Calculating amp loads

[ontheroadagain]

So I am a bit confused about amps and load and current draw calculations.<br /><br />Lets say I have a 120v 450 watt microwave connected to an inverter that is connected to a 330ah bank of 12v batteries.<br /><br />When I turn on the microwave it draws 450 watts / 12v = 37.5 amps per hour<br /><br />1.&nbsp;&nbsp; So is the load on the wire 37.5 amps at this point ?<br /><br />2.&nbsp;&nbsp; or is it 37.5 amps / 60 minutes = 0.0625 amps ??<br /><br /><br />I'm guessing @1 and that I would need a fuse larger than 37.5 amps (maybe at least 50 amps)<br /><br />3.&nbsp;&nbsp; If I run this microwave for 5 minutes at one time I would take <br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0.625 amps x 5 min = 3.125 amps from the battery?<br /><br />That doesn't seem like much for the use of a microwave<br /><br />thanks<br />rick<br /><br /><br />

 

[blars]

You seem to be confused, partly by people who call amp-hours amps.&nbsp; An amp is a rate, an amp per hour would be a rate of change.&nbsp; (like acceleration)<br /><br />Microwaves are rated in output power, input power is 50% to 100% greater.&nbsp; The inverter to convert 12v to 120v will be 90% effecient as well. So your 450 watt microwave may take 1000 watts at 12v.&nbsp; 1000/12 means you need 83 amps.&nbsp; A 100 amp fuse to the inverter would be minimum.&nbsp;&nbsp; Multiply the 83 amps times the amont of time you use it for amp-hours you need from your battery.&nbsp; (My aproximations are pesomistic, but it is not off by more than a factor of 2.)<br /><br />

 

[bk2valve]

<span id="post_message_1276029622"> (My aproximations are pesomistic, but it is not off by more than a factor of 2.)</span>

<br /><br />What would pesomistic be converted to in US dollars?&nbsp;&nbsp; (12.8 MX Peso = 1 dollar)&nbsp; <img rel="lightbox" src="/images/boards/smilies/angel.gif" class="bbc_img" /><br /><br />Sorry...<br />Bri

 

[ontheroadagain]

Multiply the 83 amps times the amont of time you use it for amp-hours you need from your battery.&nbsp; <br />

<br /><br />So if we assume your numbers are close enough for this example how many amps would be taken <strong>from</strong> my 330 amp/hr battery bank if I ran the micro for 5 minutes? <br /><br />thanks<br /><br />edit for to <strong>from</strong>

 

[bee]

Go to this link &nbsp; &nbsp;http://www.windsun.com/Batteries/Battery_FAQ.htm<br />Then you can click AH for the explanation. &nbsp;All the information on this site is very good.<br /><br /><br />Also as mentioned above 450w seems awful low for a microwave. &nbsp;You might want to consider investing in a watt meter to find out realistic draw. http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&amp;field-keywords=watt+meter<br /><br />Also if you do plan on running a microwave, then you&nbsp;definitely&nbsp;can't skimp on your battery cables.<br /><br /><br />

 

[wrcsixeight]

Fuses are meant to protect the wiring, not the devices being powered.<br /><br /> &nbsp;Fuses by their very nature present resistance in a circuit, and reducing resistance and voltage drop is why fat cables are wanted for heavy loads, so using fat cables capable of passing 300 amps at 12.x volts then putting a 60 amp fuse in line, makes little sense.<br /><br />&nbsp;With Such huge draws on a battery bank, the Peukert's law comes into effect.<br /><br />P's law basically says the higher the draw from the battery, the less the battery has to give.<br /><br />Running a microwave on batteries via an inverter is certainly feasible, but it is hard on the batteries, and if the alternator is the primary recharging source, it is hard on that too.<br />If the batteries are not regularly fully charged, expect short life, and disappointent in their performance.<br /><br />The first set of batteries will be your learning batteries, spending top dollar on high quality deep cycles just to kill them prematurely by heavy discharges and insufficient recharge is less than an optimal result.<br /><br /><br />

 

[Guest]

Fuses are meant to protect the wiring, not the devices being powered.<br /><br /> &nbsp;Fuses by their very nature present resistance in a circuit, and reducing resistance and voltage drop is why fat cables are wanted for heavy loads, so using fat cables capable of passing 300 amps at 12.x volts then putting a 60 amp fuse in line, makes little sense.<br /><br />&nbsp;With Such huge draws on a battery bank, the Peukert's law comes into effect.<br /><br />P's law basically says the higher the draw from the battery, the less the battery has to give.<br /><br />Running a microwave on batteries via an inverter is certainly feasible, but it is hard on the batteries, and if the alternator is the primary recharging source, it is hard on that too.<br />If the batteries are not regularly fully charged, expect short life, and disappointent in their performance.<br /><br />The first set of batteries will be your learning batteries, spending top dollar on high quality deep cycles just to kill them prematurely by heavy discharges and insufficient recharge is less than an optimal result.<br /><br /><br />

<br />This is the best battery post I have read yet&nbsp;because it hammered home, for me, the need for practical experience and the&nbsp;likelihood&nbsp;of mistakes. &nbsp;Thank you.

 

[josephusminimus]

Good post WRCsixeight. I'm especially glad you mentioned the alternator piece of the deal.&nbsp; It doesn't get much mention during these discussions, but I've spent a lot of time thinking about the one under my hood and thinking how much work it's being asked to do.

 

[ontheroadagain]

tara: thanks for that link I am studying up now.<br /><br />wrcsixeight: maybe I didn't word it all correctly. I am not ry to draw 300 amps at 12v.<br />I am trying to figure out the math for amp draw on a set of batteries by devices. the 450watt micro was an example so I could learn the math. I want to calc what each purchase I am considering for use in a 12v system. <br /><br />=========================================<br />Back to the original question I am trying to solve:<br /><br />So if I understand Tara's link info correctly:<br /><br />450watts / 12v = 37.5 amps<br /><br />37.5 amps x .08333 hrs = 3.13 ah<br /><br />=========================================<br /><br />It doesn't seem that that would kill a battery???<br />I am not claiming the microwave actually exists here.<br />It is an&nbsp; example but I am pretty sure someone on this site said the owned one.<br /><br />thanks for helping and being patient<br />rick<br /><br />

 

[wrcsixeight]

Your math is correct, but not taking into effect the inverter inefficiency, or peukert's law.<br /><br />Heavy loads like that on a battery and the amps consumed over time isn't simple math, and each battery, its design, health, state of charge, and &nbsp;cabling to the inverter all play huge factors in how it performs.<br /><br />Even the best battery monitors/ amp hour counters out there are basically just guessing how much is really removed from a battery when subjected to a very heavy amp draw for any amount of time, as there are sooo many variables involved. &nbsp;I basically don't trust my battery monitor after I run my angle grinder (62 amps) for any amount of time until I reset it and let the solar top up the batteries again.<br /><br />Most microwaves use twice that theoretical wattage, and can briefly double that on startup.&nbsp;<br /><br />While most MSW inverters will make a claim of up to ~92% efficiency, these numbers are likely cooked with some fuzzy math. &nbsp;All inverters have a load sweetspot where their efficiency peaks, and this is usually in the 50% of max range, but also likely varies widely brand to brand, and it is more realistic to consider 80% efficiency into any math.<br /><br />There are also reports of microwaves not liking MSW inverters and not lasting very long.<br /><br />Each inverter should also instruct which size fuse to put inline on the power cables from the battery.<br /><br />Plan your battery usage to only use 50% of the total battery capacity.

 

[peacetara]

wrcsixeight says good things again <img src="/images/boards/smilies/smile.gif" border="0" align="absmiddle"><br /><br /><br />OnTheRoadAgain: Your math is correct, assuming you are not doing a bunch of conversions in the middle.&nbsp; But like wrcixeight says, you probably are.<br /><br />If you have a 12V input microwave (i.e. it takes 12V in, and not 120V "normal" power), then you don't have to worry about inverters.&nbsp; They do make 12V input microwaves, but I'd bet they are more expensive.<br /><br />The bottom line about Microwaves is, they are generally a bad idea for running off of batteries.&nbsp; They take a LOT of electricity. Electricity isn't actually "FREE" like we have been conditioned to think when growing up and living in American socially traditional housing (sticks and bricks as so many here call it).<br /><br />The best way to run a microwave is from a generator, like those little Honda's or whatever.&nbsp; They are way better for larger loads.&nbsp; It's not to say you can't run it from a battery bank, you CAN, but when you are talking about microwaves and angle grinders and other large load appliances, suddenly 300AH worth of battery seems tiny in comparison <img src="/images/boards/smilies/smile.gif" border="0" align="absmiddle"><br /><br />With Love,<br />Tara

 

[minimotos95]

well one thing you are forgetting with your calulations is actual voltage isn't a constant 12,&nbsp;say 12.7 resting? drops down to&nbsp;xx.x&nbsp;with that load?&nbsp;and 13-15.6 running depening on your alternator,&nbsp;higher input voltage&nbsp;would lower the amperage in that formula...<br />personally i wouldn't even consider running it off just&nbsp;a battery bank.. if i had an alternator that was over the stock output by 100amps and good quality i would do it with the motor running at over 2500rpm...&nbsp;what i would do if i had the need: a 110v microwave and&nbsp;just use a generator, for the 10 minutes you need it it would use almost no fuel, something like .0x of gallon. the 12v output on generators is&nbsp;normally only 8-10 amps so won't have enough juice for a 12v microwave<br />wrcsixeight and Tara pretty much summed it up&nbsp;