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Old 27 June 2010, 04:11   #1
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Testing for current draw

To test the amp draw in order for me to ascertian if I have sufficient battery power / charging capacity fo I simply connect multimeeter in line between + terminal and lead with all turned on.

Sorry if this has been covered before but I can't get the search function to work today on the ipoone.

Glenn
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Old 27 June 2010, 04:58   #2
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There will usually be a 3rd terminal on the meter that you will have to move one of the leads to, it will be marked.
Make sure the current you expect to measure is within the range of the meter, make sure its set to dc, not ac.
And dont try to measure the starter current, unless you want to upgrade your meter.
If the meter has a needle, it will matter which way round you connect the leads. If its a digital display it wont matter, you will just get a - before the numbers if its the wrong way round.
Hope this helps, Nick.
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Old 27 June 2010, 05:27   #3
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Glen,

Its worth bearing in mind that some things will use variable amounts of power. E.g. the vhf will use much more power when transmitting at 25 W than "listening" - although of course most people won't be "talking" for long periods. Likewise fishfinders presumably use more when "scanning" the sea than sitting in "standby" on the trailer. And chartplotters / other displays will use more with the backlight on and the brightness turned up full.

Personally I'd work off the manufacturers specs (usually because yachties are senstive about power these will be quite easily found). If given as power just divide by 12 (V) to get W to to A (or mW to mA).
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Old 27 June 2010, 07:24   #4
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If given as power just divide by 12 (V) to get W to to A (or mW to mA).
Sorry being tick.... Can you explain
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Old 27 June 2010, 07:54   #5
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Sorry being tick.... Can you explain
Often the manufacturer will state the power consumption rather than the current draw.

Power (P, measured in Watts) = Current (I, measured in Amps) x Voltage (V, measured in Volts)

So if the stated peak power consumption is, say 24 W, then the current draw is 24 (W) / 12 (V) = 2 Amps.

If it is a low power draining device (or on standby) it might describe the power as say 180 mW, [i.e. 0.18 W].

The current is then 180 (mW) / 12 (V) = 15 mA = 0.015 A

Does that make more sense?
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Old 27 June 2010, 07:59   #6
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Thanks

I follow that know.

It is a very hot day and the brian just ain't quite up to it at the mo.

G
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Old 29 June 2010, 11:51   #7
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To test the amp draw in order for me to ascertian if I have sufficient battery power / charging capacity fo I simply connect multimeeter in line between + terminal and lead with all turned on.
Better have a stout DMM. Most will only read a few hundred mA directly. More than that and you blow fuses.

Depends on what you're trying to read (i.e. GPS draw, all electronics draw, starting current to the motor, etc), but a car ammeter temporarily stuck in the line would do the trick for big current applications.

For electronics, I usually go by the manufacturer's max current specs. Worst case is your friend, in this regard.

jky
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Old 29 June 2010, 13:38   #8
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Alternative suggestion

Use your multimeter in Voltmeter mode. Set to 0-20v DC.

Connect across the battery and read the voltage with nothing on. Should be 12.5 volts or above.

Turn everything on. Should hold above 12v, unless you have some major problems.

When starting, it could drop to 8v.

This is much less risky than trying to measure current. After all, other than calculating capacity of the battery required in theory, it doesn't tell you much about real life. You need to maintain a battery voltage to fire up all your toys.
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Old 29 June 2010, 14:01   #9
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This is much less risky than trying to measure current. After all, other than calculating capacity of the battery required in theory, it doesn't tell you much about real life.
I though this was precisely what he was trying to ascertain? e.g. how long he can sit at anchor with the engine off and still be able to start the battery with confidence?
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Old 29 June 2010, 15:48   #10
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Here we go!

So, how do you calculate it?

I have I have 120 watts of toys and a 60 amp hour battery.

In theory I can sit there for five hours after which, miraculously it will all go quiet.

But, in practice:

How do I know the battery is fully charged when I arrive?

How long can I discharge it and still get a start?

How has the capacity degraded due to life?

My point is that a 'stiff' supply which maintains a high voltage at standstill gives a much better indication.

It is all theoretical anyway.

The moral of the story is get a dual battery system and make it stiff enough to hold on one isolated battery 12.5v even with everything on.

It is Wimbledon week..
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