# Home of Professional Cinematography since 1996

Battery Charger Draw

__ Published__ : 14th July 2012

Where can I learn how many watts a camera battery charger draws? I am taking a 5D, 7D, and AF100 on a 5-day wilderness expedition and bringing along a 'field battery' (my term) which runs an inverter and can be re-charged with a folding solar panel. But I want to square my numbers. One of these field batteries puts out 120 watts and has a capacity of 120 watt-hours.

I've done this before with the Liberty Pak Little Genny, but it weighs 19 lbs and was more than I needed, so I am going to try the Goal0 Sherpa 120 battery. In calculating how many I need to bring, I'm trying to find info on how much juice these camera battery chargers draw.

On the AF100 charger, for example, it says its input is 22W; its output 1.75 amps. Does that mean it will draw 22 watts per hour?

I am bring enough camera batteries to theoretically get me through the amount of shooting I have budgeted (not a massive amount), but it is tight. The field battery is for back-up.

Thanks everyone, anyone...

Peter Olsen

Director of Photography

Interstate Media, Inc.

Brooklyn, NY St. Paul, MN

Peter Olsen writes :

*>> On the AF100 charger, for example, it says its input is 22W; its output 1.75 amps. Does that mean it will *

*>> draw 22 watts per hour? *

I wouldn't trust that number, inverters are not 100% efficient and there will be more power drawn from the battery than 22 watts. I'd take a good handheld meter like a Fluke 87 and directly measure the DC current going into the inverter (watts = battery voltage X battery current) from the battery while charging a drained camera battery before I assumed anything.

I bet Mitch at Abelcine knows of at least one field tested storage battery/inverter/charger/camera battery combination.

There's always the option of carrying a storage battery with a DC input custom charger that eliminates the inverter. I built one for my HV20's batteries, I can charge them from any 12 volt DC source. If the DC charger

you need doesn't already exist, it would have to be built from scratch. If I had to build one I'd first start with a standard charger and reverse engineer it. There's the possibility that a standard charger could be modified to take a direct DC input but that's an assumption, not a certainty.

Hal Smith

Engineer and Somewhat DP

Edmond, OK

Peter Olsen wrote:

>> On the AF100 charger, for example, it says its input is 22W; its output 1.75 amps. Does that mean it will >> draw 22 watts per hour?

Yes in theory .. but possibly not in practice .. When they put a power consumption figure on an item it tends to be "Worst case" (not things like light bulbs!).

So the charger will probably draw 1W or even 1/2W (possibly even less) when there is no battery in the charger .. when the battery is first placed in the charger then the consumption will rise to close to the limit ..but (if it's anything like mine) will then fall as the cycle goes on. Eventually ending at the 1/2W or so standby consumption.

The trick is to get an efficient inverter and to make sure you disconnect it the second the batteries are charged ...

So assuming that you have an inverter with 90% efficiency (and this is where the 110V system does have advantages over the 230V system) a 22W charger would pull about 25 1/2 watts from a 12V battery ... or 2

and a bit amps ...

So if you assume that the battery takes an hour to charge you would need 2.5AH in the backup battery to charge each battery (assuming you turned off the inverter when not in use , took the batteries out when charged etc ...)

So your 120Watt hour battery should in theory charge four batteries if the time is an hour ...

Hal has said the opposite to me about the power consumption .. I suggest you do as he suggests and measure the draw on the battery...

--

Justin Pentecost

Motion Control

London

http://www.portablemotioncontrol.com

+44 7973 317241

*>> On the AF100 charger, for example, it says its input is 22W; its output 1.75 amps. Does that mean it will >> draw 22 watts per hour? *

No, it draws 22 watts. This is power... if you put 120V into it, it draws 0.18A amps because current times voltage gives you power.

The battery is rated for 120 watt-hours, that means it will drive a 120 watt load for an hour. Therefore it will drive a 22 watt load about five and a half hours.

However, once you put the inverter in there, a lot of the power disappears into the inverter. So you may have 40 watts going into the inverter to get 22 watts out.

On top of that, once you get into the cold, all the numbers and guarantees from the battery manufacturers no longer apply. So add a fudge factor of two in for that.

So figure you can probably get at least an hour and a half run time safely.

Scott Dorsey

Kludge Audio

Williamsburg, VA.

Scott Dorsey wrote:

*>> However, once you put the inverter in there, a lot of the power disappears into the inverter. So you may *

*>> have 40 watts going into the inverter to get 22 watts out. *

Thanks for all the information gentlemen. Too bad there isn't a way to cut the inverter out of the loop. I am testing now, and based on the length of time it will run a 75w light bulb, I can see the inverter itself sucks some of the charge. But not too bad!

Thanks again.

Peter Olsen

Director of Photography

Interstate Media, Inc.

Brooklyn, NY St. Paul, MN

pb.olsen@verizon.net

http://interstatefilms.com

There IS a way to cut the inverter out of the loop, by using a direct charger that runs off a DC supply.

Not only that, there's a way to cut the whole loop out completely by running the camera off the external battery completely, I bet.

Scott Dorsey

Kludge Audio

Williamsburg, VA.

*>>No, it draws 22 watts. This is power... if you put 120V into it, it draws 0.18A amps because current times >> voltage gives you power.*

If the power factor is 100%, not a given except for first class professional gear.

Hal (one finger typed w/1st Grandson on knee) Smith

Engineer and Somewhat DP

Edmond, OK