# How do I measure current draw?

Subject line contains the general question, now for specifics.
What methods could be used to semi-accurately determine the current draw on
battery packs. Two specific scenarios apply in my case:
1. Standard RX battery pack driving approx 4 servos. 2. Electric flight pack feeding an ESC/BEC with two or three micro-servos.
As an aside, I have an ESC of unknown rating but was advised what battery packs to used. No problem, all works well for this basic configuration (ESC and two other channels) but it would be nice to know the correct method of calculating both current draw and an estimate of the ESCs load rating.
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The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3
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The Raven wrote:

Measuring current just involves putting a ampmeter in series with the battery pack - I rig up a JR servo plug and socket together to make it easy to put the meter in line. I've got a similar setup with Deans connectors for electric planes.
As for the load capacity of the BEC circuit in an ESC, I'm not sure how you can test it without connecting the battery pack in question and adding on servos until the BEC regulator shuts down from excess power - however you run some risk of damaging the ESC or RX depending upon how the BEC is designed.
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Is this a system that measures in flight loads or one you use to get readings from while on the ground?

Took another look at the ESC (which is hidden behind a cowling). 22Amp for a small can motor electric. Probably overkill for my application. Now all I need to find is the jumper settings and/or some instructions.
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The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3
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The Raven wrote:

22A is good for speed 400/480, but marginal for a speed 600.
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I believe I'm using a 380 or 400 but really don't know much about what the numbers mean. Is there a simple method of equating the numbers to the output (say, for can motors only)?
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The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3
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The Raven wrote:

It's only for ground readings, since it's just an ampmeter which is found on most cheap \$10 multimeters. For RX draw, you can move both sticks around simultaneously to get some idea of the current draw, although depending on how you fly, and what the control surface loads are, this isn't the most accurate test, but by far the easiest and cheapest.
For and electric aeroplane, the reading on the ground will tend to be the greatest current draw. Once the aircraft is flying, the load on the prop will reduce and the current draw will drop.

The rating of the controller doesn't tell you what the controller can handle in terms of number of cells versus number of servos.If you can identify the controller, finding the manual is by far the best way of determining what combinations are acceptable.
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An inexpensive volt-ohm meter is all you need as far as equipment goes. Radio Shack has these in stock country wide.
You need to make a cable adapter (from an aileron extension cable) that you can insert between the battery and the switch. One wire, it doesn't matter which, positive or negative, needs to be cut, prepped and then fitted with a pair of probes that will plug/fit into the front panel jacks of the Volt-Ohm Meter (VOM from hereon).
If you get the polarity wrong on the meter jacks, the needle will try to drive backwards. Not good, so be brief when testing.
Understand that the best you will be able to measure is the quiescent state, or near quiescent state of the system. Most flight commands, on a model set up with freely moving surfaces and good linkages, won't cause a heavy load to appear over the quiescent (quiet) reading. Seeing a significant increase in current at any time after the baseline current has been established in grounds for an in depth investigation as to what new thing is going on in the system.
I used to fly a model named Peppermint Pattie. It was a miniature pattern ship with a near midwing configuration. There wasn't much room in the fuselage for radio gear or fuel tank.
Pattie was powered by an HB .15 R/C engine. The flight pack consisted of a Digital Commander 7 channel receiver and four Bantam Midget (all Ace gear) plus a 100 mah square battery pack. The 100 mah pack was cutting it very close in terms of capacity. I charged the system with a fresh 6 volt lantern battery and guestimated at how long to charge it. I checked the charge with an ESV. This system never let me down.
I flew Pattie for a couple of years. She used to blow by a full house Curare with retracts and a piped 60 like a demon on fire. 8^>
TAS
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I always call a multimeter a VOM. Incorrectly, I might add. Sorry about that.
TAS
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The Raven wrote:

Clamp on ammeter is good if you can find one that will do DC. Sears I think. For flioghtpower anyway.
Probbly best to use a normal ammeter on te battery pack. Servos draw about 250-500mA apiece when moving..
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It just so happens that I published a neat way to build a simple current measuring device in the current edition of our newsletter. Go to www.brcm.org/sw.pdf and see page 3.
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Thanks for that, very interesting stuff.
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The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3