Selfmade powerpack?

[George Gearless]

I'm often frustrated that my 'targets' of the evening are hidden behind my neighbours big tree or my own house. So, I've been doing some reconnaissance around my neighbourhood to find a suitable site for setting up shop. I've found a spot a good 1 km away. Needless to say, power will be an issue.

After doing some research into how much power I'm going to need, I figured I'll be needing a rather large powerpack. Mount, heatbands, Stellarmate/laptop (I'm currently without a Stellarmate because I dropped it and broke it 😰).

As you are no doubt aware, a 'proper' powerpack designed for this type of use, is rather expensive. They are, as far as I can tell, Lithium based for easy lightweight handling. But since I'll be hauling my equipment in the trunk of my car anyway, I figured a car battery would serve the purpose. I have a brand new car battery (660 Wh/55Ah) that my girlfriend bought by mistake and could not return. That would give me all the power I need. And then some. So if I fitted two 12v sockets on that, I could plug in my mount and my heatbands directly to the battery. I also have a 12v DC to 230V AC inverter, which I could potentialy use for powering my Stellarmate (when I get a new one) or extra power for a laptop.

But I am worried that a car battery will mess up the electronics of my mount motor. Also, if the voltage of the battery isn't precisely 12v, what will that do to the siderial tracking rate? The question is, if the controlbox of the stepping motor can handle a somewhat unequal voltage? Or will that not matter significantly?

Anyone got any experience with this?

[theropod]

Hey,

Car starting batteries are not designed for deep cycling, and will have a very short life in that application. Starting batteries are designed to provide short bursts of high amperage, and to have the power removed to start the engine rapidly replaced.

Your mount has internal voltage regulation and should accept direct current voltage up to a specified point, and down to a certain point. Your owners manual should detail these specs. Stepping motors will work with a supply voltage that can vary quite a bit, but whether the driver will accept these voltage swings is another matter. Again, the user instructions should  make this clear.

I would look for a battery pack designed for the task, reference the user manual and be sure to install an in-line fuse rated about 20% over the expected running load to protect your mount from those “Oh no!” moments. A battery SHOULD provide the smoothest direct current possible for your mount. Just be sure what the mount will accept.

[George Gearless]

22 minutes ago, theropod said:

Hey,

Car starting batteries are not designed for deep cycling, and will have a very short life in that application. Starting batteries are designed to provide short bursts of high amperage, and to have the power removed to start the engine rapidly replaced.

Your mount has internal voltage regulation and should accept direct current voltage up to a specified point, and down to a certain point. Your owners manual should detail these specs. Stepping motors will work with a supply voltage that can vary quite a bit, but whether the driver will accept these voltage swings is another matter. Again, the user instructions should  make this clear.

I would look for a battery pack designed for the task, reference the user manual and be sure to install an in-line fuse rated about 20% over the expected running load to protect your mount from those “Oh no!” moments. A battery SHOULD provide the smoothest direct current possible for your mount. Just be sure what the mount will accept.

Hmm, hadn't thought about the deep cycling issue. I suspected there was a reason why the powerpacks designed for this use, were so expensive compared to the price of a car battery. Apart from the lightweight benefits, ofcourse. But hadn't thought about how differently they operate.

I had, in my mind, considered some sort of safety in order to protect the equipment against powersurges. Was thinking a 0.5 A fuse would do the trick.

Anyway, thanks for your reply. Since I already have everything needed, I'll propably do a dry-test run at the house anyway, and see how long it'll keep things running. But with your post in mind, I find it likely that I will be browsing the web for a lithium based powertank. Thanks.

[IanL]

You can certainly do as you propose if you use the right sort of battery. I use one of these, but any sealed 12V AGM mobility scooter or golf buggy battery would do the trick:

https://www.amazon.co.uk/gp/product/B07GDPPMNP/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

Avoid any type of wet battery (traditional lead acid for example) as they're not good for transporting and can generate explosive gas when charging. Non-deep cycle batteries will die fairly quickly if used as astronomers tend to use them

To charge up I use a cheap 'smart charger' - just connect the clips to the terminals and leave it plugged in when not using and it will keep the battery charged and in good condition. You do not want the battery to discharge too much as that will also kill it (and they tend to discharge over time):

https://www.amazon.co.uk/gp/product/B01MY5UT1E/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I use a wheeled tool case to lug around all my astro gear, and the battery and charger are just fixed inside that. Just be careful that nothing metallic can sit on the battery terminals and short them out or it'll be a barbecue. You will definitely want a 12V fuse of the appropriate rating set in the positive wire as close to the battery as possible. Just add up the amps drawn by all your gear and go for the same or nearest rating up from that. Do ensure that the wiring is thick enough to take the current.

[George Gearless]

39 minutes ago, IanL said:

You can certainly do as you propose if you use the right sort of battery. I use one of these, but any sealed 12V AGM mobility scooter or golf buggy battery would do the trick:

https://www.amazon.co.uk/gp/product/B07GDPPMNP/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

Avoid any type of wet battery (traditional lead acid for example) as they're not good for transporting and can generate explosive gas when charging. Non-deep cycle batteries will die fairly quickly if used as astronomers tend to use them

To charge up I use a cheap 'smart charger' - just connect the clips to the terminals and leave it plugged in when not using and it will keep the battery charged and in good condition. You do not want the battery to discharge too much as that will also kill it (and they tend to discharge over time):

https://www.amazon.co.uk/gp/product/B01MY5UT1E/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I use a wheeled tool case to lug around all my astro gear, and the battery and charger are just fixed inside that. Just be careful that nothing metallic can sit on the battery terminals and short them out or it'll be a barbecue. You will definitely want a 12V fuse of the appropriate rating set in the positive wire as close to the battery as possible. Just add up the amps drawn by all your gear and go for the same or nearest rating up from that. Do ensure that the wiring is thick enough to take the current.

Yes, it's sealed. And yes, I've got an impulse charger that should allow perpetual 'recharging' without causing damage or overload. As I said, I'll give it a whirl and see what I can get out of it. If I can get 2-3 hours worth of operating time, I'm good.

[IanL]

16 minutes ago, George Gearless said:

Yes, it's sealed. And yes, I've got an impulse charger that should allow perpetual 'recharging' without causing damage or overload. As I said, I'll give it a whirl and see what I can get out of it. If I can get 2-3 hours worth of operating time, I'm good.

The problem won't be whether you can get a few hours out of it at first, I'd think you will. The issue will be that the battery performance will degrade fairly rapidly with repeated charging/discharging cycles. Car batteries are made to provide a lot of current for a short period of time after which they are recharged whilst still mostly full. Deep-cycle batteries are designed to be used continuously over a much longer period and will withstand repeated heavy discharge/charge cycles.

[George Gearless]

1 hour ago, IanL said:

The problem won't be whether you can get a few hours out of it at first, I'd think you will. The issue will be that the battery performance will degrade fairly rapidly with repeated charging/discharging cycles. Car batteries are made to provide a lot of current for a short period of time after which they are recharged whilst still mostly full. Deep-cycle batteries are designed to be used continuously over a much longer period and will withstand repeated heavy discharge/charge cycles.

Yeah, I understand. 

Given my poor record of having the night off when there, quite exceptionaly, are clear skies I'm probably looking at 3 maybe 4 nights until the summer sun starts ruining nightviewing. If this solution can tide me over until then, I'll have the summer to save up for a propper powerbank. The car battery is just sitting there anyway. So I might as well get out of it what I can.

[Carbon Brush]

I wouldn't worry about a car battery having a shorter life (number of charge/discharge cycles) than a leisure battery.
Your car battery is basically free and will degrade if you don't use it.

Just build yourself a decent box for fusing, switches, power outlets, etc. Make sure you use use proper battery clamp clips (not crocodile) on the battery.
Mounts do not like the power removed/reconnected rapidly that can happen with a nudged crocodile clip.

When the battery eventually expires, then you will have the experience to choose your next battery for your actual, rather than predicted, requirements.

HTH David.