"Internal battery resistance"

[JanCPF]

\"Internal battery resistance\"

Does anybody know of a good source that lists the internal resistance of some of the batteries that are relevant to us? Correct me if I'm wrong, but wouldn't this be *the* figure needed to determine which batteries would be able to drive a certain LED at a certain current? You could then draw an equivalent schematic of the battery using a resistor in series with a pure voltage source, and also easily mimic all kinds of batteries with a bench supply and a handfull of resistors. (only applies to new batteries I guess)

Jan

 

[Nerd]

Re: \"Internal battery resistance\"

The resistance of the battery changes as it drains down from full power if I didn't remember wrongly. Nothing is absolute, so it's hard to calculate exactly. Plus the voltage also varies as the battery becomse depleted. But using resistors, you won't be able to drive a LED at a fixed current throughout the whole battery's life.

 

[JanCPF]

Re: \"Internal battery resistance\"

Nerd,
Yes I'm aware of that, but I just meant the list to be a tool to determine if this or that led/current combo could be driven by this or that battery combo, or if it actually was futile in the first place due to the internal resistance of the battery. Yes your right - it would only apply to fresh batteries.

Jan

 

[Doug S]

Re: \"Internal battery resistance\"

[ QUOTE ]
JanCPF said:
Does anybody know of a good source that lists the internal resistance of some of the batteries that are relevant to us? )

Jan

[/ QUOTE ]Battery manufacturers' datasheets. Note that not all datasheets give the Ri. Even for the same manufacturer, some have it and some don't.

 

[brickbat]

Re: \"Internal battery resistance\"

JanCPF,

That's a great idea. There's one other piece of info that's needed to do a thorough design of a LED/battery/resistor circuit, though and that is the Vf/If data for the LED. It makes a good approximation to consider the Vf as a constant, regardless of the If, but in reality, it is a curve.

As to internal resistance, it's straightforward to measure it in a given cell. Measure the unloaded cell voltage with your DVM, then load the cell with your approximate operating current. Quickly measure the loaded cell voltage. Compute the internal resistance as (Vunloaded - Vloaded)/I load.

But, yes, it varies a LOT with cell temperature and charge level - especially with alkaline cells.

 

[asdalton]

Re: \"Internal battery resistance\"

This issue has been discussed extensively here.