Model Boat Mayhem - Forum

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: A 'Flat' NimH battery  (Read 1540 times)

Tug-Kenny

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 7,680
  • Location: Newport. S Wales
A 'Flat' NimH battery
« on: September 14, 2013, 05:01:33 PM »


As the title says, I have a flat (NimH) battery in my Spektrum DX7. It should show 9.6 volts but the screen is blank.

 I had left in on for a over a week  ???    <:(

I connected it up to the  'clever'  charger but the red light keeps flashing, so I assume it's not charging. I measured the voltage on the battery and it reads 1 to 2 volts

What is the clever technology that refuses to let it be charged back up please  ?


Thanks

ken

Logged
Despite the high cost of living   .......... It remains popular

Netleyned

  • Full Mayhemer
  • *****
  • Online Online
  • Posts: 8,979
  • Location: Meridian Line, Mouth of the Humber
    • cleethorpes mba
Re: A 'Flat' NimH battery
« Reply #1 on: September 14, 2013, 05:11:33 PM »

I've brought them back from 3v on a 7.2 pack.
You need a non smart charger.
Charge for a few hours then leave for an hour
charge again. Keep doing it checking the pack voltage
If the voltage is slowly rising OK if not bin.
Once you get a reading of 7or8 volts use the smart
charger.

Worked for me

Ned
Logged
Smooth seas never made skilful sailors
Up Spirits  Stand fast the Holy Ghost.
http://www.cleethorpesmba.co.uk/

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,564
  • Location: Blackpool, Lancs, UK
Re: A 'Flat' NimH battery
« Reply #2 on: September 14, 2013, 07:14:21 PM »

Basically, smart chargers work by monitoring the current and voltage.  When there isn't any current, it sees its own output voltage and concludes that the battery is charged.  If it sees too much current (or its having to work too hard to limit it) and there is a lack of volts, it concludes that the pack is a dud.
Do it like netleyned said, it might be an idea to use an ammeter (probably on a low milliamp scale) to check that current is actually going into the pack. 
If it has discharged to zero volts by being left switched on, there is a good chance of recovery, if it just flattened itself, its dead.  It could also be a failed connection.  Separate cells usually respond to a rub rotating them in their sprung holders.  A plug in pack should not have that problem.
Logged
"With the right tool, you can break anything" - Garfield

Tug-Kenny

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 7,680
  • Location: Newport. S Wales
Re: A 'Flat' NimH battery
« Reply #3 on: September 14, 2013, 08:32:29 PM »


Thanks for the response.  I'll try with the the fixed voltage idea and let you know how I get on.  It's not the cost of a new one, as they are only a tenner, but the technology that commits suicide I'm not keen on.

Cheers

Ken


Logged
Despite the high cost of living   .......... It remains popular

NFMike

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,062
  • Location: Hythe, Hants, UK
    • Plague Marine Services
Re: A 'Flat' NimH battery
« Reply #4 on: September 14, 2013, 08:41:59 PM »

If it has discharged to zero volts by being left switched on, there is a good chance of recovery, if it just flattened itself, its dead.

I thought it was the other way round.
Because the cells will never be identical, then when a pack is over-discharged one or more cells (the ones to go flat first) get reverse charged by the rest. Which is apparently not a good thing.
OTOH for long term storage I understood it was OK to allow NiCd and NiMH to go completely flat through self discharge.

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,564
  • Location: Blackpool, Lancs, UK
Re: A 'Flat' NimH battery
« Reply #5 on: September 15, 2013, 08:58:53 AM »

What I read, was that for long term storage, the cells were deliberately discharged and kept shorted, there presumably being a difference in the reactions going on inside between that and self discharge.  Probably something to do with the chemical properties of the internal self discharge path growing with use.  Then again, thinking back, it was NiCads that I was reading about, things could well be different with NiMH.  The one cell being less good than its mates is the origin of the memory myth with NiCads.
With batteries, as any other electrical component, there are just the two basic failure modes - it can either go short, or it can become disconnected, allowing that there are degrees on the way to either. 
If a pack grows a disconnection, voltage measured by a smart charger will be the full charger voltage but with zero current.  Just one dud connection between or within a cell will do that. 
For a shorting problem, it is likely to be just one cell that discharges itself all the time, and the voltage measured becomes pack voltage minus 1.2 volts.


Logged
"With the right tool, you can break anything" - Garfield

Tug-Kenny

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 7,680
  • Location: Newport. S Wales
Re: A 'Flat' NimH battery
« Reply #6 on: September 15, 2013, 09:00:07 PM »


I'll do some more analysis tomorrow

Cheers

Ken

Logged
Despite the high cost of living   .......... It remains popular

Tug-Kenny

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 7,680
  • Location: Newport. S Wales
Re: A 'Flat' NimH battery
« Reply #7 on: September 20, 2013, 11:08:44 AM »


I have rescued the battery.    :-))

I put it on a 9 volt unregulated power supply and watched it for an hour.  I then connected up with the proper charger and the red light came on, so it finished the charge correctly.

All is now well as the following day it was still charged ok.  Thank you guys for the advice.

Cheers

ken

Logged
Despite the high cost of living   .......... It remains popular
Pages: [1]   Go Up