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I need to hook up a 1000W inverter inside my van. I can't hook it up directly to my cigarette lighter because the lighter only gives about 100W-120W. I want to pick up a separate deep cycle battery to run the inverter from but I need to be able to charge it while my van is running. If I plugged the extra battery into my cigarette lighter would it create a parallel circuit with my van battery and would it charge while I drive? I have a feeling it's not that simple. If that's the case any ideas?

  • You can't just parallelize sources, as any difference in the voltages will cause a current from one source to the other. And once they are fused (and the lighter is) it will probably blow up. – Eugene Sh. Nov 16 '15 at 22:00
  • Maybe better fit for Mechanics.SE? Throwing an extra battery on a car system, with no consideration to wiring or the alternator load or anything? – cde Nov 16 '15 at 22:02
  • It is possible to add the second battery in the engine compartment, if the alternator is large enough as @Passerby suggests, and run straight from that to the inverter. But depending on what kind of load you are thinking of plugging into it (hair dryer? Microwave oven?) that could quickly drain the battery. Next time the car is started, the alternator is going to struggle recharging two batteries. Possible, perhaps. Ideal? Probably not. – rdtsc Nov 16 '15 at 22:07
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    If you must charge your battery off the cig lighter you should have some form of current limiting and something to stop the battery trying to run back when the car ign is off . – Autistic Nov 16 '15 at 22:10
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You can put the deep cycle battery in parallel to your main car battery. DO NOT DO THIS VIA THE CIGARETTE LIGHTER AS YOU MAY BLOW A FUSE. Connect it positive to positive and negative to ground (could be frame ground or the ground terminal). On your ground cable place a battery isolator (in your case a 150 amp or greater continuous solenoid). Then connect you inverter to the battery isolator. When your car is running switch the battery isolator so your deep cycle battery is connected to your starting and stopping battery. Your alternator will then attempt to charge both. You may need to upgrade your alternator.

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If you just connect a discharged battery with no control then it will initially try and charge very quickly. If you try and do it through the cigarette lighter this will blow a fuse.

There are various methods of setting up a dual battery system with different pros/cons. Almost all of them will want a higher current connection than a cigarette lighter socket can deliver.

http://www.12voltplanet.co.uk/split-charging.html

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I do this regularly. I have an ordinary secondhand car battery, that I charge through the lighter socket with nothing more than a plug, 2 crocodile clips and a 10A fuse (the car's own lighter socket has a 16A fuse, so the "charger" fuse should blow first.) The issues are as follows:

In my case, I always remove the battery in order to use it, so forgetting to disconnect it is not a problem.

When you build the adapter, get a plug, a wire (for the negative) an inline fuseholder (for the positive) and 2 crocodile clips. Make sure the only exposed metal is the crocodile clips. Also, get into the habit of connecting the crocodile clips before inserting the plug. This will prevent them from accidentally touching. I haven't blown a fuse yet.

With a leisure battery, it may have a much larger capacity than the car battery that I am using, and it may be deep cycled down to a much lower voltage. Both of these will increase the current. A 0.1 ohm resistor in 40W power rating could be useful here: at a 2V difference it would be guaranteed not to exceed 20A. Such a simple solution would of course slow the final part of the charging process. Any electronics store should carry such resistors, but be careful as they can run very hot. Getting a panel mount one and bolting it to a piece of aluminium (not wide enough to bridge the battery contacts) would help with that. I considered such a resistor but in my case it was not necessary.

If the charging rate is unacceptable for your application, you may need to install a lead under the bonnet. The most important thing is always to make sure it is properly fused (check the current rating of the wire you use.) Car batteries can produce enormous currents if short circuited and can easily start a fire. A resistor of 0.05 or 0.02 ohms will help guarantee the current does not exceed a certain value, though the resistance in the wiring and connections may be comparable to the value of the resistor.

Finally, don't leave your battery on display. Someone broke into my car and stole only the battery that was on the floor in front of the passenger seat (and while they were at it, the battery under the bonnet/hood too.) Nothing else was stolen, but my door is now bent out of shape. Now i always stuff some old overalls on top of it whenever the car is parked so that the battery cannot be seen.

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