Page 53 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

the battery for 14 h to get a full charge. While manufacturers claim
that it is OK to overcharge NiCd batteries at the C/10 rate, most
engineers recommend switching over to a trickle charge after
the initial 14 h at C/10. A trickle charge is usually rated at C/30,
or 1/30 of the battery’s capacity. A trickle charge for our 1-Ah
battery would be around 33 mA (1 A/30 33.3 mA).
Memory effect A disadvantage to NiCd batteries is the memory
effect. If one repeatedly recharges a NiCd battery before it has
completely discharged, the battery forms a memory at that
recharge level. It then becomes difficult to discharge the battery
past that remembered level. Obviously this can severely limit the
battery’s capacity. To correct that problem the battery must be
completely discharged, by leaving a load connected to the battery
for several hours. Once the battery is complete discharged, it can
be charged normally and will function properly.
Lead-acid Gelled-electrolyte battery cells (gel-cells) are similar to
automotive batteries. They are sealed, maintenance-free, lead-
acid batteries. They don’t make gel-cells in the familiar D, C, AA,
AAA, or 9V battery cases. Gel-cells are typically larger and may be
used in larger robots.
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Gel-cells are available in numerous voltage ratings, from 2V to
24V, and current capacities. These batteries may be charged
with a current-limited constant voltage or a constant current
like NiCd batteries. Typically to charge a gel-cell, one applies a
fixed 2.3V to 2.6V per cell. Initially the battery will draw a high
current that tapers down as it charges. When fully charged, the
battery need only draw a trickle charge (approximately C/500)
to maintain itself in a fully charged state.

Gel-cell batteries vary from manufacturer to manufacturer. To
safely recharge a gel-cell, you should check the manufacturer’s
recommendation. In general, a simple charging device can be
made using an LM317 voltage regulator. A fixed voltage (2.3V per
cell), constant current C/10 is applied to the battery. When the bat-
tery reaches a full charge, the constant current source is removed
and a regulated voltage is applied.

Many gel-cell batteries do not like to be deep cycled. Therefore it
becomes necessary to monitor battery voltage under load. When
the battery voltage drops by a specified amount (check the manu-
facturer’s data sheet), it needs to be charged.

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