# Internal resistance of the battery

I am having Sealed Lead Acid Rechargeable battery.This is 12Volts 7.5Ah battery.

I want to know the internal resistance value of this battery.

How can i measure?

Hi,

how to measure internal resistance of sla battery

Tom...

Can you explain with an example?

A very basic simple method is as follows :

Fully charge the battery, then leave it for a couple of hours off-charge.
Connect a voltmeter across it’s terminals to measure Vopen
Connect a current meter in series with a fixed load resistor that will draw a current of around 5 amps (say 2R2ohms)
[Vopen - Vload] / Iload represents the voltage ‘lost’ due to the internal resistance of the battery.

Note that this value will change depending upon state of charge of the battery, the thermal rise in resistance of the resistor as it heats up and the actual load current.

Make sure the resistor has a large enough power rating, it will be dissipating 65W

Much easier to use a car bulb, they are 12V loads that you'll have already in a car spare-bulb set probably.

The internal resistance of lead acid batteries changes a lot as they discharge, BTW.

Much easier to use a car bulb, they are 12V loads that you'll have already in a car spare-bulb set probably.

Easier, but the resistance of the bulb increases as the filament heats-up.

The internal resistance of lead acid batteries changes a lot as they discharge, BTW.

True, and probably not just for lead acid batteries and it may vary with the load. Although it is true-resistance... It resists the flow of current, heat is generated when current flows, and it dissipates power, etc... It's not a "resistor".

So... the resistance should be specified under certain conditions, such as at full-charge with a certain current load.

jackrae:
A very basic simple method is as follows :

Fully charge the battery, then leave it for a couple of hours off-charge.
Connect a voltmeter across it’s terminals to measure Vopen
Connect a current meter in series with a fixed load resistor that will draw a current of around 5 amps (say 2R2ohms)
[Vopen - Vload] / Iload represents the voltage ‘lost’ due to the internal resistance of the battery.

Note that this value will change depending upon state of charge of the battery, the thermal rise in resistance of the resistor as it heats up and the actual load current.

I understood what you have answered.

Also i want to know that what will be the maximum internal resistance value of 12v 7.5Ah battery?

rajansn24:
Also i want to know that what will be the maximum internal resistance value of 12v 7.5Ah battery?

I would imagine that the maximum internal resistance could be very high (infinite?) if the battery was abused, for example by overcharging or by long periods of complete discharge.

I think you need to ask yourself "what is the highest internal resistance my circuit can cope with".

JohnLincoln:
I would imagine that the maximum internal resistance could be very high (infinite?) if the battery was abused, for example by overcharging or by long periods of complete discharge.

I think you need to ask yourself "what is the highest internal resistance my circuit can cope with".

From your point, i came to know that we can't judge the maximum internal resistance value of particular battery.

The internal resistance will vary depends upon the voltage drop across battery and the load current.

Am i write?

Just for completeness' sake, why is the internal resistance important to you?

manor_royal:
Just for completeness' sake, why is the internal resistance important to you?

I have been involved in one project which has battery interfacing with arduino microcontroller.

In this project, i need to measure the internal resistance value. Thatswhy i asked.

Hi,
You need to start with manufacturers spec/data sheets and look at the varoius discharge graphs and stated internal resisitance.

This one is 25mOhms.
http://www.yuasabatteries.com/pdfs/NP_7.5_12_DataSheet.pdf

Tom....

DVDdoug:
Easier, but the resistance of the bulb increases as the filament heats-up.

Doesn't matter, you have to measure the current anyway.

rajansn24:
In this project, i need to measure the internal resistance value. Thatswhy i asked.

Is this a set question ?

No problem with that , just trying to understand why it is important.

I vaguely remember that there is a way you can measure it by quickly drawing certain amount of current and measure the voltage drop at the output of the battery to get the battery output impedance.

It's great that folks are willing to respond to questions but can they really not be bothered to read what's already been written and so avoid restating prior responses.

rajansn24:
Also i want to know that what will be the maximum internal resistance value of 12v 7.5Ah battery?

Typically about 30 milli Ohms rising to 1 Ohm at the end of life of the battery.

This is a circuit using jackrae method.i #3.

Note the three measurements cannot be made at the same time as the circuit has to be disconnected to measure V open circuit.

Normally a DMM would be used for the measurements.

An arduino can be used as a crude DMM and a search should find examples of this.
The results will not generally however be anywhere near as accurate as using a DMM.

This is the basic method so i think the question now becomes :- How do i measure voltage and current with an arduino

rajansn24:
From your point, i came to know that we can't judge the maximum internal resistance value of particular battery.

The internal resistance will vary depends upon the voltage drop across battery and the load current.

Using the above method you can measure it.
The battery manufacturer should be able to give you typical values.

The resistance itself will remain relativley constant over short periods however the terminal voltage will always vary depending on load.

You cannot constantly measure internal resistance as the load has to be removed for one of the readings.

jackrae:
A very basic simple method is as follows :

Fully charge the battery, then leave it for a couple of hours off-charge.
Connect a voltmeter across it’s terminals to measure Vopen
Connect a current meter in series with a fixed load resistor that will draw a current of around 5 amps (say 2R2ohms)
[Vopen - Vload] / Iload represents the voltage ‘lost’ due to the internal resistance of the battery.

Note that this value will change depending upon state of charge of the battery, the thermal rise in resistance of the resistor as it heats up and the actual load current.

Formula i understood.Can you explain with one example?