Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!tektronix!tekcrl!tekchips!jans From: jans@tekchips.TEK.COM (Jan Steinman) Newsgroups: sci.electronics Subject: Re: NICADs (really gel-cells) Message-ID: <1661@tekchips.TEK.COM> Date: Thu, 3-Sep-87 12:21:14 EDT Article-I.D.: tekchips.1661 Posted: Thu Sep 3 12:21:14 1987 Date-Received: Sat, 5-Sep-87 15:38:03 EDT Organization: Tektronix Inc., Beaverton, Or. Lines: 53 <<>> I inherited quite a number of these from the trash bin at a previous job, so I've had some experience reviving them. Gel-cells, being of your basic lead-acid chemistry, are charged at a constant voltage, as opposed to the constant current used to change NiCd cells. Maximum "trickle charge" voltage is 2.3 volts per cell -- gel-cells can safely dissipate an indefinite charge at this potential. In addition, there is a "maximum bulk charge rate", which is typically .2C. Extremely dead cells may go above this rate if fed from a 2.3V/cell charger. Going above this rate will cause excessive outgassing, which will eventuall dry out the electrolyte. When the electrolyte dries up, the cell develops a high impedance, refusing to accept much current at the 2.3V/cell potential. To fix this situation, you need a drill press, a 1/4" to 3/8" bit, and a syringe. Clamp the cell in the drill press vise, and set the stop in order to drill about 1/8" through the cover. Be careful! If you do not clamp the cell tightly, or do not set the stop properly, the bit will dig right into the cell, destroying the vent seal and possibly the whole cell! Once you have a neat little hole in *just* the plastic cover, use a small nail or tack to poke a small hole in the rubber vent seal. Fill the syringe with distilled water, insert the syringe into the vent seal hole, and add as much water to the cell as it will take. ****IMPORTANT**** ****WEAR EYE PROTECTION!**** If you have over filled the cell, fluid will squirt out when removing the syringe! Tilt the syringe toward you, and have some acid-resitant (like polyethelyne) backdrop to avoid damaging your work area! Using a constant-current source, charge the cells at a .1C rate. THis may require as much as 20V/cell if the cell is extremely dry. Check the fluid often, replenishing a s necessary. When the cell voltage falls to about 2.5V/cell, switch to a constant voltage, 2.3V/cell charge for about a week to absorb any free fluid left. At the end of this period, remove any unabsorbed fluid with the syringe. BE CAREFULE! THIS IS BATTERY ACID! DISPOSE OF PROPERLY! If you damaged the vent seal during the drilling operation, you can touch up with silicone caulk. Do not use too much, or the cell might develop excessive pressure during the outgassing that accompanies charging. I've had great luck with this process, although I've destroyed a few cells in developing it. :::::: Software Productivity Technologies --- Smalltalk Project :::::: :::::: Jan Steinman N7JDB Box 500, MS 50-470 (w)503/627-5881 :::::: :::::: jans@tekcrl.TEK.COM Beaverton, OR 97077 (h)503/657-7703 ::::::