18650 Li-Ion power pack

[dikman]

My new-to-me 6SE came with a Celestron "power tank" (which works fine) but on the Cloudy Nights forum there was discussion about using smaller battery packs that could be strapped to the fork, thus avoiding cable wrap. I have a heap of 18650 cells from defunct laptop battery packs, but no way of conveniently mounting them. Three in series produces 12v, so I looked on thingiverse and found a suitable holder to 3D print. The contacts are simply stranded wire wrapped around the posts inside the printed top and bottom caps. The power cable is from my supply of salvaged DC power cables, just a matter of finding one with the correct size plug.

It works well. I don't know how long they will run for in this situation but I suspect a few hours at least, as the folks on Cloudy Nights said that Celestron has very efficient electronics (unlike Skywatcher, for example).


Just an idea.:)

[sil]

do they talk about any form of regulator to use with the pack? Ive seen many other electronic in the past get fried by simlar battery packs (ie batteries + connector + box). gettng the voltages correct is easy enough but the amps are different considerably and not all devices are built to run off rechargable batteries instead of the recommended alkaline. plus one faulty cell in the pack can do all sorts of bad stuff so make sure to charge separately and all cells charge completely, if one starts to fall behind, ditch it. Also make sure your soldering is solid and contacts have firm connections, you dont want any chance of batteries arcing at all. its a workable solution, just advice for others to take care when trying this. Always pay attention to the specifications for powering the device like voltage and amp ratings and ranges especially. The device might allow for some tolerance in deviation but never assume it will.

[sharkbite]

Hi Richard...

This is a good idea....as sil said be careful of the voltages.

You might find that these drop to below what the Mount needs...
I'd considered doing this on my Meade, but with 4 26650s and a regulator to
Give exactly 12v.

You should get a few hours out of this - I drive my lx90 all night with a 2.2 ah SLA.
And your batteries should be about that.

Don't charge them in series. If one lets go or shorts it will take out the other two.

A proper charger for these can be found on eBay for not much $

[dikman]

The articles were referring to buying already made up packs with their own charging cct. No idea, of course, just how efficient they would be.


Using 3 cells is, in part, an experiment, to see just how long it can run on such a pack. Just had a thought, I should hook up a power supply and start dropping the voltage to see at what point the mount stops working.


I had thought of using 4 (or more) cells and building a 12v regulator but then it starts getting bulkier, however if 3 cells prove impractical then I guess that's my next step.


As for charging, yes, they're recharged individually as I don't like the idea of charging these things in series.

[Ukastronomer]

I think considering how expensive scopes are (mounts/drives) to replace I would rather buy a proper power supply

[dikman]

It's quite safe - as long as you know what you're doing.:) There will generally be some operating tolerance, for example my Celestron power pack reads over 13v with no load, but if the power supply is rated at 12v and is regulated then there's little danger of any problems occurring. The real danger is if someone uses a cheap unregulated plug pack as the no-load voltage on those can easily be several volts higher.


It's also quite common for people to use 12v gel and lead acid batteries and these are often 13.5v when fully charged, further proof that there is "some" margin.

[Hoges]

I powered my HEQ5 with a 3 cell LiPoly from Hobby King. It was about 12.5v off the charger and I'd rarely end up running it much below 12v - it worked very well and was much lighter, less affected by cold and the voltage didn't drop as much as lead acid. Not sure how many amps the mount needed (maybe 2?) but it was well within what the lithium batteries could supply when needed. Those 18650's are in everything - computers, all my cordless tools, flashlights, power banks and even the first generation (or two?) of the Tesla electric cars. Amazing what we can 3D print these days.

[dikman]

For anyone who may be interested I carried out a couple of tests on my Nexstar 6se. The mount will operate quite happily down to 9v, at 8v it still worked but there was a noticeable slowing of the motors (so not a good idea).


When slewing it draws 400 mA. Using my 3 cells, at a nominal 11.7v, means they should be good for quite a few hours.

[Imme]

I believe lower than optimal voltage, even though it will work, will burn the motors out.
Similar to having too much voltage drop from a long line of extensions cords joined together when using 240v.....it kills whatever you’re using

[luka]

I don't think that the motors will burn out if you run them at lower voltage.

Stepper motors have fixed coil resistance (unless hot). Running them at lower voltage will result in lower current and hence in lower torque.
Lower voltage+current also means lower power consumption and less generated heat (longer lifetime?).

However, if the voltage is too low they may start skipping steps and the controller may not like it either.