[Chaos-l] Where to buy 12V batteries?

[Michael Hrivnak]

Thanks everyone for the advice.  I ended up going over to Batteries Plus and 
getting a Werker 33Ah AGM deep cycle battery.  I'll address some points that 
have been brought up by sharing what I've learned the last few days:

While it is best to limit the discharge of an AGM battery to 50%, it is 
perfectly acceptable to discharge 100%.  You will get more cycles out of the 
battery over its life by limiting the discharges, but any decent AGM should 
get at least 300 cycles of 100% discharge.  300 observing sessions should last 
most of us a long time!

"Deep Cycle" means a battery is intended to be discharged more than 20%.  They 
are good at lots of long discharge cycles.

A "Starting" battery is designed primarily to put out lots of current in one 
short burst.  These do not last long when used in a deep cycle application.

One of the most important ways to keep a battery healthy is to keep it 
properly charged.  And for applications like astronomy, where a battery is 
likely to sit unused for weeks or sometimes months, it is very important to 
keep it on a maintenance charger.  A good automatic multi-stage charger is a 
great solution.  Here is a description of the three stages of battery 
charging:

http://www.solarnavigator.net/battery_charging.htm

On the topic of using an inverter to improve efficiency, I have a theory.  Those 
of you with science degrees, feel free to jump in and correct me!  Many 
devices, such as tracking motors, actually only use 6V.  I recently installed 
6V tracking motors on my Celestron CG-4 equatorial mount.  Rather than deal 
with 4 D batteries as intended, I decided to build a voltage regulator that 
would take any DC source from about 7.5-15V and regulate it down to a steady 
6V.  (I know, I probably could have bought one, but then I wouldn't have 
learned anything! I'm a sucker for these sorts of projects.)

I learned that the most common (and by far the cheapest) method of regulating 
voltage, a "linear regulator", involves basically throwing away a lot of 
current.  For my mount's motors, converting 12V to 6V dissipates about 50% of 
the current as heat.  A "switching regulator" is much more expensive, but can 
be >85% efficient.  Nearly all DC-DC power adapters, such as your cell-phone's 
car charger, use linear regulators.

In contrast, inverters are often better than 85% efficient.  On the other end 
you get a transformer/rectifier combination (that's built into your device's AC 
power adapter) that is also often 85% or more efficient.  With two devices in-
line working at 85% efficiency each, that comes to a combined 72% efficiency.  
Given a comparison of this vs. a 50% efficient linear regulator, we would 
clearly expect to see a performance difference!

To put this in cost perspective, the parts to build the linear regulator 
circuit cost me less than $2.  A switching regulator would have been $20-30.

In sum, the best solution here would be a switching regulator, but because of 
cost, they are difficult to find and rarely used.  Using an inverter with an AC 
power adapter is still quite good, and much easier to do because both pieces 
are very common.  Of course, my whole theory assumes that the equipment you 
are running actually operates on less than 12V and thus requires regulation.

Michael
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