Testing internal resistance as the battery discharges

A place for general discussion of battery testing.
Martin
Posts: 3
Joined: Mon Mar 17, 2014 1:01 pm

Testing internal resistance as the battery discharges

Postby Martin » Mon Mar 17, 2014 1:11 pm

Hi, I god one of Paul's chargers and am actively using it to test the capacity and internal resistance of NiMh and Alkaline batteries.

It looks like the internal resistance of the battery increases at it discharges and wanted to changer the charger code (adapt the source) to get frequent internal resistance estimation as the battery discharges but had a couple of questions before changing the code:

How is the internal resistance computed?

Would this affect the battery capacity measurements if it was frequently tested during a discharge?

What would be the smartest: adding an internal resistance column for every entry in the log or a frequent call to the internal resistance estimation procedure?

Where it the best place in the code to do that?

Thanks !
Martin

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Testing internal resistance as the battery discharges

Postby Mark » Mon Mar 17, 2014 1:37 pm

The code already calculates the estimated resistance whenever the current is turned on or off (and the voltage/current is measured)

As long as you weren't doing the resistance check too often (say at least 5 seconds between checks) I wouldn't expect that it would affect the capacity results very much.

I'd probably do it by adding an extra column - unless you were only doing the checks infrequently - in that case, showing the results in the same manner as the IR check before charging might make more sense.

If you want to add an extra column, you should have a look at LogData() in CVM.cpp You'll also need to modify the code in ProgSupport.cpp to pause the discharge at regular intervals - I'd suggest having a look at the code that does the pauses when charging - you should be able to reuse some of that code.

If you have any difficulties with making these changes, just let me know!

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Testing internal resistance as the battery discharges

Postby Mark » Wed Mar 19, 2014 11:14 am

After putting a bit more thought into this, I've decided to add this into the next firmware version.

I'll include an extra column to show the resistance that is calculated as it's charging and discharging. With discharging, I'll add a brief pause probably every 5 seconds to measure the OC voltage and the IR will be automatically calculated from that. I'll also be showing the IR on the LCD screen for the next charger version. I'll make the pause as small as possible by only doing a 12 bit measurement - should be plenty of resolution for estimating the IR and should only effectively decrease the discharge rate by about 0.1% Hopefully it shouldn't cause the voltage readings during the actual discharge to fluctuate too much after doing the OC measurement.

Please note that the code won't automatically measure the IR when discharging into an external load - just in case you're using a wire wound resistor. A wire wound resistor will act as an inductor and possibly damage the controlling FET if it's being switched on and off all of the time. The LCD charger will probably have a diode added to prevent damage to the FET in this type of situation. If you're wanting to measure the IR when discharging into an external load, it will require only a minor code change - you'll just need to make sure that you're not using a wire wound resistor, or add an external protection diode across the resistor.

Martin
Posts: 3
Joined: Mon Mar 17, 2014 1:01 pm

Re: Testing internal resistance as the battery discharges

Postby Martin » Thu Mar 20, 2014 6:07 am

This is a great news (I won't have to port my changes to your new versions :-) Thanks !
I'm hoping to discover whether all batteries are made equal regarding internal resistance as they discharge...

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Testing internal resistance as the battery discharges

Postby Mark » Thu Mar 20, 2014 6:50 am

Martin wrote:This is a great news (I won't have to port my changes to your new versions :-) Thanks !


No problem - thanks for the suggestion! Are you interested in beta testing the changes for me?

I'm hoping to discover whether all batteries are made equal regarding internal resistance as they discharge...


That's a good question.

From past experience, it seems that certain kinds of damage at least can cause NiMH cells to develop very high IR at the end of their discharge curve - at least one cause seems to be over discharging them - i.e. holding their voltage very low for extended periods of time.

I'm really not sure what differences there would be between different cells when they're new though!

Martin
Posts: 3
Joined: Mon Mar 17, 2014 1:01 pm

Re: Testing internal resistance as the battery discharges

Postby Martin » Thu Mar 20, 2014 7:13 am

Mark wrote:Are you interested in beta testing the changes for me?


Sure ! I'd love that (I don't have any equipment capable to test the electrical properties logged by the charger however).

Mark wrote:That's a good question.

From past experience, it seems that certain kinds of damage at least can cause NiMH cells to develop very high IR at the end of their discharge curve - at least one cause seems to be over discharging them - i.e. holding their voltage very low for extended periods of time.

I'm really not sure what differences there would be between different cells when they're new though!


I'll make a test with usual brands found in stores here in Belgium (e.g. duracell) and some other usual ones (e.g. eneloop) and will keep you posted.

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Testing internal resistance as the battery discharges

Postby Mark » Thu Mar 20, 2014 7:16 am

Martin wrote:Sure ! I'd love that (I don't have any equipment capable to test the electrical properties logged by the charger however).


Sounds good. I'm more concerned with bugs in the firmware causing the charger to crash or do something it's not supposed to, so you don't need anything besides the charger itself. I'll let you know once it's ready for you to try!

I'll make a test with usual brands found in stores here in Belgium (e.g. duracell) and some other usual ones (e.g. eneloop) and will keep you posted.


Sounds good!

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Testing internal resistance as the battery discharges

Postby Mark » Tue Jun 03, 2014 2:22 am

Hi Martin,

The latest firmware version includes the ability to test the IR of the cell as it discharges - you need to use the configuration utility to set it to pause the discharge however many seconds you want.

The configuration tool can be found here:
viewtopic.php?f=3&t=68

The latest firmware can be found here:
viewtopic.php?f=3&t=64


Return to “Battery Testing Discussion”

Who is online

Users browsing this forum: No registered users and 1 guest