jim@alberta.UUCP (Jim Easton) (02/08/86)
*** This article is in response to some articles which appeared several weeks ago - I'm just slow in responding. Both anode and cathode of a discharged lead-acid battery are covered with lead sulfate which has several rather unfortunate properties. First it is mechanically rather weak and crumbly and will flake off quite easily and drop to the bottom of the case. Not only does this reduce the capacity of the plate but by removing sulfate ions reduces the strength of the acid. Furthermore this layer eventually gets high enough to short out the cell allowing faily high self discharge currents. Thirty years ago battery rebuilding companies existed. The top was removed, bringing the plates with it. The case was cleaned out, the plates washed and new acid added. The result was almost as good as a new battery. In order to help harden the lead and lead sulfate other metals, primarily Antimony in the old batteries, are added to the lead. Unfortunately the additives tended to interfere with the action of the battery - in particular they catalysed electrolysis. Vents and filler caps were required. In the new batteries Calcium or Strontium is used instead of Antimony which is better with respect to catalysing electrolysis. Since electrolysis is greatly reduced they optimistically concluded that caps and vents were no longer required. The happy side effect is that the acid can't spill out all over the battery etc. In case the charging system goes berserk and overcharges a pressure relief valve, of some sort, is required as a safety measure. Another thing they did was to change the rating system from amp-hours to "cranking power" and "reserve capacity". Cranking power is the current the battery can supply for 30 seconds and still maintain a voltage above a certain value (I forget). Reserve capacity is the number of minutes the battery can supply 25 amps and still maintain a voltage above a certain level. A second unfortunate property of lead sulfate is that slowly over a period of days it will change its crystal structure to a form that is difficult to oxidise/reduce by electrolysis (ie. it won't charge). People refer to this condition as "sulfated". Lead acid batteries must therefore be maintained in a charged state. Charging Proper charging is actually a very complex subject but the basics are simple enough. The charging system attempts to apply a constant voltage, which is temperature dependent, to the battery. If the battery is fully charged it will develop a back voltage to match and very little current will flow. (N.B. we are not talking about the sealed gell types which work differently). At normal operating temperatures the regulator voltage should be somewhere around 14.4 volts. An appropriate charging voltage is also dependent on the type of driving that is done. eg. The voltage in a car which does a lot of short trips should probably be set higher than in one that does a lot of highway driving. In the old days (ie. old style batteries) a good rule of thumb was based on the amount of water it used. If it used more than about an ounce of water per cell per 1000 miles driven the voltage was probably too high. Maintenance free batteries are not so simple - they should use almost no water. eg. the battery, mentioned in an article a few weeks back, that was dry after 6 or 7 years was almost certainly being overcharged. However the voltage on most cars these days is not adjustable - that may or may not be a good thing but it can at least be checked. If it is adjustable careful adherence to instructions is recommended. Cheap home battery chargers are usually unfiltered and unregulated. Overuse of them can result in overcharging causing excessive electrolysis, heat, which may warp the plates, and excessive oxidation of the anode and separators. As far as I know, little inherent damage is done by mild overcharging except that if you can't replace the water then you are definitely damaging the battery. A battery charger should not be used all night long on a regular basis - if it's needed something else is wrong. The real damage is usually done when the water is added. Use distilled water and do not not store it or handle it in a metal container. One must also be careful to add water before the level drops below the top of the plates. I do not know the chemistry but if they are exposed to air they become unusable. The battery in my old Valiant has lasted 14 years and is still going - I believe it is because of the following; - There is an ammeter in the car which tips me off to trouble eg. erratic regulator, slipping belt or over/under charging. - it is never left for any length of time in a discharged state. - I add only distilled water. Ammeters It is incredible what an ammeter, monitoring the battery current, can tell you about the state of battery and charging system. It is far more useful than a voltmeter. For example sludge pilling up and shorting the plates can be detected with an ammeter but by the time it is detectable with a voltmeter the battery has long since quit. I notice that new cars are comming out with voltmeters. That is nice in that it provides a space on the dashboard to install an ammeter. It seems to me that the reason voltmeters are being installed is that buyers want gauges (It's the "In thing" again - at last) but voltmeters are easier to install and are cheaper - maybe as much as 20 cents. Their attitude seems to be "So what if it doesn't tell you anything (very much), it's a gauge isn't it?". Jim Easton (..!alberta!jim)