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182 ALL ABOUT BATTERIES

Figure 18- 2 This universal charger works with Figure 18- 3 Sealed lead- acid (SLA) batteries are
both NiCd and NiMH batteries. You flip a switch the largest and heaviest of the bunch, but they provide
depending on which kind you are recharging. a lot of current, useful for larger robots.

weight for the current output they provide. However, Li- ion cells require specialized recharg-
ers; both battery and recharger can be frightfully expensive.

SEALED LEAD- ACID (SLA)
Sealed lead- acid batteries are similar to the battery in your car, and they work in much the
same way. SLAs are “sealed” to prevent most leaks, but in reality the battery contains tiny
pores to allow oxygen into the cells. SLA batteries, which are rechargeable using simple cir-
cuits, are the ideal choice for very high current demands, such as battle bots or very large
robots.
They’re also pretty inexpensive for the amount of juice they pack. However, they are
among the heaviest of all the rechargeables, so use them only in robots that can support the
weight.
SLA batteries are most often sold in 6- and 12- volt packs, like the one in Figure 18- 3
(24- volt and higher packs are also available, but not quite as common). Inside the battery are
multiple 2- volt cells— the cells are combined to create the desired voltage. Three cells are used
to make a 6- volt pack, for example.

SO WHICH ONE SHOULD I PICK?
Most experienced builders select from a small palette of battery types based on the size and
application of their robot.

• Alkaline, NiCd, and NiMH batteries are the most common among small tabletop robots.
When using alkalines, you may chose between the rechargeable and nonrechargeable
types. Nonrechargeable alkalines are a convenience, but using them can be expensive if
you need to replace them often. Opt for rechargeable alkalines to save some money, or
switch to NiMH or NiCd cells.
• Midsize “rover” robots use larger NiMH or Li- ion cells; bigger robots still are ideally suited
for sealed lead- acid batteries. SLA batteries are available in a wide variety of capacities, and
the capacity largely determines the size and weight of the battery. More about battery
capacity in a bit.

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