Question about battery and power inverter

[jbd]

Just wondering how one determines how long a battery will power an AC appliance when using a power inverter?

Lets use a marine battery, fully charged and in good condition powering a device that uses 80 watts for reference.

 

[Charlie99]

Just wondering how one determines how long a battery will power an AC appliance when using a power inverter?

Lets use a marine battery, fully charged and in good condition powering a device that uses 80 watts for reference.

To get that 80 watts out, the inverter will be taking in 90 to 100 watts. 96 watts is 12V x 8 amps (4amps @ 24V.

Check the rating of your battery. If it's an 80 ampere-hour rated battery, then you will get about 10 hours of life.

 

[jbd]

To get that 80 watts out, the inverter will be taking in 90 to 100 watts. 96 watts is 12V x 8 amps (4amps @ 24V.

Check the rating of your battery. If it's an 80 ampere-hour rated battery, then you will get about 10 hours of life.

Thanks Charlie99--you are indeed a veritable fount of information.

 

[Scubaroo]

To get that 80 watts out, the inverter will be taking in 90 to 100 watts. 96 watts is 12V x 8 amps (4amps @ 24V.

Check the rating of your battery. If it's an 80 ampere-hour rated battery, then you will get about 10 hours of life.

Which would absolutely toast a regular cranking battery, and you would be up for a new one very soon.

If you're looking for a 12V power supply for regular use, do some Googling on deep-cycle batteries - lots of solar power / wind power forums have good information, as well as some 4WD/SUV forums (look for info on "dual battery" installation). Even deep-cycle batteries which can tolerate being drawn down to nearly flat (unlike regular cranking batteries), should not be drawn down more than 80%, and no more than 50% if you want to prolong their life. So ideally an 80ah battery should only have 40ah drawn from it before recharging (4 hours use), or 64ah (6.4 hours use) if you weren't too concerned about shaving a year or two off it's service life.

As Charlie indicated, inverters have inefficiencies - they draw more power than you can draw from them. Typically somewhere in the order of 80-90% efficient depending on the quality, whether they're square, modified-sine, sine wave, etc.

Deep cycle batteries will have amp-hour (ah) ratings, cranking batteries will have cold-crank-amp (cca) ratings. If you're looking to do something like power a 12v fridge, or run an inverter to run 110v/240v electrical appliances, you want a deep-cycle battery for any sort of long term use. Also check the wave form output of the inverter against the requirements of your device - laptops often won't run on square wave, and any device with a motor or compressor (eg a fridge) can have issues with square wave as well.

 

[jbd]

Scubaroo,
Thanks for the added information. I had looked through some solar/wind power websites and noted that the deep cycle batteries are best. The 80 watt example I gave above was from the packaging of an inverter I looked at yesterday. I haven't looked at any bateries yet so I'm not sure what the typical amp-hour ratings are for deep cycle batteries. I did have a look at the item I want to power, which is a small TV with built in VCR. The label on the back reads 120 v 60 Hz 90 w I wouldn't have any need to run it more than 2 hours.

The inverter I looked at was a modified sine wave. Would this work for the appliance I intend to use? Is there some way for the ordinary guy like me to figure this out?

The only other thing that I can remember about the inverter was that it was 375 watts continuous use and 600 watts peak use.

 

[Scubaroo]

Back of the napkin type calculations... assuming 90% efficiency in your inverter.

120V, 90W = 0.75A. Allowing for the inverter inefficiency, calculate for 0.8333A at 120V (0.75 / 0.9).

120V / 12V * 0.8333A = 8.33A. So for two hours use, you want 16.67Ah available. Assuming you're buying a deep cycle battery, at 50% use, you could get away with as small as a 33AH battery. Kind of rules out the Radio Shack variety - you can get very small 12V/18AH deep cycle SLA batteries here in Oz for example - this usage could be drawn from the battery, but flatten it in the process and shorten it's lifespan.

375W inverter sounds fine - if you get a smaller inverter, eg 100W, they get hot and they can cut out even though they're "rated" for the load.

Charging deep cycle batteries is another issue, which is why I mentioned AGM batteries - they're much more tolerant to casual charging without the need for stepped voltages. Might be a good idea to drop into a solar forum and put the same question to them and see if you get a similar answer - this is an area I've done a stack of research on, but haven't actually laid out any cash yet (was looking at dual batteries in a 4WD for running a camping fridge etc). You could also get better advice about whether modified-sine is appropriate for the TV/DVD.

 

[jbd]

Back of the napkin type calculations... assuming 90% efficiency in your inverter.

120V, 90W = 0.75A. Allowing for the inverter inefficiency, calculate for 0.8333A at 120V (0.75 / 0.9).

120V / 12V * 0.8333A = 8.33A. So for two hours use, you want 16.67Ah available. Assuming you're buying a deep cycle battery, at 50% use, you could get away with as small as a 33AH battery. Kind of rules out the Radio Shack variety - you can get very small 12V/18AH deep cycle SLA batteries here in Oz for example - this usage could be drawn from the battery, but flatten it in the process and shorten it's lifespan.

375W inverter sounds fine - if you get a smaller inverter, eg 100W, they get hot and they can cut out even though they're "rated" for the load.

Charging deep cycle batteries is another issue, which is why I mentioned AGM batteries - they're much more tolerant to casual charging without the need for stepped voltages. Might be a good idea to drop into a solar forum and put the same question to them and see if you get a similar answer - this is an area I've done a stack of research on, but haven't actually laid out any cash yet (was looking at dual batteries in a 4WD for running a camping fridge etc). You could also get better advice about whether modified-sine is appropriate for the TV/DVD.

For the battery, I was thinking about getting one that is used to to power the electric trolling motors on fishing boats--I have no idea what the amp hour rating is on them though.

 

[Charlie99]

For the battery, I was thinking about getting one that is used to to power the electric trolling motors on fishing boats--I have no idea what the amp hour rating is on them though.

The other common deep discharge battery is the 6V one from golf carts. It is designed for both deep discharge and vibration. Just put two in series.

 

[FIXXERVI6]

What are you wanting to power with this?

I have a fairly large bank built out crated that takes 115 ah batteries wired in parallel in groups of 3 then the groups of 3 wired in prallel, I forget what I came up with but planned on over 900 ah of battery to run an air conditioner (yea yea I know I know but it gets hot in TX and I like to camp and there is a VERY long story behind this) anyway I found the most effecient AC I could find and figured around a 60% drain over 9 hours or something like that I don't remember anyway long story short the startup motor in the AC is a 3 phase motor so while the thing listed its draw and startup draw most inverters won't start 3 phase motors without being like rated for more than 4 times the startup, pretty crazy the thing would only turn over the compressor from time to time, I ended up running extension cords to power sites in the end

 

[FIXXERVI6]

Sorry just read what you wanted to power

I use my bank for power outages, I can run my computers, Data projector, VCR, DVD player all night long on one case, 345 AH total in one case.