Sealed Lead Acid Battery Tester Strategy?

[Mike]

I've designed and built a prototype PIC based Sealed Lead Acid Battery
Tester, but I'm having confusing and conflicting results and would like some
input on whether the strategy I'm using is sensible ...

It's based around a constant-current load, controllable by the PIC chip.
The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where
C is the nominal Ah rating of the battery). The unit can monitor the
voltage of the battery. All the voltage measuring/current measuring and
current setting is appropriately calibrated, so any problems are down
to approach or duff assumptions on my part

The basic idea is a "brute force" test of the capacity of the battery: Load
it up at C/n and time how long the battery takes to reach a discharged state.
In theory, it will take "n" hours for a 100% battery (aside from the obvious
that discharging faster will reduce apparent capacity etc.)

From my research so far a fully charged SLA cell should read 2.16v at rest,
and 1.75v when discharged. So for a 12v battery, the useful capacity is
found between terminal voltages of 10.5v and 12.96v. Right?

The capacity of the battery can be estimated using this voltage range, which
I'm assuming to be a linear function (it nearly is...). So the first feature
on the tester is a battery capacity estimator, based on offload voltage.

Then loading the battery at (some constant current) begins the discharge
process, and the voltage starts to fall. I time how long it takes for the
voltage to reach 10.5v, and then stop the timer, and compute the capacity
actually achieved. It should be that simple, however ...

I have a dilemma: When things say "don't discharge the battery below 10.5v"
do they mean on-load or off-load? There is a difference!

I noticed that loading the battery, especially at higher currents, causes
the terminal voltage to fall, due to the internal resistance of the battery.

So I added a feature to try and measure this resistance at the start of the
discharge process. I did this by taking the off load voltage, the on-load
voltage at C/n, and working out the voltage being dropped across that
resistance. This gives me a figure of <100 mOhm on new batteries, and
anything up to 4-5 Ohms on older "suspect" batteries. It also gives me a
"delta voltage" to adjust the 10.5v limit figure downward by.

My tester currently records two capacities. One: Discharge to 10.5v on load.
Two: Keep discharging until 10.5v MINUS the voltage-delta calculated above.
Obviously capacity 2 is always >= capacity 1. This seems to give reasonable
figures, of capacities that are <= nominal.

The snag is that when this test terminates, the battery really *really*
should be flat. As I've compensated for the voltage-drop across the internal
resistance, I should get 10.5v when the load is taken off, and the battery
is empty, right?

Wrong.

The voltage bobs back up to an embarrassing "You have 60% left" level. That
shouldn't happen I flattened it to 10.5v on-load. I flattened it further
to 10.5v minus the voltage across the internal resistance ... how can it
still have power in there?

Trying to load the battery up further (repeating the discharge) just causes
it to immediately terminate as the voltage is too low.

Is there anything particularly wrong with this strategy?

Should I even be compensating for the voltage drop across the internal
resistance? I know it's meant to be low order mOhm, but even that can cause
a significant drop at e.g. 30A test current on large batteries. So I think
yes, I should be.

And does the internal resistance significantly change, meaning I should be
re-testing it at intervals? I thought the internal resistance was a function
of how sulphated the plates were, and would only vary if the battery was
deep discharged and left, or charged and rejeuvenated.

Why does there appear to be useful power left in the battery (from the
terminal voltage) and yet there .. isn't!?

If the discharge is genuinely flattening the battery, yet the terminal
voltage comes back up so high, how can "battery fuel gauge meters" ever
reliably indicate the state of charge of the battery? I'm beginning to
think that they can't!

Maybe I should take the load off every so often, wait for the voltage to
bob back up and settle, and if it's > 10.5v, go on a bit longer?

Here's an example of the kind of readings I'm getting. Each capacity is
stated twice, once for "discharge to 10.5v" and once for "discharge
below 10.5v by dV"

Battery: Camden Europa Plus, 20 months old, UPS battery. 7.2Ah nominal.
Internal Res: 94-126 mOhm (tested at 7.2 Amps)
Measured Capacity @ C/1 rate: 2.44Ah/3.16Ah
Measured Capacity @ C/10 rate: 5.11Ah/5.18Ah
Est. Capacity after C/1 test - 67% (UPS software claims 63%)
Est. Capacity after C/10 test - 50% (UPS software says 58%)

Spookily, I seem to have drained 33% out of the battery (2.44Ah) and 67%
is estimated to be left, which means I haven't flattened the battery yet.

Battery: Yuasa NP1512, 5+ years old, 15Ah nominal
Internal Res: 76 mOhm (tested at 15 Amps)
Measured Capacity @ C/1 rate: 9.9Ah/10.5Ah
Measured Capacity @ C/10 rate: 15.15Ah/15.0Ah
Est. Capacity after C/10 test - 18%

Battery: Sonnenschein Dryfit A300, 5+ years old, 3Ah nominal
Internal Res: 270 mOhm (tested at 3 Amps)
Measured Capacity @ C/1 rate: 1.05Ah/1.08Ah
Measured Capacity @ C/10 rate: 2.7Ah/2.7Ah
Est. Capacity after C/1 test: 50%
Est. Capacity after C/10 test: 60%

What have I screwed up?

Mike.
--
--------------------------------------+------------------------------------
Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

 

[Roger Lascelles]

"Mike" <mjb@posie.local.dom> wrote in message
news:da8b9k$v2m$1@posie.local.dom...

I've designed and built a prototype PIC based Sealed Lead Acid Battery
Tester, but I'm having confusing and conflicting results and would like
some
input on whether the strategy I'm using is sensible ...

It's based around a constant-current load, controllable by the PIC chip.
The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where
C is the nominal Ah rating of the battery). The unit can monitor the
voltage of the battery. All the voltage measuring/current measuring and
current setting is appropriately calibrated, so any problems are down
to approach or duff assumptions on my part

The basic idea is a "brute force" test of the capacity of the battery:
Load
it up at C/n and time how long the battery takes to reach a discharged
state.
In theory, it will take "n" hours for a 100% battery (aside from the
obvious
that discharging faster will reduce apparent capacity etc.)

From my research so far a fully charged SLA cell should read 2.16v at
rest,
and 1.75v when discharged. So for a 12v battery, the useful capacity is
found between terminal voltages of 10.5v and 12.96v. Right?

The capacity of the battery can be estimated using this voltage range,
which
I'm assuming to be a linear function (it nearly is...). So the first
feature
on the tester is a battery capacity estimator, based on offload voltage.

Then loading the battery at (some constant current) begins the discharge
process, and the voltage starts to fall. I time how long it takes for the
voltage to reach 10.5v, and then stop the timer, and compute the capacity
actually achieved. It should be that simple, however ...

I have a dilemma: When things say "don't discharge the battery below
10.5v"
do they mean on-load or off-load? There is a difference!

I noticed that loading the battery, especially at higher currents, causes
the terminal voltage to fall, due to the internal resistance of the
battery.

So I added a feature to try and measure this resistance at the start of
the
discharge process. I did this by taking the off load voltage, the on-load
voltage at C/n, and working out the voltage being dropped across that
resistance. This gives me a figure of <100 mOhm on new batteries, and
anything up to 4-5 Ohms on older "suspect" batteries. It also gives me a
"delta voltage" to adjust the 10.5v limit figure downward by.

My tester currently records two capacities. One: Discharge to 10.5v on
load.
Two: Keep discharging until 10.5v MINUS the voltage-delta calculated
above.
Obviously capacity 2 is always >= capacity 1. This seems to give
reasonable
figures, of capacities that are <= nominal.

The snag is that when this test terminates, the battery really *really*
should be flat. As I've compensated for the voltage-drop across the
internal
resistance, I should get 10.5v when the load is taken off, and the battery
is empty, right?

Wrong.

The voltage bobs back up to an embarrassing "You have 60% left" level.
That
shouldn't happen I flattened it to 10.5v on-load. I flattened it
further
to 10.5v minus the voltage across the internal resistance ... how can it
still have power in there?

Trying to load the battery up further (repeating the discharge) just
causes
it to immediately terminate as the voltage is too low.

Is there anything particularly wrong with this strategy?

Should I even be compensating for the voltage drop across the internal
resistance? I know it's meant to be low order mOhm, but even that can
cause
a significant drop at e.g. 30A test current on large batteries. So I think
yes, I should be.

And does the internal resistance significantly change, meaning I should be
re-testing it at intervals? I thought the internal resistance was a
function
of how sulphated the plates were, and would only vary if the battery was
deep discharged and left, or charged and rejeuvenated.

Why does there appear to be useful power left in the battery (from the
terminal voltage) and yet there .. isn't!?

If the discharge is genuinely flattening the battery, yet the terminal
voltage comes back up so high, how can "battery fuel gauge meters" ever
reliably indicate the state of charge of the battery? I'm beginning to
think that they can't!

Maybe I should take the load off every so often, wait for the voltage to
bob back up and settle, and if it's > 10.5v, go on a bit longer?

Here's an example of the kind of readings I'm getting. Each capacity is
stated twice, once for "discharge to 10.5v" and once for "discharge
below 10.5v by dV"

Battery: Camden Europa Plus, 20 months old, UPS battery. 7.2Ah nominal.
Internal Res: 94-126 mOhm (tested at 7.2 Amps)
Measured Capacity @ C/1 rate: 2.44Ah/3.16Ah
Measured Capacity @ C/10 rate: 5.11Ah/5.18Ah
Est. Capacity after C/1 test - 67% (UPS software claims 63%)
Est. Capacity after C/10 test - 50% (UPS software says 58%)

Spookily, I seem to have drained 33% out of the battery (2.44Ah) and 67%
is estimated to be left, which means I haven't flattened the battery yet.

Battery: Yuasa NP1512, 5+ years old, 15Ah nominal
Internal Res: 76 mOhm (tested at 15 Amps)
Measured Capacity @ C/1 rate: 9.9Ah/10.5Ah
Measured Capacity @ C/10 rate: 15.15Ah/15.0Ah
Est. Capacity after C/10 test - 18%

Battery: Sonnenschein Dryfit A300, 5+ years old, 3Ah nominal
Internal Res: 270 mOhm (tested at 3 Amps)
Measured Capacity @ C/1 rate: 1.05Ah/1.08Ah
Measured Capacity @ C/10 rate: 2.7Ah/2.7Ah
Est. Capacity after C/1 test: 50%
Est. Capacity after C/10 test: 60%

What have I screwed up?

There seem to be two possible themes in your post about "battery testing".
1) Measuring how charged is a battery. 2) Measuring if a battery is worn
out. Very different things. In this post I just talk generally about both
issues.


RATE OF DISCHARGE

We also use constant current drain and measure time to reach a trip voltage.

There is fast discharge, like cranking an engine, where the battery is
hardly discharged, but terminal voltage plummets because of reaction
products "choking" the plates. Then there is slow discharge, where reaction
products have time to disperse where you really get the nameplate Ah.
Manufacturer specs give different capacities at different drain currents.


BATTERY METERS BUILT INTO EQUIPMENT :

I think about it this way : I drain the battery at the current drawn by my
application, and set the trip voltage at a value which does not discharge
the battery too deeply and reduce life excessively.

If your application has a fairly constant drain, you can calibrate your
"meter" function to be meaningful at that particular drain. Simplest to get
a new charged battery and measure voltage under load - that is "charged"
point. Then discharge battery gently to your endpoint then measure voltage
under load to get "discharged" point. Connect the two voltages linearly and
you have your meter.

If your drain varies dramatically, it is more difficult, but you have to
fudge something. Battery meters can never be taken too seriously. Also,
users get used to the meter and soon work out that say "two bars left on the
meter means I have got 3 more tests in the unit", etc.


BENCH BATTERY METERS

To get consistent results: Fully charge the battery on a voltage float
charger or other programmed quality charge process. Then you rest it 24
hours. Then you perform the discharge test. You do it all at a fixed
temperature (say 26.5C water bath). Other procedures give you results
dependent on history, temperature.

The lower current discharge takes longer, which may be a problem. C/10
takes about 10 hours, and is a pretty fair measure of capacity.

I would not worry about the bounce effect. Once the voltage hits your low
trip point, time is up. That is how Ah tests are performed. A full
discharge meter by definition performs a one shot measurement, then you must
charge the battery and start again.

If you settle a battery for 24 hours at your standard temperature, terminal
voltage is an excellent measure of how charged the battery is.


DEPTH OF DISCHARGE

Going down to 10.5 V may be OK for a tester, but in use I would recommend
11.8V minimum for a decent battery life.


INTERNAL RESISTANCE

Varies between brands, types (eg gell to VRLA), time on the shelf and "how
worn out" and depth of discharge. Resistance is useful for judging battery
deterioration, but with small batteries you have to calibrate against a
particular brand and model as well as state of charge, so it can mean a lot
of work for you to get it right. Measure resistance at small signal - say
200 mA pp 100Hz squarewave forced thru battery - resistance is then
proportional to AC terminal pp voltage.

The discharge to flat time at C/10 is a good cross brand universal test of
health of small lead acid batteries and can be your standard against which
all other tests are calibrated.


Roger Lascelles

 

[john jardine]

"Mike" <mjb@posie.local.dom> wrote in message
news:da8b9k$v2m$1@posie.local.dom...

I've designed and built a prototype PIC based Sealed Lead Acid Battery
Tester, but I'm having confusing and conflicting results and would like
some
input on whether the strategy I'm using is sensible ...

It's based around a constant-current load, controllable by the PIC chip.
The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where
C is the nominal Ah rating of the battery). The unit can monitor the
voltage of the battery. All the voltage measuring/current measuring and
current setting is appropriately calibrated, so any problems are down
to approach or duff assumptions on my part

The basic idea is a "brute force" test of the capacity of the battery:
Load
it up at C/n and time how long the battery takes to reach a discharged
state.
In theory, it will take "n" hours for a 100% battery (aside from the
obvious
that discharging faster will reduce apparent capacity etc.)

From my research so far a fully charged SLA cell should read 2.16v at
rest,
and 1.75v when discharged. So for a 12v battery, the useful capacity is
found between terminal voltages of 10.5v and 12.96v. Right?

The capacity of the battery can be estimated using this voltage range,
which
I'm assuming to be a linear function (it nearly is...). So the first
feature
on the tester is a battery capacity estimator, based on offload voltage.

Then loading the battery at (some constant current) begins the discharge
process, and the voltage starts to fall. I time how long it takes for the
voltage to reach 10.5v, and then stop the timer, and compute the capacity
actually achieved. It should be that simple, however ...

I have a dilemma: When things say "don't discharge the battery below
10.5v"
do they mean on-load or off-load? There is a difference!

I noticed that loading the battery, especially at higher currents, causes
the terminal voltage to fall, due to the internal resistance of the
battery.

So I added a feature to try and measure this resistance at the start of
the
discharge process. I did this by taking the off load voltage, the on-load
voltage at C/n, and working out the voltage being dropped across that
resistance. This gives me a figure of <100 mOhm on new batteries, and
anything up to 4-5 Ohms on older "suspect" batteries. It also gives me a
"delta voltage" to adjust the 10.5v limit figure downward by.

My tester currently records two capacities. One: Discharge to 10.5v on
load.
Two: Keep discharging until 10.5v MINUS the voltage-delta calculated
above.
Obviously capacity 2 is always >= capacity 1. This seems to give
reasonable
figures, of capacities that are <= nominal.

The snag is that when this test terminates, the battery really *really*
should be flat. As I've compensated for the voltage-drop across the
internal
resistance, I should get 10.5v when the load is taken off, and the battery
is empty, right?

Wrong.

The voltage bobs back up to an embarrassing "You have 60% left" level.
That
shouldn't happen I flattened it to 10.5v on-load. I flattened it
further
to 10.5v minus the voltage across the internal resistance ... how can it
still have power in there?

Trying to load the battery up further (repeating the discharge) just
causes
it to immediately terminate as the voltage is too low.

Is there anything particularly wrong with this strategy?

Should I even be compensating for the voltage drop across the internal
resistance? I know it's meant to be low order mOhm, but even that can
cause
a significant drop at e.g. 30A test current on large batteries. So I think
yes, I should be.

And does the internal resistance significantly change, meaning I should be
re-testing it at intervals? I thought the internal resistance was a
function
of how sulphated the plates were, and would only vary if the battery was
deep discharged and left, or charged and rejeuvenated.

Why does there appear to be useful power left in the battery (from the
terminal voltage) and yet there .. isn't!?

If the discharge is genuinely flattening the battery, yet the terminal
voltage comes back up so high, how can "battery fuel gauge meters" ever
reliably indicate the state of charge of the battery? I'm beginning to
think that they can't!

Maybe I should take the load off every so often, wait for the voltage to
bob back up and settle, and if it's > 10.5v, go on a bit longer?

Here's an example of the kind of readings I'm getting. Each capacity is
stated twice, once for "discharge to 10.5v" and once for "discharge
below 10.5v by dV"

Battery: Camden Europa Plus, 20 months old, UPS battery. 7.2Ah nominal.
Internal Res: 94-126 mOhm (tested at 7.2 Amps)
Measured Capacity @ C/1 rate: 2.44Ah/3.16Ah
Measured Capacity @ C/10 rate: 5.11Ah/5.18Ah
Est. Capacity after C/1 test - 67% (UPS software claims 63%)
Est. Capacity after C/10 test - 50% (UPS software says 58%)

Spookily, I seem to have drained 33% out of the battery (2.44Ah) and 67%
is estimated to be left, which means I haven't flattened the battery yet.

Battery: Yuasa NP1512, 5+ years old, 15Ah nominal
Internal Res: 76 mOhm (tested at 15 Amps)
Measured Capacity @ C/1 rate: 9.9Ah/10.5Ah
Measured Capacity @ C/10 rate: 15.15Ah/15.0Ah
Est. Capacity after C/10 test - 18%

Battery: Sonnenschein Dryfit A300, 5+ years old, 3Ah nominal
Internal Res: 270 mOhm (tested at 3 Amps)
Measured Capacity @ C/1 rate: 1.05Ah/1.08Ah
Measured Capacity @ C/10 rate: 2.7Ah/2.7Ah
Est. Capacity after C/1 test: 50%
Est. Capacity after C/10 test: 60%

What have I screwed up?

Mike.
--
--------------------------------------+-----------------------------------
-
Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

The terminal voltage between fully charged and flat is from about 13V to
12V.

www.yuasa-battery.co.uk/ind_vrlamanual.html

Has a really excellent technical write up on their SLA type batteries.
Use it as a good reference point for designing any high quality SLA
charger/discharger.

regards
john

 

On Sun, 3 Jul 2005 09:31:32 +0000 (UTC), mjb@posie.local.dom (Mike) wroth:

If the discharge is genuinely flattening the battery, yet the terminal
voltage comes back up so high, how can "battery fuel gauge meters" ever
reliably indicate the state of charge of the battery? I'm beginning to
think that they can't!

The better fuel gages actually measure the current rather than applying
a fixed load. The amount of charge on the battery is the difference between the
current used to charge the battery and the current drawn from the battery,
integrated with respect to time of course.

You have to fudge the data to account for energy lost in the internal
resistance and the "self discharge" rate if you want maximum "accuracy".

Jim

 

[ehsjr]

Mike wrote:

<snip>

Take a brand new known good 1.5 volt penlight cell,
and connect it in series with a 1K resistor in an
open circuit. Now, read the voltage, placing the voltmeter
from the unconnected end of the resistor to the unconnected
end of the battery. What do you read? You still see 1.5
volts, but the cell + resistor can provide only .0015 amps
instead of the MUCH higher current the cell could provide
without the resistor. Imagine that the resistor is the internal
resistance of the battery. That is what happens when a battery
discharges - its internal resistance goes up.

Next: don't discharge the battery below 10.5 - in fact, don't
discharge it to 10.5. Each discharge takes a little life
from the battery that CANNOT be retrieved. The deeper you
discharge, the more life you take.

Someone recommended a site for some reading - it would be
very helpful to you to look at that, and other sites, as well.
There's a lot of information on the care and feeding of
batteries out there.

Ed

 

Mike <mjb@posie.local.dom> wrote:

It's based around a constant-current load, controllable by the PIC chip.
The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where
C is the nominal Ah rating of the battery).

The variable current is a good idea, and C is probably as high as you
want to go for small sealed lead-acid batteries. Somewhat along this
line, I have tested larger absorbed glass mat batteries (12 V, 25 - 40
Ah nominal) being used in hybrid cars. At first the test was done
something like the test for a flooded lead-acid car starting battery:
apply a very large load for a relatively short period of time (less than
a minute) and make sure the terminal voltage doesn't drop below x. This
test is also pretty quick to do. However, we had batteries that would
pass this test, but perform poorly in the car. Testing them at a smaller
load of 10 to 20 A or so, to some final under-load voltage, and computing
the Ah produced proved to be a much better way of identifying weak
batteries.

From my research so far a fully charged SLA cell should read 2.16v at
rest, and 1.75v when discharged. So for a 12v battery, the useful
capacity is found between terminal voltages of 10.5v and 12.96v. Right?

I think this is right. I'm not sure to what extent SLA batteries have
this problem, but you might look up "surface charge" in relation to
flooded lead-acid batteries. Basically, measuring the voltage of a
battery that has just been charging, even if the charging current is
switched off, won't always give you the right answer.

I have a dilemma: When things say "don't discharge the battery below
10.5v" do they mean on-load or off-load? There is a difference!

I'm pretty sure they mean on-load. If they specified an off-load
voltage, then if you wanted your battery-powered device to be able to
know when its battery was dead, you'd have to design it to switch itself
off (or to a very low load) occasionally and measure the off-load voltage.
This is probably trickier than just putting a comparator across the
battery and shutting down when the voltage gets to some low level.

So I added a feature to try and measure this resistance at the start of
the discharge process. I did this by taking the off load voltage, the
on-load voltage at C/n, and working out the voltage being dropped across
that resistance.

I understand that commercial testers do almost the same thing, but use
on-load measurements at two different currents instead of one on-load
and one off-load voltage.

This gives me a figure of <100 mOhm on new batteries, and anything up
to 4-5 Ohms on older "suspect" batteries.

This seems reasonable. Some SLA manufacturers do mention a range of
internal resistances for their batteries; you might look those up.

The voltage bobs back up to an embarrassing "You have 60% left" level.
That shouldn't happen

If you try to draw any current, though, it will quickly crater to down
around 10.5 V. You might discharge a battery and then experiment with
how little load is required to knock the terminal voltage back down to
<10.5 V; I think you'll be surprised.

You may have done this already, but you might look at some technical
data from makers of small SLAs. I think Power-Sonic and Panasonic have
some reasonable information, both on individual batteries and general
information on charging, finding state-of-charge, etc.

Matt Roberds

 

[Mike]

Thanks for the replies ...

I am aware of "surface charge" and I am resting the batteries after
charging to make sure that they've settled down, so my start point
seems fairly well defined. The voltages for a charged, off load battery
are mostly well clustered around 12.7-12.9v for both new and used batteries.

Exceptions are duff batteries e.g. 10.78v (shorted cell?).

"Roger Lascelles" <despam_rklasl@aanet.com.au> wrote :-

The lower current discharge takes longer, which may be a problem. C/10
takes about 10 hours, and is a pretty fair measure of capacity.

I've got time to burn, so C/10 is acceptable. This is not for a production
unit, or super accuracy, but I would like it to be in the ballpark, and
consistent.

It is, sort of, (save for the estimate function being so over-enthusiastic
about my flat batteries!)

I would not worry about the bounce effect. Once the voltage hits your low
trip point, time is up. That is how Ah tests are performed. A full
discharge meter by definition performs a one shot measurement, then you must
charge the battery and start again.

Understood. It's just that, when repeating the estimate-my-battery
function at the end, I got an unexpected result. Curiosity is a bad thing.

terminal voltage is an excellent measure of how charged the battery is.

That was what *I* thought, as I said, once I've flattened the battery, the
voltage comes back up over minutes to say "70% full". After 24 hours, I'd
fully expect it to be even higher. Which is even *worse*! Hmm.

Going down to 10.5 V may be OK for a tester, but in use I would recommend
11.8V minimum for a decent battery life.

Yes this is a tester ONLY, not a battery meter for use in equipment or for
spotting when a device should shut down.

Varies between brands, types (eg gell to VRLA), time on the shelf and "how
worn out"

I can deal with those variables, by taking a reading at the start of the
discharge.

and depth of discharge.

Damn. That shoots holes through that one then If it varies, I'd need
to retest it, or ignore it.

"john jardine" <john@jjdesigns.fsnet.co.uk> wrote :-

The terminal voltage between fully charged and flat is from about 13V to
12V.
www.yuasa-battery.co.uk/ind_vrlamanual.html

I'll check out the link. The low point is one of the contentious values
that varies. Some manufacturers quote "watts available to 10v" and
other voltages up to 11.5v as a minimum point. That's a big range.

jmeyer@nowhere.net wrote :-

The better fuel gages actually measure the current rather than applying
a fixed load.

What I'm doing isn't strictly a fuel guage (part of is, just as an estimator
BEFORE the test is done). The actual tester applies a load which ensures
a constant current (and measures this also to ensure it's really happening).

ehsjr <ehsjr@bellatlantic.net> wrote :-

That is what happens when a battery
discharges - its internal resistance goes up.

OK, I'm going to have to rethink how and when to measure the resistance
if it's expected to change.

Next: don't discharge the battery below 10.5 - in fact, don't
discharge it to 10.5. Each discharge takes a little life
from the battery that CANNOT be retrieved. The deeper you
discharge, the more life you take.

Understood: 300-400 cycles for 100% discharges, 3500+ for 20% discharges
are the kind of figures I've seen for e.g. Camden Europa. This is not
a process to be repeated often.

cnctutwiler@wmconnect.com wrote :-

Voltage is not linear but is close during the first 60-65% of
discharge--after that point good luck.

From recording the voltage over a discharge, I got a linear-ish section
for the first part of discharge, then a second linear section (shallower)
for the remainder. Then a strange plummet ... that looks like the battery
is done, and I'm stopping. This may not be true across all batteries
though

Maybe....the internal resistance of the cells is best measured after
discharging at a constant rate long enough to obtain a steady voltage
at the terminals,and then quickly interrupting the discharge and noting
the instantaneous rise of voltage. The instantaneous rise divided by
the value of the current at the instant of interruption gives the
internal resistance. Another way is to suddenly reduce or increase the
current by a definite amount, and divide the instantaneous change in
voltage by the change in current.

OK, that would be a possibility: Halve the current periodically during the
test and then check the voltage change.

what size wire are you using to attach your test equipment--one foot of
#8 @50 amps will drop another .02V.

The tester has 2 independent banks of transistors. Each bank is rated for
25A max. Each is cabled with 30A cable. So far I'm keeping to load currents
to < 20A (so that's 10A per bank). The wire is 50/0.25 copper (2.5sqmm).

mroberds@worldnet.att.net wrote :-

I'm pretty sure they mean on-load.

In which case I have seen some batteries which when loaded by the
target device is almost immediately flat, as the voltage falls below
10.5v. However, discharged at a lower rate, they are still useful batteries.

So are they flat or charged?

An example I'm thinking of is e.g. a 44AH mobility battery: It won't
drive the mobility vehicle up a hill without the speed controller
kicking in and objecting to the low voltage, even when newly charged.

However, they will power a 12v emergency fluorescent for HOURS. Internal
resistance of the cell is high ... but the capacity is OK. Which is
why measuring the capacity doesn't tell the full story, and that's why
I'm also checking the resistance.

This seems reasonable. Some SLA manufacturers do mention a range of
internal resistances for their batteries; you might look those up.

Done this already, which is why I know that feature is reading figures
in the right ballpark. But I wasn't expecting it to change with discharge
level. That's not mentioned in the datasheets

Thanks to all, I shall go and have a rethink and re-write some of the
code to try and get this a bit better. At least it seems I'm on the
right track.


--
--------------------------------------+------------------------------------
Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

 

[R Adsett]

In article <dacdt6$60v$1@posie.local.dom>, mjb@posie.local.dom says...

In which case I have seen some batteries which when loaded by the
target device is almost immediately flat, as the voltage falls below
10.5v. However, discharged at a lower rate, they are still useful batteries.

So are they flat or charged?

An example I'm thinking of is e.g. a 44AH mobility battery: It won't
drive the mobility vehicle up a hill without the speed controller
kicking in and objecting to the low voltage, even when newly charged.

However, they will power a 12v emergency fluorescent for HOURS. Internal
resistance of the cell is high ... but the capacity is OK. Which is
why measuring the capacity doesn't tell the full story, and that's why
I'm also checking the resistance.

If you are doing EV (or any other highly variable load) get yourself a
copy of the Curtis battery book.

http://www.evparts.com/shopping/product_details.php?id=&product_id=1885

Robert