SLA Battery monitor

[cowana]

I want to create a battery monitor, that will use an LED to display the voltage of a sealed lead acid battery.

The device must light a series of LEDs (or extinguish LEDs, either is OK) when the battery voltage drops below 10v (roughly). I'd like to avoid using a PIC and regulator, as I'm hoping to do this with only a few components. I'd like to avoid any ICs if possible, but if I had to I could use an op-amp.

I'm more into digital design rather than analogue, so can anyone lend a hand? Possibly something using a zener and transistor? Something along these lines: **broken link removed**

I'm thinking of using a chain of resistors to divide the voltage up, then using transistors to monitor the voltage at each point along the chain. (See attached schematic). I'm not sure that idea would work, and it would certainly use a lot of components. Can anyone think of a better way?

Andrew

 

[Reloadron]

Since you want a series of LEDs I would consider using the LM3914N as it will directly drive LEDs and the data sheet gives some examples. You could also run with a LM339 quad comparator and set it up to drive 4 LEDs but depending on the LEDs you will maybe need transistor drivers for the LEDs off the comparator. I would run the comparator off a 5 volt regulator and divide the battery voltage at the inputs. This link gives a good overview of comparator circuits.

You could use the transistor circuit but again I would use a 5 or 9 volt regulator and divide since using the straight 12 volts will shift the divider outputs as the battery drops in voltage supplied.
Ron

 

[cowana]

Thanks for the input, Ron. To make the board as small as possible (it will be surface mount), Ideally I'd like to stay away from ICs. Any ideas on how something like **broken link removed** may work?

 

[cowana]

It's still a lot of resistors, but I think I'm going to go with the TL431 voltage monitor IC. Using four resistors and a SOT-89 package per LED (so each battery monitoring board will have 12 0805 resistors and and three 0805 LEDs), I will have a very good idea of the condition of each battery.

TL431 Battery Voltage Monitor - Storage

Andrew

 

[Reloadron]

That should be a good solution.

Ron

 

[Boncuk]

Hi cowana,

if you like it please PM me for the Eagle files.

All resistors and LEDs are 0805.

Board size is 23.0981X12.5413mm (0.90937X0.49375")

Regards

Boncuk

 

[Reloadron]

Nice solution Hans.

Ron

 

[Boncuk]

Nice solution Hans.

Ron

Thanks for the flowers, Ron

Sunday, and Thai TV is boring.

Hans