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Chapter 8: Batteries 185• Number of plates, and type of separators between plates• Quantity and specific gravity of electrolyte• Age of battery• Cell condition—sulfation, sediment in bottom, etc.• Temperature• Low voltage limit• Discharge rateNotice the first four items have to do with the battery’s plates and electrolyte—itsconstruction; the next two items concern its history; and the last three depend on howyou are using it at the moment. We’ll get into all the details, but keep in mind that themost truthful thing you can say about battery capacity is: it depends.Battery capacity is specified in ampere-hours. A battery with a capacity of 100ampere-hours could in theory deliver either 1 amp for 100 hours or 100 amps for 1 hour.This doesn’t help you any more than would drawing a straight line on a map if someoneasked you for a destination. You need the second coordinate, the second factor-rating.A battery’s rating is the second specifying factor. It refers to the rate at which it canbe charged or discharged. It is analogous to how fast the sink will fill up with the waterfrom the jug. In equation form:Battery Rating 5 Capacity/Cycle TimeIn this equation, the rating is given in amperes for a capacity in ampere-hours anda cycle time in hours. In practical terms, a battery with a capacity of 100 ampere-hoursthat can deliver 1 amp for 100 hours (known as a C/100 rate) would not necessarily beable to deliver the much higher 100 amps for 1 hour (known as a C/1 rate). You can onlyget the water out of the jug so fast.Requesting 10 amps from a fully charged 100 AH capacity battery reflects a C/10rate; this same request reflects a much lower C/40 rate from a 400 AH battery. In otherwords, smaller batteries have to deliver energy faster in relation to their size, or largerbatteries have lower discharge rates in relation to their capacity.Capacity of commercial batteries is standardized by the Battery Council International(BCI) into several usable figures. Two figures, a 20-hour capacity and a reserve capacity,are usually given for every battery depending on its application.• 20-Hour Capacity—This is a battery’s rated 20-hour discharge rate—its C/20rate. Every battery is rated to deliver 100 percent of its rated capacity at theC/20 rate, if discharged in 20 hours or more. If a battery is discharged at a fasterrate, it will have a lower ampere-hour capacity.• Minutes at 25 amps Reserve Capacity—This is the number of minutes a fullycharged battery can produce a 25-amp current. This is the automotive startingbattery rating that tells you how long your starter battery will power yourautomotive accessories if your fan belt breaks and disconnects the alternator; inother words, how many minutes you have to get to the nearest gas station.• Minutes at 75 amps Reserve Capacity—This is the number of minutes a fullycharged battery can produce a 75-amp current. This is the golf cart batteryrating, because 72 minutes translates to about the amount of time it takes to