I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
sailboat. I wish to test their remaining Ah capacity by applying a fixed
current load and monitoring the terminal voltage over time.
Question: I believe the Interstate table of voltage vs state of charge
is open circuit voltage, if this is true then I would have to interrupt
the discharge periodically in order to measure the o/c voltage then
resume the load until I have achieved 1/2 remaining Ah.
Does anyone have a more direct test of remaining Ah capacity?
Dick

In article , Richard Lane wrote:
I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
sailboat. I wish to test their remaining Ah capacity by applying a fixed
current load and monitoring the terminal voltage over time.
Question: I believe the Interstate table of voltage vs state of charge
is open circuit voltage, if this is true then I would have to interrupt
the discharge periodically in order to measure the o/c voltage then
resume the load until I have achieved 1/2 remaining Ah.
Does anyone have a more direct test of remaining Ah capacity?
Dick

Just keep watch of voltage under load. Forget open cell.

greg

On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
wrote:
I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
sailboat. I wish to test their remaining Ah capacity by applying a fixed
current load and monitoring the terminal voltage over time.
Question: I believe the Interstate table of voltage vs state of charge
is open circuit voltage, if this is true then I would have to interrupt
the discharge periodically in order to measure the o/c voltage then
resume the load until I have achieved 1/2 remaining Ah.
Does anyone have a more direct test of remaining Ah capacity?
Dick

Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
e.g., a 100 AH battery should be measured with a 5 amp discharge
current, and your 170 AH bank should be measured at a rate of 8.5
amps.

First step is to build a load that will draw about 8.5 amps. Some
combination of 12 volt bulbs will work.

Next step is to fully charge the batteries and leave them with no load
attached for 24 hours. The open circuit voltage at the batteries
should measure about 12.6 volts.

Now attach your 8.5 amp load and measure the time it takes for the
voltage to decrease to 11.6 volts (approximately 50% discharged).

Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.

That said, if your batteries are 5 years old it is almost certainly
time to replace them as you will probably discover when you do the
capacity measurement.

"Wayne.B" wrote in message
...
| On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
| wrote:
| I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
| sailboat. I wish to test their remaining Ah capacity by applying a fixed
| current load and monitoring the terminal voltage over time.
| Question: I believe the Interstate table of voltage vs state of charge
| is open circuit voltage, if this is true then I would have to interrupt
| the discharge periodically in order to measure the o/c voltage then
| resume the load until I have achieved 1/2 remaining Ah.
| Does anyone have a more direct test of remaining Ah capacity?
| Dick
|
| Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
| e.g., a 100 AH battery should be measured with a 5 amp discharge
| current, and your 170 AH bank should be measured at a rate of 8.5
| amps.
|
| First step is to build a load that will draw about 8.5 amps. Some
| combination of 12 volt bulbs will work.
|
| Next step is to fully charge the batteries and leave them with no load
| attached for 24 hours. The open circuit voltage at the batteries
| should measure about 12.6 volts.
|
| Now attach your 8.5 amp load and measure the time it takes for the
| voltage to decrease to 11.6 volts (approximately 50% discharged).
|
| Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
| it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.
|
| That said, if your batteries are 5 years old it is almost certainly
| time to replace them as you will probably discover when you do the
| capacity measurement.
|

I presume you do not mean 11.6V at open circuit Voltage, because I'd
consider the battery completely empty at 11.6V (after stabelizing).
wbh

WBH wrote:
"Wayne.B" wrote in message
...
| On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
| wrote:
| I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
| sailboat. I wish to test their remaining Ah capacity by applying a fixed
| current load and monitoring the terminal voltage over time.
| Question: I believe the Interstate table of voltage vs state of charge
| is open circuit voltage, if this is true then I would have to interrupt
| the discharge periodically in order to measure the o/c voltage then
| resume the load until I have achieved 1/2 remaining Ah.
| Does anyone have a more direct test of remaining Ah capacity?
| Dick
|
| Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
| e.g., a 100 AH battery should be measured with a 5 amp discharge
| current, and your 170 AH bank should be measured at a rate of 8.5
| amps.
|
| First step is to build a load that will draw about 8.5 amps. Some
| combination of 12 volt bulbs will work.
|
| Next step is to fully charge the batteries and leave them with no load
| attached for 24 hours. The open circuit voltage at the batteries
| should measure about 12.6 volts.
|
| Now attach your 8.5 amp load and measure the time it takes for the
| voltage to decrease to 11.6 volts (approximately 50% discharged).
|
| Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
| it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.
|
|
|

I presume you do not mean 11.6V at open circuit Voltage, because I'd
consider the battery completely empty at 11.6V (after stabelizing).
wbh


No, I think Wayne B meant 11.6 v on load as he did not specifically
mention disconnecting the load prior to measuring the voltage. I have
asked him privately where he got the loaded voltage to state of charge data.
Dick

"Richard Lane" wrote in message
...
| WBH wrote:
| "Wayne.B" wrote in message
| ...
| | On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
| | wrote:
| | I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on
my
| | sailboat. I wish to test their remaining Ah capacity by applying a
fixed
| | current load and monitoring the terminal voltage over time.
| | Question: I believe the Interstate table of voltage vs state of
charge
| | is open circuit voltage, if this is true then I would have to
interrupt
| | the discharge periodically in order to measure the o/c voltage then
| | resume the load until I have achieved 1/2 remaining Ah.
| | Does anyone have a more direct test of remaining Ah capacity?
| | Dick
| |
| | Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
| | e.g., a 100 AH battery should be measured with a 5 amp discharge
| | current, and your 170 AH bank should be measured at a rate of 8.5
| | amps.
| |
| | First step is to build a load that will draw about 8.5 amps. Some
| | combination of 12 volt bulbs will work.
| |
| | Next step is to fully charge the batteries and leave them with no load
| | attached for 24 hours. The open circuit voltage at the batteries
| | should measure about 12.6 volts.
| |
| | Now attach your 8.5 amp load and measure the time it takes for the
| | voltage to decrease to 11.6 volts (approximately 50% discharged).
| |
| | Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
| | it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.
| |
| |
| |
|
| I presume you do not mean 11.6V at open circuit Voltage, because I'd
| consider the battery completely empty at 11.6V (after stabelizing).
| wbh
|
|
| No, I think Wayne B meant 11.6 v on load as he did not specifically
| mention disconnecting the load prior to measuring the voltage. I have
| asked him privately where he got the loaded voltage to state of charge
data.
| Dick

I'd be interested to know this too. I've never seen any data on this, but I
guess it should be fairly easy to find out the loaded voltage for various
loads at a given state of charge, by first determining the state of charge
from the open circuit voltage after stabilizing, then load with say 2 amps
and check voltage after (10 ?) minutes, increase load to 4, check voltage
after 10 mins. etc. It won't be accurate because the battery is being
further discharged in the process, but I suppose it would give a pretty good
idea.
Wout

WBH wrote:
"Richard Lane" wrote in message
...
| WBH wrote:
| "Wayne.B" wrote in message
| ...
| | On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
| | wrote:
| | I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on
my
| | sailboat. I wish to test their remaining Ah capacity by applying a
fixed
| | current load and monitoring the terminal voltage over time.
| | Question: I believe the Interstate table of voltage vs state of
charge
| | is open circuit voltage, if this is true then I would have to
interrupt
| | the discharge periodically in order to measure the o/c voltage then
| | resume the load until I have achieved 1/2 remaining Ah.
| | Does anyone have a more direct test of remaining Ah capacity?
| | Dick
| |
| | Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
| | e.g., a 100 AH battery should be measured with a 5 amp discharge
| | current, and your 170 AH bank should be measured at a rate of 8.5
| | amps.
| |
| | First step is to build a load that will draw about 8.5 amps. Some
| | combination of 12 volt bulbs will work.
| |
| | Next step is to fully charge the batteries and leave them with no load
| | attached for 24 hours. The open circuit voltage at the batteries
| | should measure about 12.6 volts.
| |
| | Now attach your 8.5 amp load and measure the time it takes for the
| | voltage to decrease to 11.6 volts (approximately 50% discharged).
| |
| | Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
| | it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.
| |
| |
| |
|
| I presume you do not mean 11.6V at open circuit Voltage, because I'd
| consider the battery completely empty at 11.6V (after stabelizing).
| wbh
|
|
| No, I think Wayne B meant 11.6 v on load as he did not specifically
| mention disconnecting the load prior to measuring the voltage. I have
| asked him privately where he got the loaded voltage to state of charge
data.
| Dick

I'd be interested to know this too. I've never seen any data on this, but I
guess it should be fairly easy to find out the loaded voltage for various
loads at a given state of charge, by first determining the state of charge
from the open circuit voltage after stabilizing, then load with say 2 amps
and check voltage after (10 ?) minutes, increase load to 4, check voltage
after 10 mins. etc. It won't be accurate because the battery is being
further discharged in the process, but I suppose it would give a pretty good
idea.
Wout




Two thoughts. First, the published OC
voltage is to be measured after the
surface charge has been depleted. Run a
10 amp load for 10 minutes or so and
then measure the OC voltage. Otherwise,
you will get an inflated measure of
state of charge.

There are generic curves available on
the internet showing the voltage under
various loads at various states of charge.

The typical battery load test consists
of a load at the CCA rating of the
battery for 15 seconds. Voltage under
that load at the end of 15 seconds is
measured and, depending on temperature,
will tell you whether the battery passes
the load test. Not sophisticated, but
used almost universally to determine
battery health. A 50 or 100 amp tester
is sometimes used instead. Harbor
Freight sells a functional 500 amp
tester for about $40 when it is on sale.

Not really an answer to your question,
but food for thought.

Chuck

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Chuck wrote:

There are generic curves available on the internet showing the voltage
under various loads at various states of charge.


Chuck, do you have a link to the generic curves? My boatyard offered to
loan me a CCA tester which they use to decide on battery health instead
of Ah/20 load test, perhaps when I return to my boat next April I should
accept their offer.
Thanks, Dick