LSUMatt1514

Don't laugh, but I want to make sure my math is right on this. I've got a couple of threads going right now on batteries, and I want to get someone to check my math for me.

The battery I'm looking at getting has a reserve capacity rating of 95 minutes. This is based on a continuous 25A load before it is completely discharged. Now, since I'll be running only a 20" TV off my inverter which only pulls a maximum of 2A it serves to figure that....

25A/2A=12.5 Reserve capacity multiplier

95*12.5=1,187.5 minutes which is 19.8 hours.

Of course I realize that I cannot completely drain the battery and expect to start the truck back up. The inverter and the voltmeter in my dash alarm at 10.5V telling me I better start the truck in a hurry.

I guess my question is based on a 2A load off of this battery which has the said rating, how long can I expect to run the TV before the battery hits 10.5V? I have no idea what the reserve capacity of the stock battery is, but I can only run the TV for about 20-25 minutes with the truck off before the alarms sound and I've got to crank it. Usually at that point, I just leave the thing running. I want something that will power this thing for at least an hour before cranking (maybe more) but I have no idea if I'm in the ballpark or not with this new battery I'm looking at. Someone give me some feedback here. Thanks!!!

Matt

superjoe83

one way to find out is to try it, and adding another battery will extend your time also

msw0085

I know the Optima Yellow top is designed to be run all the way down and recharged..its meant to be used for cars with alot of electronics

LSUMatt1514

Found my answer....Here's the example I found....

After determining the size of inverter needed, you need to determine the size of battery (or batteries) required for the running time you want. The storage capacity of batteries is measured in "amp hours" (abbreviated "AH".) The definition of "amp hour" is the ability to provide 1 amp of power for 1 hour. To convert from total watts of load to AH, first use the formula Watts ÷ Volts = Amps. That will tell you the continuous amperage rating of your load. Then multiply that times the number of hours you want to run, and you have the required "amp hours" required from your batteries.

Example: Looking at the table above, you'll see that a laptop computer might require about 70 watts of power (at 120 volts.) So we first multiply 70 watts x 1.15 (to add 15% for the consumption of the inverter) and get 80 watts. Dividing 80 watts by 120 volts = .67 amps of constant load. A mid-size Optima deep cycle battery like the model D34 is rated 55 AH (amp hours) @ 12 volts. Because the inverter will be converting the battery's 12V current to 120V (which is greater by a factor of 10), we divide the battery's 55AH rating by that same factor of 10, and the result is that the D34 battery has a rating of 5.5AH @ 120V. If we then divide that 5.5AH battery rating by the laptop's consumption of .67 amps, we see that the Optima D34 deep cycle battery will run the laptop for 8.2 hours through an inverter.



If I get the Optima D34 Yellow Top, I could run the TV for over 8 hours. I'll have to call Napa tomorrow to get the Amp/Hr rating for their Exide battery.

Matt

bdeakin

My wife and I are EE's. You've got the right idea on the situation, but something to remember about batteries is that how much juice you can get out of them really depends on how you drain them. You may need to add another fudge factor aside from the 15% for the inverter, the battery (depending on how/who made it) may only give you 60-80% the advertised capacity. When in doubt... get a few more batteries! If you can separate the power needs from your main battery, then get a nest of 10AH gell cells from your local NPS (or equiv.). There you can get small 10AH sealed lead acids for $4 each. They're about 3"x6"x4". I'd grab a rubbermaid, toss a level of 8 batteries in the bottom, fab a "floor" and mount the inverter and the charging circuit on the "floor" and voila, portable power for use in your car as well as in the house when the power goes out.

Just an idea.

I'm going to be doing something larger scaled for my house :-)

98LimitedCustomized

Your inverter only pulls 2 amps? so it's a 30 watt inverter? check that out, either I've had too many beers or something is up with that.... I'm no EE but I'm darn there an ME... If you really want to you be able to run your stuff with your vehicle off you should invest in an isolater and a second battery... once your fun battery goes dead flip a switch, start it up, and charge it up.... my inverter pulls up to 60 amps and it's not all that much of a monster.... even the cigarette lighter one i have pulls 12 amps. Recheck that figure, cause that will dictate all your math...

LSUMatt1514

No, it's an 800W inverter, but all I really run off of it is a 20" TV which is only 60W and pulls like 0.6A (this includes adding 15% for loss in the inverter). That's where the figure came from.

Rick F.

Get a separate deep cycle battery. I've ran a 36" TV, sound system, DVD/VCR player and a video game system off of 2 small deep cycles paralleled to my inverter for over 4 hours continuous one year due to a storm.

bamachem

Matt:

Aren't YOU an EE???

LSUMatt1514

Yes sir I am, just wanted someone to double check me. I know power systems (ie electricity grid) but wanted someone to check me on my DC math.


Here's one for you...The guy at a local car stereo shop told me this morning that an inverter alone (without any load plugged into it) would pull 20A-30A by itself. Is this true? I figured you'd have to factor in like 10% or 20% as far as an amperage draw from the battery (for the load plugged into the inverter) since it's going through the inverter, but the inverter itself pulling 20A-30A???????

bamachem

i don't believe that for a minute.

Flamedx4

:pat: Nope. But what it does draw will depend on the inverter itself. Some have an idle mode that will power down when there's no load and only draw maybe a couple milliamps (basically its occasionally pinging the load line to see if it needs to wake up.) Simpler ones don't have a feature like that so they stay powered up - but if there's no load - where would 30amps GO? Powered up but not supplying a load still shouldn't draw much, half an amp at most I'd guess.

98LimitedC - an inverter will only "draw" enough battery power to meet the supply, plus inefficencies - quality of power conversion, thermal losses etc. So even a 3000 watt inverter *could* only draw 2 amps if that's all it needs to meet the load (assuming it has the ability to operate at such low output...)

snap-on

I skimmed most of the posts and i didnt see this mentioned.

What your really battleing is battery TYPE, not size.

Lead acid car batteries do not do a good job of supplying a constant voltage over there entire "life" A NiMh bat or lithum battery supply a nearly constant voltage over their entire "life" dropping off very quickly at the end.

So you have to ask yourself. Do you need the voltage in the car bat (how FAST the starter spins) or the overall current required (torque) to turn it over.

deathrunner

With all due respect, your and electrical engineer and you're taking advice from the dude at the stereo shop??? I don't even trust those guys.

It just struck me as funny, no offense to you, man.

LSUMatt1514

Deathrunner,
Not taking advice. I called to ask them something else and we started talking about it and he threw that out there. I knew it was bogus as soon as he told me but juuuuuuuuust to make sure, I figured I'd hit you guys up.

Unhappy99

Power(Watts) = Voltage(Volts) X Current(Amps)
So, a 60 watt device rated for 12 volt input consumes 5 amps.
A 60 watt device rated for 120 volt input consumes 0.5 amps.

Taking your particular laptop example, 70 watts, and assuming a reasonable 15% inverter loss.
70W X 1.15(inverter loss) = 80.5 watts needed.
80.5 watts / 12 volts = 6.7 amps current draw on car battery.

The battery has an amp-hour rating. Simply take that amp-hour rating, divide by the 6.7 amp draw, and you get the number of hours your laptop can operate before draining the battery. Keep in mind your battery is also supplying power to the various truck electronics, such as clock, stereo, ECU.....all which shorten your run time.

Unless something is terribly wrong, an inverter does not draw 20 amps with no load attached. There is a minimum current draw, probably 0.5 amps or less.

LSUMatt1514

Unhappy,
Come on dude....Ohm's Law

4chucker

hey man, just following the thread here. once sure way to find the amount of current being drawn is to put an amp meter inline on the positive lead of your inverter. i would "bench" test this. as you know, when voltage goes down, current(amps) go up. so,the amount of current will always be changing. if this was me doing this i would go out and get the biggest deep cycle battery you can fit as an aux. battery(with isolator not a solenoid to parralel ) charge it up and run a few test cycles to see what the avg. run time is. its really the only way to find out . as for reserve and ah ratings, i find them a little tricky to work with and predict. proof is in the puddin`! thats what my 15 yrs as a stereo dude tells me. hey we`re not all bad =)