I'd like to design a battery testing rig to do the following:
Calculate the actual capacity of a battery (wH and/or mAh)
Calculate the actual C rating of a battery(current the battery is capable of)
Measure the internal resistance of the battery (it'd be nice to plot it over time during the discharge of battery)
Measure the voltage of the battery and it's individual cells during discharge
I'm sure this is to much to ask for the entire solution so I wont, but if I can get a few pointers on where to direct my research and maybe some of the math needed for the calculations that would be very helpful.
I think I want to use a RC helicopter motor and esc to use as my load. This way the test will take less than 10 minutes and the sound of the spinning motor will be cool.
Calculate the actual C rating of a battery(current the battery is capable of)
This requires a short circut across the battery. This is NOT recommend! Look at the battery factory specifications for the maximum RATED current. This will be considerably less than the max current it is capable of.
Trying to get the max capable will also likely to cause excess heat and smoke may escape.
Internal resistance is usually measured during the charge cycle
Get yourself a Revolectrix Powerlab PL8 charger. It'll do virtually all you want including a calculation of C
The C rating is the AH capacity of a battery, not the maximum current it's capable of delivering.
Once you have the C value and the internal resistance values you can estimate its current discharge capability.
jackrae:
The C rating is the AH capacity of a battery, not the maximum current it's capable of delivering.
Not with the sort of LiPo packs he wants to test.
A C rating a multiple of how much current the battery can deliver compared to delivering the battery's total capacity in one hour. The maximum current possible is divided by this one hour discharge current to determine the C rating.
A 5Ah battery with a C rating of 20 can deliver 100A of current. A 5Ah battery with a C rating of 1 can deliver 5A.
There isn't a safe way to determine a battery's C rating.
I agree with the suggestion to purchase a charger with these sorts of features built in.
Definitely don't try to test C, and furthermore don't take a battery to the limit - although
it will nominally be able to supply the current when freshly charged and new, it won't do
the battery life any good, and you are at a much higher risk of fire.
Think more about measuring the voltage droop at a percentage of the C rating, such as 25%,
to get a measure of the cell's internal resistance.
So for instance the 5Ah with a C of 20 could be given a 25A load and the voltage
difference measured as load is applied and released - if it was 0.2V difference, then you could
assume an internal resistance of 8 milliohms (= 0.2 / 25), which would mean 5W dissipated internally
at 25A and 80W internally at the full C rating (P = I^2 R). (No idea if these actual figures are realistic).
Then you have characterised the battery/cell without too much stress.
Perhaps the non-obvious thing is that C-ratings are very arbitratry - it depends on how much
damage you are willing to tolerate. Say the battery can do 200 recharge cycles at C=1 before its
capacity halves, it might do 100 cycles at C = 2, and 10 at C = 10, or 2 at C=20 - where do you
draw the line between peak power and battery life? The manufacturers tend to quote recharge
cycles at low currents only and C as large as they feel they can get away with, because that's what
sells batteries to naive purchasers.
Obviously the "C" word in RC speak doesn't mean the same as it does in 'conventional' battery speak.
A lead acid battery (conventional) is usually specified at either C5, C10 or C20 rating which means its capacity in AH if discharged over a period of 5, 10 or 20 hours respectively. The C value has nothing to do with how many amps you can get out of it if you try and "abuse" it.
So in the OP's case the C value (discharge current) will presumably be determined by the battery capacity in AH divided by the battery's total internal resistance plus the external load resistance.
Obviously the "C" word in RC speak doesn't mean the same as it does in 'conventional' battery speak.
No, its the same use.
A lead acid battery (conventional) is usually specified at either C5, C10 or C20 rating which means its capacity in AH if discharged over a period of 5, 10 or 20 hours respectively. The C value has nothing to do with how many amps you can get out of it if you try and "abuse" it.
Beg to differ but C in RC speak appears to be the product of AH x C eg 10AH x C8 = 80amps discharge capability
Whereas C in lead acid speak (and I agree it should be C/x) is AH rating at the number of hours in the denominator so 10AH at C/8 will deliver 1.25amps for 8 hours
Wow that is a lot of good stuff. I do have a Powerlab 8 Charger and it is a wonderful charger. My motivation is I own a RC supply company and wanted to have a demo rig that would demonstrate the batteries I supply and individuals could test their batteries and see how they compare.
So of my desired features what of them would be feasible? Maybe do an IR test at the before the battery is discharged then start the test? I know I could put them on my charger and measure it but I want to showmanship affect of a all in one testing unit.
Is there a way to measure the effective mAh of a battery while monitoring the battery's total voltage and individual cell voltage?
I was getting ready to suggest various ways of getting data out of a meter which didn't provide serial output but then I decided to Google "Powerlab 8 Charger".
If so, you're in luck. The charger will output serial data.
Serial data output: 19.2kbps, 8 bits, 1 start bit, 1 stop bit, no parity
This makes getting data from the device easy.
kodapa:
Is there a way to measure the effective mAh of a battery while monitoring the battery's total voltage and individual cell voltage?
I'd think the answer to the above is yes.
You just need to decide on what sort of display you want to use and how you want the data displayed. An appropriate microcontroller could then read the serial data and display the data. You could even produce graphs of the charge/discharge cycles. "It's just a small matter of programming."
No need to programme. The serial device supplied by Powerlab (called FUIM3) comes with all the software required. It truly is plug and play and the graphs on-screen are good. So far I haven't managed to get good print-outs but by using an application called 'skitch' the screen displays can be cut and pasted to produce annotated printouts.
