Advice on using an inverter and Amp usage - Fiberglass RV


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Old 09-26-2012, 06:56 PM   #1
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Advice on using an inverter and Amp usage

I bought an inverter and it came in the mail today. I hooked an old style 13" TV, indoor antenna amp, and a digital converter to it. Testing the amps with everything running, it was right at 1 amp.

In about 30 minutes time I measured the voltage drop on the 12V battery and it went to 12.2 volts from about 12.4 at the start. It's a marine battery rated at 85 amp hours. Theoretically, shouldnt I be able to get 85 hours at 1 amp draw? Also, is it common for voltage to drop that fast?

I know I really need a true deep cycle battery, and I plan to install a solar panel one day, but for now I just need one nights television use on our upcoming boon dock trip. I have already installed LED lights.

Thanks for the help.
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Old 09-26-2012, 07:06 PM   #2
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Some good info here: Inverter FAQ - DonRowe.com - Frequently Asked Questions about Power Inverters

You don't want to deplete a battery more than 50% so an 85 amp hour battery gives nominally 42 amp hours.

Was the 1 amp draw at 12 volts or 110 volts?
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Old 09-26-2012, 08:29 PM   #3
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Tom is asking the right questions. A few points.

Yes the 85 Amp hours theoretically means 1 Amp at battery voltage for 85 hours. Reality of course is different.

12.4 is not a fully charged good battery. Either your battery is getting a little old or it was not fully charged. I generally think of 12.4V as about 80% of full power. 12.2V as about 60% of full power. I would not go below 12V because these batteries do not like going way down as Tom pointed out. Personally, I try to stay above 12.2V

If you were running 1 Amp at 110V for a 1/2 hour that would roughly be 5 Amp hours at 12V (depending on the inefficiency of the inverter and wiring among other things). If your battery is many years old and/or has been cycled too often, and/or deep cycled too far down that would make some sense with what you are seeing (although it seems a little too fast going down for a good battery).

In addition to if the 1 Amp was on the 110 or 12V side, I would want to know everything that was hooked up to 12V and 110V and what was drawing power. If the 1 Amp was the TV and lights and other items were on that would explain why the battery voltage is going down faster.

I hope this helps. Let us know if you have questions.
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Old 09-27-2012, 07:55 AM   #4
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Good question guys! Didnt think about the difference in which side I was getting the reading from. The 1 amp reading was on the 12v side going from the battery to the inverter as it was on and running those 3 devices I mentioned earlier.

The only other thing that runs on 12V in the camper is 2 LED lights, and they were off.

The battery is less than a year old.

I dont know how to take an amp reading on the 110 side. Is that where I should be reading?
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Old 09-27-2012, 08:01 AM   #5
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Originally Posted by bicklebok View Post
Good question guys! Didnt think about the difference in which side I was getting the reading from. The 1 amp reading was on the 12v side going from the battery to the inverter as it was on and running those 3 devices I mentioned earlier.

The only other thing that runs on 12V in the camper is 2 LED lights, and they were off.

The battery is less than a year old.

I dont know how to take an amp reading on the 110 side. Is that where I should be reading?
What you are interested in is amp hours from the battery, so yes, reading amps from the battery is what you want.

1 amp to run all three of these devices at once is suspiciously low. Especially an older CRT television. After all, that is 12 volts times one amp = 12 watts - almost nothing.
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Old 09-27-2012, 08:13 AM   #6
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Tom is asking the right questions. A few points.


If you were running 1 Amp at 110V for a 1/2 hour that would roughly be 5 Amp hours at 12V (depending on the inefficiency of the inverter and wiring among other things). If your battery is many years old and/or has been cycled too often, and/or deep cycled too far down that would make some sense with what you are seeing (although it seems a little too fast going down for a good battery).
Can you explain this calculation for me please. Im still learning.
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Old 09-27-2012, 08:18 AM   #7
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Can you explain this calculation for me please. Im still learning.
The power is proportionate on either side of the inverter, neglecting the inverter efficiency.

So, 1 amp at 110 volts is about 10 amps at 12 volts. So pulling 10 amps for 1/2 hour is about 5 amp hours of battery use.
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Old 09-27-2012, 08:22 AM   #8
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Volts x Amps = Power
12V x 1A = 12W

This gets more complicated on AC, but still basically applies.
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Old 09-27-2012, 02:18 PM   #9
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I will top the battery back off with the charger when I get home, and then do another test.

Any suggestions on a good value true deep cycle battery? They are quite expensive!
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Old 09-27-2012, 02:38 PM   #10
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Any suggestions on a good value true deep cycle battery? They are quite expensive!
Costco. I bought two group 24 batteries for about $75 each. The group 27 was about $99.
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Old 09-27-2012, 02:48 PM   #11
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Yea, Costco or Walmart both sell the so called Marine battery - not true deep cycle, but sort of a hybrid deep cycle / starting battery.
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Old 09-27-2012, 07:04 PM   #12
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Thanks for all the help guys. Im actually using a marine battery now, but would like to get something better. I looked at the specs on my marine battery(cant remember them now) and it's actually pretty good.
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Old 09-27-2012, 07:46 PM   #13
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I would try to figure out what your real loads are first before buying a new battery. Taking a voltage reading on the battery is only a rough guide to the batteries state of charge. Measure the amperage draw of the TV on 110v then measure the 12V current draw. Compare the two according to the formula David provided. Battery maintenance and operation are more important than battery brand or even type. Your battery is not that old unless it was badly abused by drawing it down until dead over and over. Use your head not your wallet.
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Old 09-27-2012, 09:46 PM   #14
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Quote:
Originally Posted by David Tilston View Post
Volts x Amps = Power
12V x 1A = 12W

This gets more complicated on AC, but still basically applies.
Same thing with AC. The key is power (Watts) stays the same.
120 Volts x 1 Amp = 120 Watts.
12 Volts x 10 Amp = 120 Watts.
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