Internal Resistance of Battery

Thread Starter

Tejasvi471

Joined Jun 9, 2020
17
Dear all,

I have HW battery (rated 9V) (image attached), how much should be the internal resistance of this battery ?
I have measured terminal voltage of my battery = 6.45 V
Voltage drop across 470 ohm resistance = 2.032 V
so internal resistance = 1022 ohms

is it OK ?
is my battery worn out ?
 

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AlbertHall

Joined Jun 4, 2014
10,018
There is no solid answer to that, it will depend on what you want to use it for. That 6.45V battery would still work powering a few CMOS logic chips. However it won't make an LED glow brightly for very long.
 

OBW0549

Joined Mar 2, 2015
3,446
How much should be the voltage on battery for it to be considered flat ?
That depends entirely on the minimum allowable battery voltage requirement of the circuit it is supplying. 8 volts? 6? 4? It all depends. This Energizer spec sheet uses 4.8 volts as the arbitrary end point in its mAH capacity graphs.

I've no idea what internal resistance to expect from that type of battery; but whatever it is, internal resistance is not the source of your problem: your battery is simply dead. Throw it out.
 

MrChips

Joined Oct 2, 2009
21,325
As Albert says, it depends on what you want to power with the battery.
I have a 9V batttery that is now showing 6.5V under load.
My load is a switch mode power supply that is powering a 3.3V circuit that takes 5mA while running an <1μ in sleep mode.
 

Audioguru again

Joined Oct 21, 2019
1,577
All half-decent 9V batteries are Alkaline chemistry. Your battery is so cheap that it does not say its chemistry (probably old carbon-zinc).
Your dead battery voltage drops to 2.032V with a 470 ohm load then the current is only 4.3mA. A good 9V alkaline battery can provide hundreds of mA. Here is the performance of Duracell and Energizer 9V alkaline batteries:
 

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Audioguru again

Joined Oct 21, 2019
1,577
My smoke detectors use a Name Brand 9V alkaline battery. They beep as a warning that the battery power is low. When I measure the voltage of the "dead" battery with no load, it measures close to 9V but the battery cannot supply much current. You MUST add a suitable load when measuring a battery voltage.
 

crutschow

Joined Mar 14, 2008
25,116
How much should be the voltage on battery for it to be considered flat ?
A 9V alkaline or zinc battery uses 6 cells in series to get 9V nominal.
I find that the no-load voltage of alkaline cells falls somewhat proportional to their remaining capacity, and generally consider a battery essentially dead when a cell reaches a no-load voltage of 1V, so that would be 6V for a 9V battery.

But it really depends upon what the battery is powering.
Some circuits will complain that the battery voltage is low, well above that value.
 

BobaMosfet

Joined Jul 1, 2009
1,118
Dear all,

I have HW battery (rated 9V) (image attached), how much should be the internal resistance of this battery ?
I have measured terminal voltage of my battery = 6.45 V
Voltage drop across 470 ohm resistance = 2.032 V
so internal resistance = 1022 ohms

is it OK ?
is my battery worn out ?
Realistically, you should pull a datasheet, and _understand_ it. Here is a datasheet from a different manufacturer for the same model of battery:

https://data.energizer.com/PDFs/6F22SIL_EU.pdf
 
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