Build Your Own Combat Robot

94 Build Your Own Combat Robot
Hawker brand SLA batteries (www.hepi.com) have peak current delivery up to 40
times the 20-hour capacity. For example, the Hawker 16Ahr battery can source
680 amps for 5 seconds—or about 42 times the 20-hour capacity.
Charging is accomplished by applying 2.45 volts per cell and limiting the current
to the battery manufacturer’s recommended charging current. The exceptions to this
rule are the Hawker batteries, which do not require current limiting. A 12-volt
SLA battery has six cells, so the charging voltage is 14.7 volts (14.7 = 6 × 2.45 volts).
If you are leaving the battery on the charger for an extended period of time, the
charging voltage should be reduced to 2.27 volts to 2.35 volts per cell. When storing
SLA batteries, you should be sure to charge them fully every six months. A good
automatic automotive charger will work well for fast charging; however, it is important
to use a battery that can handle fast charging and to use a charger that
does not force charge into the battery after it is fully charged.
Following are some of the advantages of using SLA batteries:
■ It’s the least expensive rechargeable battery type, so it’s easier to purchase
more than one battery at a time.
■ Up to 300 charge/discharge cycles to 80-percent capacity are possible.
■ When stored at 25 o C, it loses less than 1 percent of its charge per day.
■ It can supply the highest current of any battery type.
■ The wide range of battery capacities makes it easy to size the battery to
the job.
■ It gives some advance warning before going dead. For a 12-volt battery,
the voltage gradually lowers from 13.2 volts (full charge) to 10 volts (empty),
making it relatively easy to tell how much charge is left in the battery.
■ It handles fast deep-discharge better than other battery types. This is true
as long as the battery is placed on a charger quickly after the discharge.
■ The Hawker brand Cyclon and Genesis and Odyssey SLA batteries can be
charged in about 30 minutes to about 1.5 hours depending on how large
the charger is.
Following are some of the disadvantages of using SLA batteries:
■ It has the highest weight of any recommended battery type.
■ The 6-minute rating drops the effective Ahr rating more than any
rechargeable battery type.
■ Because of gas venting problems, most SLA batteries cannot be fast charged.