sci.electronics.basics
[Top] [All Lists]

## Re: 9V battery testing; Thevenin equivalent; car headlamps.

 Subject: Re: 9V battery testing; Thevenin equivalent; car headlamps. Adam Funk Tue, 1 Aug 2006 14:11:59 +0100 sci.electronics.basics, sci.electronics.repair
 ```On 2006-08-01, Joop van der Velden wrote: > Adam Funk wrote: >> I recently tested a 9V alkaline battery by measuring its open-circuit >> voltage (9.0 V) and then measuring it with a car headlight lamp (R = 1 >> Ohm) across the terminals (4.0 V). The lamp lit up brightly and got >> warm, but from the significant voltage drop I conclude that the >> battery is basically dead. Correct? > > No, 1,25 ohm ESR (Equivalent Series Resistance) for a 9V battery is > quite good. For a 1,5V "D" cell i would consider it too high. Interesting. I though a 44% voltage drop sounded like a lot, but as you and others have pointed out, the load resistance I've used is very low. What sort of resistance do I really need for this sort of test? >> - Vb + Rb >> -----|||||-----/\/\/\----- >> | | >> | | >> o o >> | | >> | Rl | >> -----------/\/\/\--------- >> >> >> With the load removed, and assuming the voltmeter is an open circuit, >> Vb = 9.0 V. With Rl = 1 Ohm in place and the voltage across o-o >> measured as 4.0 V, the loop current is 4 A. So Rb is 1.25 Ohm. >> Correct? > > Yep. But remember that the 1 ohm of the lamp is measured in cold state. > At 9V it is probably a lot more. Right. I measured the lamp's resistance with an ohmmeter, which of course puts very little current through it. But I took the measurements by clipping the voltmeter (actually it's the same meter) leads onto the battery terminals, reading the open-circuit voltage, then pressing the lamp's terminals against the battery terminals (the spacing was convenient --- that's where I got the idea from) and immediately reading the loaded voltage (before the lamp heated up). >> Is there a rule of thumb for judging a battery as "still OK" or >> "dead" based on the calculated Thevenin resistance? > > It depends of the size, technology and voltage. A large "D" cell will > have in its new state an Rb of about 0,1 ohm or even less. > A new 9V battery might give you somathing like 1 ohm. Thanks! ```