SLA: circuit to check if the battery needs to be replaced - Electronic Circuits Projects Diagrams Free

logicnibble · 23rd June 2008, 05:26 PM

Hi!

I have a pcb that is being powered by mains and backed-up by a 12V SLA rechargeable battery.

Due to charge and discharge cycles, the SLA battery might lose its properties.
So I would like to if there are any methods to check if the battery needs to be replaced or not.

I have an ADC so I can read SLA battery voltage and do some calculations.

I guess there is a method of applying a constant load, pulling a certain amount of current from the battery and by reading its voltage it might be possible to determine whether the battery is ok or needs to be replaced.

Is anyone aware of such methods?
Any papers about this and the theory behind it?

**Krumlink** · 23rd June 2008, 07:27 PM

Charge the battery to full and then measure the voltage under no load. SLA 12VDC batteries are actually 13.8VDC but if its fully charged and reads around 13.4VDC (like mine) they are probably old.

RadioRon · 23rd June 2008, 07:53 PM

Quote:

Originally Posted by logicnibble

Hi!

I have a pcb that is being powered by mains and backed-up by a 12V SLA rechargeable battery.

Due to charge and discharge cycles, the SLA battery might lose its properties.
So I would like to if there are any methods to check if the battery needs to be replaced or not.

I have an ADC so I can read SLA battery voltage and do some calculations.

I guess there is a method of applying a constant load, pulling a certain amount of current from the battery and by reading its voltage it might be possible to determine whether the battery is ok or needs to be replaced.

Is anyone aware of such methods?
Any papers about this and the theory behind it?

I believe the simple algorithm that you describe is at the heart of automotive battery "exerciser"-type chargers. I'm no expert, but an increase in the internal resistance of an SLA battery would be a pretty good indicator of wear. The question then is, how much is too much?

I got some pretty good education at this site:
VARTA Automotive*-*Battery know-how

google for "load testing lead acid batteries" and you will get a lot of hits

eblc1388 · 24th June 2008, 01:02 AM

The following method would give a good indication of the heath of the battery:

1. Fully charge the battery

2. Apply a constant current discharge load of "C" to the battery. e.g. a 12V 20AH batery would be a 20A and a 12V 6AH battery would be 6A discharging current.

3. Record the battery terminal voltage 10 seconds after start of discharging, and then again after a minute. Try this method first with a few new and fully charged batteries so you can get yourself some reference figures of what to expect.

4. Most bad batteries will fail to last the duration of the test or show up with a very large drop in terminal voltage.

logicnibble · 25th June 2008, 11:36 AM

Hi!

Thank you all!

I've done some more research (thanks RadioRon, I guess I wasn't using the right search strings) and here is a summary of what I've got:

Heavy-load test:
Battery must be at least 75% charged
During the test the voltage should not fall below 9.6V. Otherwise, replace the battery.
The test should last 15 seconds the maximum
The correct load should be 1/2CCA or 3xAh

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23rd June 2008, 05:26 PM	(permalink)
logicnibble	SLA: circuit to check if the battery needs to be replaced Hi! I have a pcb that is being powered by mains and backed-up by a 12V SLA rechargeable battery. Due to charge and discharge cycles, the SLA battery might lose its properties. So I would like to if there are any methods to check if the battery needs to be replaced or not. I have an ADC so I can read SLA battery voltage and do some calculations. I guess there is a method of applying a constant load, pulling a certain amount of current from the battery and by reading its voltage it might be possible to determine whether the battery is ok or needs to be replaced. Is anyone aware of such methods? Any papers about this and the theory behind it?

23rd June 2008, 07:27 PM	(permalink)
Krumlink	Charge the battery to full and then measure the voltage under no load. SLA 12VDC batteries are actually 13.8VDC but if its fully charged and reads around 13.4VDC (like mine) they are probably old. __________________ MechTronics World of Warcraft is a disease

24th June 2008, 01:02 AM	(permalink)
eblc1388	The following method would give a good indication of the heath of the battery: 1. Fully charge the battery 2. Apply a constant current discharge load of "C" to the battery. e.g. a 12V 20AH batery would be a 20A and a 12V 6AH battery would be 6A discharging current. 3. Record the battery terminal voltage 10 seconds after start of discharging, and then again after a minute. Try this method first with a few new and fully charged batteries so you can get yourself some reference figures of what to expect. 4. Most bad batteries will fail to last the duration of the test or show up with a very large drop in terminal voltage. __________________ L.Chung

25th June 2008, 11:36 AM	(permalink)
logicnibble	Hi! Thank you all! I've done some more research (thanks RadioRon, I guess I wasn't using the right search strings) and here is a summary of what I've got: Heavy-load test: Battery must be at least 75% charged During the test the voltage should not fall below 9.6V. Otherwise, replace the battery. The test should last 15 seconds the maximum The correct load should be 1/2CCA or 3xAh