Internal Resistance of cells ..

Internal resistance gives an indication of how efficient a cell (or battery) will be in giving back what you put in .

If 1WHr of electricity is put into two separate cells, the one with low resistance will give back more of that energy ... the one with higher resistance will waste more in heating up the cell and give less back in useable electricity.

How much will be the difference will depend on the charging and discharging currents .... the lower the currents the less will be lost ..

Let's consider 2 cells , one with 100mOhm ...another with 300mOhm Int res.

charging at 1A the loss in the cell is I squared R = 100mW and 300mW ... this is lost in charging and discharging.

If the cell voltage is 3.7 , thenpower going into the cell @1A is IxV = 3.7W

So If 3.7W is put into both cells the one with
100mOhm int res. gives back 3.5W....5.4% islost...
300mOhm cell gives back 3.1W.....16.2% islost ... this lost energy is what heats up the battery pack ...

At 0.5A charge and discharge the % loss is halved to 2.6% and 8.1%

At 0.25A charge and discharge the % lost is ...............1.3% and 4.05%

The higher the temperature the lower the internal resistance , ... You may measure the internal resistanceat room temp , 25*C, but if the cell is kept outside and cycles at 10*C the resistance will be double and so will the losses.

So if energy has a high value to you, reject cells with high resistance .... if you have excess solar capacity and sometimes powergoes to waste , then keep all but the very worst cells.

I'f you're a real perfectionist you could put those with high resistance on the outside of the pack where the heat can get out more easily ...

Although high cell temperature does increase efficiency (reduces heat in cell) it is stressful for the cell ...temps below 10*C should be avoided when cycling (not for storage) as resistance is high.

How do you go about measuring the internal resistance of a cell?

There's AC resistance and DC resistance, most of the chargers seem to measure AC resistance while what we need for calculations is DC resistance.

Elmo said:

How do you go about measuring the internal resistance of a cell?

There's AC resistance and DC resistance, most of the chargers seem to measure AC resistance while what we need for calculations is DC resistance.

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My solar charge controller and the Powerlab both use high frequency PWM with buck and or boost converters, so there is an AC component, which brings capacitive and inductive impedance into play Right? Even if your power supply or charger smooths the output there will most likely be ripple.

Every PWM circuit I've ever designed and most of those I've seen has had capacitors to smooth ripple, perhaps not entirely but AC should be only a minor component of any current going to/from your battery.

My own procedure for measuring internal resistance has been to use a two step load, a major load to set the output voltage and then a smaller load that can be switched in and out, measure the voltage swing as the switch is flipped (I was doing it graphically on a monitor) and you can tell the relative change in output voltage with a smallish step in current which should give you a reasonable idea of the internal resistance of the cell.

V=IR

R = (V1-V2)/(I1-I2)

What is your proposed dischargecurrent? 1/2 Amp?
Then discharge at 1/2 amp and 3/4 amp and measure thevoltage at each current.

Then calculate the resistance using the above equation.

An example

I1=.5 V1=3.9V
I2=.75 V2=3.8V

R = (V1-V2)/(I1-I2)
R= (3.9-3.8)/(.75-.5) = .1/.25 = 0.4 ohms

Easy

Or you can do it this way, with OCV and voltage for one load.





http://physicsnet.co.uk/a-level-phy.../electromotive-force-and-internal-resistance/

for the kinds of loads you guys are likely to be drawing (your wiring adds a ton of inductance) i would imaging your more after the sub 100Hz ESR, so closer to the DC ESR, in truth the batteries i have seen start resistive, around 5-7Khz become capacitive, then over 35Khz become very inductive.

ESR is not truly linear vs current drawn, its more like a decay line, where the closer you get to max draw, the higher it gets, they have a positive temperature coefficient, and the ESR increases with temperature, and to top it all off, ESR generally decreases as the state of charge increases.

As for measuring, a step change in load is the better method as it removes the issue of surface charge making the readings lower than they actually are (they have a small amount of natural capacitance due to there constructions)

If you have over 4000 cells to test how time consuming are these above ideas?

depends on how your willing to approach it, in reality it should not be more involved than measuring the voltage of the cells after letting them sit for a week,

Measure voltage, push button to turn on load, measure voltage, done,
If you use something like a 100mA current sink, you can directly read off the change in voltage,
You would likely have your current sink wires wrapped around the test lead tips to keep the contact path small.

That procedure would be,
Press leads to cell ends,
Press relative on multimeter
Turn on load (foot switch maybe?)
Read off ESR

Move on to next,

so if 100mA drops 40mV, you have an ESR of 0.4 ohm.

FiremanDIYPowerwall said:

If you have over 4000 cells to test how time consuming are these above ideas?

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Beats me. One needs a set of parameters to give you a Go / No Go Spec. I am still trying to work out what failure mechanisms / life cycle indicators etc are relevant to this application.

I look at it in the aspect that my main build will be 14s300p and the amperage going into each cell would be so small that ir shouldn't have much effect if any.

FiremanDIYPowerwall said:

I look at it in the aspect that my main build will be 14s300p and the amperage going into each cell would be so small that ir shouldn't have much effect if any.

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So you are saying that if a cell isnt obviously knackered. Shorted. 60 percent charge capable. Etc you will incorporate it in a project?

When you look at cells the cells that have a high IR are towards the end of their life so therefore are most likely going to test poorly on capacity. So for my wall if the cells test high enough with their capacity then it lends to reason that the IR should be low and they are good enough for the Powerwall.

Muzzlehatch said:

FiremanDIYPowerwall said:

I look at it in the aspect that my main build will be 14s300p and the amperage going into each cell would be so small that ir shouldn't have much effect if any.

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So you are saying that if a cell isnt obviously knackered. Shorted. 60 percent charge capable. Etc you will incorporate it in a project?

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I think that's what fireman is doing ...and to me it sounds like the best plan ... as long asthey don't lose too much voltage when left overnight the cell is not draining the pack ...and the more cells you have in your firewallthe lower current they will run at ...more efficient ..longer life ... The most important thing is for daily use , don't charge to over 3.9v then should get over athousand cycles ..

How much is too much to lose overnight ??? Depends ... if you cycle them daily maybe a drop from 4 to 3.90 over night might be acceptable .... I check each cellin this way first , then build a pack of 20p and checkwhat that drops by overnight , normally 4 Vdrops to 3.99 that's excellent ...

New cells do have a low internal resistance perhaps 50 , old ones perhaps 100 mOhms... so each cycle old cells will waste electricity more electricity

If charge and discharge is 0.5A ....100mOhm will waste ....I squared R ... =25mW

@ 0.5A disharge of 2AHr cell takes 4 hrs , loss is 100mWHr when the cell has delivered 0.5 x 3.7 x 4 = 7.4 WHrs

In a thousand cycles one cell delivers 7.4KwHrs of power , and 0.1 KwHrs is lost

Value of electricity is 12c/kwhr ..one cell processes 88c worth of electricity every 1000 cycles , and loses 1.2c if cell old 0.6c if new ...

If only charged to 3.9 a new cell will give over 3000 cycles and will process $3 worth of electricity in it's life... the losses in an old cell will be slightly greater , but not too much(1%) difference in efficiency comparing old to new at low currents.

Would be nice if the price of electricity was 12c. It's about 21c here in Brisbane.

FiremanDIYPowerwall said:

Would be nice if the price of electricity was 12c. It's about 21c here in Brisbane.

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Here in Germany it's 25-29c so... Could be worse. However it makes solar and batteries more profitable.

I'm also thinking about the internal resistance thing and it's importance. I've built an 6-cell Arduino based charger/discharger. It works great apart from the 1-2% error in voltage readings (even though I'm trying hard to calibrate it) due to a huge impact of the TP4056s. They cause offsets of ~100mV (but not always 100% the same) each when turned on/off. I haven't figured out yet why.

It's also capable of measuring the internal resistance but due to the errors it's worth nothing. Measuring each cell's resistance individually seems to be a labour intensive job...

So do you really thinks it matters? Maybe for the life span? Or do you agree with Fireman that if you only discharge at 100-300mA at max it doesn't matter?

I'm really undecided about that topic

Sohax said:

So do you really thinks it matters? Maybe for the life span? Or do you agree with Fireman that if you only discharge at 100-300mA at max it doesn't matter?

I'm really undecided about that topic

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At those sort of currents ...even up to 0.5A , I don't think we need to worry about internal resistance , since even the worst cells are really not too bad ... in electric bikes where the current is many amps per cellit's good to use cells with lower resistance...

I really started this thread to work out if int. resistance was important, how much energy was wasted in high resistance cells..... I convinced myself it's not really important ... except, perhaps, as an indication of how close to the end of life a cell may be.

Even on ebikes internal resistance isn't such a big deal until you get into high performance particularly acceleration or climbing serious hills on a hub motor. My recumbent ebike ran great with various packs of used laptop cells but I don't live in serious mountain territory, this is more rolling hills. I had as much top speed as I felt comfortable with on the flats and could outright terrify myself going downhill. Hill climbing was only so so but that pushed me into pedaling harder up hills than I would have otherwise. Keep the speed up and a hub motor stays fairly efficient, bog it down to a crawl uphill and it turns into a heater with a slight side effect of locomotion. Playing with the Grin Technologies hub motor simulator it's clear that there are times when one watt of pedaling will save five or more watts of motor power in certain hill climbing situations.

Okay. Thanks! I came up with a new implementation but if it doesn't work I'll just ignore it

Sohax said:

Okay. Thanks! I came up with a new implementation but if it doesn't work I'll just ignore it

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If you have a way to measure current, you can measure it at start of charge and end of charge passively, just note the difference in voltage before you begin, to when it is charging, vs the change in current, gets you the flat ESR, them do the same when you stop charging, and you get your charged ESR (pretty much your best and worst case).

This way as long as your just monitoring it, it doesnt add any time to the setup.