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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Teun on June 11, 2020, 12:18:02 pm

Title: Defect battery? Voltage higher than rated
Post by: Teun on June 11, 2020, 12:18:02 pm
Hi Guys,

I have replaced a rechargable battery recently, which was rated 3.6V but gave 4.045V.
http://www.farnell.com/datasheets/1695897.pdf?_ga=2.188866731.1823348601.1591876850-2138108918.1589272900&_gac=1.43556759.1591599898.CKzHx6Pt7tICFQkUGwod0boJ2w (http://www.farnell.com/datasheets/1695897.pdf?_ga=2.188866731.1823348601.1591876850-2138108918.1589272900&_gac=1.43556759.1591599898.CKzHx6Pt7tICFQkUGwod0boJ2w)

I know charged batteries give a little more but this seems very much to me.

It came off the EERPOM module of a CNC press brake. Since the battery is probably at least 20 years old is this a more common phenomenon?
Does anyone have a decent explanation for this?
Title: Re: Defect battery? Voltage higher than rated
Post by: ni.st on June 11, 2020, 01:11:33 pm
This battery consists of 3 NiMH cells in series, the open circuit voltage can therefore be as high as 3*1.4V=4.2V.
So it is most likely not defective. If you connect a 1 kOhm load, the voltage will probably drop significantly.
Title: Re: Defect battery? Voltage higher than rated
Post by: mikerj on June 11, 2020, 03:47:03 pm
The datasheet you linked to shows the terminal voltage could be close to 4.5v at a charge rate of 0.3C, and starts at around 4.2v on the discharge curves.  1.25v is simply the nominal NiMh cell voltage, it varies significantly from fully charged to full discharged.

This is the same as Li-ion cells being rated at 3.7v when they vary between ~3.0 to 4.2v depending on load and state of charge.
Title: Re: Defect battery? Voltage higher than rated
Post by: cliffyk on June 11, 2020, 06:10:23 pm
This is the key behind Dewalt and others touting their 20V(Max) Lithium-ion cordless tools, as being allegedly superior to the plain 'ol 18V tools such as Ryobi's. 5 * 4.2 (fully charged, which drops very rapidly under load) = 21 V ; 5 * 3.7 (nominal) = 18.5 V.  Check a freshly charged "18 V" Ryobi battery and you'll see 20 V and a bit same as the other's "20 V" systems. It's just a typical marketing department white lie...
Title: Re: Defect battery? Voltage higher than rated
Post by: Teun on June 12, 2020, 11:13:31 am
Ah ok. I'm not really into battery technology.

But it did measure 4.045V under load however.
Title: Re: Defect battery? Voltage higher than rated
Post by: tunk on June 12, 2020, 12:14:08 pm
A guess is that the battery is used to store volatile data
when your device is off, and that it is being charged when
it's on.
Title: Re: Defect battery? Voltage higher than rated
Post by: cliffyk on June 12, 2020, 05:04:34 pm
Quote from: Teun on June 12, 2020, 11:13:31 am
Ah ok. I'm not really into battery technology.

But it did measure 4.045V under load however.

What was the magnitude of that load--must have been quite small...