9V battery and 15V power supply and its mA - Page 1
Forum section: electronics / general

author: ElectroElvis
author_rank: Regular Contributor
timestamp: December 22, 2011, 11:40:52 pm
content: Hello folks,I have a question regarding with normal DC 9V batteries non rechargable.I bought 2 new XXX brand for few € and decided to test them with my analog multimeter.Both show the next mA readings:first 2 seconds hooked up it showed little over 1A then it started to drasticly fall in next 10 secs all the way to 150mA. After few testings it now shows only about 50mA. Is this normal?Second question is regarding 15V 800mA powersupply:First of all ony my meter it reads almost 24V and not 15V as it is labeled, any clues?Second first measure i did for mA it showed over an 1A since the needle hited the end on 1A scale, but second later it fell all the way to the 50mA and its staying there wont go up or down (not even to 800mA as it is labeled)?Did i do somthing wrong or is it just broke or somthing(powersupply)?Thanks

author: Unknown
author_rank: Unknown
timestamp: December 22, 2011, 11:51:35 pm
content: Current is typically measured in series with the load, eg. battery + -> meter -> lamp -> battery -. If you're directly connecting the meter across battery terminals, you're effectively shorting the battery, hence the high momentary current, which will quickly drop because a wimpy 9V cell can't sustain it for long. Same for the power supply. You likely tripped some sort of protection. It either blew a fuse, just tripped a resettable fuse or damaged a component.The 24V reading was probably because it was unloaded, some cheap power supplies have a much higher voltage when unloaded, especially if they're unregulated. Try measuring will drawing a few hundred (<800) mA.

author: vtl
author_rank: Regular Contributor
timestamp: December 23, 2011, 02:34:04 am
content: Short-circuit current testing is not the way to test a power supply or battery. You can blow a the fuse in your multimeter if you hooked it up something powerful enough. To test power supplies or batteries you need to hook up a dummy load which draws a controlled amount of current. You can also hook up a resistor as a dummy load as well. With your multimeter youre just shorting out the terminals of your battery/power supply. You can damage batteries and melt unregulated power supplies.Now something that I sometimes do is test the short circuit current for certain batteries (NOT lithium ion). But this is mainly to test the peak current avaliable for those short bursts. If you want to test this only do it long enough to get a reading.Also you can test solar cells short circuit current and that is the corrent way to test them.Dummy load:http://www.eevblog.com/2010/08/01/eevblog-102-diy-constant-current-dummy-load-for-power-supply-and-battery-testing/

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2011, 01:15:25 pm
content: I'm astonished at how many nubes make this mistake. It's something I wouldn't have even thought of doing even when I first started playing with electricity at the age of 5.The mA rating of a battery is it's capacity. The mA rating of a power supply is the maximum current which can safely be drawn before it shuts down, blows a fuse or is damaged..Short circuiting a battery is always a very silly thing to do. You're lucky you didn't do this with a car battery.

author: saturation
author_rank: Super Contributor
timestamp: December 23, 2011, 03:13:40 pm
content: To help visualize what others have already explained here's a discharge curve for alkaline chemistry batteries:Shorting the battery terminals is a curve far worst that the 545mA you see above, so it will be almost a straight line down; how well the battery recovers depends on how much electrolyte was expended. Generally, as the curves show, the useful life of this battery, even if it did recover, is far reduced. The reason why such batteries can recover is the internal resistance is often high enough to prevent all the cells to expending electrolyte to ~ zero, but it also prevents potential injury, such as if you did this with a NiMH or Li cell with known low internal resistances that can explode or get white hot.To repeat what others have already said on the second question, the AC-DC supply you measured is open circuit, non-load voltage. In poorly or unregulated supplies, they are used for a designed load, the output will appear to be high until the right load is placed. If the supply now fails to function its because its either damaged or its overload or short circuit protection has tripped. The only hopeful news is some supplies have self resetting protection, all you need do is wait and it should reset itself if it has no visible reset buttons. If it has an internal fuse or equivalent, you'll have to open the unit to find and replace it.Good luck!