Need help: Electronic calculations (i.e. Amps, Hertz, Watts)

[TomcoPDR]

Here's the situation, I'm trying to run some light gagets off of those 12v DC to AC invertors... However, there's no cigerette lighter (or a vehicle) avaliable, and I'm looking to retrofit a battery pack to run these invertors off of (i.e. from a 12v drill, Radio Control batteries, etc...)

The question is: I just want to know what battery is required to perform the task.

Invertor available for use:
1 (first unit). Input 12v DC, 8A max; Output 115v, 60Hz, 70w, 8A (that's the info given)
2 (second unit). Input 12v DC; Output 115v, 60Hz, 240w, 2.1A continuous

Items being powered by invertor:
1. 118v, 60Hz, 70w, 8A
2. 120v, 40w


The safe bet is using a car battery, however, the point is I'm trying to down size into a more compact battery source.

TIA

[nobb]

I would go for a deep cycle battery. You dont want to be running down a normal car battery regularly...thats going to kill it. Since you said compact, maybe you should look at wheelchair batteries. The size is going to be related to how long you want to be running all that equipment and at what average current draw.

[Grogador]

You know, I typed all this up, then I thought "uhmm, why not just charge a computer UPS at home and bring it with you?". My 1200VA APC fits under your (strong) arm and provides ~25mins of runtime at half load, actually right now it claims I'm using 200W and it can back me up for 33mins. That sounds about right with two 8Ah batteries. It's relatively efficient, you can plug whatever you want into it, not very expensive and they come in all sizes. Just make sure none of your devices will mind a "stepped sinewave"... they shouldn't... probably cleaner than most automotive inverters anyway.

---

It's 3AM and I have teh dumb so he's a little branial aneurysm for you. Numbers are probably wrong and I'm making all sorts of assumptions, anyway: Power (W) = Current (A) * Voltage (V) or...

P
AV

Inverter #1
Input -- 12V * 8A = 96W
Output -- 115V * 0.6A = 70W
(73% efficiency)

Inverter #2
Input -- 12V * 26A = 312W (assumed 80% efficiency for beefier unit and easy round numbers)
Output -- 115V * 2.1A = 250W

Load #1
118v * 0.6A = 70w
(8A makes no sense, maybe a 12V drill battery rated 8Ah or some 8A motor/drill?

Load #2
120v * 0.33A = 40w

So off the bat you can see the inverters will waste 20-30% of your battery power as heat. If you're just plugging wall-warts into the inverters (converting your 115Vac back to DC and generating more heat in the process) to charge some devices, skip the inverters if possible. Batteries provide very clean power, the alternator is the filthy one. You can filter and regulate if your devices are picky. You can get DC-to-DC converters which can convert fluctuating (usually 10.5 to 14) car battery juice to the 9 or clean 12 or 17 or 24 or whatever Vdc you need, at ~95% efficiency. Here's the first sales-type link from Googling "buck boost dc converter".

So, how long will it last? Well, how heavy of a battery do you want to carry around? Anything from deep cycle 100Ah (amp*hours) at 1A/hr to various smaller UPS batteries (sold in sets of 2 or 4) from 8Ah and up (rated for quicker discharge too...)

Keep in mind your inverters' efficiency will vary depending on load. At full tilt, your 250W inverter (drawing 26A) and that massive deep-cycle 100Ah battery would last 2 or 3 hours (the discharge rate given is for 1A/h, the 20 hour and 5 hour ratings will be quite a bit worse). Since your devices are only 110W let's say it's at half load and you get 5hrs.

[Grogador]

Originally posted by Grogador
right now it claims I'm using 200W and it can back me up for 33mins. That sounds about right with two 8Ah batteries.

You can add more batteries to the UPS as well, or swap them out with some charged ones.

[nobb]

UPS might not be a good idea. They usually have tiny heatsinks and are not designed for operation for hours at a time at high loads. The batteries are also not usually true deep cycles either.

[Zero102]

Something is wrong with the math above:

Invertor available for use:
1 (first unit). Input 12v DC, 8A max; Output 115v, 60Hz, 70w, 8A (that's the info given)
2 (second unit). Input 12v DC; Output 115v, 60Hz, 240w, 2.1A continuous

Items being powered by invertor:
1. 118v, 60Hz, 70w, 8A
2. 120v, 40w

Been a while since I did any electrical calculations, but...

118V * 8A is over 900W.

Also, the first inverter, the 8A is the input current, with an output power of 70W it can make just over 0.5A output, not 8A. You may need to double check the inverters you are listing, because I think you are about to be in for a disappointment.


If you are looking for batteries for these inverters, remember that most inverters require an input voltage of at least 11.0-11.2V, and if your battery dips below this they will cut out, so if you are using small batteries (under 10Ah) then you will likely see the voltage drop down below that threshold under moderate loads (5-10A) and this could cause you further grief.

My advice would be deep cycle lead-acid batteries. Usually these are sold as marine or RV batteries at walmart and other similar places. They normally have capacities of 50-70Ah, although they do weigh quite a bit.

Now, if you actually only need 40W and 70W, and not 900W like listed above, then the requirements are MUCH smaller, some little 5-10Ah batteries from UPSs or princess auto will be just perfect for the job.

[Grogador]

Originally posted by nobb
UPS might not be a good idea. They usually have tiny heatsinks and are not designed for operation for hours at a time at high loads. The batteries are also not usually true deep cycles either.

Run the UPS at half-capacity so the inverter doesn't have to work very hard, like my 1100 in the example above. You could also wire some nice deep-cycle batteries to it, although you'd need to verify that the charging voltage is compatible!!

[TomcoPDR]

Yea, I could be wrong with the written numbers, it's kinda dark. I'll try out different battery sources.

Thx for the help