Batteries for Mobile Power

[rackdude]

Here's a syndicate of an email I sent to some battery place. I think you guys may be able to help too.




Hello, sorry for the odd question but I hope you guys are the battery experts. I am trying to have some equipment such as mixers and preamps be mobile for a sound engineering gig for video. However, all of my equipments plugs in to the circuit. At first I thought of using a car battery with a power inverter, but I don't know how well that would work, the price, etc. So here's exactly what I need:

Must be able to run sound equiptment for an extended period of time (say, two+ hours?) . One of the stuff is a 9v 2a piece, while other possible items may be as large as a computer.

Must be able to charge using AC

Portable in a sense.

What do you think will be best for this situation?

[synthoid]

I have some rackmounted UPS'es that can operate my racks for long periods without wall power. I charge them up fully, and then go. Easy squeezy.

-synthoid

[Shadow_7]

As far as I can tell, you need to add up the actual amps of the gear that you will be running from that power source and choose accordingly. There's some formula out there that converts that ampage, plus the inverter ampage and relates it to the batteries ampage to determine run time / life expectancy. Your average UPS type system is rated 5 to 20 minutes for a desktop computer, which may not be good enough for your needs. But it could probably run a laptop for two hours. Or a 2 channel field recorder for 6 hours. It really depends on what and how much of it you have drawing current.

[rackdude]

What I will be running most of the time is just a DMP3 which has a 9v 1 amp power supply. How does this relate to watts and computer power supplies? Is it just P=VxI, and therefore it's 9 watts, which is 20x less than a normal computer supply (the apcs are rated off of a 200 watt computer)

[Shadow_7]

Originally Posted by rackdude

What I will be running most of the time is just a DMP3 which has a 9v 1 amp power supply. How does this relate to watts and computer power supplies? Is it just P=VxI, and therefore it's 9 watts, which is 20x less than a normal computer supply (the apcs are rated off of a 200 watt computer)

If that's all you'll be running, then is there some reason you want the DMP3 plus battery plus inverter, instead of another option like the sonosax SX-M2(?) that is a battery powered and higher end preamp? By the time you buy a quality inverter($1K), you've already paid for the sonosax. And you're likely to get more charges out of traditional rechargeables than a UPS.

IIRC, part of the formula is Amps times 120Watts, to determine the Watts of the inverter you'll need. I think my laptop is 1.7 Amps and requires a 200+W inverter. I really should re-google this stuff. I only glanced at it since the prospect of carrying three pieces of gear for 1 purpose proved less than ideal. Plus the weight of car batteries and similar kind of turned me off. Not that big of a deal in a rack on wheels, until you have to lift it higher than a bumper to get it in the van. Or walk out on stage with it in hand. In addition to your instrument, props, and uniform accessories.

[rackdude]

Does it have to be that quality of a converter? How do the cheap APCs compare?

[Shadow_7]

From what I've heard, it has to be a pure sine wave inverter, which are costly. Otherwise you introduce some noise issues into your audio gear, and suffer some reliability and other issues. The cheaper stuff might work, but if it doesn't you just wasted that money. And it might prove more costly than it's worth to use. So how many times do you want to buy the gear, and how well do you want to sleep at night?

I've been on road trips where an inverter was used to power a TV + VCR in the passenger van. And one of two inverters (in the other van) didn't have enough Watts to power the gear, and otherwise did nothing more than get hot.

Another alternative is a battery that subs in for your power cable. You avoid the DC/AC and AC/DC conversions that route. Although you get stuck with limited selection. batterygeek.net has one with several connectors that fit a lot of devices. Not quite as costly, but not cheap either. And a little better suited for specialized tasks like the one mentioned. But you loose the flexibility to plug in any device. And it's difficult to know which battery for which device with what connector.

[t_chance]

Taperssection.com

Taperssection.com - Index

The tapers have a "remote power" thread. A lot of good reading there.
Have fun

[wax808]

I've got lots of experience running banks of batteries for large CB Radio Amplifier installations, I have used these same banks of batteries to broadcast a pirate FM station also.

The way I ran the pirate station was via an inverter. It ran for an entire weekend, the transmitter, amplifier, and Ipod all ran no problem off of two Optima Yellow Top gel cells.

The problem is that banks of batteries are expensive, and so are the large alternators to charge them up. I've seen people use Solar power to keep their batteries topped off during the day.

If price is not an issue go with a bank of batteries and a large inverter. If the gig is only a few hours and you have a chance to test all the gear a few Yellow tops should be enough. If they aren't then add another and pre-charge them all. Remember those that these batteries cost between $130 and $200 each. Try and go with the Optima Yellow tops though, they wont spill or discharge too much gas. You can completely drain them without killing them also.

If it is feasible though go with a gas powered generator. Will last way longer than batteries and shouldn't leave you hanging. If noise is an issue try and isolate it with a long extension chord run.

[HughH]

Hello,
If you choose the inverter route, be sure to spend the extra for a "real" sine wave unit. The less expensive units and almost all the UPS units make what some call a "modified sine wave" or other brand-specific nomenclature. It gives you a 3-step square wave ( 0v, plus big volts, 0v, minus big volts, back to 0v ) that wreaks havoc with linear power supplies and emits quite a bit of noise due to the square-ish waveforms. Can also put lots of noise in your mic preamps, depending on the situation. I never knew a DA88 power transformer could actually melt.
The nice units cost quite a bit so you have to be sure you want to do it. If it's just your dmp3 and a laptop maybe a whole lot of C cells for your dmp3 and spare batts for your laptop would be ok.

Let's play with some C-cell battery figures for a moment to give you an idea.

Your dmp3 power supply is rated at 9v/1A. The unit doesn't draw that much but the supply has to be overrated. Let's assume worst case, that this unit draws 750mA. I'm betting less but this will give you a start.
Do you have a voltmeter? It would be nice to check the voltage into the dmp3 - if it's a transformer power supply without regulation the input may be as high as 12-13v which will be good to know.

The lowest voltage you want into the unit is 9v. If the unit is ok with 12-13v input (see voltage measurement above) then 8 cells in series will do nicely -
- new batteries (1.6v) = 12.8v
- 9v dropout voltage = 1.125v

Now let's figure current. The typical alkaline C-cell is rated at 8350mAh, but this is at absurd test conditions - very, very low current draw and 0.8v cutout. You'll be working it hard, so a more realistic estimate is 2000-3000mAh.
Let's say you want to run your dmp3 for 5 hours (test + show).
Assuming our original 750mA and battery capacity of 3000mAh/cell, we get a capacity of 4 hours per serial bank of cells. In reality you won't get close to that as our 3000mAh rating guess is high because we're working it so hard. If you parallel another series bank (16 cells total, 2 banks of 8) you'll have some power headroom.

If this is something you plan on doing regularly, you may decide to invest in rechargeables and a charger. The same basic calculations apply as above, except that rechargeable batteries have lower voltage and less capacity. Therefore a "brick" type may be more economical and easier.

Hugh

[synthoid]

An decent online UPS is exactly an inverter + battery(s) + charger. Already packaged up, rackmountable if you want, with current capacity figures already worked for you etc. And it also functions as a UPS when you're not using it for remote power!

-synthoid

[Remoteness]

Yes, but not all UPS' are true sine wave devices.

Originally Posted by synthoid

An decent online UPS is exactly an inverter + battery(s) + charger. Already packaged up, rackmountable if you want, with current capacity figures already worked for you etc. And it also functions as a UPS when you're not using it for remote power!

-synthoid

[HughH]

Hello,
UPS devices are (usually) intended for computer applications, and computer power supplies are all switching supplies that have no issues with the squared waveforms that most UPS units put out. It's when we extend their use to linear supplies and noise-sensitive applications (mic pres) that we have issues. The packaging is certainly convenient however.
Hugh