I was trying to figure out what my new charger is doing in terms of IR calculation, and wondering what chargers in general are doing with IR. The concensus seems to be that their numbers mean little or nothing, and they all give different numbers. But what is the number? It must come from somewhere...

I was just noticing that it doesn't seem likely that the charger is intermittently discharging a pack during the charge cycle to measure IR...which it would have to do if it is giving a discharge IR...which is what everybody wants.

But what the charger is constantly doing is monitoring it's output voltage (into the cell), individual cell voltage, and current (into the cell). That's all the info needed to figure resistance to taking a charge?


Any thoughts about the relationship of these different IRs? Is the resistance to taking charge the same as resistance to discharging?

I'm sorry if this opens up a whole can of worms...I know people debate about charge rate and how that affects battery life...so the idea that charging at 1C is critical implies that charging IR is much higher than discharge IR?

Later today I'll resurrect my IR testing rig. I just got a new charger with IR read-out, and I'd like to find out if I can replicate the results...maybe better understand what it is telling me. I figure the number means something, but it doesn't seem to mean what everybody expects it to means...since testing results between different devices lack consistency.

Only discharge matters

I'm not asking which one matters...but thanks for your opinion.



I am asking about the relationship between the charge vs discharge IR measurement.

Is IR the same for charge vs. discharge? Or is it different?

conventional wisdom says to charge slow, which implies a high charging IR compared to the discharge IR.

Chargers are estimating IR by measuring voltage difference between what they are outputting through the main leads and reading through the balance port and taking into consideration the charge/discharge current.

The main reason you get a huge variation in IR on the same device (or even worse comparing the same battery on different chargers) is because the IR of our LiPo's is very small (1-5 mOhm range) and even small changes in battery temperature, charge level, calibration of voltage measuring, resistance of the connectors (worn out or corded connectors) can easily cause few mOhms of additional resistance. To be more exact all other external factor can contribute more to total resistance than the actual IR of the battery!

Industry grade low resistance meters cost >1000 USD and have a strict procedure how to do the measuring process (an example of such device of resistance measuring of grounding for your home).


The procedure for low resistance measuring is as follows:
A+ ---wire--- B+ - battery - B- ---wire--- A-

A+- are the main battery lead and B+- are balance leads.
1. You measure battery voltage with no load (V_noload) and also compare if the voltage is the same on A+- and B+- (to rule out interference or bad connections).
2. You push/pull some known amount of current I_load (lets say 1A) between A+ and A- terminals (your main leads) and measure the voltage between A+- and B+-. They should be very close and the difference is actually shows losses because of wires/connectors/IR of your measuring equipment. Voltage V_load is measured between B+-.
3. From V_noload, V_load and I_load you can calculate the internal resistance of the battery:

R_battery = (V_noload - V_load) / I_load

This is a very simplified explanation of LiPo battery IR. In practice LiPo IR is not constant but:
1. increases with increasing current
2. decreases with rising temps (up to a certain limit, min IR for LiPo's is when the cell (not pack!) is at about 50 degrees Celsius, so the pack warmup used is stock racing makes sense (but it's just a small difference))
3. decreases with rising voltage of the cell (IR at 4.2V is lower than at 3.2V)