At what point do you guys replace your batterys for racing?

When they puff, or short. But that's because i'm slow, and my batteries don't make a big difference to me.

A guy I talk to at the track quite a bit, uses Ir as his determining factor. That's probably a good place to make your judgement.

once a year usually. Only if the IR is higher.

What is the threshold for IR per cell (for a 2S battery) which will make it worth to be replaced?

Good question ... I think 4mOhm per element is a no-go for me...

how di you measure that? Idst charger? Before charging or after charging?

IR numbers vary by charger though. (My icharger reads lower IR numbers than my buddies) As long as you note the IR of your pack when it's new and keep an eye on it, that's what really matters. You can't go off someone else's numbers.

iCharger 308, before the charge

That, is a problem. Ir is a determinable, value. It shouldn't vary between devices. That shows a problem with the devices doing the testing... And or testing methodology.

Have found that different wires to battery cause different readings.

They do. A proper measuring setup would be done with 4wire contacts, carrying the current on one set of wires, and then getting the voltage from the other. I suppose 6, depending on how much you're testing at once.

When measuring miliohms... that stuff matters. https://en.wikipedia.org/wiki/Four-terminal_sensing

I've seen 7miliohms due to a bad ballance lead. :-/

When they take forever to charge on the charger...like stuck at 0.5amps and it never finishes.

How many batteries do you have for each class you run in?