Batteries for CCD Astronomy and the like....
- I don't take credit for all of the information provided here, but have found it to be useful in understanding the problems you
must deal with under conditions such as those we experience were I live - Minnesota, -20 to -35 Fahrenheit in winter... but the
skies are clear<g>:
Lead calcium batteries are the type used to start automobiles. They can supply high current (amps) for short periods of time,
stand mild overcharging without losing electrolyte and not be used for weeks or months and still retain most of their stored
charge. Unfortunately, they can not tolerate being heavily discharged. If they are repeatedly drained by more than 20 percent of
their capacity, their life will be severely shortened.
Lead antimony batteries can be repeatedly discharged from 50 to 80 percent of their capacity, so they are the choice for
operating your astronomy equipment. However, even though they are designed for deep discharge, their life will be longer if
discharge is kept at or above 50 percent of capacity.
Deep cycle batteries range in size from the typical 12 volt RV and marine trawling motor batteries to huge 2 volt cells designed
for industrial applications. These low voltage batteries can be wired in series to get higher voltages and in parallel to get
higher amp-hour capacity, or a combination of the two.
The size of the battery your equipment requires depends on the storage capacity required, the maximum discharge rate at any time
(peak load), and the temperature of the batteries. When considering a battery for your system, all of these factors should be
considered and the situation requiring the largest capacity should dictate battery size.
The storage capacity of a battery, the amount of energy it can hold, is expressed in ampere hours, or amp-hours [A-H]. At any
given voltage, if one amp is supplied for 100 hours, then 100 amp hours have been supplied. Ten amps supplied for ten hours is
also 100 A-H. It is as simple as that. Your battery should have sufficient amp hour capacity to supply needed power during the
longest expected period of continuous use without being discharged more than 80 percent.
As has been indicated, the storage capacity is expressed in amp hours at a certain discharge rate. The discharge rate of a
battery is the amount of time a battery is designed to deliver its full capacity. For example, a 115 amp hour battery (like
mine) with a 23 hour discharge rate is designed to deliver 5 amps. Every hour it will deliver 5 amp hours and in 23 hours it
will have delivered 115 amp hours (if it could be completely drained). If the battery is discharged faster, its capacity will be
less and if it is discharged more slowly, its capacity will be greater. If this same 115 amp hour battery was discharged at 2.5
amps instead of 5 amps its capacity would be 46 amp hours.
Charging a battery too fast (too many amps) causes excessive heat to build up in the battery, which can shorten its life.
Normally, a typical deep cycle RV or marine battery can be safely charged with a current (amps) equal to one tenth of its amp
hour capacity, or less. Industrial batteries can be charged faster than this. The slower a battery is charged, the greater the
charging efficiency. This is understandable because any heat caused by rapid charging is energy lost from the system. Using an
automatic charger will avoid too rapid charging, and keep the electrolyte from being overheated, which can put thermal stresses
on the battery case as well as potentiall cause a leak.
Battery temperature must be considered in the design, particularly here in Minnesota. The capacity of a battery (amp hour) is
correct as stated when the battery is 80°F. When the same battery is 40°F, its capacity is only 75 percent of its rating and at
0°F, its capacity is half of its rating. If your battery will be used in an unheated area, the lowest temperature should be
considered in considering the size of the battery youll need (in amp-hours, of course). In my case, using the same 115 amp-hour
12 volt battery which is at, say 0 Fahrenheit, I have to consider that it will only deliver about 50% of the original 46 amp
hours at 2.5 amperes, which is usually going to be more than enough for an entire night's observing.