Inverters (and yes, I've performed a search)

Submitted: Thursday, Mar 13, 2003 at 14:37
ThreadID: 3829 Views:1865 Replies:9 FollowUps:8
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There seems to be lot of discussions and info on inverters here, and in the archives, but a few questions haven't been asked/answered. Maybe I'm the only one that doesn't know these things. Can someone enlighten me, please?

When using an inverter, is the current draw on the battery equal to the inverter's potential output, or only what the inverter is driving? For example, if a 300 watt inverter is driving a 100 watt appliance, is the current draw on the battery 300/12 = 25 amps per hour or 100/12 = 8.3 amps per hour?

Can the inverter and appliance be run with the vehicle engine running? If so, and if the alternator was pumping, say, 35 watts back into the battery, the above example would result in a zero net draw from the battery? Yes?

Can an inverter be hooked up directly to an alternator, with appropriate solanoid/regulator, and be used to run appliances/charge batteries while driving the vehicle?
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Reply By: Voxson - Thursday, Mar 13, 2003 at 16:46

Thursday, Mar 13, 2003 at 16:46
08 83498332 = Pecan Engineering....

They manufacture and supply solar panels and invertors.
Ask for Bruce... Between 9am & 3pm...*************************************
Just countin the days till July 5th. *Cape York Trip*
*************************************
AnswerID: 15095

Follow Up By: ExplorOz Team - David - Thursday, Mar 13, 2003 at 22:52

Thursday, Mar 13, 2003 at 22:52
Tell he to list his business for free in the search engine under 4WD Accessory partswhen you next speak to him.Regards
ExplorOz Team - David
--------------------------
Always working, not enough travelling ;-)
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Follow Up By: Voxson - Thursday, Mar 13, 2003 at 23:23

Thursday, Mar 13, 2003 at 23:23
Ok i will do that.......*************************************
Countin the days till July 5th. *Cape York Trip*
*************************************
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Follow Up By: Rick - Friday, Mar 14, 2003 at 20:12

Friday, Mar 14, 2003 at 20:12
Hey Voxson,

Any chance that your posted pics are at Robe?
C U there at Easter?

Cheers

Rick
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FollowupID: 9253

Follow Up By: Voxson - Friday, Mar 14, 2003 at 22:22

Friday, Mar 14, 2003 at 22:22
Yep Rick... Robe - Beachport...
Good eye me boy.........

_____________________________________________
Countin the days till July 5th. *Cape York Trip*
_____________________________________________
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Reply By: sean - Thursday, Mar 13, 2003 at 19:11

Thursday, Mar 13, 2003 at 19:11
I have used lots of inverters but always cheap ones. I found they all draw current even when there is no load and the bigger the rating the more waste there is. They all drained the battery if left conected, even if not powering anything.

I have about 5 or 6 of the things and never use them for camping/touring. I have found the Piranhas the best by far. Why do you want one may I ask.

Sean
AnswerID: 15112

Follow Up By: Member - Rohan K - Friday, Mar 14, 2003 at 09:19

Friday, Mar 14, 2003 at 09:19
Sean, I have two small 7.2 amp 12 v sealed lead-acid batteries that I use to run an electric water pump and also want to use to run several LED light arrays. I have a simple 4 amp 240 v battery charger that I want to use to keep the two small batteries charged up. I'd also like to use it to allow me to charge my digital camera battery that runs on 240 v and 23 watts. Then I figured I could use it for a few other similar purposes like recharging the lap-top battery or, if I go for a pure sine wave inverter, even run the PC straight from the inverter. My battery needs are minimal at the moment as I done run a fridge. Until then I thought a small-ish (say 150 to 300 watt) inverter would be a cost effect solution.

Rohan
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Follow Up By: sean - Friday, Mar 14, 2003 at 11:55

Friday, Mar 14, 2003 at 11:55
Rohan

I use them for similar uses. I have sealed lead acid batteries (ten 17 A/h batteries and one 34 A/h battery. The 34 A/h is a portalac and its a ripper).

I reckon that the Piranha inverter will suit your needs and I found these to be the best of the cheapies as far as efficiency and reliabity goes. Its not pure sine wave but I have run notebook computers (lots of) off inverters for 6 years now and never had a problem with the computer.

Hope this helps some.

Sean

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Reply By: GaryInOz - Thursday, Mar 13, 2003 at 19:38

Thursday, Mar 13, 2003 at 19:38
Most inverters are not 100% efficient. They have a baseline current draw, and are only about 70-80% efficient at the "real" conversion of the rest of the power from 12 volts dc to 240 volts ac. Count on an efficiency of 70% and you shouldn't go too far wrong ie. 70 watts (small TV) will draw about 100 wats from your inverter (12 volts x 8.3 amps) , which will need 120-140 wats (12volts x 10-12 amps x 1 hour) to recharge your battery.

If you must have it get as much "native" 12 volt gear as you can as it saves the capacity in you batteries. It must be said that inverters make good heaters on those cold and frosty nights ;-)
AnswerID: 15114

Reply By: Mick - Thursday, Mar 13, 2003 at 21:14

Thursday, Mar 13, 2003 at 21:14
Rohan, I echo sean's question - why do you want one?? With excellent 12 volt lights available, 12volt or gas fridges, gas heating, gas cooking I can't see any reason for using such an inefficient energy source. Maybe you have thought of something that I haven't but I charge my mobile phone and video camera, run my PC and GPS, light my camp, cook my meals and refrigerate my food and drink without 240volts! Cheers!
AnswerID: 15138

Reply By: Mick - Thursday, Mar 13, 2003 at 21:14

Thursday, Mar 13, 2003 at 21:14
Rohan, I echo sean's question - why do you want one?? With excellent 12 volt lights available, 12volt or gas fridges, gas heating, gas cooking I can't see any reason for using such an inefficient energy source. Maybe you have thought of something that I haven't but I charge my mobile phone and video camera, run my PC and GPS, light my camp, cook my meals and refrigerate my food and drink without 240volts! Cheers!
AnswerID: 15139

Reply By: paul - Thursday, Mar 13, 2003 at 21:49

Thursday, Mar 13, 2003 at 21:49
Just to confirm that inverters draw power while attached and turned on - not turned off, but you know your power point at home draws current when the telly is turned off, just an electricity thing. As for your conversions you only need to use ohm's law and not get confused by the wrong question, it is not comparing watts but comparing watts to volts. If it is a 240 volt appliacnce then you it will take out of an inverter X amps at the watts rating divided by its volts rating - 240 of course or you would not be using an inverter. Therefore, a 100 watt appliance reliant on a 240v power source will require current at A = W/V which would be 100 watts divided by 240v = 0.41amps. The conversion of 12v pulse to a 240v wave is a separate matter governed by the efficiency of the inverter. That uses current itself as people have pointed out, the higher the 240v current is required the less the efficiency of all inverters and current consumption rises. that's where quality of pure sine wave and modified sine wave inverters and respective build qualitites come into play.

So use an inverter with a power (watts) rating greater than your applicance, but to measure current draw you then forget about the inverter rating (like an accelerator, only as much current is drawn thru the inverter as is want to be consumed by the appliance) and measure the watts against the voltage source it is operating off.

Watts don't get pumped into batteries, except as a function of current, so it goes round. I run my inverters off stationary batteries not even in the car - i just recharge them the next day, so yes you can run an inverter off a wagon 12v socket without the vehicle running. As with any applicant, 12v or 240v off an inverter, you should be conscious of running your battery out of amps (which you can measure by the state of the voltage).

An inverter cannot be hooked up to an alternator because alternators themselves put out unregulated current and pretty uneven voltage levels, governed primarily by the person operating the throttle. Every decent inverter has a fuse say 20 amps, my alternator pumps out at mid throttle about 90amps, 90 into 20 means blown fuse and inverter not operating.

I think everything i have said it right and trust someone will correct me if i have slipped up.

Voltage = watts / amps
Amps = Watts/ votlage
Watts - Voltage multiplied by Amps

Inverters are great, just use them when you need them and disconnect the when you don't, or monitor your batteries and recharge as soon as possible.
AnswerID: 15148

Follow Up By: GaryInOz - Thursday, Mar 13, 2003 at 23:03

Thursday, Mar 13, 2003 at 23:03
Two minor points

1. All mains powered items with remote controls will never be truly "off". They require a source of power to run the reciever for the remote control and other "backup" items (clocks/last channel/volume/brightness/etc. memory).

2. The preferable way to run an inverter IS WITH THE ENGINE(ALTERNATOR) running charging the battery across it. This MAY cause induced interference in some electrical equipment attatched to the inverter depending on the type of inverter used. Sinewave inverters wil be less likely to cause interference than square wave, but are a little less eficient due to the power dissipated to "round off the corners" of the output wave. You will not "force" 90 amps into a circuit drawing 20 amps, The fuse is there to protect the battery if the inverter develops an internal short or is heavily overloaded, just as the alternator fuse protects the battery if the alternator develops a short .

One other formula you may like to play with that will explain 90 amp assumption as false:

Using your 12 volt 20 amp inverter consumption 240 watts (realistically a "200 watt" inverter),

Ohms = volts/amps
or ? = 12/20
? =0.6 ohms

but you suggst "forcing" 90 amps through it

0.6 = ?/90, transposing (multiply both sides of the equation by 90)
0.6 x 90 = ?
? = 54 volts!! from a 12-14 volt alternator?

The other more realistic assumption is that the resistance changes with the load (ie. dynamic), therefore to get 90 amps through the 12 volts you would have a resistance of 0.13333 ohm and have a load connected of 12 x 90 = 840 watts, or over 4 times the rated load for your inverter. If your inverter had a 20 amp fuse then it would blow the fuse to save itself and/or the battery.

For clarity of explanation inverter efficiency has been disregarded.
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FollowupID: 9185

Reply By: peck - Friday, Mar 14, 2003 at 17:37

Friday, Mar 14, 2003 at 17:37
Power (watts) = Voltage (volts) times Current (amps).
Power out = Power in times efficiency.
So if a 100 watt appliance is connected to the inverter, the current draw at 12 volts (input to the inverter) is given by (100/12)/0.8 amps (10.4)assuming an 80% efficient inverter.
Relatively cheap inverters are available from Jaycar Electronics with efficiencies of around 90%. It is a lot safer if these are installed with a switch in the input side (12v) and hence the problem of idle current flattening the battery is avoided. They can be connected straight into the vehicles electrical system without much trouble. I use one for charging video camera batteries, digital camera batteries and running and charging laptop battery with no problems. It beats purchasing and carrying around the extra charger for each device to connevt to the 12v car supply.
AnswerID: 15204

Reply By: paul - Friday, Mar 14, 2003 at 18:30

Friday, Mar 14, 2003 at 18:30
To Gary in Oz, in retrospect i should not have attempted to answer a technical question late at night after a bottle of red wine, apologies and am glad to be corrected, of course a battery only takes from an alternator what its internal resistance permits - but the question was "can i connect my inverter directly to my alternator" not "to my battery which is charged by my alternator" - so i would like to see what an alternator would do connected directly to an inverter which has no internal resistance other than a relatively small fuse.

And yes an inverter can, and it is better for the battery if the, be connected to the battery while the alternator is charging it.

Cheers all.
AnswerID: 15207

Follow Up By: GaryInOz - Friday, Mar 14, 2003 at 20:00

Friday, Mar 14, 2003 at 20:00
The simple answer is that alternator should not be run without the battery attached, it will damage the diodes in the alternator.
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FollowupID: 9252

Reply By: Member - Rohan K - Tuesday, Mar 25, 2003 at 12:43

Tuesday, Mar 25, 2003 at 12:43
Thanks for all the advice folks. I've picked myself up a small 140W unit from Jaycar. I've tested it out on the biggest current drawing appliance it will be used on - the PC battery charger, and it works fine. I didn't measure the current drop but it took about the same time to recharge the battery (to full according to the PC) as through mains so I'm guessing its reasonably efficient.

Another useful addition to the gear, I reckon.Be good, or be quick.
Rohan
AnswerID: 16062

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