Any idea what is good value for IR?
It varies.
For Lipos anything under 5 miliohms per cell.
Li-ions that I have tested where much higher.
You would need to test all your cells to get an average.
As namasaki said, you should test all of your cells and look for outliers. If anything is abnormally high comparatively then you have a bad cell.
Thinking about stripping down my pack and rebuilding when I get a run of wet days and time. Idea will be to better match the batteries into packs and see if there are any with high IR so that I can swap them out with a couple of spares I have. I have an iMax B6 so will use this for the discharge/charge and IR. Iāll document how I do this on my build thread when I get round to it.
Do not get a knock off b6. They are known to overcharge batteries and will damage them in the long run. If youāre going to buy a b6 make sure its the real thing from skyrc (itās usually about $40-50)
Thatās only if youāre building a pack from scratch with new batteries. In this case he already has the batteries wired in.
You cannot measure battery IR with a multimeter. How an rc charger does it is by rapidly increasing the current draw from 0A to 5A and measuring the voltage drop. R = V/I. They do this at several currents and get the average.
Another reason not to get a fake charger is sinse their voltage sensors are crap, you never know if the IR measurements are accurate.
Mixing new and old cells is bad. I would never recommend doing this.
@Tuomalar. It looks like you had some shitty welds which is probably the most common cause of drifting cells. I recommend you get the charged cells back in line with the others and then re welding the pack.
I only recommend you ride with a BMS as if this happens and you ride your battery until itās fully depleted then the three cells that have to compensate for the missing cell will be at a much lower voltage and probably damaged. @Eboosted had several cells go all the way down to 0V a while ago because of the exact same issue and not using a BMS.
Tried to quote but Iām on mobile. If he has the skyrc 12V switching power supply he can buy the imax for $20. Thatās what I was getting at. Also he had taken the batteries out of the pack, so I was saying he might as well match them.
I never rode battery pack below 36.5V and so far charged batteries only 6 times. All cells seems to be okay. New welding paid 10e with 10mm/0.2mm nickel strips from local battery store.
Problem with BMS is that massive size. I could use it as a deck 
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Quick question. Iāve just taken apart my 10s4p as I suspect one of the parallel packs has died. I havenāt had chance to take them all apart but before I do, what would you suggest for next steps? I have 2 new batteries - Iām thinking of building a test rig and marking them all up by capacity and IR then rebuild the battery. If 4 cells are a complete loss. Would you go 9s4p, 10s3p (current thought) or order another few batteries and rebuild as a 10s4p.
How would they have died?
I think 2 of the solder joints detached when I went over some rough ground and so the V on the other cells over discharged. Hot to touch when I got back 
Were you using a bms?
No. Probably a lesson learnt there too.