Portable Power Solutions for Astronomers: Part 2, Just One Night

This Part of the Series reviews battery types and then explores the question: what does it take, in terms of battery power to get though a night of astronomical observations? Not having enough power to get through a night’s observing plan can be a big disappointment considering the amount of effort required to field a telescope for a single night. The power that is required to get through a night, in short, is a function of the capacity of a battery that ideally exceeds the power needed by all the devices that are drawing power from the battery for the duration of an observing session.

There are many battery types, but deep cycle batteries are the choice of astronomers. Deep Cycle batteries allow for continuously supplying power for long periods of time.  They can also discharge more of their stored energy.   Deep cycle batteries are also constructed with thicker plates so they can withstand repeated charge and discharge cycles. 

Breaking it down further, there are different types of deep cycle batteries:  Flooded Lead-Acid (also called wet cell), Gel, absorbent glass mat (AGM) and Lithium.

The flooded lead-acid battery is the older of the designs.  For example, traditional marine and car batteries use flooded lead-acid batteries.  These batteries require maintenance involving keeping each of their cells filled with water, and cleaning terminals.  They must be kept upright and can be heavy.  However, they are less expensive than Gel and AGM (per amp hour cost).   

The Gel and AGM batteries incorporate improvements over the flood lead-acid batteries.  They do not have water cells which eliminates need to maintain water levels in cells.  The Gel batteries use an electrolyte Gel, and the AGM uses electrolyte absorbed in a fiberglass mat.  Their advantage is they will not release gases when charged.  They are sealed so they reduce spillage and do not have to remain upright.  They are lighter in weight.  They are more expensive than the flooded lead-acid battery. 

Lithium batteries are expensive, but they have many advantages.  They require no maintenance, can be discharged more deeply and are light weight.  They also provide more power throughout discharge cycle, and they are not damaged if operated or discharged at a lower level of discharge.

As for how a given battery gets an astronomer through a night, voltage is usually the first consideration on both sides of the equation as both the power source and the devices that it powers must have the same voltage rating. By far the most common battery used for astronomy in the field supplies 12 volts of direct current, or 12 vdc. Most astronomical equipment runs on this voltage, or other voltages are derived from the 12 vdc battery.

Derived voltages may be encountered on occasion. The Losmandy goto system, for instance, runs better on 18 vdc than 12 vdc, so a boost converter is used to get the higher voltage. Jim fields two of these systems.

Voltage is the amount of force or pressure available to create current flow. A battery’s voltage is measured with a voltmeter by placing leads on the positive and negative battery posts. Fully charged, a 12-volt battery’s actual charge will usually read around 13.2 volts. During use, a battery’s voltage drops as it is depleted toward some lower limit, which varies by battery type. It is also important to note that a battery’s charge will slowly begin to dissipate after it is removed from a battery charger, even though there are no powered devices attached to draw down the battery’s charge. Some batteries can dissipate their charge at up to 3% per month.

There is usually a specified lower limit to how far a specific battery can be drawn down. For instance, Kendricks specifies 11.6 vdc as the lower discharge level for the sealed batteries in their Power Packs. The lower limit varies by battery type and manufacturer. Drawing a battery below the recommended limit can damage the battery or limit its useful life. Reading the instructions that came with the battery is highly recommended.

Amperage (amps) comes into play as well. Amperage is a measure of how many electrons (how much charge) flows past a given point in a circuit per unit of time. Terms like current, load, and draw are used to describe the current that a device uses, or alternatively, at what rate and for how long a battery can supply power before being depleted.

Powered devices have a rating in amps that describes how much current flow is required to operate them. A dew heater strap at its highest temperature setting, for instance, is likely to draw more power (amps) than a small camera. Most astronomical device literature describes the maximum amps that a device might pull, but depending on the device’s state, it might be drawing a lot less power than specifications suggest. As an example, a mount control system might draw much less than an amp while the telescope is tracking at a low speed, which is most of the night, but it could draw well over an amp for several seconds while it is slewing at a high speed from one target to the next.

Knowing the actual amperage load of a of all the devices connected to a battery is important, because it is the total draw over time that depletes a battery’s charge. Knowing the battery’s capacity is also important because this determines how long a given load can be sustained before the battery’s charge is fully depleted.

Battery capacity is specified in amperage hours (amp hours, or AH). Many batteries will give their amp hour rating based on 20 hours of capacity based on an average amp draw per hour.  The formula to assess a draw against a battery is amps (summed for all the devices connected to the battery) multiplied by time.  For example, a 7 AH battery can sustain a draw of 1 amp for 7 hours, 7 amps for 1 hour, or some other tradeoff of amps and time in between.   

Most battery specifications will describe Amp Hours at 20 hours.   This translates into how many amps (or fraction of amps) can be drawn per hour from a battery for it last 20 hours.   For example, the 7 AH battery would last 20 hours while drawing .35 amps per hour.  So, if you are drawing .35 amp hours your battery lasts 20 hours.  If you use 1 amp per hour your battery will last 7 hours.  Hence, by knowing your power requirements in amp hours you can determine how long your battery will last using 20 hours as a benchmark. 

This kind of analysis could seemingly help an astronomer compare the total equipment suite’s load to battery capacity to assess sufficiency of the battery for a full night of observing, but it is difficult to perform because of several factors. On the powered accessory side of the equation, it is difficult to know the actual draw of each device as its operating state changes over the course of a night. On the battery side, cold weather and battery age could reduce capacity.

A way around this limitation is to conduct dress rehearsals at home. An astronomer could engage in a night’s observing, just as if the observing site were away from home. Avoid connecting anything that will be taken to the field to household current. Power up everything, to include dew heaters and the laptop as well. If an ammeter is available to measure the load in amps, then connect it as the first device on the positive side of the battery and take notes of the amps being drawn. Also take notes of the length of time that the equipment is powered up, and on the how the battery depletes over time during the night. Here is an example log:

Battery Unique Name: Kendrick 33AH
TimeEventBattery VoltageAmpsDuration (Min)
2030Equipment powered on13.22.9 
2100Battery check12.82.930
2115Dew heater on12.73.245
2200Battery check12.63.290
2245Battery check12.53.2135
Sample Battery Usage Log

A power log like the one above can provide valuable insight into how telescope, accessories, and battery perform as a system. The Battery Voltage is a voltmeter reading of the voltage present at the time of the log entry, and the Amps column is an ammeter reading of the total draw in amps that was. If the log suggests that the battery capacity is inadequate to power an observing session through the night, there are alternatives to consider: add battery capacity, reduce the load, or plan for less observing time.

Most astronomers have a lot to learn about powering their equipment in the field. If you have helpful information, please leave comments below regarding your battery performance experience.

© 2021 Jim Johnson and Doug Biernacki

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