Cell Internal Resistance Meter - more useful than 'C' ratings - Now available.
This is a product development thread. The final device will typically go on commercial sale on my site ( http://nqrc.com
) but I like to show the community what's happening during the development process which is useful because invariably it results in a superior end-product.
Have you ever noticed that the 'C' ratings for lipo cells (and Ni*) are generally meaningless thumbsuck values in most cases?
Your 20C from one brand is probably a 15C or 12C in another brand, really there's just no standard and it's mostly luck that you get what you need.
So, being the "scratch the itch" sort of person, I've started working on my internal resistance meter - and yes, it will be commercially produced.
What is "internal resistance" you might ask, well, it's the resistance value of the actual battery cell, this is a very useful figure because from it you can work out things like your voltage-drop under load and how much power is being burned up inside the cell as heat, also it finally lets you -really- compare between packs, rather than just hoping the 'C' values are the same.
As an example, if you have a cell with a 50mR resistance inside, it's a 2S pack, so that's 100mR for the whole pack, we'll ignore the lead and connector resistance for now (though they can an DO cause problems at times - witness people using JST connectors at 10~15A... meltdown).
Okay, so we've got our 100mR pack, we want to draw 9A from it... what's the voltage real voltage (from 8.4V) and how hot will that pack get?
V = I * R
V = 9A * 0.1R
V = 0.9V drop at 9A, so that means your 8.4V pack only gives you 8.4 -0.9V, 7.5V at the motor/ESC
Now, how about how much power/heat is being lost in the pack itself...
P = I * I * R
P = 9A * 9A * 0.1R
P = 81 * 0.1R
P = 8.1W ... that's quite a considerable amount of heat for a small pack. Won't take long to push it over 45~55'C at that rate.
So, that maths is all very good - certainly shows you why knowing the internal resistance is a lot more useful than the 'C' rating doesn't it!
Here's a couple of photos of the first prototype I've built.
(please note, to budding electronics people out there - I know there's a lot of ways of testing cell resistances, including techniques such as impedence spectroscopy, however for now I'm keeping things simple with a static load for a fixed period ... at least in the first prototype
Features (thus far)
* Single cell test, all
cell types that are up to 5V (per cell).
* Externally powered by 7V+ (9V battery etc)
* Performs 3 different tests (1kHz, and two separate static loads)
* Displays the internal resistance computed from each test
simple to operate, just place the cell to be tested onto the 2-pin connector.