lithium ion battery internal resistance+ how to measure? - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 02:04:35 pm
content: ive a failing ebike battery,i suspect dead cells,all measure roughly the same voltage per cell,can i use a capacitor esr meter to check the internal resistance of the cells?,my esr meter is from the 5 transistor design from j diddy on here.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 02:38:04 pm
content: First: I'm no expert with Li-Ion. Never used a big battery pack.Normally ESR meters are used with discharged capacitors, and adding the DC voltage of the battery itself may or may not work, depending on how the ESR meter is build internally.You could try to put a bunch of low ESR capacitors parallel, and then put that in series with your ESR meter. Some of the designs already have this.You can also get an estimation of the ESR of individual cells, by drawing a low steady current out of them (for example 1A) and then use a 555 with a MOSfet and a power resistor to draw short high current (10A or so) pulses out of the whole battery pack. Then you can measure the AC voltage over each cell individually.What do you mean with "roughly the same"As far as I know, 200mV difference is a big deal in Li-Ion battery packs.You can also discharge the pack, and then when the bad cells start to fail, the voltage of those cells collapses.Bug big battery packs with li-Ion are very common, there must be lots of specific information about for them.

author: tkamiya
author_rank: Super Contributor
timestamp: October 05, 2020, 02:43:51 pm
content: How about something like this? I have no experience with Lithium Iron battery but this is pretty much a standard way to measure internal resistance.Measure the no load open voltageAttach a load, as big current as you can.Measure the voltageMeasure the currentHave the exact measurement of the load in term of ohmPut that into ohm's law and this is not going to match. The difference is the internal resistance. You can calculate backwards, what the missing resistance needs to be to make this equation work.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 02:56:06 pm
content: I prefer to measure DC ESR because that relates to the actual cell capability of delivering current, can be used to calculate losses and temperature rise. In my experience, < 0.2Hz is DC enough. One method is to do two full discharge cycles with different currents; DC ESR at each point is dV/dI. Another method that can be used with a single (or even partial) discharge cycle, is alternating the discharge current. I prefer three-step method, so that,During t1=10s, I1 = 0.9*IavgDuring t2=10s, I2 = 1.2*IavgDuring t3=10s, I3 = 0.9*Iavg,which keeps the average current constant,Measure V1 right at the end of t1 (just before adjusting current to I2), and similarly for V2 and V3.DC ESR = dV / dI, where dV is avg(V1,V3) - V2, and dI = I3-I2.

author: ucanel
author_rank: Regular Contributor
timestamp: October 05, 2020, 03:11:26 pm
content: Ric means R internal,deney A, deney B means,experiment A, experiment B.You may calculate internal resistance of a power sourceor a battery by doing this two measurements and then solve the equation.

author: Miti
author_rank: Super Contributor
timestamp: October 05, 2020, 04:08:58 pm
content: What tkamiya said with the exemption that the no load must be small load, 100 mA is enough.