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Old Jan 08, 2009, 02:16 AM
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PLD
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PLD
Magicsmoke maker
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Australia, QLD, Charters Towers
Joined Dec 2003
17,428 Posts
Hi there everyone,

icrashrc, sure hope I am a millionaire soon, been trying for a long time... so long as it's not Zimbabwean dollars.

kgfly, nice to see someone from Australia (I'm a bit further north ). The fields are just the raw ADC (analogue-to-digital-converter) values (10-bit, 5V reference, so something like '809' means V = 809/1024 * 5 ~= 3.95V ). I've since updated the software to convert them back to proper voltage values. I was somewhat "excited" to get things out to the public to see so I just took the very first LCD outputs up.

The algorithm isn't anything profoundly exciting, I simply first test the open-circuit voltage and store, then I connect the load and check the voltage again to get the loaded voltage. There's also a sample taken on the other side of the load to negate any effects of the switching MOSFET involved.

Essentially you end up with the following data (examples);

Voc = 4.2V
Vload = 4.1V
Vfet = 0.01V

Since the resistor value is fairly constant and known, you can then derive the internal resistance. The very nice thing is that you don't even have to convert the ADC values to get the result, which saves a lot of rounding/conversion errors creeping in.

Right now it's only showing a single cell resistance as picked off from the balance plug.

Moving to multiple cells brings in a lot more challenges and that's something I need to tackle in a benchtop/precision unit, the cost of such a version would be quite a lot more.

Right now I consider being able to pick off per-cell resistances from the balance plug as being the most 'useful' starting point, since it won't be hard for people to add these up for the 'pack' resistance, it also lets you pick out dud cells. You can furthermore compute the wiring loom resistance with a trick (measure between cell-1's +ve terminal and the main battery -ve lead (eg, deans). If you already know the cell resistance is 40mR and then the battery->lead reads 45mR, then you know your lead/plug has ~5mR resistance.

The resolution/precision of this device is +/-5mR at the moment (that's my personal aim) without calibration, the biggest limits are the load resistor, of which I've got special 1% tollerance ones arriving tomorrow, the resistance of the terminals used to probe the balance plug and the built in error factor of the ADC conversions.

Paul.
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