BATTERY SIZE DETERMINATION
by Edwin J. Somers, CAS

How do you determine how big a battery you need to externally power your sound equipment? First, you will have to make some decisions. Exactly what equipment do you want to externally power? How long do you want the equipment to operate before the battery must be recharged? What battery chemistry do you want to use?

Let's talk about battery chemistry. You have four choices:
Gel-Cell Sealed Lead-Acid Battery - least expensive, heaviest, very tolerant of abuse, lasts longer if frequently recharged
NiCad, Nickel Cadmium - has flat discharge curve, about 30% more expensive, about 40% lighter, exhibits a so-called memory effect that requires reconditioning for long life, and requires a battery specific charger
NiMh, Nickel Metal Hydride - recent technology, 50% more capacity than NiCad at a bit less weight, has no memory effect, usually doesn't require reconditioning, you can often use the same NiCad chargers
Li-ion, Lithium Ion - brand new technology, much lighter than NiMh, very expensive, requires an expensive charger

Make a list of everything you want to externally power and list the current consumption of each item in amps, or in milliamps (for example 800 milliamps is written as .8 amps). Check to make sure everything works on 12 volts DC. If you can't find out the current consumption, contact the manufacturer or LSC. Maybe we can figure it out. Add up the total current drawn by all the equipment.

Deciding how long the equipment must operate before recharging will be based on your style of operation. How long do your typical jobs run each day? How often do you want to recharge? How much weight are you willing to carry around? How much do you want to spend?

To determine how big a battery you need, just multiply the total amperage by the total running time. For example, let's say you determined that you needed to operate 4 hours and you needed 1.8 amps: 4 x 1.8 = 7.2 Ah (batteries are rated in ampere-hours or "Ah"). Many battery manufacturers make 7 ampere-hour gel-cells that are small enough to carry around in an equipment bag; however, they weigh over five pounds.

Well, there you have it; you just learned to use Ohms Law. Who said that electronics was complicated? Now look into what battery packs are available to fit your needs, or have LSC build a custom battery pack.

Here are some specifications of NP-1 batteries compared to Gel Cells:
Li-ion NP-1 2.8 Ah, .86 lbs.
NiMh NP-1 3.9 Ah, 1.5 lbs.
NiCad NP-1 2.3 Ah, 1.5 lbs.
Gel Cell 2.3 Ah, 2.0 lbs.

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