Model Boat Mayhem
Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: polaris on December 06, 2012, 10:33:42 pm

Dear All,
Am working on an idea (shortly to be in practice), whereby I have something that is 12v, and has a draw of 600w. What I would like to ask those who know about such things re batts., is, per 100ah. of any given batt. source, what will be the opers. time span per 100ah. of the batt. supplying the 12v unit. Viz., if the batt. is 600ah., how long will it power a 600w draw, and, likewise, if the batt. is 300ah., is the opers. time in proportion?  likewise 200ah..
I thank in advance for advice.
Regards, Bernard

Bernard this is quite easy your 600w is simply divided by 12 thus giving you a Current draw of 5Amps.
So a 12v 7amp hour batt for example should be able to run it for 1hour and roughly 10 mins or so,obviously it will depend on the batt,if the capacity of the batt is less than 5 amps then it simply will not last.
Dave

how big is the boat, 600ah  thats a huge lorry battery size or bigger, 120Ah is a car battery size, are you thinking mAh?
Grendel

Bernard this is quite easy your 600w is simply divided by 12 thus giving you a Current draw of 5Amps.
Surely it's 50 Amps
12v 7Ah wont last any time providing 600 watts. Less than a minute and probably a very hot if not exploded battery.
Your battery will feel like its been shorted.
John

Dear All,
Thankyou for your thorough replies.
The batt. will indeed be apx. 600ah., and it is a heavy duty agri. type batt. (and they are heavy!). Stavros's 5 amps. equation is very very useful to know, since the draw unit is permanently 'self standing' and I don't want to have to move the batt. every couple of days for recharging if I can help it!!!!
Thankyou again.
Regards, Bernard

As it is indeed 50A (600/12) per hour, you will have approximately 12 hours of capacity in your 600Ah battery, however conventional wisdom generally says you should not exceed discharging much past the halfway point so you would need to charge after about 6 or7 ours continuous use, for the continued health of your battery. assuming it is a conventional lead acid type battery.
Grendel

maybe permenant mounting on a small trolley would assist in moving the battery.

Dear Grendel,
Thankyou for your very prompt reply.
I am in the process of buying a small piece of eqpt. (2 off), from the States, and have the option of 600w or 200w draw. The 200 might be better, but maybe won't quite do the job  until tested. So, have emailed the people asking to change the order from 2 x 600 to 1 x 6 and 1 x 2, but whatever be the case to delay until this eve.  if they haven't already despatched the 2 x 6 that is! (They are always very efficient). This is non model related I should add, but, the principals are very useful to know anyway.
Must dash out now, I am running late!
Regards, Bernard

Hi Bernard another way of looking at it is 200 watts is about 1/4 bhp and the other is 3 times this at 3/4 bhp. A factor of 3 is quite a difference.
Just a thought but when you start your car the starter motor reduces the 12 volt battery down to 8 or 9 volts under load.
Maybe this is too much information!
regards Roy

Just to confuse "ohms Law" a little %) ......[V= IxR] :X
I have witnessed at the auto electrician...my wifes 12 volt car battery being subjected to a 135 amp current draw when cranking the engine in an attempt to start the engine
Naturally the starter motor was 'poling' and needed replacement at great cost >>:( <*< ....Derek

I have been thinking about this as well Derek. All those Amps from a 12 volt supply. Curiouser and curiouser.
Imagine the wire thickness required. %)
ken

Most lead acid batteries are happy enough if the current figure is below 1/10 of the capacity figure. 50 is less than 600/10=60, so its within reason. Run time theoretically is 12 hours, but good sense suggests derating by about 40% partly to keep the battery happy, partly because the motor will probably start noticeably losing power beyond that. Call it 7 hours or so. It might well be a deep discharge battery designed to live through that kind of use and abuse.
If the load is to be within jump lead distance of the battery, heavy duty jump leads are the cable weight required, any further will need heavier wiring to avoid excessive loss in the cable.
Treat batteries of that capacity with respect  DC doesn't let go, and there is an awful lot of power there. Not voltage to jolt you, but a short will result in, at best, an impressive shower of sparks composed of bits of flying white hot metal as a result of the almost infinite current that will be offered. Don't ask.

Yes Ken......I needed to re check the grey cells....but the Wikki people have confirmed that "ohms Law" is still the same as it was when I first studied it {)
(http://rpmedia.ask.com/ts?u=/wikipedia/commons/thumb/f/fb/Ohm%27s_law_triangle.svg/130pxOhm%27s_law_triangle.svg.png) (http://en.wikipedia.org/wiki/File:Ohm%27s_law_triangle.svg)
The average Nissan 4 cylinder motor vehicle battery connection wire is probably about 4 mm sq* and after 30 seconds of 135 amps the wire was way TOO hot to touch........
It will be interesting to understand what our member Polaris is building %) ........ Derek

OOPSY spot MY del MISTAKE .......well maths wasnt one of my strong subjects feell a right PRAT now yes of course it is 50 amps and NOT 5 sorry for the confusion
Dave

you get used to numbers in the range you expect the answer to be, so easy to slip a decimal place, one reason I checked on the 600Ah figure thinking it might be 600mAh.
Grendel

Hi I know there are large batteries involved but they will need charging with some pretty frightening currents, perhaps there is already a charger available?
Interesting though.
regards Roy

I'd like to know how 'big' this 'small piece of equipment is.
I think a nice big Diesel Genny on wheels would be preferable
to humping a 600AH battery home to be charged each day.
Is Bernard building the ultimate electric chair coming from Stateside :police: :police:
Or an electric fence to stop the Welsh Cobs emigrating %% %%
Ned

Thought this might help
(http://i1248.photobucket.com/albums/hh485/tommy3781/images.jpeg)
P= Power in Watts
V= Volts
I= Amps

This is worth the bandwith :))
http://www.electronics2000.co.uk/download.php#assistant (http://www.electronics2000.co.uk/download.php#assistant)
Leds ,regulators,voltage dividers,resistor codes,resistor values,
Pick your poison ,or keep guessing :D

Hi Polaris,
If this project is still on the boil, I can probably help, I deal with this sort of thing every day, 12 volt motors with 425 amp fuses etc Got some engines here that need 1200 CCA batteries to turn them over :o
Appears that Polaris hasnt been on the forum since Dec 17 though {:{
Cheers
Nick