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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hi guys.
So I've been foolin' around with my teensy 3.1 in the arduino IDE...
And came up with this program;
My only question is, did I create the proper equation for figuring out the capacity?
From my understanding
Capacity = time x load current, which is what the equation I'm using equates to. I'm not looking for 100% superb accuracy, just a rough estimate of it. If anyone could double check my work I'd be super appreciative.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Assuming the load resistor you are using is 4.7 ohms then your code is roughly correct, although will under estimate the capacity of the battery a fair bit.
The formula you give (capacity = time * current) is only accurate in the case where you discharge the battery with a constant current (as many other chargers do). It looks like you are using a resistor to discharge the battery, so the discharge current will inherently be changing as the battery voltage decreases (starting around 4.2v/4.7ohms = 0.9amps and stopping at 2.7v/4.7ohms = 0.5amps). You are currently using the current at the end of the battery discharge to calculate the capacity, so you will be underestimating the capacity by somewhere between a factor of 10-50% depending on the initial charge of the battery.
Ideally you would integrate the current, for example if you run the loop at 25hz (your code will run slightly slower than that) you could say capacity = capacity + (current * 0.000111hours). Of course you would want to initialize 'capacity' to zero before the main loop.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I changed the code a little, but it's more or less the same. I instead went with a basic constant current source (using an op amp, transistor and a sense resistor).
It draws 530 mA (roughly) from the battery all the way down to 2.7 volts, so the mAh curve should be much more accurate now.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Would this work for my pile of LiNiMn cells? Culled from a dead e-bike (fried controller) pack, most of the cells look OK but unfortunately I'd never find ones with the same remaining capacity or impedance to rebuild it into a safe pack. These are structurally similar to Li phosphate but a higher terminal voltage (3.6V) so a single schottky 120mV diode will allow them to be charged from a standard Li charger.
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Arduino Battery Capacity Tester
