Battery undervoltage warning circuit · 29 October 06
Jerry has asked me to support him on this site by posting articles. This is my first time so please be patient while I get my act together.
There are big wires, big batteries and big currents in an EV. Sometimes things can go wrong and the sooner you know about it, the better.
Last year when I was a green EV-er I just thought it was OK to connect all my cables to the battery terminals without checking them or even cleaning the oxide off the lead. Everything was OK – for a while. Then there was a pop and a fizzle on day and one of my terminals had melted and dropped molten lead onto the battery plates below. I was barely able to make it home and had to spend some time patching the hole in the battery and fishing bits of lead out.
I have been thinking ever since about some sort of device which will give me early warning about either weak cells and/or weak connections, that is, a high resistance link somewhere in my 16 long battery string.
Each of my batteries at rest is supposed to be at 6V (about 6.37V and a bit of fluff on top if they are fully charged). Under load they will “sag” because of their internal resistance down to, for my car under load, about 5V. If there is a weak battery, it will sag faster and lower than it’s friends. If there is a weak connection between the batteries, it will also cause a “sag” and heat up.
Some people connect a voltmeter to each of their batteries so that they can watch the condition of each battery from the cabin. I actually wanted a much more immediate kind of warning which involves a light lighting up when each battery and it’s next connection sag under load. 16 lights in all.
If the light was to light at about 5.5V, all lights should be out when the battery is unloaded showing that their resting voltage is good. Any light now would show up an uncharged battery. When applying load, all the lights should come on at about the same time, showing that all those batteries are stressed to the same level. One light on early or staying on for a while after the others spells trouble.
Here’s the circuit. It has two potential dividers using 3.3V Zener diodes and 100 ohm resistors and an LED between them This gives you two reference points, one 3.3V above the battery negative, one 3.3V below the battery positive. The difference between these is the sag from 6.6V. When the battery is at 6.3V it means that there will be 0.3V across the LED. Not enough to light it up. When the battery is busy and at 5V there will be 1.6V across the LED. that’s bright light! There are sixteen of these circuits (click on the diagram), one for each battery.
I have opted to disconnect my battery monitor from the batteries using lots of 12V relays. It’s crude but I don’t want my batteries drained by the monitor. The relay is controlled by the ignition signal and by another switch so that the monitor is only on when the ignition is on and the other switch is on.
The battery monitor is wired into the car using CAT5 ethernet cables. They are rated to take the kind of voltages and currently involved. Each wire has a 250mA slow blow fuse near the battery. At the moment only 10 of the 16 batteries are connected up.
On the picture here you can see how I have used 2 banks of 4 RJ45 sockets to connect to the batteries and to connect to a smaller board with 16 orange LEDs on it for the display.
When the accelerator is pressed the orange LEDs on the dashboard display light up.
Here’s what it looks like installed in the car:
Now to see what improvements everybody can make to this design!James May in the UK