Measuring cell voltage - under load?

[Russel]

Yep, it was my poor test jig that caused the excessive voltage drop! :sigh:

The CBA II battery tester has a couple short leads of #13 wire (as I understand it) to a powerpole connector. I simply took a foot of red wire and a foot of black wire and soldered the powerpole connector on one end of each, stripped the insulation off of about a half inch of the other end of each wire an tinned the ends. Then clamped the tinned wire ends to the Eneloop with a big wooden clamp. Not pretty, but it worked well. Here is the result of battery number five, which tested exactly like battery number one with the previous test jig, tested at 1000ma and 2000ma.

Looks like my batteries are OK after all. :twothumbs And I learned a few things!

OK, If I understand things correctly:

(1.347v [static after test] - 1.298v [1000ma discharge]) / 1A = .049 Ohms

or about 50 milliohms.

Is this correct?

 

[Mr Happy]

OK, If I understand things correctly:

(1.347v [static after test] - 1.298v [1000ma discharge]) / 1A = .049 Ohms

or about 50 milliohms.

Is this correct?

Aye. :)

 

[Russel]

First, let me apologize for hijacking the thread. The more experienced people here know this stuff, so forgive me for posting yet another graph of what some may see as redundant information. But, I learned from this, and maybe other will too.

Anyway, here is Eneloop number 5 load tested at 1000ma first with my lousy jury rigged setup with thin wire and too many connections, then with good connections to the battery and thicker wire.

 

[Hoggy]

Anyway, here is Eneloop number 5 load tested at 1000ma first with my lousy jury rigged setup with thin wire and too many connections, then with good connections to the battery and thicker wire.

WOW! It's amazing to see what a good connection can do!
Quite informative.

 

[TakeTheActive]

Ok, I'm thinking aloud here, perhaps you can tell me if this will work ... :thinking:

If you measure a cell's voltage with a DMM, you'll get a higher reading because it's not under load. Using a tester that puts a load on it, you get a more accurate reading.

If I want to determine a cell's true voltage, can I just slip it into a slot on the C9000, select discharge and a low rate such as 200 mAh, then wait for it to cycle to the voltage? Wouldn't this be the voltage under load, and mean that I wouldn't have to buy a tester to do the same function?

Wow Mr Happy that is great idea, thanks :):):)

Thanks for sharing your thoughts fireguy! :thumbsup:

And, look at all the neat graphs you inspired Russel to create. :twothumbs

I have to re-boot my P266 w/160MB RAM and Win98SE now because my Swapfile overflowed and my HDD is trashing. Good Stuff (this 'thinking out loud')! :D

 

[Russel]

I have to re-boot my P266 w/160MB RAM and Win98SE now because my Swapfile overflowed and my HDD is trashing. Good Stuff (this 'thinking out loud')! :D

My old Pentium 400 only has 64mb of RAM, but it works well running FreeNAS.

 

[sbdwag]

I think you would be better served to calculate internal resistance under load as a indication of health of a battery.

Internal resistance can be defined as the change in voltage divided by the load.

For example suppose you go to Radio Shack and buy a 10 ohm 10 watt resister and place the resister on the negative and positive terminal of the battery. Say your resting voltage is 4.15 and when you connect the resister your voltage drops to 3.75. Your delta V is .4

The current is 375 ma and Internal resistance is .040 ohms. This number .040 is the only true measure of a batteries health. As the battery gets older its internal resistance goes up. Indirectly its also a measure of how much current a battery can supply a load is this case this battery can supply 375 ma's to a 10 ohm load

I might add a resistance in this range would be quite bad my new aw 16340's internal resistance is about .014 and will deliver 400 ma's to the same 10 ohm load

regards
sbdwag

 

[kananga]

I've recently started doing this! Just for background, I'm running two XR-E's off of four AA's, total of about a 3.5 watt load for about two hours every day on the bike. That's my headlight, so I'd rather catch bad cells before they kill the group out on the road somewhere.
Sometimes one amp is the difference between a couple hundredths resting voltage difference throughout a battery group and a .3 volt sag on a bad cell! I've started logging my AA voltages under load once a month to prevent some lazy slacker cell from dragging a group into the grave with it. (yes, my AA cells have names :shakehead )

Last month, fully charged resting voltage: 1A through 1D all between 1.35 and 1.36.
Ten minutes in, plugged into a one amp load (aka the Amp Hog Dinnotte): 1A=1.024, 1B=1.132, 1C=1.235, 1D=1.087! The month before, 1A showed less than .1 volt deficit, while the other 3 were grouped within a .05 of a volt. This is an old group anyway, so I let it slide in the name of science just to see how abusive one slacker cell in a group can be to the rest. 15 cycles was enough to answer that question!

I've seen worse, but this is bad enough. This group's retired from pack service. B and C are destined for backup camera batteries, A and D get remote control duty. I think two years of every-other-day use as a pack is good enough!

 

[SilverFox]

Hello Russel,

You have learned an excellent lesson, but to continue on a little...

Keep in mind that you should measure the voltage drop in your CBA set up. I believe the new program allows you to offset that so you can get accurate voltages on the graphs. Otherwise, you will have to make adjustments manually. At low current rates these don't amount to much, but they can add up when you get above 5 amps on a single NiMh cell.

It is nice to see that your Eneloop cells have been saved from the crap pile by better connections. :)

Tom

 

[bob_ninja]

I tested a bunch of cells and found 2 Sanyos to be weak. After Refresh/Analyze one 2500 and one 2300 both were under 2Ah
So the tes works well. I'll try breakin on 2500 next.

However, there was another older Radio Shack cell, around 1.9Ah I think. Open voltage was 1.4V, then tried the discharge test (300 mA) and it promptly said Done still showing 1.4V?!?! That is odd???

So I placed into baby monitor and measured voltage, still 1.4V. Not sure what is the load in baby monitor; I supposed mid/low load.

If a cell is holding the voltage so well why would C9K think it is done???? Unless that short 1A pulse dropped the voltage a lot but then as soon as the load was stopped it bounced back so quickly that the display never displayed the real voltage under load.

 

[Mr Happy]

If a cell is holding the voltage so well why would C9K think it is done???? Unless that short 1A pulse dropped the voltage a lot but then as soon as the load was stopped it bounced back so quickly that the display never displayed the real voltage under load.

It is possible. I once had an AAA cell do that to me on the C9000, although I did momentarily see the voltage drop to 0.9 before DONE appeared. In the AAA cell case I tried again and was able to discharge it successfully at a lower rate.

 

[oldpal]

I think you would be better served to calculate internal resistance under load as a indication of health of a battery.

Internal resistance can be defined as the change in voltage divided by the load.

For example suppose you go to Radio Shack and buy a 10 ohm 10 watt resister and place the resister on the negative and positive terminal of the battery. Say your resting voltage is 4.15 and when you connect the resister your voltage drops to 3.75. Your delta V is .4

The current is 375 ma and Internal resistance is .040 ohms. This number .040 is the only true measure of a batteries health. As the battery gets older its internal resistance goes up. Indirectly its also a measure of how much current a battery can supply a load is this case this battery can supply 375 ma's to a 10 ohm load

I might add a resistance in this range would be quite bad my new aw 16340's internal resistance is about .014 and will deliver 400 ma's to the same 10 ohm load

regards
sbdwag

Better look at this again, sbdwag. Looks like about 1 ohm internal resistance to me.

Hugh

 

[Russel]

Hello Russel,

You have learned an excellent lesson, but to continue on a little...

Keep in mind that you should measure the voltage drop in your CBA set up. I believe the new program allows you to offset that so you can get accurate voltages on the graphs. Otherwise, you will have to make adjustments manually. At low current rates these don't amount to much, but they can add up when you get above 5 amps on a single NiMh cell.

It is nice to see that your Eneloop cells have been saved from the crap pile by better connections. :)

Tom

Hi Tom,

The eneloops were never in any real danger of being discarded. They have been sitting for several weeks since their last use and recharge, but during their last use they were used every day for a month with excellent performance. When I realized the excessive voltage drop, I was pretty sure the spagetti that I was using to connect everything was the cause.

I've just recently began to play with the CBA II pro software (had to purchase it before the price went up for current owner's on 2/15) and I haven't calibrated the voltage yet. Fortunately, when measuring voltage drop under load, exact voltage isn't critical as consistency. Last night I wanted to try the charge monitor feature so I connected the CBA in parallel with the BC-900 and a Eneloop AA. I had to run the BC-900 off of a battery with an inverter to isolate it from the computer with the CBA connected to it. I figure the voltage readings are about .9volts high, but it does show the Delta V at the end of the 1000ma charge cycle nicely.

Russ

 

[PeAK]

Better look at this again, sbdwag. Looks like about 1 ohm internal resistance to me.

Hugh

I agreee with Hugh:

delta-V=0.4V
delta-I=0.375A
Ro = 0.4/0.375=1.06 ohms

You can also check the 0.04 value by summing the voltages through the resistor under load to see if they sum up to the open circuit value:

Rl*0.375 * Ro*0.375 = 10*0.375 + 0.04*0.375
= 3.75+0.015
=3.765

If the value of 0.04 is replaced by 1.06, the numbers add up (to 4.15V).

PeAK

 

[PeAK]

.
.
.When I realized the excessive voltage drop, I was pretty sure the spagetti that I was using to connect everything was the cause.
.
.
... but it does show the Delta V at the end of the 1000ma charge cycle nicely.
.
.

I'm not familiar with the harnessing of wires in the CBA but if the voltage is sensed "near the battery", then you can keep your spaghetti wiring and get accurate measurements using something called the 4-point probe technique. This has also been referred to as a Kelvin Contact scheme. By placing the voltage probes near the battery, all the drops in the sphaghetti wires are ignored (either due to high current or high resistance). If long leads come from the voltmeter, the small currents in these wires result in little to no drop/error in the voltage measurement.

Last, in your charge graph, I can see the plateau, but I think the drop is due to the charger shutting down the current and the smooth portion of the curve showing the settling of the voltage to its resting value.

PeAK

 

[Mr Happy]

I asked about the four point connection at the foot of post #19, but no one picked up on it. From Silverfox's comments about calibrating for the resistance of the test leads, I am assuming that the CBA II does not provide this facility.

 

[Russel]

I asked about the four point connection at the foot of post #19, but no one picked up on it. From Silverfox's comments about calibrating for the resistance of the test leads, I am assuming that the CBA II does not provide this facility.

Actually, you can calibrate the CBA II to compensate for your test leads, that is what Tom was telling me earlier. My last chart showed my result without calibration.

Russ

 

[Mr Happy]

Actually, you can calibrate the CBA II to compensate for your test leads, that is what Tom was telling me earlier. My last chart showed my result without calibration.

That helps of course. But what I am talking about is to have four test leads. Two leads connect to the battery to source or sink the current, and the other two leads connect separately to the battery to measure the voltage. Since the voltage sense leads do not carry any current their resistance is unimportant and you always get the true voltage at the battery terminals.

 

[Anders]

Hello.

Maybe this also is Off Topic, Sorry
When we are talking about
Internal resistance, did you see this thread from Rcgroups:
Cell Internal Resistance Meter - more useful than 'C' ratings



Maybe IR measurement need an own thread?


Anders

 

[Russel]

I'm not familiar with the harnessing of wires in the CBA but if the voltage is sensed "near the battery", then you can keep your spaghetti wiring and get accurate measurements using something called the 4-point probe technique. This has also been referred to as a Kelvin Contact scheme. By placing the voltage probes near the battery, all the drops in the sphaghetti wires are ignored (either due to high current or high resistance). If long leads come from the voltmeter, the small currents in these wires result in little to no drop/error in the voltage measurement.

Last, in your charge graph, I can see the plateau, but I think the drop is due to the charger shutting down the current and the smooth portion of the curve showing the settling of the voltage to its resting value.

PeAK

I was just playing with my CBA II and the new "Pro" version of the software for it. A better way (than the CBA) to monitor and graph the voltage would be with my Fluke multimeter with optical computer link. That way, as you describe, I could measure the voltage at the battery without significant current flowing through the voltage test leads, and graph it with a computer with greater accuracy.

It's always fun to play with new toys!

Russ

Edit: Thanks for the "4-points probe" link. Lots of cool information there!