# DC to AC to DC question (Use of Battery & Power Inverter)

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
Hi everyone. I have a trade show I need to attend in a few days. I just found out they will not have power available for me to run my lighting. I am thinking about running everything off of a car battery with an inverter but I can't seem to come up with the what run time I should expect from my setup.

Battery: Marine battery w/ 800 cca (my research says it should be about 45Ah)
Inverter: I have two. I can use a 100w or a 400w inverter both cut off at 10.5v)
Lighting Source: 120v LED bulbs - totaling 44w

My main concern is how to factor in the power that the inverter will consume in the conversion to AC and what will be lost in the circuitry of the LED bulb in the conversion back to DC. I know there will be some loss here but I can't figure out what it would be and if it is significant to worry about.

What do you think my realistic run time should be?

#### #12

Joined Nov 30, 2010
18,167
Eleven hours.

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
Will the fact that the inverter cuts off at 10.5v be a problem or will I get a good 11 hours out.

#### #12

Joined Nov 30, 2010
18,167
10.5 volts has little to do with this. A led acid battery is basically kaput at 12.0 volts. That's what I calculated for.

I might be mistaken because I don't know anything about the conversion back to DC, but if the bottom line is 44 watts, I am probably right, or at least a lot closer to right than a wild guess.

#### tcmtech

Joined Nov 4, 2013
2,868
A common 50 AH deep cycle battery (Group 22 size) will easily keep your system alive for about 12 hours at a time but given variable and unknowns plus you would be running the battery down to its lower charge limits I would recomend going one size larger to a group 24 ~80 AH capacity unit which for basic deep cycle battery costs that s roughly $60 -$70 Vs $75 -$85 for the added 30 Ah extra capacity.

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
I just went rummaging in my shop and found two group 24 starting / deep cycle marine batteries. Both are between 6 months and 18 months old. Volt meter says 11.8 volts. I have a charger on them now to bring them back up. My trade show is 3 days so I will have to charge one / use the other and then swap each day. I should also mention that I only need the power for 6 hours so I am thinking based on what has been said, I should be ok since I am only using the battery for half the life I can get out of it.

I'm still worried about the efficiency loss in the cheap 400 watt harbor freight inverter I have but I'm assuming that I should not be too worried about it.

#### tcmtech

Joined Nov 4, 2013
2,868
Even the cheap inverters still run at 85+% efficiency at high load and cool to the touch at no load.

Hook it up and do a test run for a day and see what happens. Either the batteries last or they don't and you will have to swap one out for the other midway though.

#### shortbus

Joined Sep 30, 2009
7,234
Since it seems your in the USA,(the reference to harbor freight), my question would be what kind of "trade show" wouldn't have 120V power available? If you don't have the 120V power, how are you going to recharge your batteries?

#### #12

Joined Nov 30, 2010
18,167
I'm still worried about the efficiency loss in the cheap 400 watt harbor freight inverter
Worry as much as you want. Two different, well experienced, electronics nerds have told you basically the same answer. If you asked the experts in order to doubt the answers, test it yourself. A much more likely failure is that your batteries aren't as good as new. Finding a lead-acid battery at 11.8 volts after a year of sitting alone indicates that it is sulphated and might not recover, let alone act like brand new.

#### crutschow

Joined Mar 14, 2008
23,756
One test beats a thousand analyses.

#12

#### KL7AJ

Joined Nov 4, 2008
2,224
Hi everyone. I have a trade show I need to attend in a few days. I just found out they will not have power available for me to run my lighting. I am thinking about running everything off of a car battery with an inverter but I can't seem to come up with the what run time I should expect from my setup.

Battery: Marine battery w/ 800 cca (my research says it should be about 45Ah)
Inverter: I have two. I can use a 100w or a 400w inverter both cut off at 10.5v)
Lighting Source: 120v LED bulbs - totaling 44w

My main concern is how to factor in the power that the inverter will consume in the conversion to AC and what will be lost in the circuitry of the LED bulb in the conversion back to DC. I know there will be some loss here but I can't figure out what it would be and if it is significant to worry about.

What do you think my realistic run time should be?
Well...they're a lot better than they USED to be. However, there should be a pretty simple spec for that, which should be published.

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
Since it seems your in the USA,(the reference to harbor freight), my question would be what kind of "trade show" wouldn't have 120V power available? If you don't have the 120V power, how are you going to recharge your batteries?
Good question. The event was moved last minute and the new place it is being hosted at decided that only the people in certain spots on the floor will have power. The hosts of the event say that they can't run power to everyone else because it is some "code violation".

Thank you all for your assistance. I really appreciate it.

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
Hello All.

So I ran some tests today. I wasn't able to bring the two old batteries back to a full 12.5 volts so I went out this evening and bought a new group 24 - 80Ah deep cycle battery. I also bought new bulbs to low my wattage to 32 watts total. Hooked everything up and I got exactly 3 hours and 15 minutes before the battery read 10.5v and the inverter quit on me.

I'm really scratching my head on what to do now. Looks like I'm losing a large amount somewhere. Would I be better just going with something like this: https://www.amazon.com/gp/product/B017386GLS/ref=pd_sim_121_1?ie=UTF8&pd_rd_i=B017386GLS&pd_rd_r=2Q7S09TEWN55P2AKPN9B&pd_rd_w=l4HB9&pd_rd_wg=cgALZ&psc=1&refRID=2Q7S09TEWN55P2AKPN9B and rigging it right to the battery?

#### #12

Joined Nov 30, 2010
18,167
"code violation"
There is more than one code that says you can't run extension cords all over a public gathering.
Would I be better just going with something like this: https://www.amazon.com/gp/product/B...rd_wg=cgALZ&psc=1&refRID=2Q7S09TEWN55P2AKPN9B and rigging it right to the battery?
That was my next idea.

This is where we ask for a drawing and the labels on the parts. You can't be that different from the estimates without having some bad information in the brew.

Did you have the new battery fully charged before you started or did you just trust the vendor to have it topped up?

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
There is more than one code that says you can't run extension cords all over a public gathering.

That was my next idea.

This is where we ask for a drawing and the labels on the parts. You can't be that different from the estimates without having some bad information in the brew.

Did you have the new battery fully charged before you started or did you just trust the vendor to have it topped up?
I just picked the battery off the shelf. It read 12.5 volts. Battery is this one: http://www.interstatebatteries.com/p/interstate-batteries/interstate-batteries-24m-xhd-24m-xhd?dsNav=N~2147384903
Inverter is this: http://www.harborfreight.com/400-watt-continuous800-watt-peak-power-inverter-61479.html
Bulbs are these: https://www.walmart.com/ip/Great-Value-GVRLBR30827KND-Great-Value-LED-Light-Bulb-8W-65W-Equivalent-Soft-White-BR30-Reflector/51496376

#### tcmtech

Joined Nov 4, 2013
2,868
So I ran some tests today. I wasn't able to bring the two old batteries back to a full 12.5 volts
So what did you charge them with and for how long?

I just pulled a near dead battery out of our big forklift last week that showed about 10.6 volts and had no cranking amps left and I let it sit on the 10 amp charger at 15 volts for 3 full days before it charged up properly to where it now has some pretty decent cranking amps and as tested reserve power again.

The first 24 hours didn't do much for it's voltage stability or reserve power so I just let it sit and trickle for next 2 to see if it would burn some of the sulfation out or kill it all the way.

#### shortbus

Joined Sep 30, 2009
7,234
So I ran some tests today. I wasn't able to bring the two old batteries back to a full 12.5 volts so I went out this evening and bought a new group 24 - 80Ah deep cycle battery. I also bought new bulbs to low my wattage to 32 watts total.
If this is just for lighting of you display, why not go to a RV store and buy some 12V camper type light bulbs? They have an Edison type base and will fit your 120V sockets, and the fixture wiring will work fine. This will eliminate the losses from the inverter and the need for 120V AC.

EDIT. Just Googled about 120V led bulbs. For what your doing they aren't a good fit. Inside they have various different circuits, depending on the brand. All of these are adding up to more losses that you don't need. http://www.powerelectronictips.com/teardown-60-w-equivalent-led-bulbs/

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Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
EDIT. Just Googled about 120V led bulbs. For what your doing they aren't a good fit. Inside they have various different circuits, depending on the brand. All of these are adding up to more losses that you don't need. http://www.powerelectronictips.com/teardown-60-w-equivalent-led-bulbs/[/QUOTE]

That was the thing I was worried about and why I asked the question. I had a feeling there was going to be a loss. I asked a few other people and was told that the internal circuitry of the 120v LED causes harmonic distortion within the inverter and ends causing an extra load from heat within the inverter.

This morning I ordered some 12v LED RV bulbs and will modify my wiring so I can hook it up to a 12v source.

Thread Starter

#### thebigcheese

Joined Nov 14, 2016
10
So what did you charge them with and for how long?
10 Amps at 15 volts for 12 hours. Brought it back to 12.5 but it discharged over a few hours down to 12.25.

#### tcmtech

Joined Nov 4, 2013
2,868
10 Amps at 15 volts for 12 hours. Brought it back to 12.5 but it discharged over a few hours down to 12.25.
How much run time did you get from the battery after charging for 12 hours?

I've came across many batteries that didn't have all that great of open circuit voltage yet when put in use had all around good service life and working capacities just the same. Not 100% new capabilities but still well into the upper half of their original ratings.

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