14 replies to this topic

[asaint]

Portable Power Primer Article

[rboe]

After moving to Arizona I found they like Hot Cranking amps. Go figure. The heat down here just kills batteries: But that can be an article unto its' self.

I didn't know about the upper limit on battery drain and why you don't want to buy a huge battery if your needs are small; expectin bigger to be better. The article was worth the time to read, if only for that tid-bit.

There are gizmos' that will take your car battery voltage and make it good enough for your laptop without going up to 120V and having your laptop cut it back down to what it needs. Laptops can vary in their needs a bit so it's best to check it out.

I have used my car battery to power my scope since I've had the scope; but when you're a weenie when it comes to observing and five hours seems like a long time it's not a problem. Now I have the laptop and camera so this article is VERY timely. I have to thank John for a well written article and the juicy bits of info I can put to practicle use.

Thanks John!

[jrcrilly]

Thanks for them kind words, Ron.

I keep forgetting that many laptops only need 18 volts or so; my Compaq just has an AC line cord. The charger's built-in.

[jrcrilly]

Welcome, Sam.

Yes, I and many others have used such battery packs with the LXD55. You'll need a cable with a cigarette lighter plug on one end to go into the battery pack and a coaxial power plug on the other. For the LXD55 (or any other current Meade instrument) that'll be 5.5 mm outside diameter, 2.5mm inside diameter, center positive. Such cables are available from Meade dealers, Radio Shack, and www.scopestuff.com . http://www.scopestuff.com/ss_cig1.htm

[David Knisely]

OHH Boy, this article left out a big "no-no". You do *NOT* want to use the battery in your vehicle for long-term constant current drain applications, as it will eventually damage or even kill the primary function of that battery, namely the high surge current needed to start your car. Low constant drain will cause the lead in the cells to actually fall off of the thinner plates of a starting battery (although the process happens gradually). Occasional use for things like GO-TO telescopes is one thing, but depending on the car's internal main battery for laptops and telescopes is probably not a good idea. Check out the article "Zen and the art of Astro-batteries" on the NexStar 11 page: http://www.nexstar11...tutorials.shtml Clear skies to you.

[scopefreak]

A very well written piece. I use 2 portable battery packs. I have the Celestron Power Tank 7Ah for my telescopes and the Powerpack 400 plus from Sears that has a built in inverter for my laptop. The only down side is I can only get about 5 hours out of the Powerpack before it needs a charge. I am thinking of building a battery box with a 105Ah deep cycle battery and inverter just for the laptop. How much time do you think I would get out of that?

Kevin

[Tom T]

Kevin

A typical laptop computer draws around 1.5-3 amps depending on if it's charging the battery or not (check your docs to be sure). If the 400 plus is a 17amp hour gel cell, and you are getting 5 hours before it's telling you you need to recharge, then you are probably drawing about 1.7 amps / hour. Remeber on these you really only use 1/2 the capacity of a battery before you should recharge the unit. (Deep Cycles can obviously go a bit lower) When taking this to the deep cycle battery, don't forget to figure in some loss for the inverter - 10% is fairly common. But the bottom line is the 105A/h deep cycle would give you tons of time.

Tom T.

[scopefreak]

By tons of time do you mean like 10 to maybe 15 hours?

Kevin

[Tom T]

By tons of time do you mean like 10 to maybe 15 hours?

Kevin

Kevin

If that's *all* you are running: and you really are drawing 1.7 amps / hour, and it's a 105 aH battery... Without figuring in the inverter (running the laptop off DC), 105/1.7 = ~61 till it's TOTALLY dead, (not recommended) and thus about 30 hours till it's at 50% (or ~45 hours at 80% for a deep cycle).

Tom T.

[scopefreak]

WOW, Thats a long time. I found a 105 Ah deep cycle for $79.95 without a charger. Most chargers I've seen go for around $19.95. I found this site for building a battery box
http://www.twcac.org.../bcastro_01.htm that looks fairly easy and cheap.

What do you think


Kevin

[jrcrilly]

What do you think
Kevin

That's a ton of capacity; should be great. Just don't neglect it - careful charging/discharging plus some means of equalizing the cells are necessary to keep that capacity available.

[scopefreak]

What do you think
Kevin

That's a ton of capacity; should be great. Just don't neglect it - careful charging/discharging plus some means of equalizing the cells are necessary to keep that capacity available.

How do you mean "equalizing the cells" . What does that mean

Kevin

[jrcrilly]

How do you mean "equalizing the cells" . What does that mean

Kevin

Hi, Kevin.

As discussed in the article referenced (but usually neglected in astro battery discussions) there are issues involved when using multiple cells in series. Unless periodically charged or discharged at a substantial rate the individual cells will gradually achieve varying states of charge. The usuable capacity of the battery is limited to that of the least-charged cell, so capacity will drop off. This is why a simple "bigger is better" approach isn't necessarily the correct way to approach battery choice.