Design Another Freakin' Battery Powered Practical

[T1NM4N]

Hey All,

So a few months ago, I asked about powering lights in a box with car battery. Well, it worked. Thanks to all who helped! Now, I get to play the game again and I want to get a second opinion on my math.

So, DC power... Amp-Hours...

My battery cranks at 180 Amps at freezing temps. Obviously not that cold.

The amount of current for the Lamp, Show Baby, and Portable Dimmer (Leprechaun) is 32.25 Amp-Hours.

Am I correct in understanding that my battery will power these pieces of equipment for a little over 5 hours?


theT1NM4N

 

[robartsd]

Not enough information. You only specified the power rating of the battery (actually current rating, but I'm assuming 12V car battery as cold amps is a very common marketing number for them), not its capacity. I highly doubt the battery is designed for 180 Amps continuous for an hour (even if it has 180 amp-hour capacity) - it would probably overheat.

 

[T1NM4N]

So then how do I accurately figure out the time the battery can run 32.25 Amp-Hours? I mean it needs to run the stuff on and off for a show that runs 90 mins. Any other suggestions would be greatly appreciated as well. A turn table prevents me from hard wiring it as the lamp sits on the turn table.

 

[robartsd]

Also your load would be in amps, not amp-hours. You need to know what the capacity of the battery is (typically rated in amp-hours). Also car batteries really don't like to discharged for very long - I'd either plan on keeping the discharge fairly shallow or using a deep cycle (marine) battery if you want to maintain useful life in the battery. With similar capacities, car batteries will provide more amps for a short period of time and deep cycle batteries will provide better longevity as the battery remains in a state of discharge for a period of time.

If you've already got the equipment, I'd recommend testing to see what you'll actually get rather than rely on marketing numbers.

 

[WooferHound]

Are you using 12 volt lights, or are you using an inverter to get 120 or 220 volts AC ?
To work out the math you need to know the battery rating in Amp-hours, Cold cranking amps is really a useless number.
A car battery is about 35 amp-hours.

 

[BillESC]

What size turn table? How much weight?

 

[T1NM4N]

@WooferHound : I will check on the equipment when I get back in today and post what specs I have for each piece.

@BillESC : It is a steel deck turn table about 20 feet in diameter. EXTREMELY heavy.

 

[RC4Wireless]

Hey hey. Well I gotta tell ya... "Freakin' Battery Powered Practicals" are pretty much all I do.

In terms of battery power, most comments here are correct: cold-cranking amps is not a useful specification for what you doing.

As also mentioned, a marine battery (i.e. deep cycle) battery is a better choice than a car battery. Car batteries are designed to deliver one big whollop of starter-motor current, then be trickle charged the entire time you're driving. Marine batteries are designed to run a small refrigerator for hours while you're out in the water. Go with the battery intended to run things for sustained periods.

Lead-acid batteries are less efficient with larger loads. Thus, lead-acid batteries rated in Amp-Hours (A/h) are tested over a 20-hour period. A battery rated for 20A/h will deliver 1A for 20 hours. But --- surprise, surprise! -- it only delivers about half it's rating when you draw a full 20A. So you have your choice of 10A for an hour, or 20A for less than 30 minutes (roughly).

Let's look at the spec sheet for a good battery:
http://www.optimabatteries.com/en-u...d34m-bluetop-marine-boat-deep-cycle-starting/

Click the "specs" tab and you'll see:

Capacity:55 Ah (C/20)

That "C/20" indicates what I said above -- it's rated at a 20-hour discharge. You're going to get about 25 minutes of running time when you connect a 35A load to it.

I'm happy to help you further with this. But please have a look at what we make at RC4Wireless. Our RC4Magic DMX4dim-500 can directly drive a 500W 12V load, taking care of everything you need with one convenient little box (provided you use 12V lamps and motors). RC4Magic provides better data security and system privacy than Show Baby. It's smaller, more reliable, about the same price, and we sell it with a Lifetime Warranty. Our stuff is used by Disney, Cirque, and many others, for the reasons noted here.

Good luck with your project!

Jim
RC4

 

[T1NM4N]

Thanks @theatrewireless ! I do have a little bit of a budget for this project so will check it out at least to rent. So your product can receive DMX wirelessly and can be powered by an inverter and Deep Cycle Battery? If it draws less than 30 amps (Leprechaun Portable Dimmer we are using draws 30) we might be in business.

 

[Les]

If it draws less than 30 amps (Leprechaun Portable Dimmer we are using draws 30) we might be in business.

Hang on a second - I may have missed where you told us what was going to be connected to the dimmer, but does it really draw 30 amps? That may be what the capacity of the dimmer pack is but it only actually draws as much as you hook up to it (a small amount greater due to thermal losses). A 100w incandescent lamp only draws 0.83 amps. You can get that way down if, say, a 100w equivalent warm-white dimmable LED lamp would be acceptable. They only draw around 18w.

TL;DR: Your dimmer pack only draws 30 amps when you plug 30 amps worth of lights in to it.

 

[T1NM4N]

Thank you everyone for the help! Running a stress test of my rig and so far so good. I'll post the results tomorrow!

 

[robartsd]

Hang on a second - I may have missed where you told us what was going to be connected to the dimmer, but does it really draw 30 amps? That may be what the capacity of the dimmer pack is but it only actually draws as much as you hook up to it (a small amount greater due to thermal losses). A 100w incandescent lamp only draws 0.83 amps.

Assuming a 12V battery, it would require 8.3 amps to power 100W. I'd plan on 15 amps (probably actually closer to 10 amps unless the inverter is way oversized) at the 12V battery if using an inverter and dimmer with a 100W / 120VAC load.

 

[Les]

Assuming a 12V battery, it would require 8.3 amps to power 100W. I'd plan on 15 amps (probably actually closer to 10 amps unless the inverter is way oversized) at the 12V battery if using an inverter and dimmer with a 100W / 120VAC load.

Absolutely - that is likely what the inverter will draw. I was more picking apart the 30-amp draw of the dimmer pack itself.

 

[Jay Ashworth]

And, as always, keep in mind you want at least 150% of your show duration, preferably 200%

 

[RC4Wireless]

Thanks @theatrewireless ! I do have a little bit of a budget for this project so will check it out at least to rent. So your product can receive DMX wirelessly and can be powered by an inverter and Deep Cycle Battery? If it draws less than 30 amps (Leprechaun Portable Dimmer we are using draws 30) we might be in business.

Oops! Sorry for taking a month to reply

Our devices run from 6V - 35VDC, not from 120V AC inverter output. If you can find a way to do the job with 12V lamps, like MR16s and MR11, or other products for the auto and RV market (like headlight bulbs, etc.) then running at 12V will be more efficient and you'll get longer battery life.

If you must use 120V fixtures, then use our RC4Magic DMXio transceivers for wireless DMX, and send the receiver data into a portable "power-bar" style DMX dimmer. The power to the dimmer, and then out to the fixures, is the AC out of the your inverter. Did I mention you need a good quality sine-wave inverter? The less expensive "modified sine wave" inverters won't work well with AC dimmers, which watch the AC line for clean zero-crossings to do their job.

Run at 12V. Way simpler, way easier, may more efficient.

Jim
RC4