Battery Monitor Circuit

[rob81]

Hi all, new here so hopefully you can help.

I'm looking for a simple circuit to allow me to test the level of a rechargeable NIMh 1.5v AA battery. Preferably using an LED Bar Graph display.

Any help would be appreciated, Cheers

Rob

 

[Nigel Goodwin]

There really isn't one - you can measure the voltage across it, but it's a VERY poor indication of the state of charge.

 

[rob81]

How can there not be one, there must be some way of doing it??

 

[Nigel Goodwin]

How can there not be one, there must be some way of doing it??

Not that works with any degree of usefulness?.

 

[JimB]

How can there not be one, there must be some way of doing it??

The discharge curves for NICAD and NiMH batteries are rather flat, ie the volage does not change much between fully charged and nearly completely discharged. A voltmeter is of limited use.

If you want get a better indication of the charge in the battery you will have to integrate the charge/discharge current with respect to time. This is getting away from the idea of "simple".

There are integrated circuits which implement a "gas gauge" function for use in laptop batteries etc, try searching the Maxim website, I think they do one or two of these things.

JimB

 

[Nigel Goodwin]

There are integrated circuits which implement a "gas gauge" function for use in laptop batteries etc, try searching the Maxim website, I think they do one or two of these things.

Have you ever taken a laptop battery apart?, there are often 8 or 9 SM IC's in them - pretty serious things!.

 

[Sceadwian]

Testing a batteries state of charge is not that complicated. Open circuit voltage is as Nigel said relativly useless by itself. But you can get a very good idea of the state of charge of a battery by comparing a no load voltage with a loaded voltage. A good 'load' current to put a cell under is 1/10th of it's rated capacity. So for a 2000ma NiMh, put it under a 200ma load and compare the voltages. You'll get a pretty decent representation of the battery charge.

If you say compared 10ma 100ma and 500ma load voltages with the no load voltage I'm sure you could calibrate a 'gas gauge' style meter within 5% accuracy.
If you've ever seen those built in battery gauges on AA's (I think Energizer and Duracell both did this for a time) when you press the side of the battery it shorts the battery across a thermal strip and the relative current through it causes it to heat up and show a charge state, same general concept.

 

[Nigel Goodwin]

If you've ever seen those built in battery gauges on AA's (I think Energizer and Duracell both did this for a time) when you press the side of the battery it shorts the battery across a thermal strip and the relative current through it causes it to heat up and show a charge state, same general concept.

And shortens the battery life every time you do it! - good marketing ploy by Duracell!.

 

[Sceadwian]

Yes, but a solid state digital circuit such as perhaps a PIC or other compareable micro controller with an ADC can read the loaded voltage (switched easily via a transistor) in less than a milisecond. At 1 reading a second that's 1ma average draw for a 1 second update on battery capacity with a 100ma draw... Check it once a minute and the wasted current is almost less than self discharge. I wouldn't recommend anything less than a 1ms 'short' to the load before sampling simply because internal surface charge of the batteries chemistry can artificially 'liven' the loaded voltage for a brief period of time.

 

[JimB]

Have you ever taken a laptop battery apart?, there are often 8 or 9 SM IC's in them - pretty serious things!.

No I have not, but that is the point I was trying to make by my earlier statement:-

If you want get a better indication of the charge in the battery you will have to integrate the charge/discharge current with respect to time. This is getting away from the idea of "simple".

It is not SIMPLE.

JimB