Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!wuarchive!uunet!optilink!elliott From: elliott@optilink.UUCP (Paul Elliott x225) Newsgroups: sci.electronics Subject: Re: Why Are 9v Rechargables Not 9v?????? Summary: Varta, Everready, Cylindrical cells, Button cells Message-ID: <5597@optilink.UUCP> Date: 12 Feb 91 19:51:17 GMT References: <1991Feb11.005844.1630@julius.cs.uiuc.edu> Organization: Optilink Corporation, Petaluma, CA Lines: 39 In article <1991Feb11.005844.1630@julius.cs.uiuc.edu>, totty@flute.cs.uiuc.edu (Brian Totty) writes: > > I just bought a set of rechargable batteries and a charger for them. > To my surprise, I found out that the 9v batteries were actually only > 7.2v in a "nine-volt size". I was wondering why they couldn't make > NiCd batteries in the same size that could store 9v. Other brands on > rechargeables also seem to be 7.2v. This seems to be a pretty serious > degradation from 9v, and I am surprised that these batteries would > even work in some applications. One person has already mentioned Varta as a source of 9V "9V" batteries, and Everready also sells (or did sell?) one. This may be a re-labled Varta. There are two basic types of "9V" NiCd batteries: The 7.2V ones are made of 6 cylindrical cells. At 1.2V per cell, this gives 7.2 Volts. Varta (and others?) make a 7-cell model, made of stacked oval "button" cells, yielding 8.4 Volts. From what I've seen, the button cells also have a higher Amp-hour capacity; about 100 mAh, as opposed to about 65 mAh for the cylindrical cells, making the 7-cell battery an all- around winner. Actually, the lower Voltage of the 6-cell (and 7-cell) batteries isn't really much of a loss, compared to the Carbon-Zinc or Alkaline 9V batteries. The NiCd discharge curve is *much* flatter that the non-rechargables. The non-rechargables spend as much time *below* 7.2V as above it, during discharge. Your battery charger will probably charge the 7-cell NiCd batteries just fine. They are typically a constant-current charger, with some taper as the battery Voltage reaches overcharge. A charger for 6-cells should work, but the time to full-charge may be increased a bit. You can measure the charging current to verify this, or measure the open-circuit output Voltage of the charger to get a clue. -- Paul M. Elliott Optilink Corporation (707) 795-9444 {uunet, pyramid, pixar, tekbspa}!optilink!elliott "If I had known it was harmless I would have killed it myself." - P.K. Dick