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Webster Telescopes Forum :: View topic - When is 12v not 12v?
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When is 12v not 12v?
http://webstertelescopes.com/forum/viewtopic.php?f=7&t=592		 Page 1 of 1
Author:  Eric [ Thu Apr 07, 2011 6:35 pm ]
Post subject:  When is 12v not 12v?
When is 12v not 12v? When we are discussing 12v batteries.

It is springtime and I've had a few calls about clients "getting back out observing" but having their GOTO move a few inches then stall, move a few more inches, only to stall again. If we are talking about the Servocat, there are usually only two likely reasons: the scope is out of balance, or the battery is giving less than 13 volts. I hear you say "13 volts?! I thought we were talking about 12v batteries?!"

12v lead acid batteries are actually "dead" if they only put out 12v volts! If you hooked a voltmeter up to the battery, you would find:

12.9v 100% charged - full
12.4v 75% charged
12.2v 50% charged
12.0v 25% charged - dead

In fact, fresh off of the charger, your battery will probably read 14v! You have probably noticed that the voltmeter on your dashboard reads 13.8v when your battery is healthy.

So what happens over the winter is that the battery gets neglected and probably has not had a charge in many months. Fresh off the charger it reads 12.9v, but after it sits by itself for two hours it falls to 12v - the battery is dead.

Can this be prevented? Yes, keep your battery on a Float Charger any time you are not using it. Float chargers "top off" the battery every few hours, but do not allow the battery to become overcharged. Float Chargers use very little electricity.

Can I fix my now dead battery? Maybe. If your battery is a "jump-pack" or a Gel Cell, you are out of luck. If your battery is a Deep Cycle Marine Battery it can be fixed by adding warm Epsom salts. There are detailed directions all over the web on how to do this.

Can I use a 12v power supply rather than a battery when I'm at home? Yes, but make sure the power supply actually delivers 13.8v volts or you will have the stalling problem (especially on our larger scopes). We recommend the Pyramid PS15KX 10A 13.8-Volt Power Supply with Cigarette Lighter Jack. It works with the Cigarette Lighter Plug we include with our scopes; no adapter needed.
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