Although my main observing sites are blessed with AC power, most amateurs rely on batteries in the field, as I sometimes do. For much more advice about portable electric power, see How to Use a Computerized Telescope. Here I'll review the basics.
Most telescopes operate on 12 volts DC and can be powered from the battery of your parked car. Doing this can be risky; what if you drain the battery and the car won't start when you're ready to go home?
It's much better to use portable lead-acid batteries, either gel cells or deep-cycle boat batteries. Check the battery voltage periodically during use and don't let it get below 12.0. Recharge the battery before storing it.
The capacity of a rechargeable battery is rated in ampere-hours (amp-hours, AH), thus:
Battery capacity in ampere-hours
Current drain in amperes
If possible, you should measure the current drawn by your equipment using an ammeter. If you have to guess, a telescope draws about 1 ampere, a dew heater system draws 2 or 3, and an autoguider or laptop computer may draw 2 to 5 amperes. That's enough to run down a commonly available 17-AH battery pack very quickly.
In such a situation, a 60-AH deep-cycle battery is worth paying for, even though it isn't cheap. Better yet, put the telescope on one battery, the laptop on another, and the dew heater on a third. This will prevent ground loop problems (see p. 118) and will ensure that a mishap with one piece of equipment doesn't disrupt everything.
Don't use a car battery; it is not designed for deep discharge, and if you use it within its rated limits, it won't do you any more good than a much smaller, lighter gel cell pack. If you discharge it deeply, it will wear out prematurely.
The most convenient way to power a laptop computer in the field is to use its own battery, which is good for two or three hours; you can bring a fully charged spare or two. To save power, turn down the LCD brightness, reduce the CPU speed if you have that option, and turn off the network adapter.
Second choice is to equip the laptop with a 12-volt power supply, such as you would use in a car or aircraft, and connect it to the main 12-volt battery.
Third choice, distinctly less efficient, is to use an inverter to convert 12 volts DC to 120 or 240 volts AC, and feed that to the laptop's AC line power supply. The problem is that every voltage conversion wastes energy; even the best switchmode converters are not 100% efficient. But a big advantage of DC-to-AC inverters is that they provide transformer isolation, eliminating ground loops (see next section).
The same goes for the camera. My own approach is to bring three or four fully charged batteries and swap them as needed. I can keep them in my pocket (in insulated wrappers, of course) so that they don't get too cold. The alternative is to use an external power supply for the camera. Generally, these require 120 or 240 V AC, but some enterprising experimenters have built versions that operate on 12 V DC.
The lithium-ion (Li-ion) batteries commonly used in cameras and laptop computers have a big advantage over earlier NiCd and NiMH types: they retain much more of their charge while unused. You can charge a battery, put it away, and expect it to work without recharging a month later.
Li-ion batteries always require a "smart charger" rather than just a source of regulated voltage or current. The smart charger senses the state of the battery to charge it rapidly without overcharging.
This means no harm will result if you "top up" your already-charged Li-ion batteries before an observing session. In fact, I usually do so. This is done by putting them in the charger in the usual way. In a few minutes, it will report that the battery is fully charged, but keep the battery in the charger anyway; it continues to gain charge for as much as an hour after the full-charge indicator comes on.
As batteries age, they lose capacity. There is no quick trick for restoring old Li-ion batteries, but sometimes, all they need is another charging cycle. One of my Canon batteries became almost unusable, but I ran it through the charging routine twice, and since then, it has worked fine. Perhaps one of the measurements used by the smart charger had come out of calibration.
If you use one battery for several pieces of equipment, you may have problems with ground loops. A ground loop is what happens when two devices are tied together through "circuit ground" (typically at USB or serial ports), and also share a power supply, but the voltage of circuit ground relative to the power supply is not the same in both devices.
Here's an example. If you power a laptop through a 12-volt power supply, its circuit ground will be tied to the negative side of the battery. But circuit ground in a classic (non-GPS) Meade LX200 telescope is a fraction of a volt higher than the negative side of the battery; the current-indicating circuit (LED ammeter) is between them. If you connect the laptop to the telescope, serial port to serial port, you'll have a ground loop. Fortunately, in this case only the LED ammeter will be disrupted.
Fortunately, ground loops are not common. You should suspect a ground loop if you have equipment that malfunctions only when it shares the battery of another piece of equipment to which it is connected.
One way to eliminate ground loops is to power each device from a separate battery. Another is to use an AC inverter. Despite wasting a few percent of the energy, an AC inverter provides true transformer isolation between battery and load. In fact, one inverter can power several accessories, all of which are isolated by their own AC power supplies.
Think of any rechargeable battery as something like a fuel canister. It contains a tremendous amount of energy, and if short-circuited, it can produce intense heat, explode, and start a fire.
It's obvious that a large lead-acid battery deserves respect; it's big, heavy, and full of sulfuric acid. There should always be a fuse between a lead-acid battery and whatever it is powering, preferably a separate fuse for each piece of equipment, as in the electrical system of a car or boat.
Small rechargeable batteries for cameras or laptops are also potentially perilous. Always keep camera batteries wrapped in insulating material. If you drop an uncovered DSLR battery into your pocket along with your keys, you may soon find your clothing on fire. I keep camera batteries in thick plastic bags, and I only put them in my pocket if the pocket doesn't contain anything else.
The AC line at your observing site, if there is one, must be protected by a ground-fault circuit interrupter (GFCI). This does not totally prevent electric shock, but it does protect you in situations where electricity tries to flow through your body from the power lines to the earth. Remember that there will be heavy dew during the night; keep high-voltage connections dry. Also remember that AC from an inverter is almost as dangerous as AC from the mains; the only difference is that the output of the inverter is isolated from the earth.
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