12Ah battery -> inverter -> power supply -> device

[loogie]

I have a friend who is doing a project with a mini LED projector, an Optoma PK120:

http://www.optomausa.com/products/detail/Pico-PK120

Its a remote location so no AC.  A generator is not an option.  What he's tried is to plug a 12Ah battery into an inverter and then that into a power supply (UK 240v) and that into the device.  The power supply provides 5v@2A.  The inverter is 150W@240v. 

The internal battery of the PK120 is 1200mAh and is good for between 50 and 90 minutes between charges.  He needs more time.

It works, but for how long.  My back of the envelope calculation suggests that even with 50% loss he'll be able to operate for several hours.  50% is arbitrary.

What do you think?

[PRR]

> power supply provides 5v@2A.  The inverter is 150W@240v

10 Watt load, 150 Watt inverter. Inverters always have idle losses, and at 1/15th of rated power the self-loss power may be greater than the load power.

Does the 150W inverter run warm?

Can find a 25W-50W inverter?

> a 12Ah battery
> The internal battery of the PK120 is 1200mAh and is good for between 50 and 90 minutes between charges

On the face of it, from 1.2AH to 12AH "should be" 10 times the run-time, 9 to 15 hours.

You wanna derate the 12AH for repeated discharges, say 5 hours.

I'm still concerned about the idle-loss in the over-size inverter. I would not bet on 4 hours.

Curious: what is the internal battery voltage? I assume it isn't anything like the 6V/12V of standard fat batteries.

[loogie]

3.7V

[Ritchie200]

Do any of your buddies have an old UPS laying around the house?  Well, I should say a GOOD old UPS! 

Jim

[loogie]

I doubt it.  I told him to grab the battery out of the VW, but he didn't seem to go for that.  Carrying it on the Tube might be problematic.  He does have an extra battery, though. 

[sluckey]

Have you considered eliminating the inverter and power supply? And simply connect the 12Ah battery to a 5 volt regulator to the device?

[loogie]

That looks like a great idea.  I wonder how its efficiency compares to the current rig?  As I read about this stuff it looks like maybe a switching regulator would be the ultimate solution.  I'm in Ohio and he's in the UK so we can't do much collaboration.  Plus I think he's under the gun timewise.

Nice thought, though.

[sluckey]

I would think the efficiency would be better than converting the DC battery to AC then converting the AC back to DC. And you don't have any of the issues associated with the inverter. In terms of Efficiency = P_out/P_in x100%, I can't say. But the beauty of this is that the regulator circuit can be built in an enclosure about the size of a pack of smokes. The enclosure can then be taped to the side of the battery. Now you only have one piece of equipment to carry with the Optoma. The current rig requires three pieces with a gaggle of wires.

Anyhow, it was just a thought. If your friend has no electronics experience, it's probably better to stick to COTS equipment.

[PRR]

Dropping 12V to 5V with a linear regulator is, at best, 5/12 or 41.7% efficient.

It is unlikely to be better efficiency than a modern 12V:230V into the wall-wart. (Modern small inverters are good efficiency because it reduces heatsink size and thus cost.)

It has the great virtue of simplicity.

I'd be inclined to look for a 6V battery (often found in emergency lighting) and a 6V charger. Then one or two fat power diodes drops to 5.4V or 4.8V.

[super&plexi]

There are also small batteries about 8 inches by 6 by 3, for RC cars sort of like the same thing as a motorcycle battery which might even be better. Don't have stats on them right now. But RC battery I just saw looks like it would run something small for a long while, and
not too heavy

[loogie]

The battery lasted about 6 hours before it had to be swapped so all went off without a hitch.  Thanks.

Next project involves trying to find very flat speakers that can be mounted on a board and then covered with carpet allowing a child to crawl in and be surrounded by sound.  Some thoughts have been given to using piezo elements strategically mounted on a substance that will reinforce vibration.  Frequency response is not critical as long as it covers the frequency range of a buzzing bee sound.

I was thinking of experimenting with fragments of thin wood or possibly dampened sheet metal.  Again, this will all happen thousands of miles from here in the UK.  I'm a remote consultant.

http://binvestigators.com/credits/

[super&plexi]

just saw $3.oo piezo/transducer elements last week I'll try to find them on net, and pass along. people wrote that they tried them on a whim, and were blown away with fidelity-can mount w/double stick tape-turns anything,...walls, ceiling, sheet metal,  any surface into a 'speaker'.

[Shrapnel]

If you have all the info, AND the given battery has a high enough voltage, why bother with an inverter to bump it up to a higher AC voltage just to knock it back down again and make it DC?

A switching power supply takes the incoming AC voltage, rectifies and filters it to a DC voltage. The voltage regulation is done then by switching on a transistor (simplest form) for x-amount of time, and then switched off for y-amount of time. This is done at a high frequency and on only long enough to charge and hold the output at the specified voltage using caps to hold the charge and help deliver some current.

A modern PC power supply takes line voltage, converts it to DC and then runs it through the switching circuitry to drop it down to 3.3V, 5V, 12V, and -5V and -12V (These last two are the least used and have very low current out) They do this with better efficiency than they did years ago. So, the trick is to design your switching power supply that runs off the battery as efficiently as possible to minimize losses. (Now days, a good PSU in a PC will run at about 80% efficiency, or  better under most loads. Cheap power supplies? Well... think a 5v ceramic pumped up on 120 or 240, when they get severely overloaded. Most of the time before delivering their rated power out.)