Car Battery Charger | If I attached a normal car battery to a 600w inverter and to the battery I attach a battery charger?

3 Responses to “If I attached a normal car battery to a 600w inverter and to the battery I attach a battery charger?”

1. vrrJT3

   It would be easier to just use a 12V power supply that you can plug into the wall. Or use the battery to power your device, then charge it afterward.

   If you are trying to charge the battery from itself, look up “perpetual motion”. Perpetual motion machines do not work.

2. TechGuy

   Your question is not quite as easy to answer as it may seem on the surface. Several assumptions and generalizations must be made. I’ll try to give you a rough estimate and explain my answer.

   Assume for a moment that the inverter is 100% efficient at converting the 12VDC input to 110VAC output (that means there are no conversion losses due to internal device power consumption or heat dissipation). A 600W inverter at full load (defined as an electrical device connected to the inverter output and drawing the maximum rated power of 600W, or “max current draw”) would draw approximately 50 amps from the 12 VDC car battery. How this is calculated: In a DC circuit, P in watts = I (current in amps) times E (voltage in volts), assuming no resistance to current flow. To solve for current (I), you divide P by E, so 600/12 = 50. So theoretically, a 50 amp charger would be required to maintain the battery’s charge state with a discharge rate (load) of 50 amps on the battery.

   HOWEVER, nothing is 100% efficient. A 600W inverter will have some power conversion loss (due to internal power consumption and heat dissipation). For the sake of argument, let’s assume a 75% efficiency rating for the inverter at full load (even though it could actually be as little as 50% efficient).

   At 75% efficiency, the 600W inverter (under full load) would draw approximately 25% more current than if it were 100% efficient, or about 12.5 amps more, for a total current draw of roughly 62.5 amps.

   So again, theoretically, an automotive (car) 12V battery charger capable of supplying about 63 amps continuously would be required to meet your criteria for your experiment to succeed – if you want to both fully power the inverter AND maintain the battery’s charge state (not let it run down over time).

   In the case of your experiment – you are essentially making an UPS (uninterruptible power supply) circuit, where the battery is supplying the power (voltage and current) to operate the AC inverter load, and the battery charger is supplying the power to prevent the battery from discharging. In the event that the AC power supplying the battery charger failed, the battery would continue to supply power to the inverter until it discharged to the point where it would be unable to supply the inverter with enough power to continue to operate. Once the AC power to the charger was restored, it would start to recharge the battery. This is the basic principle of an UPS power supply.

   Note: A 12VDC battery charger actually puts out approximately 13.7VDC because, in order to charge a battery, you have to have a difference in potential (voltage) as well as a complete path for current flow between the power source (the charger) and the load (the battery).
   This difference in voltage (13.7VDC instead of 12VDC) does not substantially affect the amount of current that the inverter draws. The input voltage regulation circuit of a good inverter normally clamps the incoming voltage at 12 to 12.5 V, so as to minimize source voltage fluctuations (variations), thus producing a more stable (cleaner) AC output.

   Answers to Asker’s “Additional Details” questions:
   “If I use a power supply directly to the inverter what input –amps/watts should I get.” The power supply should be able to provide 12VDC at approximately 62.5 amps continuous output rating. The input power requirement for the DC power supply would be 110VAC @ approximately 8 amps, or 220VAC @ approximately 4 amps. Note: The actual amount of current the power supply will draw at maximum output load depends on the conversion efficiency rating of the power supply.

   As a side query–would a 12v supply directly to my inverter work and will i still need a 62,5 watt output for my inverter? Yes, a 12VDC supply connected directly to the inverter will work, as long as the 12V supply source output rating is approximately 62.5 amps or greater.

3. wondering

   Tech guy is correct in that what you are wanting to do is essentially make an UPS. Specifically you are making a “dual-conversion on-line” UPS as it is mains-12V-mains. If you do this please use a good true deep cycle battery and not a “normal car battery” as SLI (starting, lights, and ignition) batteries are not designed for deep discharging and will be ruined in less than a dozen full discharges.

   Note that all consumer UPSes that are around 600W capacity are of the “standby” type meaning that mains in goes directly to mains out (and a float charger) until the power fails and then a relay disconnects the mains in and substitutes the inverter output instead.

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