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Old 12-04-2006, 05:23 PM   #1
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Default Battery Runtime / IR calculations

Hi there,

Being a newbie to racing, I'm buying alot of used batts from sponsored and/or unsponsored fellas at my club (RCOttawa ).

As such, some packs are good, some are soft. After seeing a battery discharge chart showing the discharge curve, I started thinking that I could generate these curves using datalogging equipment I use for my work.

At this time, I am able to monitor and record the voltage of each cell in a pack as well as the pack total 4 times a second. I do this using a 30A linear discharge (Indy 0-30) discharging each cell individually, and I can also do the exact same measurements discharging the pack at 30A using a Indy Reactor 30.

So, I got the curves (and found some bad cells), I have the data....what I am missing is a little more information to help me match my used cells to be able to use my RC dollars as wisely as possible.

I'm not sure how to calculate runtime for each cell. I can figure out the runtime to any voltage, but I don't know what the industry standard is for this. At what voltage is the battery considered dead...1.1V? 1.0V, 0.9V?

Internal resistance - I know the average volts over a certain period of time, I know the discharge amps, should I simply use V=Amps x Resistance?

I realise that there is a possibility that some professional matchers will consider my questions taboo. I simply happen to have equipment that gives me the data. I just want to mess around - this is what the hobby is all about right :P

Once my 4 1/2 year old and yet to be born child are off on their own, I'll stick to used batts and do what I can with them.

Any input/information is greatly appreciated.
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Old 12-04-2006, 06:36 PM   #2
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The industry standards for matching are as follows.

30 amp discharge
.9 volt cut off

Cells are matched by the time it takes to discharge down to .9 volts at 30 amps (Alot of companies are now doing 35 amp discharge rate).
Then they are matched by their average voltage during that time.

The internal resistance is measured using a different means. I believe it is a 1khz at some voltage, but I don't know the specifics of that.
It does not necessarily correlate to the performance of the cell, although lower is better. But if a cell had 0 IR, it would be no good at all.

You cannot calculate the IR using a 30 amp load and the output voltage. Using your formula, I believe you would basically have to dead short the cell through a meter that could handle maybe 100 or more amps. Then you could figure out what the IR is by how many amps flowed and at what voltage.

The best way for you to match up your cells, if you have the time, Is do do basically what you are allready doing. Charge and then discharge the cells 30 amps. Monitor each cell individually and record their voltages down to .9 volts, then STOP discharging. Match the cells by the time it took to get to .9 and then by their average voltages. Thats about as good as you can do without a turbo matcher.
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Old 12-04-2006, 06:51 PM   #3
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Thanks for the input PitCrew - it confirms my tought processes. Much appreciated.
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Old 12-04-2006, 07:55 PM   #4
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I'm taking an electronics class and we discussed (briefly) the internal resistance of batteries at the beginning of the term...when the class was still fun...LOL

Anyways, if I recall correctly, you can calculate it through the voltage drop when you put the load on the battery. I think he said that the voltage drop itself is because of the internal resistance.
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Old 12-05-2006, 08:30 AM   #5
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Holy crap, going to school for engineering is actually going to be useful for a change...

The formula for calculating internal resistance of a battery is;

Rb = (Vs - V / I) - Rl

Where .. Rb is the IR of the battery, Vs is batt voltage w/o load, V is voltage under load, I is the current supplied by the battery, and Rl is your total resistance in the circuit.

In theory if you have a known load source that is carbon pile or resistor based you can know what our circuit resistance is.

However unless you've got good testing equipment or are willing to build a decent load source your best efforts will probably be +/- 0.1-0.2, which almost defeats the purpose.

I would not worry about matching IR. The packs your getting have already had their "goodness" worn off already and are a little soft anyway, worrying about IR is a waste of time. Just do what PitCrew said, and what you are already doing ... that will get you close enough and you can have some decent club race packs that way..its how I got through my first year of racing myself before I realized I wanted/needed *really* good packs and started buying new.
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Old 12-05-2006, 10:09 AM   #6
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Quote:
Originally Posted by PitCrew
The industry standards for matching are as follows.

30 amp discharge
.9 volt cut off

Cells are matched by the time it takes to discharge down to .9 volts at 30 amps (Alot of companies are now doing 35 amp discharge rate).
Then they are matched by their average voltage during that time.
Are you sure about 30 amps? All the matchers I know are using 35 amps. When people compare battery numbers, we are all assuming 35 amps.
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