The old batteries are 6 and 7 years old, and there was little doubt they were starting to show their age. Both are the wet lead-acid type, which is a little messy and a tad bit risky to haul around. They're also heavy, each being rather large (Form 27) and weighing about 55 pounds. I could justify replacing them with nice new AGM sealed batteries if they proved to be in poor shape, which raised the question of how to test a battery's capacity.
Both batteries have reserve capacity (RC) values of 175 and were advertised as being deep-cycle. I can't attest to their deep-cycle ability, but they served me through several Iowa Star Parties and four Nebraska Star parties.
RC is the number of minutes a battery at 80°F can provide 25A before it is fully discharged (it's open circuit voltage falls to 10.5V). RC is not the same as amp hours, and certainly not the same as usable amp hours.
To see why, let's pretend we can convert RC to Ah directly:
Amp hour capacity = amps provided • elapsed hours = 25A • (RC / 60) = 0.417 • RC
An RC of 175 gives a capacity of 72.9 Ah. Seem easy, doesn't it? The catch is that you almost never want to drain a battery that far-it will gradually decrease the battery's capacity. If you only discharge it about 80% you probably won't notice any loss of capacity until you're well past 200 discharges. 200 discharges is a lot of nights out imaging! So that 72.9Ah is really more like 58Ah. Sorry!
(Much of the literature about deep cycle batteries is written for solar power users who discharge their batteries nightly. In their case a maximum discharge of only 50% or less is needed if the batteries are to be economical.)
But wait, it gets more complicated! Astrophotography seldom requires anything like 25A. (For example, my setup for imaging with a DSLR requires only about 3 to 4A; it jumps to about 6A when using my CCD) Batteries are more efficient at providing power when the current is lower, meaning that the "real" Ah capacity in that case is higher. My batteries also have a stated alternate RC value of 200 based on 23A draw, which gives a full drain capacity of 23A • (200 / 60) = 76.7Ah. Two amps less and you get 3.8 more Ah. Great!
We're not done yet, though. What you're powering also will come into play. As the battery is drained its voltage falls, and as that happens voltage converters and inverters may have to work harder to provide regulated power. Some devices (Kendrick dew controllers, for example) may shut off. So draining a battery by 80% may not work for you.
Possibly the "best" way to determine how a battery will perform is to test it yourself. I recently found a nice way of doing this that employs a simple AC electric clock, AC lamp, and an inverter with a low voltage alarm. An inline DC power meter can be useful, too, but it's not necessary.
I used a 40W bulb to better match my typical current demand. (The calculation is easiest if we use amps = power / volts, so in this case 40W / 12V = 3.33A.) The inverter and clock draw power, too, so My inline meter suggested that inverter, bulb and clock used about 43 watts and would draw about 3.6A initially. My inverter is programmed to sound an alarm when the loaded voltage drops to 11V; in reality it appeared to cut off at 11.2V
As a reality check I tested my relatively new and well cared for 35Ah AGM battery. I was unable to find an RC value for it that I could trust, so I took values from its spec sheet and (with the help of a spreadsheet) interpolated a capacity of 31.65Ah at a drain of 3.6A.
The battery powered the inverter for 399 minutes, and the meter reported 28.5Ah were provided, but a closer look at current and voltage measurements suggests this should be adjusted by a factor of about 0.93--so 28.5 becomes 26.5. given the uncertainties, I'll use that as the output of the battery. 26.5 is 83% of the estimated 31.65Ah capacity.
Even though all of these numbers are fairly approximate, I think it's safe to conclude that the relatively healthy 35Ah battery was able to provide something like 80% of its capacity before the voltage fell low enough to shut down the inverter. Given that, the test can be repeated with the other batteries; if they are in perfect shape they could be expected to each deliver about 80% of their 73Ah, which is 58Ah. This is very likely an underestimate because we drain at a much lower amperage than 25A.
Battery One, which is seven years old and was once allowed to go dead, provided power for only 213 minutes. That means it provided only about 3.6A • 213 minutes • (1 hour / 60 minutes) = 13Ah. Awful!
Battery Two, which is six years old and was better maintained, provided power for much longer and ended up delivering 26.5Ah, which is still less than half of what a new battery its size could be expected to provide. It should be replaced, too.
The options are 50Ah batteries, $100 ($2/Ah), 30 pounds, or 75Ah, $140, ($1.90/Ah). I think I'll go with the 50Ah ones! Time to place an order!
