Friday, March 23, 2012

Compact field power

Many years ago when I was using my Giro mount with Tech 2000 GiroDrive, I have bought a 12V 7Ah Sealed Lead Acid battery to power everything.  It has enough juice for a whole night of observation and it even supplies power to my Sony Vaio U3.  Then I moved to a Meade LXD55 and I continue to use the same battery.  It works well, too.  Until then, I have bought a Takahashi TG-SP II for wide field imaging and I bought another 6V Sealed Lead Acid for it.

After so many years, I eventually settled myself with a GOTO Mark-X mount with a modified Carton tracking motor (the one with Tele Vue System Mount), and now it takes 9V.  With its very energy efficient controller and motor, now I only need a small battery.  In order to improve the overall portability, I've sold all my Sealed Lead Acid batteries together with the charger, and the transformer for notebook computers.  I turned myself to a Lithium battery pack.

This battery features a solar charger which works even under overcast sky, and in strong direct sunshine, the solar panel gives 0.8W which is not bad.  It provides 4.5V/5.5V/6.5V and 9V and the maximum output current is 800mA.  Now I just leave it near the window and it will gather enough juice for my weekend observations (which is usually one to two hours at most).  Of course, if I plan for a longer observation session, I will charge it via USB.

This battery is very light and compact.  Its rated capacity of 2600mmAh is probably real according from my test.  I have tried to use it with my Mark-X and it works for over 5 hours without problem.  Theoretically, it will work over 8 hours.  However, given the efficiency drop due to voltage stepping and also discounted capacity, I would expect around 5 hours.

This is a very good battery for an energy efficient mount.   However, it does have one problem.  One day with strong sunshine while I was doing solar imaging, I found that the controller was complaining low power but the battery should be charged.  Why is that?

I found that when the solar panel was working, the internal battery will be switched to charging mode and it will not delivery power.  And in that case, only the solar panel will be providing power and at 9V, the 0.8W panel at fully efficiency, will be just giving 88.9mA which is just half of what is required (160mA) by my motor and controller!  No wonder it was complaining.  In order to verify such hypothesis, I turned the solar panel upside down and then the controller stop complains at once.

This is no good.  I was thinking that I could harvest solar energy while imaging but then I found it's not supported by this battery pack.  Anyway, it is not too critical.

Another side note about this battery pack is that, it features three bright white light LED and they could be nice for us to setup and tear down the setup on site.

The bottomline is that, given its relatively low capacity, we should better have two such battery pack or we will need some short of backup power source.  While the solar panel is a interesting thing, we just could not rely on it.

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