Battery Capacity The capacity of a battery is measured in ampere- hours.  The  ampere-hour  capacity  is  equal  to  the product of the current in amperes and the time in hours during  which  the  battery  is  supplying  current.  The ampere-hour   capacity   varies   inversely   with   the discharge  current. The  size  of  a  cell  is  determined generally by its ampere-hour capacity. The capacity of a cell depends upon many factors, the most important of which are as follows: 1. 2. 3. 4. 5. The   area   of   the   plates   in   contact   with   the electrolyte The  quantity  and  specific  gravity  of  the electrolyte The type of separators The general condition of the battery (degree of sulfating,  plates  buckled,  separators  warped, sediment in bottom of cells, etc.) The final limiting voltage Battery Ratings Battery  ratings  were  developed  by  the  Society  of Automotive Engineers (SAE) and the Battery Council International (BCI). They are set according to national test  standards  for  battery  performance.  They  let  the mechanic compare the cranking power of one battery to another. The two methods of rating lead-acid storage batteries  are  the  cold-cranking  rating  and  the  reserve capacity rating. COLD-CRANKING   RATING.—The cold- cranking  rating  determines  how  much  current  in amperes  the  battery  can  deliver  for  thirty  seconds at   0°F   while   maintaining   terminal   voltage   of   7.2 volts  or  1.2  volts  per  cell.  This  rating  indicates  the ability  of  the  battery  to  crank  a  specific  engine (based   on   starter   current   draw)   at   a   specified temperature. For   example,   one   manufacturer   recommends   a battery with 305 cold-cranking amps for a small four- cylinder  engine  but  a  450  cold-cranking  amp  battery for  a  larger  V-8  engine.  A  more  powerful  battery  is needed to handle the heavier starter current draw of the larger engine. RESERVE   CAPACITY   RATING.—The reserve  capacity  rating  is  the  time  needed  to lower  battery  terminal  voltage  below  10.2  V  (1.7  V per cell) at a discharge rate of 25 amps. This is with the  battery  fully  charged  and  at  80°F.  Reserve capacity  will  appear  on  the  battery  as  a  time  interval in  minutes. For example, if a battery is rated at 90 minutes and the   charging   system   fails,   the   operator   has approximately 90 minutes (1 1/2 hours) ofdriving time under minimum electrical load before the battery goes completely dead. Battery Charging Under  normal  conditions,  a  hydrometer  reading below  1.240  specific  gravity  at  80°F  is  a  warning signal that the battery should be removed and charged. Except  in  extremely  warm  climates,  never  allow  the specific   gravity   to   drop   below   1.225   in   tropical climates.   This   reading   indicates   a   fully   charged battery. When a rundown battery is brought into the shop, you should recharge it immediately. There are several methods  for  charging  batteries;  only  direct  current  is used  with  each  method.  If  only  alternating  current  is available, a rectifier or motor generator must be used to convert to direct current. The two principal methods of charging   are   (1)   constant   current   and   (2)   constant voltage  (constant  potential). Constant  current  charging  is  be  used  on  a  single battery  or  a  number  of  batteries  in  series.  Constant voltage  charging  is  used  with  batteries  connected  in parallel.  (A  parallel  circuit  has  more  than  one  path between the two source terminals; a series circuit is a one-path  circuit).  You  should  know  both  methods, although the latter is most often used. CONSTANT   CURRENT   CHARGING.—With the constant current method, the battery is connected to a  charging  device  that  supplies  a  steady  flow  of current.  The  charging  device  has  a  rectifier  (a  gas- filled  bulb  or  a  series  of  chemical  disks);  thus,  the alternating  current  is  changed  into  direct  current.  A rheostat (resistor for regulating current) of some kind is usually built into the charger so that you can adjust the  amount  of  current  flow  to  the  battery.  Once  the rheostat is set, the amount of current remains constant. The usual charging rate is 1 amp per positive cell. Thus a  21-plate  battery  (which  has  10  positive  plates  per cell)  should  have  a  charging  rate  no  greater  than  10 amps. When using this method of charging a battery, you  should  check  the  battery  frequently,  particularly near the end of the charging period. When the battery is gassing  freely  and  the  specific  gravity  remains constant for 2 hours, you can assume that the battery will take no more charge. 2-5