# Building Portable AC power supply

#### alex ezorsky-lie

Aug 27, 2015
27
I have some hobby experience building my own small li-ion power packs for small electronics projects, but now I've come across the desire to use li-ion to power more common devices such as laptops, video projectors (especially) and sound systems etc.

This led me to the thought of creating a huge li-ion pack and using an inverter to make it AC. Essentially the goal would be to make one of these "400W li-ion solar power generators" for 1/4 the cost. Also, while I would be looking for a similar 400W max output (350W for mid-sized video projectors) I wouldn't need the 33AH, more like 3-7AH

The quick math suggests 100 X 3.7V 3860's at roughly 3AH could supply at least 350W for 3hrs which would fit my needs. Of course after heat loss and other inefficiencies caused by DIY cheap lipo-pack building there is some loss to the AC inverter, which brings me to my biggest question.

If my plan is to only use this DIY power pack for creating AC, what voltage setup would be best to start at before converting to AC? Of course arranging it in a 12V array would allow me to use commonly available commercial inverters, but why would it not be better to arrange the batteries into a 110V array, wouldn't that require less work of the inverter?

Sorry if my question too clearly gives away my lack in true electrical engineering knowledge.

#### Audioguru

Sep 24, 2016
3,650
If you use 110VDC to make an inverter then its output will be a crude squarewave that you would not want to use to power an audio amp or a video projector. Mains electricity is a pure sinewave with few harmonics and its peak voltage is 155.5V for your low voltage 110VAC or the peak voltage is 170V for my normal 120VAC.

#### alex ezorsky-lie

Aug 27, 2015
27
Is this more of a commentary on the use of DC-AC inverters for projectors or of using very high DC voltages to invert?

Does an inverter with a large voltage ramp-up somehow use that ramp as room to round the sine wave?

Thanks

#### BobK

Jan 5, 2010
7,682
To invert directly to 110V AC you would need 155V + some headroom, so maybe 170V DC. Inverting from that voltage will be more efficient than going up from 12VDC. You can make it a sine wave inverter by using a PWM signal that approximates the sine wave then filtering with inductors and capacitors.

Basically what you need is:

160-170V DC source (batteries)
PWM controller w voltage controlled by feedback
an H-bridge
filters

This is simpler than a 12V system which needs a large transformer.

Bob

#### BobK

Jan 5, 2010
7,682
However.

If whatever you are powering (this is certainly true of laptops and all low voltage devices) already is powered by a transformer that steps down and rectifies the voltage, you are better off to convert to that voltage directly with a DC-DC converter instead of first converting to 110AC and then back down.

Bob

#### alex ezorsky-lie

Aug 27, 2015
27
I'm an AV guy and my greatest goal is getting a video projector to project for at least 2 hrs via a battery.

I recently ran a test with the following:

1. A newly charged standard size car battery (not a new battery though)

2. A 400W Inverter from autozone

3. A video projector rated 300W

The projector ran for only 10 minutes before the low voltage alarm on the inverter went off and shut itself down. But why?

Even if the car battery only had 30 amp hours my 300W projector should only be drawing 25 amps at 12volts and thus run around an hour, right?

Does anyone else have a suggestion for why this occurred? Do I just have a crappy battery?

I did notice that after the inverter was disconnected the batteries voltage went back up to 12.3 which isn't great but also not totally dead.

I guess I'm curious to know if my problem here is around the batteries AH capacity or it's ability to sustain a certain wattage at all (which I don't know how could differ from AH+Volts).

Huge thanks to all the help!

#### alex ezorsky-lie

Aug 27, 2015
27
However.

If whatever you are powering (this is certainly true of laptops and all low voltage devices) already is powered by a transformer that steps down and rectifies the voltage, you are better off to convert to that voltage directly with a DC-DC converter instead of first converting to 110AC and then back down.

Bob

Yes I definitely thought of this, sadly we live in AC and most projectors dont have DC inputs

#### davenn

Moderator
Sep 5, 2009
14,150
The projector ran for only 10 minutes before the low voltage alarm on the inverter went off and shut itself down. But why?

Even if the car battery only had 30 amp hours my 300W projector should only be drawing 25 amps at 12volts and thus run around an hour, right?

Does anyone else have a suggestion for why this occurred? Do I just have a crappy battery?

because the battery isn't going to stay at the 12V for very long with a high current draw .... its going to start dropping immediately
little by little, as you discovered by this statement ......

I did notice that after the inverter was disconnected the batteries voltage went back up to 12.3

as soon as the battery voltage drops to the alarm voltage of the inverter, things are going to stop working

#### alex ezorsky-lie

Aug 27, 2015
27
because the battery isn't going to stay at the 12V for very long with a high current draw .... its going to start dropping immediately
little by little, as you discovered by this statement ......

as soon as the battery voltage drops to the alarm voltage of the inverter, things are going to stop working

I figured it couldn't drop instantly for all amp draw, or else all inverters would be useless. This "high current draw" seems relative so I googled and found that indeed 10 minutes is the predicted life of a typical car battery at 25amps draw. I need a battery with a much more stable current draw curve!

http://www.steveduncan.net/html/discharge_testing_slas.html

#### BobK

Jan 5, 2010
7,682
The capacity of batteries are typically stated for a discharge over 20 hours. Higher discharge rates result in lower capacity, sometimes much lower.

Bob

#### Audioguru

Sep 24, 2016
3,650
If your projector has an incandescent light bulb (a heater) then its current when it is turned on is maybe 250A! When its filament reaches 2 thousand degrees C then its current is much lower at maybe 25A.

A 12V car battery can deliver 600A to a starter motor at 8V in winter for about 10 seconds.

Replies
6
Views
841
Replies
1
Views
1K
F
Replies
0
Views
788
F
F
Replies
0
Views
981
Fake email
F
Replies
3
Views
683