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Cold weather observing extenders

Battery extenders

Or how to keep your batteries providing power when the temperature drops.

See this PDF for more details: keeping batteries warm - website.pdf

Completed box

To the right is the finished box with a 35ah battery inside (dimensions of this box are about 8"x10"x10"). With an internal heater using 4.5 watts of electrical power (300ma @ 15vdc), it was able to keep the sealed lead acid battery at 70 degrees f even though it was 15 degrees F outside. 

Now 300mah is relatively insignificant compared to the 35ah capacity of the battery that it's keeping warm - and functional in the cold weather while driving the 2 amp peak loads of my mount, dew heaters and ancillary equipment. Without the insulation and the heater, this battery died at about two hours into an observing session when it was about 15 degrees f outside. With the box and heater, I can get more than ten hours of observing time at these low temperatures.

The inner foam core box with 53 ohms of nichrome wire (nominal 3.3watts at 12vdc):

Reflectix surrounding the foamcore:

Insert this box into a box made of 1" Styrofoam as shown at top.

Some data:

The following graph shows what happened one night last winter. The temperature was 25 degrees and expected to drop further.I took all components (except the battery) outside to let it them reach ambient temperature.

  • I had two lM335 temperature sensors connected to an Arduino sending temperature measurements back to my pc once a second.
  • The vertical axis is temperature and the horizontal axis is time.
  • The red trace is the ambient temperature. 
  • The White trace is the absolute temperature inside the heater box.
  • The yellow trace is the temperature differential between the inside of the box and the outside air temp.
  1. After the inside of the foamcore reached ambient, I turned on the heater at 12v (3.3watts) and let the inside reach equilibrium. The temp rose to about 15 degrees.
  2. I then put the foamcore heater box into the reflectix box and again waited for equalibrium which occurred at about 50 degrees.
  3. The last step was to put the reflectix/foamcore box into the styrofoam box. At equilibrium (approx 60 degrees), I turned up the power to the heater until I was at 70degrees. The power at this point was 4.5 watts. It took no more than 4.5 watts to raise the internal temperature of the box from 15F to 70F.
  4. At the end of the test the heater was turned off and the inside temp took an hour and a half to drop from 70 degrees back to near ambient.

Now if the battery HAD been in the box at 70degrees, the mass of the battery would have allowed it to stay warmer longer JUST due to the insulating factors of the box. It's quite possible, I could have gotten four or five hours out of the battery instead of the previous two just by using the box with no heater.

Also, due to this mass, I think the amount of heat generated can be reduced to conserve even more battery power. These are tests to run next winter. What I do know is the box and heater kept me running much longer last winter than without it. 


Hand controller warmers:

After all, these babies control the mount and have to be left outside in the cold. Below about 40 degrees f the displays get dim and sluggish. Any colder and they are unusable. They NEED to be kept warm.

Layout your heater to cover the upper half of the handcontroller where the temperature sensitive LCD display is located.

Use some soft foam for support and protection.

Wrap with reflectix

Surround all with 1" Styrofoam - Open faced for easy insertion and extraction of the controller.

Laser Warmers

Lasers don't work well below about 60 degrees f and certainly not below about 50 degrees f. So adding heat and insulators is required if you want your laser to work in the cold. Now if you're just using it to point to things, then keeping it in your pocket close to your body should keep it functional all night long. But if, like me, you use it as a finder on your scope, then you know what kind of a pain it is to insert it, align it, use it then remove it whenever you want to move your scope to another object.

I finally got around to creating a warmer for my laser so I could leave it in its holder all night long. I figured six watts ought to keep it warm so I worked up a grid of twelve 330 ohm 1/2 watt resistors. All are in parallel with four rows of three each. This puts three watts around the electronics end of the laser and three watts around the battery compartment. I laid these out on a small sheet of construction paper wrapped around the laser:

Around this I wrapped some of that Hobby Lobby 18 inch Foam sheeting:

And finally, I wrapped that with some scrap Reflectix insulation (With "dew shield" snoot):

So on the first power up test it became evident I might have overestimated how much power I needed to keep this thing warm.

I put the final assembly in the freezer which is usually between 5 and 10 degrees f and let the laser get down to about 25 degrees. I turned on the heater and when I came back 15 minutes later, the internal temp was already 117 degrees. YIKES! So now I have a solution to keeping my laser warm, I just need to refine it a bit.


We've had some cool, high dew observing nights so I used my heated laser. Results were very encouraging. In use, it becomes apparent when the temperature is affecting the laser. The beam starts to get dim and that's a que to turn on the heater. It doesn't take more than 5 minutes long to bring it back to full brightness then I was good for about a half hour.

Update II:

I was at the 2015 Illinois Dark Sky Star Party (IDSSP) and things were pretty chilly and VERY "dewy". Since I have a DewBuster to control the dew heater on my C8 and ED80 scopes, I figured what the heck and I plugged the laser into the extra controller port for the C8 heater.

The DewBuster is set to keep the C8 corrector about 8 degrees warmer than ambient. I slaved the laser heater to the C8 heater so that whenever the DewBuster turned power on the C8 heater, it would turn power onto the laser heater. It worked like a champ! I monitored the laser temp by feel and I never felt anything "Too warm". The laser worked perfectly all night long even though the ambient temp got down to about 40 degrees.

I'm not happy with this arrangement as there is no temperature monitoring so I'm going to work on controlling the heater with a thermistor controlled switch. That and a small thermometer so I can monitor the laser temperature.

"Blankets for Scopes"

For my page on "Blankets for Scopes", another cold weather protection device, click on THIS or the image.

Unfortunately, with the demise of google+, these annotated images are no longer available. I am furiously working on a new page to contain this information. Sorry for any inconveniences. In the meantime, try the link below. View the individual images for additional comments attached:

New link to Scope Blanket Album

Link to Scope blanket Album

Grant Martin,
May 16, 2015, 11:45 PM