Adding a Secound Battery to Power a Inverter - Mercedes-Benz Forum

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post #1 of 13 (permalink) Old 02-15-2011, 09:30 PM Thread Starter
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Adding a Secound Battery to Power a Inverter

I have a 1987 300SDL. Im putting a laptop stand in and will be adding a power inverter to power the laptop. I was wondering if i may be able to add a secound battery to the car to help power the inverter when the car is not running. Does any one have any sugestions about doing this. Can i power two batteries off of one alternator. Can i hook up the secound battery where i only pull from it for the inverter and not to power or start the car. What kind of options do i have.

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post #2 of 13 (permalink) Old 02-15-2011, 11:15 PM
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Sure you can. Any RV story will find you battery insulator, although I had bad experience with them leaking current and prefer power combiners.
Than how big is your laptop? I am using 14" for GPS and it is on my truck running all the time. With about 80W draw I can have it running on parking lot for 4 hr without worrying about truck battery discharge. Than the laptop battery on its own will run for over 1 hr.
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post #3 of 13 (permalink) Old 02-15-2011, 11:37 PM Thread Starter
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Well i have a 17" laptop but most of the time i will probly be using my 15" laptop in the car. I also have a cell phone i will be powering on the inverter running a wifi app to provide internet service to the laptop. I inmagine the car will be running a lot of the time any way to supply AC but im not really sure how to figure out the batterys compacity and how long it will run what i will be running while still having enogh power to start the motor. I dont think i would be using the inverter for more then 2 hours at a time before running the motor again. I plan on installing a fairly large inverter with at least 3 110 outlets. 1 for the laptop, 1 for the wifi providing cell phone and a 3rd for a secound cell phone for making calls. The laptop and wifi cell will be running all day but the 2nd cell would only be plugged in if it was dying. Do you think my battery would hold up to that kind of power draw for up to two hours while still turning over the motor.

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post #4 of 13 (permalink) Old 02-16-2011, 08:50 AM
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I am pretty confident it will. Those things take minimal power. I am occasionally running my laptop off 80W inverter in the car. The 80W is marginal supply and with low computer battery on hot days the inverter will overheat, when automatic switch shuts it down, but for occasional use the 80W is all it takes. The 250W inverter I am using in the truck has a fan running constantly what not only takes extra power, but makes annoying noise, so choose your inverter carefully.
If you really want to be safe, you can build the independent battery system. Don't need big battery for what you use, so the smallest car battery should do. Find the space for it, connect to the main battery via at least #8 wire and install combiner on this wire. Combiners are pretty expensive, but other way is to install regular relay, that would be activated with ignition on. Not too complicated.
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post #5 of 13 (permalink) Old 02-16-2011, 11:26 AM
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OH. You might know that almost all inverters do have low voltage shut down switches. Meaning they should shut down before your car battery will go too low for starting the engine. I wouldn't take their word for it and test it, but IMHO you don't really need second battery, just a good one in the car.
Worse come to worse, you can always carry second, well charged battery in your trunk with jumping cables before you become confident the system works to your expectations.
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post #6 of 13 (permalink) Old 02-16-2011, 11:39 AM
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You have probably considered using a power supply that plugs into a cig lighter, but I'm curious why you don't go that route? Cheap and easy.

Does anyone know if adding a second battery would require changes to the alternator?
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post #7 of 13 (permalink) Old 02-16-2011, 05:19 PM
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Does anyone know if adding a second battery would require changes to the alternator?
Why would it? Motorhomes often have dual battery banks with 4 to 8 batteries and still run on alternator supplied with the chassis, what on older models was like 40-60 amps
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post #8 of 13 (permalink) Old 02-16-2011, 10:39 PM
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Originally Posted by Kajtek1 View Post
OH. You might know that almost all inverters do have low voltage shut down switches. Meaning they should shut down before your car battery will go too low for starting the engine. I wouldn't take their word for it and test it, but IMHO you don't really need second battery, just a good one in the car.
Worse come to worse, you can always carry second, well charged battery in your trunk with jumping cables before you become confident the system works to your expectations.
Carry an extra and use jumper cables? Really?

You can run two batteries with one alternator. It will charge both just fine.

You can also sit for hours on end without it killing both of the batteries. The laptop & phone chargers won't use much power at all. Just make sure to fuse in between the batteries and run 4 gauge power cable. Keep ground cables as short as you can and ground to the frame!


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post #9 of 13 (permalink) Old 02-17-2011, 09:05 AM
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You get 2 main type of batteries, cranking batteries and deep cycle batteries. If you are going to add a second battery for anythiing other than starting the vehicle then you would be better off adding a deep cycle battery. Often found on boats and RVs.
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post #10 of 13 (permalink) Old 02-17-2011, 03:09 PM Thread Starter
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Ok well ill give it a try with one battery and see how well it works. I have one of those jump packs ill carry that with me should be enogh to turn over the motor if my car battery should die. I do have one of those cigerete liter plug in inverters which works good but i only have the one liter plug in the car and i use it to power my gps system. so i wanted to get a inverter to power the lap top and two phones i could hook stright to the battery.

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