cameronjames@snoc01.enet.dec.com (James Cameron) (05/15/91)
Wanted: documentation on curing nicad cells that are apparently dead.
I've got some nicad cells in sets that I use for assorted purposes.
When the series connected cells drop below their operating voltage, and the equipment
starts dying, I check the voltage of each cell. Sometimes I find one cell is dead and the rest
are fine. Sometimes the dead cell is at a reverse voltage.
My father told me of a way to cure these sick cells, it involves
1) giving them half second bursts of 12VDC from a car battery or 5A power supply, until
they keep a 1.2VDC level, monitoring
a) the terminal voltage, ensuring that it does not increase beyond 1.6VDC,
b) the cell temperature, ensuring that it does not get "hot".
2) following up with a three minute ten-times-normal charging current; again keeping to
the limits above,
3) charge up to normal level,
4) discharge to zero volts and charge again.
I would like to know if this procedure, or one like it, is documented anywhere.
And if so where? Feedback by mail welcome; if there is sufficient interest I will post
a summary.
--
James Cameron
Digital Equipment Corporation (Australia) P/L
(cameronjames@snoc01.enet.dec.com)agodwin@acorn.co.uk (Adrian Godwin) (05/17/91)
In article <22682@shlump.lkg.dec.com> cameronjames@snoc01.enet.dec.com (James Cameron) writes: > >My father told me of a way to cure these sick cells, it involves > > 1) giving them half second bursts of 12VDC from a car battery or 5A power supply, until > they keep a 1.2VDC level, monitoring I've used this procedure successfully in the past - I used a large electrolytic capacitor rather than a continuous source. I think it was documented in 'Wireless World' around 1975 - that's when I started doing it, anyway. The theory was that some sort of crystals grew in the electrolyte and shorted out the cell (though this won't be tha case if a cell is reverse-charged). The high currents supposedly melted the crystals, removing the short. Given the amount of rubbish that seems to be talked about NiCd failure modes this may well be totally untrue - I'd certainly be interested to know ! However I can confirm that the procedure works : but not in every case, and the repaired cells don't seem to stay repaired forever. I haven't blown my limbs off trying this ..... yet (!) -adrian -- -------------------------------------------------------------------------- Adrian Godwin (agodwin@acorn.co.uk)
bob@miranda.inmos.co.uk (Bob Green) (05/20/91)
Buy a new one ! -- | Bob Green Inmos Ltd, Bristol | EMail(UK) ukc!inmos!bob |---------------------------------------| or bob@inmos.co.uk |The opinions above are my personal | Internet: bob@inmos.com |views and do not reflect Inmos policy. | UUCP:(US) uunet!inmos.com!bob
george@wombat.UUCP (George Scolaro) (05/23/91)
In article <7137@acorn.co.uk> agodwin@acorn.co.uk (Adrian Godwin) writes: >In article <22682@shlump.lkg.dec.com> cameronjames@snoc01.enet.dec.com (James Cameron) writes: >> >> 1) giving them half second bursts of 12VDC from a car battery or 5A power supply, until >> they keep a 1.2VDC level, monitoring > > >The theory was that some sort of crystals grew in the electrolyte and shorted out >the cell (though this won't be tha case if a cell is reverse-charged). The high >currents supposedly melted the crystals, removing the short. > >Given the amount of rubbish that seems to be talked about NiCd failure modes this >may well be totally untrue - I'd certainly be interested to know ! >However I can confirm that the procedure works : but not in every case, and the >repaired cells don't seem to stay repaired forever. GATES (a manufacturer of batteries) has a very good handbook which explains how and why nicads and gell cells work. Also explained in detail is charging techniques pros/cons and lots of other very useful information. The Handbook is: Gates Energy Products, Sealed Rechargeable Batteries Application Manual. I got mine from one of their distributors. Regarding the above article (and roughly quoting the handbook): ----- There exists 3 wearout mechanisms: - Deterioration of the electrolyte Primarily due to increasing carbonate concentration due to oxidation of organic compounds in the separator, seals and vents. Loss of electrolyte caused by excessive venting due to overcharge of cell reversal. - Deterioration of the separator <- this causes the shorts Sealed nicads contain a porous synthetic-fabric separator. This material provides electrical insulation and mechanical isolation between the +ve and -ve electrodes. Shorts are primarily due to a degradation of this separator. A conductive path forms through the separator shorting the cell. The rate of separator degradation is a function of the integral of cell temperature over time. A shorted cell may not always appear to have an electrical short, a small short may allow enough charge current to flow to the electrodes and charge the active material, while causing the cell to self discharge quickly on completion of charging. If the conductive path through the deteriorated separator has a low resistance, the charge current is partially shunted and the cell will not be able to charge. If the cell voltage can be increased enough by high charge currents, the shunting current is sometimes sufficient to burn off or clear the shorted contact, providing a 'brief' extension to life (sometimes colloquially referred to as zapping cells). - Deterioration of the seal or vent integrity The electrolyte and oxygen gas generated during overcharge (this happens after the battery is charged, but the charging continues) are normally contained within the cell. The sealing material is made of plastic and is not completely immune to chemical attack. Eventually the seal sufficiently deteriorates to the point where oxygen escapes causing the slow dryout of the electrolyte. Another section of the book describes the loaded storage of nicads, i.e. the battery is totally discharged and left connected to the load. When a nicad's voltage is held at zero or close to zero for an extended period of time, the nicad is in a situation where creep leakage may occur. Under these conditions there is a small but continuing growth of an electrolyte film on the sealing surface of the +ve terminal. This creep leakage, an electrochemical phenomenon, may ultimately cause small amounts of the electrolyte to seep around or through the vent seal. When this occurs small amounts of white crystals (potassium carbonate) appear on the outside surfaces of the +ve terminal. This white material is minute particles of the potassium hydroxide electrolyte which have reacted with carbon dioxide in the air to form potassium carbonate. In rare cases, this leakage may be large enough so that other components of the device may be exposed to potential corrosion damage from the electrolyte. --- All in all the handbook if very interesting and a must for anyone wanting to design a charger for nicads or gell cells. It contains over 230 pages of very informative text and diagrams. best regards, -- George Scolaro george@wombat.bungi.com [37 20 51 N / 122 03 07 W]