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AC to DC power question

Old 01-07-2015, 03:18 PM
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Question:
If I were looking to run a piece of hardware that has a 120v power source and consumes 250 watts at startup and then 55 Watts (Average) while running. Approximately how long would said piece of equipment be expected to run using a 600w inverter wired to 2 group 27 deep cycle batteries (90ah each) wired in series?

Is more information needed?

Not looking for an exact figure just an approx. length of time ( 1 hour, 2 hours, 6 hours? ;?

Elect conversion math is not my thing..
Old 01-07-2015, 03:30 PM
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about 35 hours.
Old 01-07-2015, 03:57 PM
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That's about 2 amps on startup and about a 1/2 amp continuous AC current. A 600 watt inverter will draw about 25 amps dc at full load and supply 5 amps AC. Backing into this you will use appox 2.5 amps dc for your 1/2 amp AC. Figure with the inverter losses your at 3.5 amps. That puts you at about 50 hours, excluding startup draw.

Now are the batteries really in series (24vdc) or in parallel (12vdc). I based it upon parallel.
Old 01-07-2015, 04:20 PM
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Originally Posted by ReelNauti View Post
That's about 2 amps on startup and about a 1/2 amp continuous AC current. A 600 watt inverter will draw about 25 amps dc at full load and supply 5 amps AC. Backing into this you will use appox 2.5 amps dc for your 1/2 amp AC. Figure with the inverter losses your at 3.5 amps. That puts you at about 50 hours, excluding startup draw.

Now are the batteries really in series (24vdc) or in parallel (12vdc). I based it upon parallel.
series or parallel should be inconsequential in this case since you're inverting 12V in parallel or 24V in series.... BUT the total amp*hours are 180 in parallel; and remain 90 in series. The math works out about the same.
Old 01-07-2015, 04:47 PM
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I'm now expert but most say not to deplete your batteries by more then 30%, for sure not over 50% or yo can damage them. So whatever the batteries are rated at, figure 30-50% of that. Again please correct me if I'm wrong.
Old 01-07-2015, 05:01 PM
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Originally Posted by ryan darrow View Post
about 35 hours.
Originally Posted by Boataholic View Post
I'm now expert but most say not to deplete your batteries by more then 30%, for sure not over 50% or yo can damage them. So whatever the batteries are rated at, figure 30-50% of that. Again please correct me if I'm wrong.

No expert either, but I think the 35 is high. I found this calculator and it gives a 15 hr time period based on 180 ah at 20 hr rate, using a 5.52 dc amp draw - using a 50% drain state for the battery. I don't know how the efficiency of the inverter is being taken into account

http://www.batterystuff.com/kb/tools...to-a-load.html
Old 01-07-2015, 05:11 PM
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Actually Ryan's math is ballpark correct in that he has an estimation (or maybe a real calculation) of the inefficiency of the inverter and then comes up with a realitic time frame. I say he is spot on. Then IMHO I would "budget" myself at 30 hours to be on the conservative side of when power loss would occur.
Old 01-07-2015, 05:34 PM
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And there's the actual capacity of older (fully charged?) batteries other than new rated capacity and potential line losses from improper wiring and poor connections. How often will it 'start up' during service...just once? Is the 'hardware' temperature sensitive? Many inexpensive inverters are terribly inefficient.

The math is a good place to start but I personally wouldn't use it as an optimistic figure.

After you get the weight, space needed and cost tallied up, you may find a lightweight, quiet generator will best suit your needs.
Old 01-07-2015, 05:34 PM
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Yup, about 33-35 hours (inv efficiency of 85%, 55w/12v).

But, if you want the batteries to last, I wouldn't go more into about half the 90ah so say maybe 16 hours of use then recharge.
Old 01-08-2015, 03:55 AM
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Originally Posted by ryan darrow View Post
series or parallel should be inconsequential in this case since you're inverting 12V in parallel or 24V in series.... BUT the total amp*hours are 180 in parallel; and remain 90 in series. The math works out about the same.
But it does matter to his inverter which is why I mentioned it.
Old 01-08-2015, 07:29 AM
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Sorry guys, my bad I meant to say the 2 batteries would be wired for 12v in parallel. The batteries would also be fairly new.

A little more detail. This is not for marine use, so being stranded with dead batteries is not an issue. It is also not a regular or continuous use, so draining the batteries repeatedly killing them is also not an issue. It would be emergency backup power sorce for my home heating system in the event of a temporary power loss.

The hardware being run is a Direct vent kero heater that is used as primary heat source in my home, it requires clean power to run and the batteries and pure sign wave inverter would be the backup power supply to run that heater in the event of temporary power loss during the heating months.
My present generator is a Honda but and older model and is modified sine wave so it will not run the heater.
I have everything needed to set this up as described with the exception of the inverter itself, so cost will be minimal.

It sounds like it is not unreasonable to expect to get as much as 20 to 30 hours of run time out of the 2 batteries wired in parallel . If this is the case i have no issue because the run time on the heater is not constant and 20 hours of run time on average could take 2 days, and my present generator can run a charger to keep the batteries partially or fully charged as needed. Power loss in my area is not very a common occurrence in the heating months but it does happen. when it does, it is typically only been a few hours at a time, but has been as long as a day or 2.

Thanks for the help, you have confirmed what I was hoping would work..
Old 01-08-2015, 08:42 AM
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Originally Posted by madhatter1 View Post
Actually Ryan's math is ballpark correct in that he has an estimation (or maybe a real calculation) of the inefficiency of the inverter and then comes up with a realitic time frame. I say he is spot on. Then IMHO I would "budget" myself at 30 hours to be on the conservative side of when power loss would occur.
Ya, I gave him a 90% efficient inverter and it came out at like 35.7...something, 90% would be a higher end inverter so I figure anywhere between 30 and 35.
Old 01-08-2015, 08:55 AM
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You could double the amp hours with 1 group 4D deep cell - 4x with two in parallel.

That way you could run your heat and a bunch of other things off the inverter... TV during emergencies, small reefer, lights...

I swapped 2 #27's with 2 4D's on my last boat - well worth the few extra $$

Last edited by Jughed; 01-08-2015 at 09:05 AM.
Old 01-08-2015, 09:31 AM
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You have a genny, have it charge a battery (ie battery charger), run the inverter orf the DC battery until you run out of gas!
Old 01-08-2015, 09:50 AM
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Originally Posted by ericinmich View Post
You have a genny, have it charge a battery (ie battery charger), run the inverter orf the DC battery until you run out of gas!
Doubt I will ever run out of gas for the genny. The boat is stored in the back yard all winter and holds a 3 week supply of gas for the genny.

Last edited by On The Edge 1; 01-08-2015 at 10:03 AM.
Old 01-08-2015, 10:11 AM
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Ok here is another question about the same system. Is there anything stopping me from charging the batteries via a battery charger & genny while the battries are connected to the inverter and being used?
Old 01-08-2015, 11:06 AM
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Originally Posted by On The Edge 1 View Post
Ok here is another question about the same system. Is there anything stopping me from charging the batteries via a battery charger & genny while the battries are connected to the inverter and being used?
That was what I was suggesting. Nope, no problem... unless you are pulling more watts than your battery charger is using.
Old 01-08-2015, 11:44 AM
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Originally Posted by ericinmich View Post
That was what I was suggesting. Nope, no problem... unless you are pulling more watts than your battery charger is using.
That's what I thought.
When the heater is running it may use a little more then the charger is putting out but the heater does not run constant and considering the capacity of the batteries I am fairly confidant I can get way more time out of the system then needed for any normal power outage up to a week or so. I also have 2 older batteries that are descent (Not new) that I can swap out if needed. Even if the older batteries are not 100% I should be able to get a day or more out of them on the charger while the new good batteries are being completely recharged..

Like I said I already have everything needed including 2 sets of batteries, cables, chargers, and the genny. The only thing I need to buy is an inverter that is 80% to 90% efficient and i'm good to go..

Thanks for the help..

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