Cell Internal Resistance Meter

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I guess we're gonna like it -- not a lot -- but we're gonna like it! :D

(And maybe Mr Daniels gets tired of the jokes...)
 
Ah Mr Happy, would you believe a lot of people don't get that anymore :( ( For those who think we're going insane, he's a magician and that's one of his favourite lead up lines to his tricks).

Of course, Rowan Aitkinson also has proclaimed "Paul Daniels" as being of God status too ;)

Now, on to the IRM, worked on the casing a bit more today, more parts arriving tomorrow, should have attempt #2 on the front panel tomorrow but here's a link to today's (somewhat dismal) attempt;

http://www.pldaniels.com/electronics/albums/irms-20090401/mpicDSCF0014.JPG

Tomorrow the mini-XLR plug/sockets should arrive along with everything else I need to theoretically finish up this prototype. . . . then it's where the electronics fun starts again as I try to also come up with an ideal 4-wire kelvin test rig.

Paul.
 
Connector and front panel

Okay, here's the next stage in the quick-hack job. I've managed to fit a faux front face and such, as well as powered up the unit, so here's a quick photo of how things might look on the final version;

spicDSCF0027.JPG
 
Working on a new circuit board today, alas these prototypes are very time consuming (typical 3x1.5" board can take upwards of 5~8 hours to go from the printout to the finished assembly... assuming I don't make some silly mistakes). Actual board -design- in the CAD takes days if not weeks depending on various things, often you can't find a way to make things 'fit right' (which is what has been the problem here with the PIRCAT) and then it hits you with a flash of inspiration one evening :)

Paul.
 
pldaniels said:
Well, here's the 3rd prototype...

Here's a shot showing the mini-XLR connector. I have to say, it's a real pain cramming those wires through it, I'll probably change the two voltage-sense wires to 24 or 26 AWG rather than 22AWG that they are now.

Sometimes when I ran into a problem like that I've found a piece of standard cable, like 4-wire telephone, that made it easier.
Click to expand...
 
Just a couple of interesting results with some batteries that I have lying around;

AAA Generic brand 'A' NiMH: 203mR (73% eff)
AA Generic brand 'A' NiMH: 310mR (64% eff) (older, a lot more cycles than the AAA)
AA SONY Enercycle: 84mR (87% eff)

AA Supermarket brand 'C' alkaline cell: 170mR (77% eff)
AA Supermarket brand 'D' alkaline cell: 432mR (56% eff)
AA Supermarket brand 'E' alkaline cell: 645mR (26% eff)

AA OLD standard Energizer: 175mR (76% eff)
AA NEW standard Duracell: 150mR (79% eff)

Big difference with the Sony vs 'generic' NiMH.

I'll try grab some more Ni* cells tomorrow.
 
What these things are going to cost...

The sale price of these units is going to initially be $139 USD ($195 AUD).

I was at one point thinking of putting a AA cell holder on the face and removing the 4-wire socket to save on the costs but it's a bad idea ultimately. Over time the AA holder will start to develop a non-negligable resistance and the only way to fix that would be to replace the holder and solder in a new one from the inside.

Keeping with a 4-wire interface plug/socket/lead system, while adding to the price does mean that the unit remains practical irrespective of the cell types used.

This unit hasn't been made with corner-cutting in mind, it's been made with the idea of picking the approriate parts for the task and is being hand-assembled and calibrated to millivolt precisions.

I should point out that the front-panel will actually look a little different to the one shown now, as they're being professionally manufacturered, when the final artwork is signed off I'll post up a picture for all to see.



Paul.
 
Interesting numbers Paul.

I recently checked a Sanyo Eneloop( date code 0707 but new purchase and cycled 3 times) on Hyp. 720i during a 2A charge and it ran 82-90 mOhm with cell at 80F.
While cycling a couple of 6cell GP 3300 NiMh batteries(5 years old) charged at 3amps. I noted 63 mOhm for one and 67 mOhm for the other. This was after 3 cyles 3amp. charge and 3 amp. discharge.

Note 720i only give pack IR. Eneloop was a single GP3300 6cell packs.

Charles
 
Turbo Guy said:
Interesting numbers Paul.

I recently checked a Sanyo Eneloop( date code 0707 but new purchase and cycled 3 times) on Hyp. 720i during a 2A charge and it ran 82-90 mOhm with cell at 80F.
Click to expand...

Nice to see that figure match up. I'll run with the fact that the Sony CycleEnergy's are roughly the same as the Enerloops. What I'm very curious to see is how some Sub-C's start stacking up.

While cycling a couple of 6cell GP 3300 NiMh batteries(5 years old) charged at 3amps. I noted 63 mOhm for one and 67 mOhm for the other. This was after 3 cyles 3amp. charge and 3 amp. discharge.
Click to expand...

Assuming all things being equal, that puts the iR at about 10.5~11mR for each GP cell, seems quite about the right ballpark.


Paul.
 
Hello Paul,

My GP 3700-SP cells come in at about 0.006 ohms, Saft 3200's come in at 0.007 ohms, as do my IB 4200's. Eneloop cells come in at about 0.025 ohms.

I am using a Kilter KT-97.

Tom
 
I believe the internal impedance measured with AC tends to come out much lower on NiMH cells than the internal resistance measured with DC. The AC value is often less than half the DC value; however, when working with DC loads the DC value is of more significance.

With AA eneloops I have not seen the DC internal resistance come out lower than 40 milliohms, with 50 milliohms or greater being more typical.
 
I just bought a fresh pack of AA Enerloops and they're posting 80~88mR with the following figures;

1.30V open,
1.12V loaded,
85mR 1kHz
86mR static (87% efficiency)
2.07A load current


Paul.
 
I'll try some alternative averaging algorithms to see if anything interesting comes up with the figures (though I do already supersample).

Additionally I'll put the new multimeter on it and track the min/max voltages and currents to see how they compare.

Paul.
 
pldaniels said:
I just bought a fresh pack of AA Enerloops and they're posting 80~88mR with the following figures;

1.30V open,
1.12V loaded,
85mR 1kHz
86mR static (87% efficiency)
2.07A load current
Click to expand...
Interesting. Internal resistance also increases with storage, so those eneloops might show lower values after a few discharge/charge cycles.
 
Here's the amusing thing about creating new designs... you find that you have to start making -other- new designs to help you calibrate/validate the first design.

In this case, it'd be nice to make a (near) perfect source cell and then plug in various internal resistances and see how the meter responds.

I suppose the simplest option is just to do something like get 10 high capacity D cells in parallel or better yet, something like a 200AH 2V lead acid cell (I can pick them up for about $250 AUD), I'd dare say the internal resistance on that thing would be quite low (better fuse it, just in case ;) )

Paul.
 
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