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macona
06-24-2011, 12:09 PM
I am working on a battery pack I will be able to use to power my telescope, laptops, cameras, etc when I go for star parties and the like. Most things will run on 12v, the scope runs on 18 through a laptop car supply, and I will need to run an inverter for the laptop docking station and the camera power supplies.

For batteries I have two battery packs for a APC UPS system. They have 8 12v, 7.2AH batteries in each unit. They are currently wired in series for 96v per pack.

So, I am trying to figure out the best way to wire these things up to get useful power. I could wire them in parallel to get 12v out of each pack and run direct. I could also leave them how they are and use a DC/DC converter to drop it to 12v. This would give me a stabler output as the batteries loose their charge. But it would also make charging more difficult as I would need a higher output charger. I could also do a series-parallel connection to the batteries to get a 48v output which would make charging a little easier and dc/dc converters are more common in that voltage range.

Any opinions?

winchman
06-24-2011, 12:25 PM
Seems like it would make sense to have the using and charging systems completely separate with a switch to select between them. In use position, the batteries would be connected with the other components to provide what you need. In charge position, they'd be connected in whatever way made charging easiest.

danlb
06-24-2011, 01:29 PM
Seems like it would make sense to have the using and charging systems completely separate with a switch to select between them. In use position, the batteries would be connected with the other components to provide what you need. In charge position, they'd be connected in whatever way made charging easiest.

I like that idea, but it's easier said than done, since you'd have to set up positive and negative connections for each of the 16 batteries. 32 pole double throw switches are hard to come by.

Then I realized this is a machining forum, and that it's not that hard to make a mount that allows a single handle to actuate a bunch of switches at the same time.

Dan

Evan
06-24-2011, 01:44 PM
I decided to make it easy and bought a 1 kilowatt pure sine wave inverter to produce 120 vac power. I have plenty of spare batteries inculding two 55 amp hour wheelchair valve regulated fiberglass mat lead acid batteries to power it. I also picked up a crystalline cell 55 watt solar panel that will produce about 70 watts at higher altitudes. That isn't really needed but it would be nice to avoid running the truck to charge batteries.

The inverter was on sale for under $200 so that was a pretty good deal. Beats having to fiddle around with all the different voltages and I can use the panel and the inverter to run lighting in my house. I like the idea of storing up sunlight and releasing it later even if the efficiency is terrible.

jkilroy
06-24-2011, 10:17 PM
How many amp hours do you need? I get battery packs made for my products all the time, and might be able to give you some ideas?

whitis
06-25-2011, 03:29 AM
Be careful you don't spend a lot extra on DC-DC converters and inverters to accomodate the specifics of your batteries. It sounds like your batteries are only 1382 watt hours total or comparable to a typical deep cycle battery. So don't spend more than a standard deep cycle or gel cell 12V battery would cost to make use of the free? batteries - especially if you will be going with a 12V battery when these die or try to run off your car (be careful not to deep discharge). At the kind of power level you are dealing with 12V is more standard.

Before paralleling batteries, you might want to jumper them together with a resistor or lamp to let the charge/voltage equalize gently between them or at least check that the voltages are similar.

form_change
06-25-2011, 04:02 AM
If you set them up as 24 V, you would then be able to use all the gear that they make for trucks - might make the thing more versatile. Lots of industrial control gear comes in 24V so there would not be a shortage of things that could run off it.

Michael