Is there an easy way to work out "current draw"?

Submitted: Tuesday, Jun 20, 2006 at 15:34

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Scoey (QLD)

Hi all,

Further to my post 35055 about my Aux battery I am going to have to get myself a new battery and I thought I might also get myself a battery monitor while I'm at it. That's all a bit by the by but.

My question this time is, how do I work out the approximate current draw of a particular appliance? eg my phone charger running through my 300W inverter. I don't have the inverter specs on me but the phone charger has on it:

Input: AC 100-240V, 50-60 Hz, 150mA
Output: DC 5.7V, 800mA

So generally speaking, can I use the specs off the particular appliance to determine the current draw for that appliance so I can estimate battery life?
Also, if that appliance is running through an inverter to add the current draw for the inverter and appliance together or just use the end appliance's draw?

Why I ask is that I can just use the monitor to guage the battery usage but my mind is a "need to know how it works" brain!

Have I completely confused anyone? Apart from myself that is!

Oh, and if it's all to hard, just tell me! ;-) Thanks a lot!

Scoey!

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Reply By: Member - Coyote (SA) - Tuesday, Jun 20, 2006 at 15:45

Reply By: hound - Tuesday, Jun 20, 2006 at 16:01

Reply By: Mike Harding - Tuesday, Jun 20, 2006 at 16:03

Tuesday, Jun 20, 2006 at 16:03

If your phone charger needs 150mA at 240V then it's power consumption is 0.15A x 240V = 36 watts. Your inverter will waste about 6W for the privilege of converting 12V to 240V therefore the overall consumption will be 42W. At 12V that means the current draw from your battery will be 42 / 12 = 3.5 amps. Hope that helps, if not just ask for more info.

Mike Harding

AnswerID: 179479

Follow Up By: Scoey (QLD) - Tuesday, Jun 20, 2006 at 16:10

Tuesday, Jun 20, 2006 at 16:10

Actually, that seems to be a tad more logical! (sorry hound!) So can i then assume that I will be using 3.5 amps...... per hour?? ;-)

Cheers Mike!
Scoey!

FollowupID: 435675

Follow Up By: Mike Harding - Tuesday, Jun 20, 2006 at 16:44

Tuesday, Jun 20, 2006 at 16:44

Exactly. So if you had a 35Ah battery you could power your charger for 10 hours (you couldn't in reality because you'd damage the battery) at which point the battery would be totally knackered and in need of a good lie-down :)

Just work out the power consumption of other items in the same manner and if they are running at the same time add them up to obtain the total current draw. eg. if you are using two phone chargers total current from the battery will be 7A (we'll ingore the inverter loss here) and your 35Ah battery would last for 5 hours.

NB. you should not, usually, take more than 40% or 50% of a battery's capacity without recharging it otherwise you may shorten it's life.

Mike Harding

FollowupID: 435681

Follow Up By: hl - Tuesday, Jun 20, 2006 at 17:19

Tuesday, Jun 20, 2006 at 17:19

Mike,

That 35 Watts for a phone charger is a tad high....It would get bloody hot if it were really dissipating 35 watts. I think that 150mA is running it at 100 Volts.
Be that as it may, running a couple of 240V phone chargers through an inverter is probably the least efficient way to do it. A 12V charger for the phones would make a lot more sense.

Cheers

FollowupID: 435695

Follow Up By: Scoey (QLD) - Tuesday, Jun 20, 2006 at 17:38

Tuesday, Jun 20, 2006 at 17:38

Hey Mike & hl,

Thanks for the info - I only used the phone charger as an example (as that's all I had close to hand in terms of access to specs) but I was essentially after a reasonably simple formula to use to work out current draw, with a reasonable degree of accuracy anyway! I think I have that formula now! ;-)

Thanks again!
Scoey!

FollowupID: 435700

Follow Up By: Mike Harding - Tuesday, Jun 20, 2006 at 17:40

Tuesday, Jun 20, 2006 at 17:40

I agree, but this was really an exercise in how to calculate current consumption rather than the efficiency of a particular device.

Keep in mind that even a 12V phone charger will be a switch mode power supply (just like an inverter) and will have losses - I'll guess the cheap 12V chargers are only about 80% efficient but that is probably better than an inverter running a light load. Fewer wires too.

Mike Harding

FollowupID: 435702

Follow Up By: disco1942 - Tuesday, Jun 20, 2006 at 17:45

Tuesday, Jun 20, 2006 at 17:45

This shows the futility of using an inverter when 12V appliances are available. The input to the charger is 36W (240V x .15A) for an output of 4.56W (5.7V x .8A) – it is only 12.6% efficient.

Scoey – I suggest you purchase a car charger for your phone or a voltage regulator that plugs into the vehicle cigar lighter socket. You will then draw a maximum of 0.8 to 1A whilst your phone is charging. If you do that you can charge the phone whilst driving and save your deep cyclebattery to light your lights and cool your beer.

PeterD

PeterD
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FollowupID: 435706

Follow Up By: Scoey (QLD) - Tuesday, Jun 20, 2006 at 19:20

Tuesday, Jun 20, 2006 at 19:20

G'Day Peter,

Yeah, the phone charger was probably a bad example, but I don't yet have a fridge do get the specs off or anything like that, and yes if I wanted to charge it, I'd use a car charger. The real question to this post was to try and find a basic formula that I could, by using the specs found on each appliance (be it a Waeco, a laptop, or just a simple fluro light), find out how much current it _Affordable_Storage_Drawers.aspx and therefore estimate how long my Aux would last before needing a recharge, or more specifically to try and plan what sort of requirements my Aux will eventually need and try and cater for it now. Does that make sense?? I'm confusing myself again! ;-) hehe!

Cheers for your input though, any advice on how to keep my beer cool for longer is appreciated! ;-)

Scoey!

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Follow Up By: Nifty1 - Tuesday, Jun 20, 2006 at 22:05

Tuesday, Jun 20, 2006 at 22:05

Scoey,

That's about right, except that you will confuse yourself by thinking in terms like "amps per hour". A device will draw a current of so many amps, while it is operating - doesn't matter if it's for a minute, second, or day. Where this becomes relevant is when looking at battery capacity. If a device _Affordable_Storage_Drawers.aspx 3.5 amps, and is run for one hour, then that's 3.5 amp-hours of capacity. (Same as running a device drawing one amp for 3.5 hours.)

As others have pointed out, there is loss in any inverter, and of course if you take 3.5 amp-hours out of a battery, then you need to supply the battery with a bit more than that to make it up again due to the system being less than 100% efficient.

Calculations are all very well. Take the advice, and borrow or buy an ammeter and measure the actual current draw. It's the only way to find out for sure.

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Follow Up By: GaryInOz (Vic) - Tuesday, Jun 20, 2006 at 22:13

Tuesday, Jun 20, 2006 at 22:13

I would guess that the 150 mA nominated would be on the 100 V end of things, and about half that current (62mA) on 240V, given that it would be a small switchmode power supply.

So 15W + 6W = 21W at 14 V = 1.5A

Better more economical way would be to get a 12V car adaprter for the phone, usually ~$20-30, this would use at the most 800 mA, possibly less if it is also a small switchmode (~320-400 mA)

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