Battery Internal Resistance & Max Current

[Just]

Hi All,

Many LiFePo4 batteries have internal resistance of about 0.1Ohm (100mOhm). So, if you want to draw 10Amps from the battery, its voltage will drop by 0.1*10=1V (from 3.3V to 2.3V). If you would draw 20Amps, the voltage drop will be 0.1*20=2V(from 3.3V to 1.3V) - very bad!

So, what internal resistance should be chosen for batteries intended for e-bikes? Let's say I want to allow the voltage drop by just 0.5V on current peaks of 50A, so I would need to take a battery with 10mOhm...

What vendors produce such batteries? RC-toys use batteries of 30C and 100mOhm  of internal resistance - it seems their voltages drop to '0' during accelerations...

Please post your comments.

Thank you!

[MAGICPIE3FOCUSPOWER]

Are you sure you can say voltage drop is 1V at 10A?

What about the load resistance? It also affect the voltage drop.
Heat, battery lifetime (running hours) affect the current and internal resistance.

So I am not sure you can say internal resistance is 0,1 Ohm at 10A.
You have to measure (loading condition) at that current.

[Just]

Are you sure you can say voltage drop is 1V at 10A?

What about the load resistance? It also affect the voltage drop.

For a good battery, the Load resistance should not affect the voltage drop. External resistance just affect on the current amount drown from the battery.

[MAGICPIE3FOCUSPOWER]

But it's still in serie with the internal battery resistance.

[Morgen 3Eman]

Where did you get the .1 ohm spec ?   It seems awfully high. 

TTFN,
Dennis

[Just]

Where did you get the .1 ohm spec ?   It seems awfully high. 

Surely it's too high. HibbyKing's Zippy Flightmate LiFePo4 batteries are rated to 30C-40C, but they have about 80-100(mOhm) of internal impedance. I thought to use them because they are light and sexy 

So, it's extremely important to figure out about the internal impedance of the batteries before purchasing them...

What batteries do you use? I'm seeking for LiFePo4 batteries, which are light and compact.

[MAGICPIE3FOCUSPOWER]

I have Hyperion EX 45C they are okay, but price is high.
But forgot to charge them (store mode) while not using.
They  are dead now!

[Just]

Do you know their internal impedance? have you ever measured it?

[Lollandster]

It's probably of no help, but I measured my battery at about 0.14ohm internal resistance. My battery is a very cheap type 7070260 with a max continues C rating of 1C. I would think 30C batteries have a lot less internal resistance.

[Just]

I would think 30C batteries have a lot less internal resistance.

They should, but they not...

BTW, polymer batteries are very dangerous... Take care

[MAGICPIE3FOCUSPOWER]

Do you know their internal impedance? have you ever measured it?

I think the value was 1 mili Ohm, but it could be lower.
I have used them for 2KW brushless 1:8 RC motor.

[Lollandster]

I would think 30C batteries have a lot less internal resistance.

They should, but they not...

BTW, polymer batteries are very dangerous... Take care

They are protected by an aluminum case and stabilized with iron so they should be as safe as rigid cylinder cells, unless I've missed something.

[Just]

Polymer batteries have a tend being fired on any short or even on over- charge/discharge. LiFePo4 have no this trend...
Just search www.youtube.com for polymer batteries and you will find some very amazing videos.
You should be very careful while dealing with polymer batteries...

[Morgen 3Eman]

I have been doing some thinking on this question, and I have an empirical data point to add.

My Morgen has an analog Voltmeter and Ammeter built into the dash.  While accelerating at full throttle I noticed the 60 Amp Ammeter was pegged, and took a look at the Voltmeter as I let off the throttle.  The voltage went from  just under 53V to a little under 54V.  If we were to attribute the entire voltage sag to battery internal resistance (not really accurate, but not too bad) it would give about 16 milliOhms for the upper limit of the series/parallel resistance of the string of cells. That would seem to make the idea of 100 milliOhm resistance of a single cell seem untenable, unless the cell was about to die. 

I also Googled some manufacturers, and found some data that points to anything over 10 milliohms as being a failure indicator, with less than or about 1 milliOhm being a pretty good number for most cells when new.

So where did you get the .1 Ohm value?

TTFN,
Dennis

[MAGICPIE3FOCUSPOWER]

When I had crappy Hobbking lipo's ( for testing) the internal resistance were above 1 Ohm.