LED Bargraph battery monitor - part 1 · 18 May 07
About 6 months ago I wrote an article here . The idea was that I would be able to avoid my nemesis, unequalised batteries , and have early warning of bad connections like the one which caused this terminal failure in my car. Argh!
The battery monitor was fairly successful but only gave a crude qualitative measurement of voltage drop. I have been looking for something better. Something with which I can see the actual voltage each battery is at.
When I saw the battery monitor on Jim Fell’s Seicento I had to have one. It’s based on the robust LM3914 LED driver IC.
The circuit I found and adapted can be found here. National Semiconductors have kindly provided an example circuit and some very good explanation.
I adapted this circuit to show 10 lights for 6.37V and above and no lights for 5.75V and below, with the rest of lights showing about 0.6V intervals. Each interval loosely approximates to 10% DoD (Depth of Discharge). Here’s the schematic:
Here’s a table which shows number of lights shown against voltage and State of charge (100- DoD):
1. 5.75V ~10%
2. 5.82V ~20%
3. 5.89V ~30%
4. 5.96V ~40%
5. 6.03V ~50%
6. 6.09V ~60%
7. 6.16V ~70%
8. 6.23V ~80%
9. 6.30V ~90%
10. 6.37V 100% upwards
The LM3914 has 2 modes: “bar” where it will show all of the lights up to a certain light and “dot” where it will only show one (or occasionally 2 lights at a time). I started off using bar mode but I found this too bright and distracting – it lit up the whole car. Lighting up all those LEDs also drew a higher current and generally read lower due to the associated voltage drop.
You’ll see that I have designed this circuit for 6V Flooded lead acid batteries. For those of you running 8 or 12V you want to replace Rx with 33K or 56K respectively. I tested this at 12V with 56K and it works just fine.
For each meter you’ll need these components:
10 DIL bargraph display
10 uF electrolytic capacitor
22K resistor (for the 6V version)
200R precision cermet trimmer
500R precision cermet trimmer
IN4001 silicon rectifier diode or similar.
a piece of stripboard 10 holes x 16 holes (strips run along the 16 hole side)
All resistors should ideally be 1% or 2% tolerance.
The electrolytic capacitor needs to be very small and short if you want to mount all the boards together as shown below.
Small soldering iron and solder
Adjustable power supply (5.5 – 6.5V for 6v batteries)
If you have made the circuits tidily you should be able to glue the LED modules together and mount the whole thing on another stripboard.
Now to put it in the car.
I have left out all of the relays I used in the original battery monitor and the unit is now permanently on. I measured current usage and found that if you are using “bar” mode with all the LEDs on, you can expect to draw about 100mA per meter at 6V, probably about half that at 12V. If you use “dot” mode then you’ll draw about 10mA per meter. Certainly for my battery pack this is a very small loss.
I am so much more confident in my battery pack now that I can see what each individual battery is doing. I am sure that it’ll make the batteries last longer because I’ll be able to know about a discharged battery before I cause a cell reversal or over gassing.
That’s all for now! Step by step stripboard instructions coming up in Part 2.