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Subzero temperature battery

Subzero temperature battery

(OP)
I need to source a small rechargable battery pack, of ideally 7 volts (12v max) with a capacity around 3 to 5 amp hours.

It will be used for several hours at a time, often in temperatures down to about minus 10 degrees C, and will need to be completely sealed since it will be in motion tilting at various angles and maybe even upside down at times. I do not want to have to provide any form of insulation against the cold, so the battery will get very cold whilst in use.

Can someone please advise me on the best type of battery, and/or point me to a supplier of a suitable battery to meet these requirements.

Thanks for looking.

RE: Subzero temperature battery

Most batteries work down to -20C so other things will dictate which to use. You'd need to provide more details for a better suggestion.

"Small" doesn't help enough.

Is this a one off?

Is this a one-time use or a rechargeable application?

Does weight matter?

Does cost matter?

Why can't you insulate or heat the battery?

Are you able to keep on top of the battery's maintenance?

Cost matter?

Do you already have a charger for any type of battery?

etc etc etc

Keith Cress
kcress - http://www.flaminsystems.com

RE: Subzero temperature battery

The "18650" type Li-ion cell is one of the most common; used in laptop battery packs and flkashlights and elsewhere. The nominal voltage is 3.7v, so two in series might (?) be about right. Some may claim 3000 mAh, but you'd best remain skeptical. They're readily available and extremely cheap (eBay), both as cells along with any possible supporting modules (chargers, voltage converters), or as packaged battery packs.

I mention this only as a starting point. You will need to investigate fully against your requirements. As mentioned, remain skeptical of purported specifications.

RE: Subzero temperature battery

(OP)
Hi Keith, and thank you for your interest.

Here are some more details.
This is a one off requirement related to my hobby. I have an astronomical telescope with a 14" primary mirror, which will be stored indoors and taken in the car for use at a dark sky site. In winter at this place, night time temperatures can get very cold. Once the telescope is assembled on site, the mirror, which is quite a large chunk of glass needs to cool to ambient temperature in order to perform at its best. During this cooling process, which (with no forced cooling) may take a couple of hours or more depending on the temperature difference, the minute movements and deformations in this extremely accurately figured parabolic mirror surface will produce poor images. Only after the mirror has settled down to, or very near ambient temperature, will it perform well. In order to substantially reduce this cool down time, it is necessary to force air around the mirror and create an airflow up through the telescope tube. This airflow also has the advantage of helping prevent dew formation on the mirror surface. The mirror I have is mounted in a steel cell, with provision for fitting three 80mm fans for this purpose. I propose to use the following fans after consultation with the manufacturer, for their low noise/vibration qualities and the fact that they have sealed bearings specified for used in damp atmospheres down to -20 degrees centigrade...
http://www.quietpc.com/afdp-8025b

I want a battery capable of powering these three fans continuously, all night long if necessary. When I have a suitable battery, I will make some bracketry to attach it directly to the mirror cell within centimetres of the fans, in order to eliminate trailing/moving wires when the telescope points to different areas of the sky. This is why I need it to work when tilted at various angles. The weight should not be a factor since I have various removable counterweights (up to 9 kilograms) already bolted to the mirror cell, to balance equipment attached to the other end. I don't think size will be a problem for the capacity I require, but I am envisaging a battery to fit into a space no larger than about 200mm x 70mm x 70mm.

I may consider purchasing two batteries in order to have one available as a back up. I don't want to pay a fortune for these batteries, but I don't foresee them being particularly expensive anyway. I don't want to have to insulation the battery causing unnecesary bulk, and feel that this would not keep it warm for long anyway. Heating the battery which will be positioned close to the back of the mirror is not an option, as it would be defeating the object of cooling the mirror.

So, to sum up and answer your queries, here are my requirements...
A one off purchase of one or two batteries.
Cost - hopefully not a problem.
7v to 12v rechargable.
3 to 5 amp hour capacity.
Work efficiently down to at least minus 10 degrees centigrade, below that, I probably won't work :)
Sealed, and able to work when positioned at varying angles.
Weight, no problem within a size limit of about 200mm x 70mm x 70mm.
Recommendations on a suitable battery charger will be welcome, preferably to maintain these on a continuous 24/7 basis, so they are always ready for use whenever the skies clear.

Hopefully I have explained the above with sufficient clarity.

Cheers,
Pete.

RE: Subzero temperature battery

(OP)
Hi Keith, and thank you for your interest.

Here are some more details.
This is a one off requirement related to my hobby. I have an astronomical telescope with a 14" primary mirror, which will be stored indoors and taken in the car for use at a dark sky site. In winter at this place, night time temperatures can get very cold. Once the telescope is assembled on site, the mirror, which is quite a large chunk of glass needs to cool to ambient temperature in order to perform at its best. During this cooling process, which (with no forced cooling) may take a couple of hours or more depending on the temperature difference, the minute movements and deformations in this extremely accurately figured parabolic mirror surface will produce poor images. Only after the mirror has settled down to, or very near ambient temperature, will it perform well. In order to substantially reduce this cool down time, it is necessary to force air around the mirror and create an airflow up through the telescope tube. This airflow also has the advantage of helping prevent dew formation on the mirror surface. The mirror I have is mounted in a steel cell, with provision for fitting three 80mm fans for this purpose. I propose to use some fans which have low noise/vibration qualities and the fact that they have sealed bearings specified for used in damp atmospheres down to -20 degrees centigrade.

I want a battery capable of powering these three fans continuously, all night long if necessary. When I have a suitable battery, I will make some bracketry to attach it directly to the mirror cell within centimetres of the fans, in order to eliminate trailing/moving wires when the telescope points to different areas of the sky. This is why I need it to work when tilted at various angles. The weight should not be a factor since I have various removable counterweights (up to 9 kilograms) already bolted to the mirror cell, to balance equipment attached to the other end. I don't think size will be a problem for the capacity I require, but I am envisaging a battery to fit into a space no larger than about 200mm x 70mm x 70mm.

I may consider purchasing two batteries in order to have one available as a back up. I don't want to pay a fortune for these batteries, but I don't foresee them being particularly expensive anyway. I don't want to have to insulation the battery causing unnecesary bulk, and feel that this would not keep it warm for long anyway. Heating the battery which will be positioned close to the back of the mirror is not an option, as it would be defeating the object of cooling the mirror.

So, to sum up and answer your queries, here are my requirements...
A one off purchase of one or two batteries.
Cost - hopefully not a problem.
7v to 12v rechargable.
3 to 5 amp hour capacity.
Work efficiently down to at least minus 10 degrees centigrade, below that, I probably won't work :)
Sealed, and able to work when positioned at varying angles.
Weight, no problem within a size limit of about 200mm x 70mm x 70mm.
Recommendations on a suitable battery charger will be welcome, preferably to maintain these on a continuous 24/7 basis, so they are always ready for use whenever the skies clear.

Hopefully I have explained the above with sufficient clarity.

Cheers,
Pete.

RE: Subzero temperature battery

Okay, that's definitely some more info Peter.

One of the nicest 20's I've seen is a collapsible unit, sort of like a Chinese finger-trap, the secondary and spider get collapsed down to the primary for transport and extended for use. The best part though is that the scope is all open when setup so it reaches thermal equilibrium very quickly. It's also part the value of all truss based scopes these days:


Might be a little scary but you might consider laser cutting a honeycomb pattern out of the barrel of the scope for the same reason. Having a few hours more of viewing time each night would probably be very nice.

But, back to your batteries. That scope already weighs a lot so I'd probably reach for Lithium Ion batteries. I would still suggest you insulate them because they are dense and will cause long-term convection around them as they cool slower than your scope.

The fans you contemplate are pretty noisy if driven at full voltage. You might consider driving them under voltage. Because of cell voltages LiPo batteries tend to be 11.1V or 14.8V packs. What I'd suggest is using the 11.1V packs as they would drive the fans just fine but with probably a loss of 10 or 15% of flow but 60~70% less noise. Noise = vibration.

I'd snag an appropriate LiPo R/C 11.1V 5000mA battery pack and a matched smart charger.
Example:
Charger
LiPo Pack
Page of Packs
LiPo packs are tough for severe service and are rated for serious current. You should include a fuse as these packs can deliver over a hundred amps if shorted.

When using one of these packs you need to make sure you don't drain them below 9V or you could have a bad day. They tend to drop off the curve really obviously so they'll keep to around 11V for long time then plunge over the precipice. So I'd suggest you use a voltmeter judiciously until you understand the discharge pattern for your use so you stay above 9V. Tune your ear to hear that drop-off, or better, have the pack sized to easily run your fans for the entire session.

Considerably less expensive but considerably heavier are Lead Acid batteries.
There is a standard 7A 12V size. It's the most common size of them all. Downside? Four times heavier. The discharge curve is much more tapered and won't be audibly detectable. You'd want to definitely size for the whole night. If you drain a LA "gel cell" below 12V and let it sit for more than a few hours it will be damaged. The modern ones sulfate incredibly fast which essentially reduces the battery's capacity - fast. Also you will see only 60% of the AHrs out of a LA down @ -20 so you'll definitely need more capacity then the math sez at room-temp. Being even more massive than the LiPo solution the LA battery will mean more thermal equilibrium time.

If you go this route, stick with major brands; UBG, USB, Panasonic, Power Sonic, or Yuasa, cuz a lot of the others are junk. You truly get what you pay for on these batteries.
LA Battery
Excellent LA charger







Keith Cress
kcress - http://www.flaminsystems.com

RE: Subzero temperature battery

(OP)
Thanks very much Keith.

I prefer to go with your recommendation for the LiPo and charger. Much more compact and neater than lead acid. I plan to use a speed controller with the fans, to tune out any vibration.

I hadn't thought too much about the effect of heat from the battery, but with the small size of that battery, some insulation around it will not be a problem. However, you now have me wondering if even an insulated battery would still cause convection negating the desired cooling effect, after all, any heat from the battery will escape eventually even through insulation, and possibly the battery could heat up more with insulation. My initial thoughts were to have a neat battery/fan arrangement totally self contained and fixed to the back of the mirror cell, in order to eliminate trailing wires. I may still rethink my design and possibly use a battery separate from the scope with the necessary trailing wires. I just don't know how much heat the battery is likely to give off, and the fans would be drawing air in from around the battery to achieve my neat design. Any further thoughts on the likely heat output of this battery during use would be welcome.

That 20" is a lovely scope, and is the type and size I will eventually build. Is it yours?

I have been doing astronomy for over 40 years now, with many scopes previously only up to 10" in size. I'm currently doing various mods on the new 14" which has not yet seen first light. This is the model I have...


With so much light pollution here in the UK, I will be using a fabric light shroud around the open truss. Over here we tend to have smaller cars than you in the states, and therefore, in general, smaller scopes :)

Cheers,
Pete.

RE: Subzero temperature battery

Hi Pete.

I think the LiPo solution is the best too. Insulation verse no insulation.. The batteries will give off some heat as you use them due to their internal resistance, however, that is greatly a function of the draw rate. If you use a ~5Ahr pack to run a few fans the internal dissipation will be inconsequential to your operation as compared to the thermal difference provided by a room temp battery. If you insulate you reduce the thermal exchange which will reduce the convection disturbances, and as an added benefit, will keep your batteries warmer. I would go with the back-of-the-mirror-cell mount with light LiPos as a first try. I wouldn't dream of hanging a massive LA battery off that point as I can't imagine it wouldn't distort the mirror cell structure in some subtle and diabolical way. I'm not even sure the LiPos won't cause some issue in that regard but one less cable dangling in the dark around your feet is worth a try.

Let us know how it goes.

As for that 20 above, no, it's just a scope (17?) picture to illustrate the "open truss" style. It's not the really cool collapsing 20 I described. I can't find a picture of it. That one's structure had about 60 pieces so it could fold down. I had a 17 inch Dobsonian until recently. It was a mammoth pain in the butt to use. The tube alone weighed 105 pounds without the mirror. Then the base was another 140lbs and large and ungainly foot crusher. Couple those issues with living in the absolute worst town on earth for star gazing and that meant I always had to load my scope up into, no less than a truck, and lug it up somewhere that could also be shrouded in fog. I finally broke it down keeping the spider, primary, secondary, and eye piece with the vague hope of building a collapsible or at least a truss scope so I don't HAVE to have someone go with me just to help with lifting.

Yes a shroud around light is needed. Truss scopes also have a little less contrast since any off axis light tends to cause that. A shroud that can absorb any and block is a great idea - after the scope is at ambient. :)

Cheers.

Oh, keep us informed.

Keith Cress
kcress - http://www.flaminsystems.com

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