Update (January 2016): After 3 years of use, the batteries featured in the article below started to suffer from problems associated with old batteries (such as inability to hold enough charge and leakage). I am now planning to replace the cells with fresh ones.
Astrophotographers who regularly travel to remote observing sites require a reliable power source to last an overnight imaging session. In this article, I will describe how to construct a DIY field battery — the most essential component of any portable imaging setup. I will also discuss how to calculate the total power requirement of your system in order to determine the recommended battery capacity (ampere-hours) that will provide continuous power that will last overnight (longer than 12 hours).
We will use a special type of lead-acid battery called ‘deep-discharge battery‘, a type of battery commonly used in rechargeable devices like emergency lights and Uninterruptible Power Supplies (UPS). Deep-discharge batteries are specifically engineered to have thick solid plates, as compared to spongy plates typical of ‘starting batteries’ used in automobiles. A battery with thicker plates lasts longer and tolerates more the rigorous cycle of charging and discharging compared to a battery with thinner (spongy) plates. Unlike with automobile batteries which are designed to deliver high surge currents in a brief span of time, deep-discharge batteries deliver a more steady current at a much longer period — a characteristic that makes it the battery of choice for powering astronomical equipment.
Deep-discharge lead-acid batteries are rated in terms of voltage (V) and current (Ah). Typical ratings are 12 V 9 Ah. The higher the capacity of the battery, the longer the battery life (e.g., a 12 V 9 Ah lasts longer than a 12 V 7 Ah battery). To provide continuous power that will last overnight (approximately 12 hours), a battery must have enough capacity.
How Much Power is Needed
Power is measured in watts, the rate at which energy is supplied or consumed. This value is usually specified in electronic devices, but if not, you can easily calculate for power (P) by multiplying the required input voltage (V) by the required operating current (A). In symbols, P = VA.
First we calculate how much energy is needed to last an overnight imaging session. We will use my laptop as an example:
The values describe that the laptop requires an input voltage of 16 V 4.5 A. To compute for the energy consumption, we refer to the formula P = VA, which translates to 16 V × 4.5 A = 72 watts consumed for every hour (assuming that the device operates at maximum power). Its battery has an output of 14.8 V 6.6 Ah and thus is capable of supplying 14.8 V × 6.6 Ah = 97.68 Wh. This means that if the laptop operates at maximum power, it will only have 97.68 Wh ÷ 72 W = 1.36 hours of battery life. But then we know that a laptop’s energy consumption depends on a number of factors (e.g., application running, screen brightness, peripherals connected, etc.) and does not always operate at maximum power. In my case, my laptop typically lasts for 6 hours on a typical imaging session. It implies that my laptop operates at a much lower power (lower than the maximum theoretical consumption of 72 W computed earlier, and thus consumes less energy) thereby explaining the extended battery life. To determine its energy consumption, we need to compare its battery life at maximum power with its battery life during a typical imaging session, and then calculate how much energy it actually consumes.
We estimate the power rating through indirect proportion: 72 W is to 1.36 hours = P : 6 hours. The power rating would be 72 W × 1.36 hours ÷ 6 hours = 16.32 W. It means that during a typical imaging session, the laptop only consumes approximately 16.32 watts for every hour.
We need to determine the energy consumption of each equipment in our setup, get the total, and then determine the appropriate energy the DIY field battery should have in order to continuously supply power for the desired number of hours (in this case, 12 hours).
You should come up with a list like the one below. To learn how the other values are computed, click here.
Laptop: 16.32 watts
Mount: 6 watts
DSLR camera: 1.33 watts
Dew Heater (main scope): 2.4 watts
Dew Heater (guidescope): 1.5 watts
Dew Heater (finderscope): 1.5 watts
Total: 29.05 watts
This means that in 1 hour, the setup consumes energy of 29.05 watt-hours. Since continuous power is needed for 12 hours, this would translate to 29.05 watt-hours × 12 hours = 348.6 watt-hours for 12 hours.
A typical 12 V 9 Ah deep-discharge battery can supply energy of 12 V × 9 Ah = 108 Wh. This energy output of 108 Wh is far below the required 348.6 Wh. Since our setup requires 29.05 watts for every hour, the 108-Wh of energy coming from battery can only supply power for 108 watt-hour ÷ 29.05 watts = 3.72 hours. The solution would be to consider a battery with a higher capacity, or to connect a number of smaller-capacity batteries in parallel.
In my setup, I have used 4 identical 12 V 9 Ah deep-discharge lead acid batteries connected in parallel. The total current of the system in this case is 9 Ah × 4 batteries = 36 Ah. This results to a field battery with a rating of 12 V 36 Ah, which translates to a total energy of 12 V × 36 Ah = 432 Wh. It should be able to power my imaging setup for 432 watt-hours ÷ 29.05 watts = 14.9 hours.
The procedure on how to construct the DIY battery is described below. For this DIY, you need identical batteries to supply enough power, a plastic housing, some solid copper wires, a battery connector, a switch, a cutter blade, and a soldering iron.
WARNING: I will not be responsible for any damage caused to your equipment. Follow instructions at your own risk!
The DIY Field Battery
Place the batteries inside the plastic housing. I specifically stated the use of a plastic (non-metal) housing to avoid accidental shorting. Use the cutter blade to trim down the sides of the housing and fit the batteries perfectly.
Connect the batteries in parallel. Attach the battery connector (cigarette lighter socket) observing correct polarity (the central contact point is connected to the positive terminal and the outer contact point is connected to the negative terminal). Insert a switch along the electrical path. Use a thick solid copper wire and then solder carefully all the connection points.
The field battery is now ready for charging.
Charging the Field Battery
The DIY field battery must be charged with an appropriate charger. The charger should match the voltage (12 V) and should deliver enough current to the battery.
To charge the battery, connect the charger to the battery observing correct polarity: positive to positive (red), negative to negative (black). Since the field battery has a capacity of 36 Ah, based on the table, the charging time should last for approximately 6 hours.
For chargers with an ammeter, upon connecting the charger to the battery, the meter indicates the amount of current being sent to the battery at any particular instant. This is very helpful in determining the battery status during charging.
Disconnect the charger as soon as the battery is fully-charged. The field battery is now ready to use.
To power various equipment requiring voltage input other than 12 V (e.g., laptop requiring 16 V, a camera requiring 7.4 V), you may just use a DC to DC converter. (Note that while power inverters would also work, that is, changing 12 V DC to 110 V/220 V AC then plugging in the appropriate AC to DC adapter/converter to obtain the required voltage, I would not recommend its use. It is rather inefficient since a lot of electrical energy is wasted as heat. Inverters must only be used if absolutely necessary.)
Examples of DC to DC converters I use in my setup are shown below:
I have been using this field battery since November 2011 and have not had problems powering my portable imaging setup. At my current load, the field battery lasts for approximately 15 hours, more than enough power for an overnight imaging session.
Related article: Improved Field Battery
© Anthony Urbano (Manila, Philippines)