What's the correct way to measure the output of a battery or PSU - Page 1
Forum section: electronics / beginners

author: cenkjc
author_rank: Newbie
timestamp: March 31, 2016, 06:12:57 am
content: Hi, everyone,When I was a student we were told never to measure the output of a battery or a PSU by sticking a voltmeter across the output terminals without there being a (known) load present. Otherwise you were simply putting the meter in series with the source. I've seen numerous YouTube videos of people connecting the output of the PSU straight into the multimeter and taking a reading.Is this still correct wisdom and lore, or has the world moved on? Is it, perhaps, a *good enough* measurement?(As an aside, when I was a student multimeters were generally analogue devices - the AVO-8 in its brown leather case, but only if the tech in charge of the lab would let you have one of those precious things. Possible idea for a Dave vid on how to read analogue meters with a mirror scale here. And answer a question - Is a high quality analogue multimeter inferior to even a low-ish cost digital meter?)I'd be interested in your thoughts.-Kev

author: Mechanical Menace
author_rank: Super Contributor
timestamp: March 31, 2016, 06:24:03 am
content: Measuring voltage isn't a problem. The resistors are only needed when measuring current or in series resistance, both achieved by measuring voltages. If you tried to measure the resistance directly you would melt something.

author: exe
author_rank: Unknown
timestamp: March 31, 2016, 08:30:15 am
content: Quote from: cenkjc on March 31, 2016, 06:12:57 amI think it was about current. Yet there is a case where some load is desirable if you do measurements on a battery: it's when you want to check how healthy it is.(Partially) discharged batteries may "regenerate" some voltage without load ("recovery") over the time. So just measuring voltage of a battery may give you a false idea of its power capabilities. It may look healthy without load, but with load the voltage can significantly drop (because of "internal resistance"). So, some load is desirable if you are trying to figure out how much energy left in the battery.Concrete example: our car battery showed around 11.6-11.8V with no load. But when I tried to start the engine the voltage dropped down to 7V or so and... the engine didn't start. That's because the battery was dead.

author: tggzzz
author_rank: Super Contributor
timestamp: March 31, 2016, 08:38:36 am
content: Firstly you have to decidewhyyou are doing a measurement, and, derived from that,howto do the measurement. Your question hints at neither.If you want to measure whether it will provide a sufficient power to a load, then you have to attachthatload before measuring.If you want to measure the internal resistance, you need an appropriate load, a voltmeter and an ammeter.And so on.

author: kolonelkadat
author_rank: Regular Contributor
timestamp: March 31, 2016, 08:42:51 am
content: The only trouble with measuring voltage across the terminals of an unloaded power source is that it wont account for the voltage sag caused by the intended load. Just as a for instance, a completely dead battery could measure as good as new if it is not connected to a load.

author: Brumby
author_rank: Unknown
timestamp: March 31, 2016, 12:28:10 pm
content: This is the basic reason to do a 'proper' test on a battery or power supply:Quote from: kolonelkadat on March 31, 2016, 08:42:51 amWhen you are measuring the output voltage on a regulated power supply (such as a laboratory power supply), the regulation circuitry will hold the output (relatively) constant, so a straight multimeter measurement will be pretty well the same whether there is a 1mA load or a 1A load. For unregulated power sources, the 'known load' configuration is necessary if you want to get an idea of how the power source will perform in actual use.

author: broz
author_rank: Regular Contributor
timestamp: April 01, 2016, 01:21:01 am
content: Was this one of the videos that you watched? If not, it may help...