Hairsy Posted September 7, 2009 Share Posted September 7, 2009 If I have a 12 volt appliance that has a 240v transformer to supply power, I can connect it two ways: 1) Bypass the transformer and supply 12 volts from a battery 2) Use an inverter to supply the 240 volt needed by the transformer Would (2) use significantly more power than (1)? Thanks Dave Link to comment Share on other sites More sharing options...
spospe Posted September 7, 2009 Share Posted September 7, 2009 Most inverters claim about 90% efficiency these days, so in simple theory the inverter route would use more power than the direct 12-volt connection. Forgot to add: the transformer itself will be less than 100% efficient, so probably overall, the inverter method will be around 15-20% less efficient, due to the losses in both inverter and transformer. Link to comment Share on other sites More sharing options...
Brambles Posted September 7, 2009 Share Posted September 7, 2009 Hi Dave, Yes, using an invertor to supply 240 volts would have power losses in the invertor itself. Depending on type/make and size could be at a guess anythng between 70 to 90% efficient. The 240v to 12 volt tranformer will also have big losses, depending on quality and load, could be anything from a massive 75% loss to about 20% loss. Go for 20% loss if it is matched to the equipment being powered and is good quality. If it is by chance a toroidal transformer then it is about 10% loss across all load conditions. So summary is you would have about 60% efficiency converting to 240 volts and back down to 12v. As the 240 volts from invertor is not a pure sine wave there will be even higher losses. A switch mode power supply (not a transformer) will have between 85 and 95 efficiency. So say 10% loss + invertor loss of say 15% = 25% loss. Or 70% efficiency. Supplying directly from batteries, just the losses in the cable.....small. So to answer your question - Yes a lot more power would be used. Do make sure if you power directly, the equipment will cope with vehicle voltages. e,g battery under charge (14.4 volt) and voltage under load on the battery, say 10.5 volts. It also has to cope with voltage spikes that may exist if you are powering from an onboard fitted vehicle battery created when you start the engine, other equipment is turned on and off etc. One way round this is to use a voltage stabilser, 12 to 12 volts such as supplied by Amperor http://www.amperordirect.co.uk/products/12v_Stabiliser.asp . Hope this helps answer yoru question. Link to comment Share on other sites More sharing options...
Hairsy Posted September 7, 2009 Author Share Posted September 7, 2009 That's great - thank you very much for such a comprehensive answer. I've been sticking with 12 volt wherever possible and you give me the reassurance that it's the way to go when it's practical. Best wishes Dave Link to comment Share on other sites More sharing options...
Brambles Posted September 7, 2009 Share Posted September 7, 2009 Oh dear, my answer is as clear as mud and a bit French - why use 1 word when 10 words can be used!!! Link to comment Share on other sites More sharing options...
moyne Posted September 7, 2009 Share Posted September 7, 2009 The current requirements would need to be looked at. ( I ) Current is what flows on a wire or conductor like water flowing down a river. Current flows from negative to positive on the surface of a conductor. Current is measured in (A) amperes or amps. A watt is standard unit of power. most appliances will give you an indication of the power required "Used" such as a 50 watt light bulb. Why is this important to you. Well, look at your battery it will have a rating which says that the battery will supply 12 v for so many Amp hours an example would be a battery with 20 amp hours (aH) . So that battery could possibly supply 12 v for upto 20 hours. Also if your wiring is not rated high enough to supply your appliance with the required power it uses you could possibly become the owner of a chargrilled loom. Unless your appliance uses low power consumption or you are able to figure out the rating best keep using a suitable inverter or mains power. Not sure if this is confusing ! Link to comment Share on other sites More sharing options...
Pat Posted September 8, 2009 Share Posted September 8, 2009 moyne - 2009-09-07 10:54 PM The current requirements would need to be looked at. ( I ) Current is what flows on a wire or conductor like water flowing down a river. Current flows from negative to positive on the surface of a conductor. Current is measured in (A) amperes or amps. A watt is standard unit of power. most appliances will give you an indication of the power required "Used" such as a 50 watt light bulb. Why is this important to you. Well, look at your battery it will have a rating which says that the battery will supply 12 v for so many Amp hours an example would be a battery with 20 amp hours (aH) . So that battery could possibly supply 12 v for upto 20 hours. Also if your wiring is not rated high enough to supply your appliance with the required power it uses you could possibly become the owner of a chargrilled loom. Unless your appliance uses low power consumption or you are able to figure out the rating best keep using a suitable inverter or mains power. Not sure if this is confusing ! I think you are possibly confused a battery will supply 12 volts for a very long time if there is only a small load ( that's why car batteries can last weeks without the car being used, even though the alarm/clock are still drawing power!) A 20 amp hour battery theoretically should supply 12 volts with a load of 1 amp for 20 hours in a perfect world, however a figure of say 16 hours or less is the norm and the voltage unfortunately will drop over that period, and that voltage drop will cause the television connected directly to the battery to reduce the picture and even switch off. Regards Pat Link to comment Share on other sites More sharing options...
tonyishuk Posted September 11, 2009 Share Posted September 11, 2009 Word of warning ( maybe a few ) It would pay to look at other threads (using the search button on the web page) about using 12 V appliances (especially TVs) which dont like anything other than 12v. 12 V from a mains unit is usually fairly constant, 12V from a leisure batery can be anything from 11.5v ( maybe less) to 14v (maybe more). The over voltage could fry the internals of an appliance expecting a regulated 12v if connected directly to a 12V battery circuit. Rgds Link to comment Share on other sites More sharing options...
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