BATTERIES AND INVERTERS

Inverters have specified power capacities (in watts).  To determine what size you need:

• Find out what amperage or wattage the machine(s) you are powering draw.   Power requirements should be listed on the specification sheet for the appliance, and on a label located near where the power wire enters the machine. Usually the start-up power requirement is higher than that needed for continuous operation. Start-up can require 2 or more times the continuous load, depending on the machine.
• If the power requirement is listed in amps, multiply by 115V or 120V to get watts. 
• The inverter should have enough capacity to supply the start-up load (estimate 2-3 times the continuous load). Some inverters are able to handle brief peak loads at start-up. Check specifications for surge power production when shopping for an inverter.

As an example, the IDEXX sealer draws 6A, and has a power requirement of 690W (6A * 115V). If the start-up load is twice the continuous load, the inverter should have a capacity of 1500W. 

Using batteries

Deep cycle batteries are designed to last through many deep discharge cycles, unlike automotive batteries that are recharged by the vehicle and generally sustain only shallow discharge. Deep cycle batteries are rated in terms of amp-hours. Although deep cycle batteries are heavy and awkward to carry in the field, the coliform field kits rely on them.

Several components of the coliform kits can also be powered using an inverter plugged into the vehicle battery through the cigarette lighter or power port. The IDEXX sealer, however, should not be plugged into a vehicle port because it draws more amps than the fuses are likely to carry.

In the field, the sealer should be run off a deep cycle battery. A single cycle of warm-up and sample processing takes about ¾ to one hour for a small number of samples. Drawing 58A, the sealer will use 45 to 58 amp-hours in that time. A fully charged, new 125 amp-hour deep cycle battery might be able to handle two such sampling events. Because the sealer uses so much power, it is not handy to use in the field, although it can be done.

A single incubation cycle, including a one-hour warm-up, takes 25 hours. A bench top incubator like the Lab-line would need 2 or 3 fully charged deep cycle batteries to meet its power requirement. As an example, the Lab-line draws 8A on a 12V battery, and would use 200 amp-hours in 25 hours. Given the moderate amperage, though, the Lab-line incubator could easily be powered by the inverter plugged into the vehicle battery. The problem would be ensuring the battery does not run down too much to start the vehicle.

Batteries in general do not deliver the full amount of power they are rated for. A good rule of thumb is to estimate 50% of the rated number of the rated amp-hours will be available when the battery is fully charged. Batteries become less efficient as they get older, and if temperatures are cool, they will not last as long.