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Old 07-03-2018, 07:09 PM
hunter88 hunter88 is online now
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Default 120v to 12v conversion UGH

Ok, I get most conversions, but when planning to run something that's 120 volt on 12 volt by use of an inverter, I get lost. Hopefully someone could help.

I have two different TVs One is 55 watts, which I figure at .45 amps when run on 120 volts, the other is .65 amps so should be 78 watts when run at 120 volts.

I have a 150 watt inverter that I can hook to a 12 volt battery, or better yet I have 2 batteries so I could run them parallel. Each battery is a 105 amp hour battery, so parallel would give me 210 amp hours.

I understand the inverter will waste some energy, which means there will be more then either 55 watts or 78 watts being used, depending on the TV I use.

If a TV draws .45 amps, then it's drawing .45 amps in an hour. But what happens to amp hours when you're drawing off 12 volts instead of 120? Does the TV still just draw .45 amps an hour when hooked to a battery?

I've used a small 7" battery operated TV in my camper. But I have a 20" and 27" TV just sitting at the house, so I thought if I could get a couple hours use out of them in the evening, and then let the batteries recharge through a solar panel, I could have a bigger TV to watch.
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Old 07-04-2018, 10:29 AM
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10 to 1 ratio.. It's in Ohm's Law..

Example.. Your TV that uses .45 amps @ 120 v.a.c.

Find the wattage.. (120 X .45 = 54 watts) *120 being the working voltage & .45 being the current demand (Amperage)

54 watts is the objective..

12 volts X 10 = 120 volts (The 10 to 1 ratio thing)

12 volts X 4.5 amps = 54 watts (10X the amperage needed to produce the same wattage)

To produce the 120 volts a.c. at 54 watts from a 12 volt d.c. battery through an inverter, the amperage demand from the battery increases 10 times because the voltage is 10 times lower. *Inverter losses not included but they usually don't eat much..*

Quote..
"I have a 150 watt inverter that I can hook to a 12 volt battery, or better yet I have 2 batteries so I could run them parallel. Each battery is a 105 amp hour battery, so parallel would give me 210 amp hours."

*Connecting batteries with the same voltage in parallel increases the current capacity.

*Connecting batteries in series increases the voltage but the current capacity remains the same as one battery.

Quote..
"Each battery is a 105 amp hour battery, so parallel would give me 210 amp hours."

This is correct BUT remember the safe discharge level is 25% of the total. *210 divided by 4 = 52.5 aHr. reserve.

If your battery is fully charged and in good condition, you actually have ~52 aHr. of reserve to use safely to avoid any damage to the battery itself. Rounding the demand of the TV used in the example above to 5 aHr., I would say you could operate that TV from that battery with your 150 watt inverter for at least 8 hrs. safely with a healthy safety margin.. (**Always round demand up and reserve down**)

This example also explains why folks that plan to use solar power to operate standard A.C. household items requires a higher voltage battery. The higher battery voltage requires less current for the conversion..

Hope this helps..
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Old 07-04-2018, 10:49 AM
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Another reality..

Don't expect a 1 to 1 charge ratio to the battery from the solar charge system..

There is a 20% loss in the charge/discharge cycle. If you provide 100aHr. of charge to the battery from your panels to the battery, the battery will only have absorbed 80%, or 80 aHr. It's just the nature of the beast..
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Old 07-04-2018, 11:38 AM
hunter88 hunter88 is online now
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Thanks, I was kind of thinking it would be a 10 to 1 ratio, but just wasn't sure.

Quote:
12 volts X 4.5 amps = 54 watts (10X the amperage needed to produce the same wattage)
So the TV that normally pulls .45 amps plugged in at home, will pull 4.5 amps each hour from the 12 volt batteries in my camper. If used for a couple hours in the evening, I'd use about 10 amps from my battery reserve, which would be about 10% of a 105 amp hour rated battery, or 5% if I ran 2 105 amp hour batteries in parallel.

So now I'll need to decide it it's worth 2 amps per hour to go from the 20" TV at 4.5 amps, or the 27": TV at 6.5 amps.

Quote:
This is correct BUT remember the safe discharge level is 25% of the total. *210 divided by 4 = 52.5 aHr. reserve.
Having an electric golf cart taught me to never run the batteries down very far. I once saw a chart that compared batteries run down 50% to batteries run down 25%. It showed a battery bank run down 25% could be recharged 7 times more then batteries run down 50%. The guy there said dry batteries like in your phone or laptop actually should work better if run down farther, but any battery with water should never go below 70% charge if you want long life.

Guess that's why golf cart guys say, if you're not riding it, plug it in.

Quote:
Don't expect a 1 to 1 charge ratio to the battery from the solar charge system..

There is a 20% loss in the charge/discharge cycle. If you provide 100aHr. of charge to the battery from your panels to the battery, the battery will only have absorbed 80%, or 80 aHr. It's just the nature of the beast..
The batteries should have plenty of time to recharge between uses.

I've been checking on the 100 watt solar kit from Harbor Freight. The reviews have been pretty good and almost all comment this unit is much better then the old 45 watt unit they used to sell. YouTube videos have been impressive on how much it will charge even in the shade.

My use will be almost 100% the TV. I usually bring a radio so I don't use the built in radio from the camper, and I rarely use the lights.

If I could safely run the TV for 8 hours, I'd probably only use it for a couple hours in a day. So even if I stayed two days I still probably wouldn't get to the 8 hour safe level. Then after I leave the solar panels would recharge and I may not use it again for a week or more, so I thought that should give them time.

Thanks for the help.
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