evaluate residual capacity of Pb battery.

[piero]

In order to roughly evaluate the residual capacity of a 6V sealed
battery, in a remote installation, I'm on the way to proceed like this:

on a regular basis a simple check is performed in order to verify the
battery is connected and alive. The charge voltage is lowered to a still

safe value and the measured current must be provided by the battery.

Over a longer period of time, I would evaluate the capacity on the
assumption that the discharge curve has always a typical shape. The idea

is to keep the charge voltage below the value of the battery and measure

the quantity of charge provided by the battery in order to reach a
defined drop of voltage for instance half a volt.

In other words, do you think is possible to extrapolate the discharge
shape from its initial behavior?

Thanks for your worthy opinions.

piero

 

[Andrew Wade]

I am working on the design of a battery tester which uses the same sort of
principle. I have a high current fet across the terminals which i pulse on
for a very short time. I then extrapolate the volt drop / fet on current. to
give an idea of battery health.

Are you trying to determine battery charged / not charged? or battery good /
not good?

Andrew

 

[Roger Lascelles]

Yes, Piero, your idea will work, provided you draw a significant amount of
currrent for a significant amount of time.

A starting point would be to draw the full load currrent the battery must
deliver for a period of one quarter of the time the battery must hold up the
load. You might get away with half the load current, but you get the idea.
You might even draw more than the load current in order to get a quicker
test. You monitor the battery terminal voltage during the load test, and
you fail the battery at the moment the voltage falls below the cutoff
voltage.

If you get a datasheet for your battery (Yuasa website, etc) you can choose
a cutoff voltage based on how deeply you want to discharge the battery
during the test. I suggest you choose a battery which has at least twice
the required capacity - that way the battery will fail the test when its
capacity has decreased to one half of the specified capacity.

If your application is high current, be sure to draw the full load current
during the test. Lead acid batteries often fail because of internal
corrosion and only a full current will test the Pb conductors properly.

If your application is subject to large temperature variations, you might
need to adjust for this. Mostly it is a matter of making the cutoff voltage
more lenient at low temperatures.

This sort of thing is very approximate, but it really does work.

Roger

 

[Jeroen Vriesman]

Maybe you should also measure the temperature of the battery and put it in
your model.

Has a lot of influence on internal resistance/current etc.

 

[Jeroen Vriesman]

I've seen such a device, the HP disk cabinets (about 10 years old, hot plug
HVD scsi, used on HP-UX machines), use it.

And they do have a temperature sensor on the (NiCd) batteries, the
measurements are translated into software warnings/alarms, log messages.

 

[Tim Shoppa]

In order to roughly evaluate the residual capacity of a 6V sealed
battery, in a remote installation, I'm on the way to proceed like this:

on a regular basis a simple check is performed in order to verify the
battery is connected and alive. The charge voltage is lowered to a still
safe value and the measured current must be provided by the battery.

I think this amounts to measuring the internal resistance at full
charge.

Over a longer period of time, I would evaluate the capacity on the
assumption that the discharge curve has always a typical shape. The idea
is to keep the charge voltage below the value of the battery and measure
the quantity of charge provided by the battery in order to reach a
defined drop of voltage for instance half a volt.

Here I think you are measuring the shape of the "top" of the discharge
curve. Maybe you aren't; maybe you are stating a somewhat different
way of measuring the internal resistance.

In other words, do you think is possible to extrapolate the discharge
shape from its initial behavior?

No, not really. But you can determine whether the internal resistance
(or the shape of the top of the discharge curve) no matter matches that
of a good battery. This, I think, is a better-than-average test for
battery goodness. It doesn't directly tell you the full capacity, though.

Tim.

 

[piero]

I am working on the design of a battery tester which uses the same sort of
principle. I have a high current fet across the terminals which i pulse on
for a very short time. I then extrapolate the volt drop / fet on current. to
give an idea of battery health.

Are you trying to determine battery charged / not charged? or battery good /
not good?

Thanks. The problem is to determine battery still good or in critical condition.

I've already done in a simplified situation where the load was significantly
constant.
Now the current can span from 30mA to probably 400mA . The idea is to integrate
the measured current, sum of current on a regular basis, in order to get
Coulombs.
The test is performed only if the battery is fully loaded that is: current very
low at the voltage required at the measured temperature.
Yes the voltage is compensated in temperature. We had to. The original
application was a remote GSM control for traffic lights ia Saudi Arabia.

 

[piero]

Thanks for your opinions.
I believe this is a very general matter, it is no so vital to actually measure
the capacity, it's matter of there is enough or not? You can't rely on
"substitute the battery every two years".
Batteries are often the less reliable components in the system. Apart from the
energy network. piero

 

[Keith Buck]

Hi,
Use a battaery impedance test. This is very simple to implement and can be
bought off the shelf.
Regards keith buck