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Power packs and voltage conversion / Skywatcher NEQ6

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#1 John Tucker

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Posted 13 June 2018 - 12:54 PM

Fairly recently I purchased a Skywatcher NEQ6 mount and a fairly high end lithium battery pack to drive it.  Unfortunately I'm having a lot of trouble that I believe to be related to the voltage of the lithium battery dropping from its fully charged, low load value of 11.5 volts to something just below 10.  The mount manufacturer strongly recommends a power source capable of delivering at least 4 amps at 11.5 to 16 volts. 

 

I hate to replace the battery because I spent a fair bit on the lithium pack and aside from the voltage shortfall it is a nice unit.  It's specs say it will deliver 180 watts from the DC port, which would be about 15 amps at 12 volts.  There is an AC outlet on the unit that the specs claim will deliver 100  watts at 120 volts, which is about 8 amps.  Its a big manly battery with 150 watt hours of storage, it is just falling short on the voltage by 10 to 20%, and this shortfall is catastrophic for my purposes.  

 

Am considering

 

1. Buying a AC power supply for the telescope and plugging it into the battery AC outlet.  It seems ludicrous to convert DC to AC and then back to DC, but I think the battery probably has the capacity to do that and still deliver 12+ volts at 4 amps. Like I said, its a big manly battery.  

 

2. There are voltage converters online that will convert 12 volts DC to 15 volts DC, which I think would covert my 10 or so volts to about 12.5 volts.  I'd just have to pull out the old soldering iron and put it inline on my power cord.  

 

 

Thoughts on this?  Anyone with similar experience?  I worry a little bit about the AC power supply approach because I destroyed a Meade LX200 mount using a nominally 12 volt AC converted purchased on Amazon.  Apparently the quality of these varies widely and they can have voltage spikes. Wife probably won't throw for another $1700 mount. 

 

Thanks

 

JT

 

 Capture.JPG Capture.JPG


Edited by John Tucker, 13 June 2018 - 01:08 PM.


#2 Chuckwagon

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Posted 13 June 2018 - 02:23 PM

The battery is probably a 3 cell lithium design, so if each cell were 3.8v full capacity, you're only getting 11.4v total.  So right off it is a bit low on volts.  Then, 150 watt-hour is only around 13 amp-hours at 11.4v.  So it really isn't a very beefy battery.  Using the 120v outlet will give you even less capacity, as the 120v conversion is wasteful.  (Also, watts=amp x volts, so 100 watts = .83 amps x 120v, which is quite a bit less than 8 amps, so the 120v outlet isn't intended to run anything very power hungry.  Unless you meant 1000w.  :)  )

 

A buck converter/voltage regulator would likely work fine, but with conversion loss it' likely your capacity is never going to be more than 10 amp-hours or so.  That's probably enough to run just the mount for an evening or two, depending on how much slewing you do and how long your night runs.

 

So I'd say investing $10 to $15 on a converter is probably worthwhile, but I'd likely get a higher capacity main battery and use this unit as a secondary supply or backup to the main.

 

If you want more power, there are tons of threads discussing batteries, costs, chemistries,etc., and I'm sure you'll be able to find enough info to make a choice.

 

Cheers


Edited by Chuckwagon, 13 June 2018 - 02:25 PM.

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#3 John Tucker

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Posted 14 June 2018 - 07:05 AM

The battery is probably a 3 cell lithium design, so if each cell were 3.8v full capacity, you're only getting 11.4v total. So right off it is a bit low on volts. Then, 150 watt-hour is only around 13 amp-hours at 11.4v. So it really isn't a very beefy battery. Using the 120v outlet will give you even less capacity, as the 120v conversion is wasteful. (Also, watts=amp x volts, so 100 watts = .83 amps x 120v, which is quite a bit less than 8 amps, so the 120v outlet isn't intended to run anything very power hungry. Unless you meant 1000w. :) )

A buck converter/voltage regulator would likely work fine, but with conversion loss it' likely your capacity is never going to be more than 10 amp-hours or so. That's probably enough to run just the mount for an evening or two, depending on how much slewing you do and how long your night runs.

So I'd say investing $10 to $15 on a converter is probably worthwhile, but I'd likely get a higher capacity main battery and use this unit as a secondary supply or backup to the main.

If you want more power, there are tons of threads discussing batteries, costs, chemistries,etc., and I'm sure you'll be able to find enough info to make a choice.

Cheers



#4 John Tucker

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Posted 14 June 2018 - 07:37 AM

Thanks for the comments.  Will definitely go with the voltage stepup adapter.  Will let you know how it goes. 



#5 Phil Sherman

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Posted 14 June 2018 - 01:46 PM

I have a similar issue with my Atlas mount when running it from a 12V SLA battery. At the start of an evening's observing all was well but at the end of the night, when I also needed to use the battery to power the laptop, the voltage dropped to the point where the mount was complaining.

 

My solution was to use a buck/boost converter to keep the voltage at 13.2v. I've had no issues since starting to use it.


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#6 John Tucker

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Posted 14 June 2018 - 05:39 PM

I have a similar issue with my Atlas mount when running it from a 12V SLA battery. At the start of an evening's observing all was well but at the end of the night, when I also needed to use the battery to power the laptop, the voltage dropped to the point where the mount was complaining.

 

My solution was to use a buck/boost converter to keep the voltage at 13.2v. I've had no issues since starting to use it.

Aside from the noise, did you notice any issues with GoTo alignment?  I've been having all kinds of problems. I was under the impression that GoTo alignment was usually pretty smooth if you had the polar alignment correct and accurately placed the tube in home position before starting, but I had my polar alignment spot on the other night (as demonstrated by several 3 min exposure photographs) and the GoTo was wildly off.  Especially toward the end of the night after I'd repeated the alignment a few times trying to get it better, you could just look at the OTA and tell it was pointing 20 degrees or more off from the target. 


Edited by John Tucker, 14 June 2018 - 05:39 PM.


#7 carolinaskies

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Posted 15 June 2018 - 12:46 PM

In the short term that converter will allow you to continue to use the battery. But in the long run you'll want to look for a better Battery Solution if you plan to run extensively and don't want to be hassled by constantly needing to recharge or swap batteries around.

Personally I don't think running the mount under voltage is a good thing. I would want to ensure that I was powering it towards the top end of the voltage rating say around 14.5v so that by the time my observing session was over I might have only discharged to just above 12 volts. Mounts definitely will start to do wonky things when the voltage and amperage drop into weak ranges.
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#8 John Tucker

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Posted 15 June 2018 - 01:16 PM

In the short term that converter will allow you to continue to use the battery. But in the long run you'll want to look for a better Battery Solution if you plan to run extensively and don't want to be hassled by constantly needing to recharge or swap batteries around.

Personally I don't think running the mount under voltage is a good thing. I would want to ensure that I was powering it towards the top end of the voltage rating say around 14.5v so that by the time my observing session was over I might have only discharged to just above 12 volts. Mounts definitely will start to do wonky things when the voltage and amperage drop into weak ranges.

I freely admit I don't know what I'm doing here, but its a 150 watt hour/12 amp hour battery.  Assuming the mount runs at about a third of maximum draw continuously, should be good for a minimum of 6 hours. 

 

The larger of the two Celestron Power Tanks (I'm aware of their terrible reputation) is about 200 watt hours and being a lead acid battery shouldn't be discharged more than half, so about 100 usable watt hours.  A car battery is about 480 of which 240 is useable, so about 60% larger than what I have. 

 

What am I missing?



#9 wargrafix

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Posted 15 June 2018 - 01:39 PM

https://www.amazon.c... power station\

 

What about this one? Its effective 15 Amp Hours



#10 John Tucker

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Posted 15 June 2018 - 03:11 PM

https://www.amazon.c... power station\

 

What about this one? Its effective 15 Amp Hours

Ooooh, I like that.  Thanks.



#11 Chuckwagon

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Posted 15 June 2018 - 04:20 PM

What am I missing?

Probably not much.  smile.gif  But if you want to get a better understanding of some of the issues involved with batteries in general, and why this seems so confusing for so many folks, read this article;

 

http://batteryuniver...n_with_voltages

 

They do a good job of explaining how various battery chemistries typically deliver power.

 

It is because of the differences in nominal charge values that you see power packs that are listed as 12v, but only have 11.1v or less of true output.  A 3 cell design, like the one Wargrafix linked, would need to have a voltage regulator to step the voltage up to a true 12v.  And since so many makers list ratings based on a single cell's capacity (for example, 50,000 mAH at 3.7v, but only 16,750 mAH at 10.89v, which would drop even further if it were stepped up to 12v) it can be difficult to get an accurate idea of how much true capacity the power source can provide.

 

Most of these lithium based solutions that people recommend around here do work, at least well enough for the folks recommending them.  But I think there is enough variation in how well various mounts work with different voltages, that I'd shy away from any solution that I didn't see working on the exact same setup I use.

 

Plus, I don't mind the extra weight of a lead-acid battery, so I prefer to use the tried and true, more standard, and less over-hyped approach.  You can get a 35AH deep cycle SLA(AGM) battery for about $50.  That would easily get you two 8-hour nights of constant 1 amp draw (without dropping below 12v) between recharges without discharging the battery below 50%.  That type of usage would allow you use the battery for around 600 discharge/recharge cycles before the full charge capacity would drop to 60% of new.  If you recharged after every night, you could extend that to 1200 cycles.  So while the lithium based solutions likely have 3 or 4 times that many cycles, they also tend to be 3 or 4 times as expensive, and the true capacity is 30-40% less.

 

So I'm still waiting for the solutions to become less expensive and better designed.  smile.gif


Edited by Chuckwagon, 15 June 2018 - 04:33 PM.

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#12 carolinaskies

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Posted 15 June 2018 - 04:35 PM

That second listed battery generator has a much better output at 12 volt and also had 19 volt available depending on which Port you were using and the raining on that 12 volt was 5 amp which is more than enough to run them out and it does have a 15 amp hour rating. So it's better than the original one listed. The problem we have right now is that most of these lithium ion battery generators are insanely expensive on the open market. So the choice of the sealed lead acid battery is more economical. For those that are going to be observing only a few hours I think the lithium-ion isn't a bad option but for those who are out Imaging for 6 or more hours a night they definitely are more maintenance heavy and having to recharge them for the next use. Along with the inconsistent ratings as was described above it makes it difficult to have to do the math to figure out what is the true capability.

For recreational use these are fine but for a dedicated system I definitely would suggest building a more robust box that will not only Power the mount but any accessories placed on it including do heaters cameras Etc.
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#13 t-ara-fan

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Posted 16 June 2018 - 03:18 PM

 when I also needed to use the battery to power the laptop

Your laptop (and mine) draws about 5A from the battery. As you say, at the end of the night.  That makes the output voltage of the SLA battery drop a lot. 

 

How big is your battery?  My 33Ah SLA was too small, the 75Ah is a lot better.  Now I have both, I can dedicate the 75Ah to the scope, cameras, dew heaters etc, and keep the 33Ah just for charging my laptop.




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