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nuckolls.bob(at)aeroelect
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 Posted: Sat Jun 27, 2020 5:53 pm Post subject: Measuring battery internal resistance |
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At 07:27 PM 6/26/2020, you wrote:
| Quote: | Hi Bob,
Thanks for the plot information...Maintainers and their profiles.
Your details brought up a question.
Would you have a suggestion for a simple setup for measuring internal cell/battery resistance, IR, of lithium based cells.
Modelers use a lot of Lipo based batteries. And there is a strong need to keep track of their condition and it seems like tracking IR could be the best option. A practical way to run the tests is sorely needed.
Your suggestions are always appreciated. |
Hmmmm . . . internal resistance can be
measured 'instantly' as opposed to doing
a total-discharge cap-check. The results
of a 'severe load' test considers
a combination of chemical potential
(capacity) -AND- internal resistance
(ohmic losses). An internal resistance
test can be conducted at lower energy
levels than the load-test. Of course,
all three tests require some instrumentation
and process.
The hammer-n-tongs way to measure cell
impedance is to load with some handy value
resistor, measure the voltage then increase
the load by say, double or tripling it
and read the voltage again.
Example: Suppose your battery under test
puts out 12.35 volts with a 10 ohm load.
Your base current is 12.35/10 = 1.235 amps
Temporarily add a second load resistor . . .
any practical value, let's say another
10 ohms. Let's assume the new reading is
12.220 volts. Okay, total R across the
battery is now 5 ohms. 12.22/5 = 2.444
amps.
So, for a delta current of 2.444A
we read a delta-volts of 0.015V
.015/2.444 = 6 milliohms
One could craft a test box containing
the necessary resistors, push-button
and meter connections to simplify the
setup but you'd still have to get out
the calculator and do the math.
I designed a direct reading battery
resistance meter about 20 years ago.
It featured a constant current load
system that was switched with a
multi-vibrator at about 10 cycles
per second.
It toggled between 1 and 11 amps.
A pk-to-pk reading voltmeter would show
the ripple voltage created at the battery
terminals as the electronic load oscillated
between the two current values.
Internal resistance was nearly direct
reading where 10 mv pk-pk equated to
1 milliohm of resistance. I've probably
got those drawings around here somewhere.
In my recent studies of on the garden
tractor battery . . .
http://www.aeroelectric.com/Pictures/Battery_Tender_Jr_Performance/03_BTJr+CCCY_Energy.jpg
I was able to command the CBA IV battery
analyzer to deviate between two accurately
known load values while accurately reporting
the battery voltage. That delta-E/Delta-A
deduced a rather high internal resistance of
about 60 milliohms.
So there's three ways to go about it ranging
from the hammer-n-tongs to poke-n-read.
Bob . . .
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skywagon185(at)gmail.com
Guest
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| Posted: Sun Jun 28, 2020 1:02 pm Post subject: Measuring battery internal resistance |
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Thanks Bob and Dick for the details of measuring IR...
I will pursue a simple test setup....
Dave
On Sat, Jun 27, 2020 at 6:50 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 07:27 PM 6/26/2020, you wrote:
| Quote: | Hi Bob,
Thanks for the plot information...Maintainers and their profiles.
Your details brought up a question.
Would you have a suggestion for a simple setup for measuring internal cell/battery resistance, IR, of lithium based cells.
Modelers use a lot of Lipo based batteries. And there is a strong need to keep track of their condition and it seems like tracking IR could be the best option. A practical way to run the tests is sorely needed.
Your suggestions are always appreciated. |
 Hmmmm . . . internal resistance can be
 measured 'instantly' as opposed to doing
 a total-discharge cap-check. The results
 of a 'severe load' test considers
 a combination of chemical potential
 (capacity) -AND- internal resistance
 (ohmic losses). An internal resistance
 test can be conducted at lower energy
 levels than the load-test. Of course,
 all three tests require some instrumentation
 and process.
 The hammer-n-tongs way to measure cell
 impedance is to load with some handy value
 resistor, measure the voltage then increase
 the load by say, double or tripling it
 and read the voltage again.
 Example: Suppose your battery under test
 puts out 12.35 volts with a 10 ohm load.
 Your base current is 12.35/10 = 1.235 amps
 Temporarily add a second load resistor . . .
 any practical value, let's say another
 10 ohms. Let's assume the new reading is
 12.220 volts. Okay, total R across the
 battery is now 5 ohms. 12.22/5 = 2.444
 amps.
Â
 So, for a delta current of 2.444A
 we read a delta-volts of 0.015V
 .015/2.444 = 6 milliohms
 One could craft a test box containing
 the necessary resistors, push-button
 and meter connections to simplify the
 setup but you'd still have to get out
 the calculator and do the math.
 I designed a direct reading battery
 resistance meter about 20 years ago.
 It featured a constant current load
 system that was switched with a
 multi-vibrator at about 10 cycles
 per second.
 It toggled between 1 and 11 amps.
 A pk-to-pk reading voltmeter would show
 the ripple voltage created at the battery
 terminals as the electronic load oscillated
 between the two current values.
 Internal resistance was nearly direct
 reading where 10 mv pk-pk equated to
 1 milliohm of resistance. I've probably
 got those drawings around here somewhere.
 In my recent studies of on the garden
 tractor battery . . .
http://www.aeroelectric.com/Pictures/Battery_Tender_Jr_Performance/03_BTJr+CCCY_Energy.jpg
 I was able to command the CBA IV battery
 analyzer to deviate between two accurately
 known load values while accurately reporting
 the battery voltage. That delta-E/Delta-A
 deduced a rather high internal resistance of
 about 60 milliohms.
 So there's three ways to go about it ranging
 from the hammer-n-tongs to poke-n-read.
 Â
 Bob . . .
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skywagon185(at)gmail.com
Guest
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| Posted: Sun Jun 28, 2020 4:19 pm Post subject: Measuring battery internal resistance |
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Bob,
Majority are 3 to 6 cell packs.
The highest would be a 6 cell pack. Thus, 6 x 4.2 or about 25 vdc.
The lowest would be a 3 cell pack. About 12.5 vdc.
Dave
On Sun, Jun 28, 2020 at 3:11 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 03:58 PM 6/28/2020, you wrote:
| Quote: | Thanks Bob and Dick for the details of measuring IR....
I will pursue a simple test setup....
Dave |
 What are the highest and lowest voltage battery
 packs you want to test?
 Bob . . .
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