battery sizing question

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On the primary side, if the inverter is running at 50% efficiency and you've got a 14 watt load on the secondary, the inverter will need 28 watts in. Since

P = IE

then

P 28W I = --- = ----- = 2.33A E 12V

With an 18AH battery, if you suck 2.33A out of it, it should last for

C 18AH T = --- = ------- ~ 7.73H A 2.33A

depending on the rate of discharge.

That is, if it's rated for 18AH at C/10, (1.8A *10H) then since you'll be taking 2.33A from it you won't get 7.73 hours out of it, you'll get something less. You can tell how much less by getting the manufacturer's data sheet for the battery. If I had to guess, I'd think you could get 6 hours out of it. To find out for sure you'll need to get the battery data sheet and the data sheet for the inverter. Specifically, what you'll be looking for is the inverter's efficiency with a light load like 14 watts on it.

Get that data and come back with it and we can give you a better estimate.

-- John Fields

Quick worst-case ballpark calculation:

14 watts output @ 50% conversion efficiency = 28 watts input

2.3 amps * 6 hours = 13.8 amp/hours

Batteries are usually rated based on a 20 hour discharge rate - for an

Since conversion efficiency is typically higher than 50%, the actual load current could be less than 2 amps, which might give you 5+ hours.

I would strongly suggest testing the setup before going "live".

John

I did the math very quickly, and found that it should last about 6 to 7 hours. This is providing that everything is ideal, and your battery is in very good condition to meet the amp hours rating that you are mentioning.

Try it for curiosity, and email us all back if it works.

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Jerry G. ======

If so, could you give me some input as to how long you think I could operate the following setup:

I will operate a vcr player that has a 14 watt rating at 120 volts. I am considering an 18 Amp-hour AGM type battery. The power inverter is rated for up to 350 watts and the no load current draw is given to be 0.4 Amps.

I need to be able to get about a 5 to 6 hour runtime. I do not plan to do a test prior to giving it a go the first time.

You must test the kit for a short period at least. We've had a a lot of problems running computers/laptops from mains inverters. The cheaper types can be very noisy. You might get noise on the tape if recording.

I have a Vector Manufacturing 350 watt Max SST power inverter

As far as the battery is concerned, I will have to get back to you on that. I haven't discussed this issue with the seller of the battery.

I did wonder whether it would be better to go with a smaller rated power inverter, because even the small 80 watt ones would have more capacity than the vcr needs. Does anyone know if they would save me alot of power useage or perhaps their effiency is lower than the bigger ones, like the 350 watt version. There is a 175 watt Vector inverter but product information claims it has 0.4 amp no load current draw just like its big brother, the 350 watt version. And the tech support guy said they both use about the same energy. My reasoning for going with the bigger inverter is I figured to get a 5+ hour continuous runtime, heat could become an issue and you want something that has enough heat absorbing capacity to make heat not be a problem.

Thanks for your responses. They were helpful.

Thanks John,

Yeah I'll probably test it and see how it does once I get everything.

I am using a Vector Manufacturing

Don't have specs on the battery at this time.

Thanks for your help.

Hi John,

With an ammeter hooked in series, I see the Vector 350 watt inverter pulling a constant 0.6 Amps at no load. (Packaging says it is 0.4.)

When the vcr gets plugged in, it goes to 1.10 with the vcr power on at idle and goes to a constant 1.47 amps when I start recording. This is because it takes more power to record than to do nothing except be on. At any rate, I have an amplified antenna that brings the above totals slightly higher to 1.23 and 1.54 amps respectively. The amplified antenna doesn't introduce much extra load.

It appears I am looking at about a constant 1.54 amp draw off the battery. I was testing with a car battery as I do not yet have the AGM Power Sonic battery that this place said they sell. All I know it is an

Here is my conclusion though so far from testing: I want to try some lower rated power inverters to see if I can get that no load draw down from 0.6 amps into something like maybe 0.3 or even 0.4. Vector Manufacturing sells a 175 watt model and a really small one at like maybe 120 watts. I am sure the really small one would probably consume less power and increase my run time and possibly the 175 watt model might do the same.

With this tactic, I think the 18 amp hour battery could work. Especially knowing that I am only pulling 1.54 amps, which is less than what we thought it might be. Also, if I power down for commercials either fully or simply halt the recording process this might help save precious amps but I am not sure how much it would help because cycling the power introduces 3 second or so current demand spikes that go up to around 2 amps momentarily and then go back down to the normal readings I gave.

Thanks.

John,

The only specs I see for the Vector

My testing shows a current draw off the battery of 1.54 amps. At that rate, that's what I need to know is how long the 20 Amp hour battery will run. I will discuss this with the people that sell this battery. Hopefully they will understand the issues you have brought up here with respect to C/10, C/20, etc.

I wish I could move up to get some extra capacity but weight is kind of a critical thing. I can't go much above this particular battery.

I did do some small testing and found that the recording quality was very acceptable. There is very little extra noise that I noticed that was introduced by the power inverter. Sometimes if the cables are too close to each other, you can get a little noise but I guess my power inverter did a fairly good job with noise.

--- Here's what I found on the PowerSonic site:

PERFORMANCE SPECIFICATIONS Nominal Voltage...12 volts (6 cells in series)

Nominal Capacity

Approximate Weight... 12.6 pounds (5.7 kg)

Energy Density (20 hour rate)... 1.54 Watt-hours/cubic inch (94.0 Watt-hours/l)

Specific Energy (20 hour rate)...16.5 Watt-hours/pound (36.6 Watt-hours/kg)

Internal Resistance (Fully Charged Battery)... 12 milliohms (approximately)

Maximum Discharge Current ( Here is my conclusion though so far from testing: I want to try some

--- From the data above, at C/10 you can get 17AH, so assuming you could also get 17AH with a 1.54A load (safe assumption, BTW) the battery would last for at least 17AH/1.54A ~ 11 hours even if you never turned anything off.

-- John Fields

depends on the inverter. 90% isn't to bad. all it means is it takes 10 Amps form your

d. smith wrote: