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Portable Power Primer
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Originally published in the August 2003 "Night Sky" (newsletter of the Astronomy Club of Akron, Ohio)
Between imaging equipment, laptops, dew heaters, and computerized telescopes there's a growing demand for field power for amateur astronomers. Coincidentally there are many products being introduced these days that address this issue nicely. I'd like to help folks decide what will best fit their needs with a brief discussion of lead-acid batteries.
First we should discuss numbers. Lead-acid batteries are rated in voltage, ampere-hour capacity, and peak available current (often referred to as "cold cranking amps"). We will begin with the presumption that 12 volts DC is what is required. Most dew heaters, imagers, laptops, and automated telescopes operate from a nominal 12 volts. One popular instrument that varies from this is the Classic LX200, which needs 18 volts DC. For the purposes of this discussion we'll say that the user has a converter to run it from 12 Volts; these are common and inexpensive. Laptops requiring 120 VAC may be operated from an inverter. We can safely ignore "cold cranking amps"
The number we need to look at when choosing batteries is the ampere-hour rating. This defines the actual energy available from the battery. We can readily see that a given battery will supply a small amount of power for a longer time than it can supply a large quantity. The amount of power consumed at a given voltage varies with the current drawn by the device. A device that draws one ampere for one hour has used one amp-hour of capacity. Imaging cameras usually draw an ampere or two. Dew heaters and automatic telescopes draw varying amounts of current at different times; I usually figure on an average of .7 ampere for a telescope and one ampere for large (8" & up) heaters.
It's easy to begin with the notion that "bigger is better" - after all, a battery with 100 amp-hours of capacity will run a telescope and heaters for days, right? For a number of reasons, it's not quite that simple. Remember (or discover for the first time) that a lead-acid cell generates a voltage of 2.2 volts DC, Cells in series add their voltage - thus to get to a nominal 12 volts DC we need six cells in series. We can't get to each cell; the only external connections are to the string as a whole. This means that we can charge the battery only as a complete unit and can't control the relative charge in each cell (battery terminology lesson #1: a battery is a group of cells).
When batteries are gently charged and discharged over a period of time, the state of charge begins to vary among the cells. Since the actual usable capacity of the battery is limited by the weakest cell, this results in degraded battery performance. In automobiles, the very high starting current serves to equalize the cells. In the field we don't have anything to fulfill that role. A battery that is never discharged or charged at a rate exceeding 10% of its capacity will deteriorate fairly rapidly. There are ways to equalize the cells in a battery, but let's just say they are expensive and dangerous and let it go at that. This gives us a rule of thumb to begin sizing our requirements.
We need a battery that can supply the expected current demand for the time period we require - but one with an amp-hour capacity not more than 10 times the expected maximum current demand. At the other extreme, for maximum battery life it's best not to discharge the battery much below 50% capacity any more often than we must. Thus, we have two outside limits.
For a modern computerized telescope and no other load, this limits us to less than 20 amp-hours capacity (as the maximum current drawn by common instruments while slewing in both axes is less than 2 amperes). Thus, car batteries (usually in the 50 to 60 amp-hour range) and deep-discharge marine batteries (usually even bigger) are inappropriate for this class of load. The minimum capacity we require is the average current drawn by the load multiplied by the duration we want. For a typical automatic telescope, we can figure on .7 amp average. A five hour observing session would use approximately 3.5 ampere-hours. We now have a target range: at least 7 amp-hours (double the 3.5 amp-hour required), but less than 20 amp-hours. Similar calculations may be used to size a battery for whatever load you anticipate.
For whatever reasons, small "jump start" batteries are becoming available at a rapidly increasing rate and at decreasing costs. Coincidentally, the two most common sizes are 7 amp-hours and 17 amp-hours. They must have gotten hold of an early copy of this article! The 7 amp-hour units sell for $40-$50 (unless you buy the one Celestron is now marketing for $100) and the 17 amp-hour jobs sell for $50-$80. They are available in the automotive department of stores such as K-.Mart, Walmart, Sam's Club, and Target. The ones I like best of all are available at our local Harbor Freight store.
All the jump start batteries have internal gel cells. This is good because they are sealed; they can't leak unless something happens to them. They are enclosed in a plastic housing and have cigarette lighter sockets already installed (and usually a light). Some also include an air compressor or an inverter. They come with a charger sized for safe operation. The Harbor Freight units even have a "'smart" charger which can be left connected indefinitely without overcharging the battery.
Now you know why the jump start batteries make perfect sense. Not only are they much safer, handier, and lighter than car batteries - they are actually better and will last longer. If your calculations reveal a need for more than 17 amp-hours of capacity, just divide the load and get two.
I never discussed using the battery in your nearby car. That does address all the issues in this article - it's already sized for its primary job, equalization is no problem, and there should be plenty of reserve capacity for this "side" job. On the other hand, if something goes wrong, you'll need a "jump start" battery to get home, so you may as well have one along anyway. The portable battery also eliminates long cables, which present both electrical losses and tripping hazards.
Trivia for the day - The one-letter abbreviation for a unit of measure named after a person should always be capitalized -this includes both V (volt) and A (ampere). This explains why you may see some crazy-looking mixtures of upper and lower case - such as mAh (milli-amp-hours).
- Francis Clouston likes this