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Light-weight, inexpensive 12V power source for a small dew heater

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#1 db2005

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Posted 19 August 2019 - 12:09 PM

Hello all,

 

Have you ever found your grab-and-go scope dewing up because your observing session lasted longer than you first expected?

 

Have you ever felt that needing to use a dew heater precludes convenient grab-and-go observing because of the extra weight and parts to bring outside?

 

Have you ever gotten tired of carrying heavy 12V batteries or power tanks for just powering a small dew heater for a 3-4" grab-and-go refractor?

 

I know I did, and I couldn't find any products on the market suiting my requirement for low weight and low cost. But, being a bit of an equipment tinkerer myself, I wish to share this very simple, low weight and low cost idea for a 12V power supply which can power a small dew heater strip.

 

power.JPG

 

It is made of 8 rechargeable 2000mAh AA batteries connected to a 12V car power adapter which I bought at a local electronics store. Total price for all components is around 40 EUR, and total weight is just under 10 ounces.

 

In use, I wrap it up in a plastic bag with rubber bands around it and put it in a small equipment bag on the mount. Given the fairly low current I use to run my dew heater (something like 200mA to 400mA) the batteries can last for several hours, more than enough for my observing sessions which very rarely exceed 1-1.5 hours.

 

One word of caution if you decide to try this yourself: Be careful to avoid short-circuits as rechargeable batteries can deliver a lot of current. I am considering adding a 1A fuse at some later time.

 

CS,

Daniel

 

EDIT:

based on comments below, I will add the following info: The above power pack delivers around 10V under load, which is perfectly fine for my dew heater controller which works with a lower voltage (down to around 8V as far as I can remember). The total theoretical capacity is 2000 mAh (since the rechargeable cells in the battery are serially connected), and somewhat lower if the power is drained quickly. I wouldn't exceed a current drain of 500mA with my dew heater strips, but 1A will probably work fine too.

 

I did consider LiPo cells too, but I decided to pass on them because (1) AA NiMH cells are much easier to obtain, (2) LiPo cells require special chargers and (3) their extremely low internal resistance makes me pretty nervous in a dew-prone environment in case of short circuits... there's potential for causing real damage


Edited by db2005, 19 August 2019 - 01:06 PM.


#2 junomike

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Posted 19 August 2019 - 12:49 PM

Can I assume the total output is 16,000mAh?

Also what is the maximum draw (Amps)?



#3 TomTTuttle

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Posted 19 August 2019 - 12:56 PM

Cool project.  Great for something like a dew shield, but not anything that actually requires 12V.   It's only 9.6V on rechargables. 12 if running normal AAs. I was thinking about it recently about 3S vs 4S LiPos and tended to go to the 4S size. 3S is only ~11.1V and that was bothering me. But it was only 86 grams (850mAH I think)



#4 RaulTheRat

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Posted 19 August 2019 - 12:56 PM

Can I assume the total output is 16,000mAh?
Also what is the maximum draw (Amps)?


No they're in series, so it'll be 2000Mah. Maximum current again will be the same, since they're in series it'll be going through all of them - I think anything over about an amp wouldn't work so well with small cells like that, but it's all you'd need anyway for the suggested application.

I think a 3S lipo pack (like used in RC applications) would work great for this as well.

#5 db2005

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Posted 19 August 2019 - 01:07 PM

Thanks for all the response so far. I have edited my original post to address your questions.




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