Watts Help Needed.

[Community Alumni]

I am trying to figure out how long a battery will last. I have Googled it and read a few sites but I still have a few questions.

I have a small TV that I plugged into my Kill a Watt meter. That shows 22 watts. I plan on using a 1000w inverter hooked to one 12v battery. I havn't purchased a battery yet so i'm not sure how many AH I will have to work with.

At 12v that would be 1.8 amps.
at 124v that would be .18 amps.

Which amp usage would I use, the 1.8 amps or .18 amps?

Thank You
Jim

[time2roll]

Very short review posted in original thread.

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29853063/gotomsg/29890403.cfm#29890403

[BFL13]

time2roll wrote:

jbfoster wrote:
I was going to get a 1000 watt generator but I am now getting a 1800 watt generator. It's rated at 15amps so I don't want to use a charger more than 10amps.

My small, light, compact and low noise 700 watt propane generator will easily put 20 amps into a 12v battery.

Check Ryobi at Home Depot.

Time2roll, so you did get that one! Are you ready to report (in your own topic of course) on that gen yet? Eg, what its real max in VA is, etc.

[time2roll]

jbfoster wrote:
I was going to get a 1000 watt generator but I am now getting a 1800 watt generator. It's rated at 15amps so I don't want to use a charger more than 10amps.

My small, light, compact and low noise 700 watt propane generator will easily put 20 amps into a 12v battery.

Check Ryobi at Home Depot. Runs a long time on 20# propane cylinder that you might already carry.

[philh]

1,000 watt inverter for a TV is an awfully big unit. I went with 150 watts. I could rationalize 300 watts, but the wiring I used was not up to the task of 300w. Bigger inverter has bigger overhead (wasted energy). IMO, 1,000 is a wasted size, too small for serious usage, too big for small usage. If you're truly going to 1k and need that much, your batteries are going to need some evaluation too.

[BFL13]

My 100amp PF corrected charger pulls 1910VA from the generator according to my Kill-A-Watt.

The Honda 2000w inverter/gen is rated at 13.3 amps, so this 1800w gen that is rated at 15 amps must be one of those very noisy open frame gens?

OP still does not understand Peukert or the divide by 10 for his invertercalculations. 8 amps on a 100AH battery is way more than the 5 amp 20 hour rate, so time will be less to 50% (and it will not be 50% of 100, but 50% of some AH number less than 100).

[pianotuna]

Hi jbfoster,

An 1800 watt generator is capable of doing 12 volt charging at up to 120 amps, if the charger is power factor corrected. It will easily work with a 45 amp charger with a power factor of 0.7.

Charging a lead acid battery comes with an "acceptance" charging rate. For a 100 amp-hour battery at 85% of fully charged that is approximately 12.5 amps. As the battery gets closer and closer to 100% the charging rate drops.

Which 1800 watt generator are you considering purchasing?

jbfoster wrote:


I was going to get a 1000 watt generator but I am now getting a 1800 watt generator. It's rated at 15amps so I don't want to use a charger more than 10amps. I don't have an RV. We will be tent camping for a while.

Thank You
Jim

[Community Alumni]

MrWizard wrote:
you have the use time figured correctly

however it 'ALWAYS' takes more energy than what was used to replace the used wattage aka ampHrs and its not linear
it won't be 10amps for 5 hours, it will start out higher amps and decrease/get less the closer the battery gets to being fully charged
the reversing of the chemical reaction that makes electricity uses power while doing the conversion/restoration

thats why you want a 30-45 amp charger
it will start out high and go down,
you will get charged faster
a 10amp charger will take 7 to 12 hrs to get too full charge
replacing 50 ampHrs
it will start at 10 and go down, making for a long charge cycle

I was going to get a 1000 watt generator but I am now getting a 1800 watt generator. It's rated at 15amps so I don't want to use a charger more than 10amps. I don't have an RV. We will be tent camping for a while. We are inbetween RV's right now due to finances being tight. This is all I can afford for now so I will have to deal with what I can do with what I will be getting.

Thank You
Jim

[MrWizard]

you have the use time figured correctly

however it 'ALWAYS' takes more energy than what was used to replace the used wattage aka ampHrs and its not linear
it won't be 10amps for 5 hours, it will start out higher amps and decrease/get less the closer the battery gets to being fully charged
the reversing of the chemical reaction that makes electricity uses power while doing the conversion/restoration

thats why you want a 30-45 amp charger
it will start out high and go down,
you will get charged faster
a 10amp charger will take 7 to 12 hrs to get too full charge
replacing 50 ampHrs
it will start at 10 and go down, making for a long charge cycle

[Community Alumni]

Let me see if I am understanding all this.


I'm forgetting the coffee maker. At this point I will use the battery for my TV(22 watts) and two cell phones at about 7 to 10 watts each and maybe my latop at 40 watts. Total about 82 watts.

Let's say I get a battery with 100ah. I assume I could run 1 amp for 100 hours but that would take the battery down 100%. The best from what I have read was use down to 80% but I could do down to 50% but that the battery would not last as long.

Based on my 82 watts that would be 6.8amps. Lets throw in another amp and round it to 8amps.

80% of the battery would give me 20ah so I could run for 2.5 hours using 20 amp hours. Would charge time = 2 hours at 10amps and 4 hours at 5 amps?

50% of the battery would give me 50ah so I could run for 6.25 hours using 50 amp hours. Charge time = 5 hours at 10amps and 10 hours at 5amps?


Thanks everyone
Jim

[pianotuna]

650 watts /10 = 65 amps

Peukert for a 100 amp-hour battery @ 65 amps is 30 minutes to 50% state of charge. The battery would be totally dead in about 54 minutes.

The 3 minute run at 65 amps is going to cost the battery about 4 amp-hours of the capacity.

This is NOT recommended.

Doubling the battery bank to 200 amp-hours gives a result of 66 minutes to 50% state of charge.

Doubling again to 400 amp-hours gives a result of 150 minutes.

My own bank of 556 would, in theory, give me 222 minutes @ 65 amps.

Real life numbers may be a lot lower, but won't be better than the above results.

[BFL13]

2oldman wrote:

jbfoster wrote:
Almost forgot the 650 watt coffee maker to make a cup or two a day. That shouldn't be too much at 600 watts for about 3 minutes a cup.

Maybe.. 50a is a lot for one battery. You're likely to see a huge voltage drop.

Yes. Usual advice is to limit charging and discharging to 25% of battery AH in amps. So 25 amps with one 100AH battery would be with a 250w appliance on inverter. (The 25 limit is to give time for the internal chemistry changes in the battery to keep up with things)

I did run a test of higher amp inverter draws with one battery, but the 100AH battery was an AGM. Toaster drew 69 amps--got a 1.3v drop

Tried it with a much used 12v T-1275 Flooded (perhaps worth 125AH at the time) and same toaster drew 71 amps with a 1.2v drop.

[2oldman]

jbfoster wrote:
Almost forgot the 650 watt coffee maker to make a cup or two a day. That shouldn't be too much at 600 watts for about 3 minutes a cup.

Maybe.. 50a is a lot for one battery. You're likely to see a huge voltage drop.

[azrving]

You also have to carry the load of the RV itself which can be an amp or so. My 2000 watt xantrax idles at .5 amp. I also don't think inverting to the coffee maker on one battery is good.

[BFL13]

Airdaile wrote:
You want to look at a battery's reserve capacity rather than amp hours.

The RC of a 12v battery is at the 25 amp rate not the 20 hour rate used for AH, so the RC is less time (unless the 20 hour rate is 25 amps). The 25 amp rate is even farther out of whack than the 20 hour rate is with his 2.5 amp draw -- Peukert.

A 100AH battery with its 20 hour rate of 5 amps is only 66AH at the 25 amp rate used with RC.

[BFL13]

The 22w with inverter drawing 2.5 amps is at half the 20hr rate of a 100AH battery, so you get longer time than just using down to 50% (50AH) due to Peukert.

You get "inverter amps creep" upwards as battery voltage falls when the inverter tries to maintain the watts, so that 2.5 amps could end up as 3 amps after a while. This will reduce the Peukert difference from the 20 hour rate as time goes on. Still more time though.

The big inverter vs small inverter for small loads was gospel, but when I measured for that in some tests I posted about a few years ago, it turned out not to matter. It was because the small inverter was on thinner wires to the battery than the big inverter, so it was a wash. Especially if the small inverter is plugged into the Winegard 12v socket.

Also you can't compare inverters in inverter/chargers with ordinary inverters. Those inverter/ chargers have big draws to run their fancy features just on standby, that ordinary inverters do not have.