the pack and your motor controller. This is the last one before the motor controller so is only one which needs precharger.


Display during precharging

So when starting the vehicle, the EVMS will first close the auxilliary contactors and enable the precharger, bypassing the main contactor through a resistor network. When it detects a sufficiently low current flowing through the precharge resistors, it then closes the main contactor.

The precharge process takes a couple of seconds, and there's an audible buzzer and progress bar on the display while it's happening.

Leakage Detection

Something not commonly seen on EV conversions yet, but in my opinion fairly valuable. Most EVs have the traction circuit "floating" with respect to the vehicle body. This means you can touch any one part of the traction circuit and body without getting zapped. However if an unexpected conduction path occurs between the body and traction circuit (e.g insulation wears off a cable and the copper touches the vehicle body), it becomes possible for humans to get an electric shock, or for fires etc to start if a second conduction path occurs! So the safest way to go is to have a device which can detect the first current path between traction circuit and body before it is able to injure someone or create a short circuit (via a second leakage point).

This leakage detector "weakly ties" the body to the centre voltage potential of the battery pack through 100kOhm resistors. It is sensitive down to fractions of a milliamp, and will raise an error (instantly opens contactors) if more than about 20mA flows i.e a low enough resistance path to potentially harm a human. (This may actually cause problems in some vehicles with high quiescent leakage - time will tell.)


Leakage display: Scale roughly 0-20mA

Temperature Sensor

It is common practice to run motors in EVs above their nameplate rating for a while, but if it is done continuously the motor will heat up, e.g when climbing long steep hills or driving around a racetrack. In such cases it is useful to keep an eye on the temperature!

The EVMS currently has one temperature probe which is to be fastened to the motor. The microprocessor calibrates this into degC, and will display a warning + alarm if the motor gets too hot. I'd prefer to have two temp sensors (e.g one for motor & one for controller), but have run out of analogue inputs on the microcontroller.. Not a huge issue as the controller will have its own thermal protection, but maybe something for the future.

Charger Interlock

In many conversions, it's possible to turn the key and drive away with the charger still plugged in! Or in some cases, you actually have to have the key on to charge (due to the need to close pack contactors).

So I figured what is needed is a charger interlock. Via a contact switch ideally wired to the fuel/charger door, the EVMS can detect when you may be charging. It then closes the auxilliary contactors, but makes sure the main contactor is open so the controller does not have power. (The charger gets wired in just before the main contactor.) This way the pack can be charged without the need for the ignition key, or the possibility of the car being driven, until the charge door is closed again.

And of course, the current going into the pack is monitored during the charge for an updated SOC in the battery gauge.

Errors and Warnings


The EVMS includes textual display of various error and warning conditions if/when they occur, with an audible alarm/buzzer to make sure it doesn't go unnoticed!

An "error" condition is one where the vehicle must cease operation ASAP, such as over-current or excessive "earth leakage" to chassis (which may be a human getting electrocuted). Contactors will open instantly. Error conditions can only be cleared by restarting the device.

A warning such as low battery voltage or a hot motor is less serious so the EVMS will leave contactors closed, but brings it to the driver's attention via on-screen message and (less irritating) alarm. Warnings may be cleared by turning the key off (and/or closing the charge door).

 

Update: Development on the EVMS continues and its on its way to being a complete, robust product you can buy for your EV! Stay tuned..

-Ian Hooper

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