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186 ALL ABOUT BATTERIES

The faster the discharge, the higher the heat that is generated. With few exceptions, bat-
teries are not engineered to dump all their current in a short period of time. So manufacturers
provide an idealized specification that more accurately represents the typical use of their
wares.

Capacity Ratings for Smaller Batteries
Smaller batteries are not capable of producing high currents, and their specifications are listed
in milliamp- hours. There are 1000 milliamps in 1 amp. So a battery that delivers half an amp
is listed with a capacity of 500 milliamp- hours, abbreviated mAh (or less accurately as mA).
Even larger batteries might be rated in milliamp- hours, as is the case with rechargeable
NiCd and NiMH cells, where it’s not uncommon to see them listed as 2000 mAh— that’s the
equivalent of 2 amp- hours. Amp- hour ratings are typical in sealed lead- acid batteries.
Amp- hours is commonly abbreviated as Ah or, less accurately, as A. For example: 500 mAh
(500 milliamp- hours) or 3.5 Ah (3.5 amp- hours, or 3500 milliamp- hours).
Very occasionally, and for some applications, batteries may be rated in watts, though this
is an imperfect measure. Technically, wattage is calculated at voltage times current, or V * I
(I stands for current— for the time being, don’t worry about why; that’s just the way it is). So
a battery operating at a nominal 12 volts, delivering 2 amp- hours, is rated at 24 watts.

UNDERSTANDING INTERNAL RESISTANCE
Batteries, like humans fighting the Borg, resist— in this case, they resist giving up their charge.
This phenomenon is called internal resistance; the higher it is, the less current a battery can
deliver at any moment. The internal resistance of a battery determines the maximum rate at
which current can be drawn from the cells.

• Alkaline and lithium- ion batteries have a high internal resistance. These can still deliver
current, but they cannot “dump” all of it within a very short period. Think of them as
long- distance runners.
• Lead- acid, NiCd, and NiMH batteries have low internal resistance. If necessary, these
batteries will empty their charge within minutes, depending on the demands of the load.
They’re the sprinters of the track- and- field team.

Bigger batteries have more surface area inside them, which also affects internal resistance.
That’s why big batteries of any given type can power higher loads.
For most robotics applications, internal resistance isn’t a major issue. When it really mat-
ters is for those tasks where extreme (and short- lived) current draw may be required. Examples
are combat robots and the battery used to power the electric propeller on a self- guided drone
plane.
Rapid, high current discharge of any battery can result in the cell overheating, fire, even
explosion! That’s why you never want to intentionally short out the terminals of a battery. This
causes the battery to disgorge all of its current as quickly as it can.

UNDERSTANDING BATTERY RECHARGE RATE
Most batteries are recharged more slowly than they are discharged. A good rule of thumb
when recharging any battery is to limit the recharging level to one- half to one- tenth the

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