How adding voltages together works

[CoggesRailway]

Hello,

Sorry a daft question here. I have built a 14.4v battery from 12 1.2v nimh cells - in fact 2 batteries to run a 2 motor Alco. When charged "fully" the overall battery registers 17.5v and even after prolonged running over two days on the weekend it hasn't gone below 15v. Why might this be? Is 1.2v just a nominal label as opposed a designed in actual voltage...?

Ian

 

[Gizzy]

NiMH cells are 1.2 Volts.

Have you tried measuring a single cell?

Maybe your voltmeter is out of calibration....

 

[nicebutdim]

I've actually noticed this before too, same set up with 12 1.2v aa's. I plug in the charger and it can register up to 17 volts. To be honest I thought it was a peak battery reading after charging, but now you have me thinking twice

 

[Gizzy]

From http://en.wikipedia.org/wiki/Nickel–metal_hydride_battery


Discharging
A fully charged cell measures 1.4?1.45 V (unloaded),[http://www.gscalecentral.co.uk/wiki/Wikipedia:Citation_needed < Link To citation needed][/sup] and supplies a nominal average 1.25 V/cell during discharge, down to about 1.0?1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal). This voltage varies depending on the discharge rate of the cell (lower discharge loads result in an increased voltage output for longer periods, approaching the 1.4 V unloaded cell voltage).[http://www.gscalecentral.co.uk/wiki/Wikipedia:Citation_needed < Link To citation needed][/sup]
Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts;http://www.gscalecentral.net/#cite_note-17 < Link To [18][/sup] some measure almost 1.5 volts.[http://www.gscalecentral.co.uk/wiki/Wikipedia:Citation_needed < Link To citation needed][/sup] This voltage falls rapidly to about 1.25 volts at 10% depth of discharge (DOD) and then remains almost constant until the cell is over 80% discharged. The voltage then falls rapidly from about 1.2 volts down to 0.8?1.0 volts at which the cell is considered "flat" in most devices. Mid-discharge at a load of 1 ampere, the output is about 1.2 volts; at 2 amperes, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms. Nickel metal hydride batteries provide a relatively constant voltage for most of the discharge cycle, unlike a standard alkaline where the voltage falls steadily during discharge.http://www.gscalecentral.net/#cite_note-18 < Link To [19][/sup]

It seems that a fully charged NiMH cell can have upto 1.5V when Off Load, contrary to my original thoughts and earlier post....

 

[CoggesRailway]

Thanks Gizzy great answer! I would never have known to look on Wikipedia. So that explains it then?

 

[Gizzy]

Google and Wikipedia are always my first port of call for research Ian....

 

[CoggesRailway]

Ok ok Ross. I should have googled before I asked. I do think you could chillax a bit... There was one member that had wondered about this and Gizzy who thought it was a defined value too- so now they know as well. However in future I will endeavour to find things out privately to reduce traffic on the forum and save all of our precious time...

 

[yb281]

Ok ok Ross. I should have googled before I asked. I do think you could chillax a bit... There was one member that had wondered about this and Gizzy who thought it was a defined value too- so now they know as well. However in future I will endeavour to find things out privately to reduce traffic on the forum and save all of our precious time...

Plus - by asking a question on an open forum and then receiving a sensible answer as above now we ALL know. What forums are all about really.

 

[stockers]

receiving a sensible answer

Lucky as well!

But very true Mel.

 

[yb281]

receiving a sensible answer

Lucky as well!

But very true Mel.

Shot in a million mate.

 

[JRinTawa]

That's very handy to know, thanks guys