Club 80-90 Forums

So i've read and reread and rereread the topics on headlight upgrades, wiring faults, and the arguments about heaters, fridges and all other things electric. However, I still dont really understand how electrics work! I think its back at the very basics I am getting stuck.
Somethings I want to know are how long will it take a 48 amp 12V device will drain a 12v battery?
Or if I leave my CD player on in my car with these speakers will it drain it more quickly?
Or whether when on electric hookup my 12V leisure battery charges, and stuff plugged into the 12V system wont drain the battery? Or do I have to just use the two, 240v style plug sockets?
And what the hell do I do with a relay!

I can follow diagrams usually, except half the time I cant find out what parts I already have are?


So my question (if you have managed to read this far!) do you know what I can ready/ study to learn a little more of this?

there are books out there on basic vehicle electrics, and a 48 amp device will drain a 12v battery in no time at all!

Can you recomend any though?

Yea, my electric cool box is that. We worked out it drains quickly the hard way!

Batteries have capacity written on them e.g 70Ah, thats 7A for 10 hours or maybe 14A for 5 hours, not 70A for 1 hour but you get the picture, higher currents cant get the chemical reaction to happen fast enough
a 48A device will drain a 70Ah battery very fast as said

Sounds to me like you need to read up on batteries not electrics

my old physics teacher had a great way of describing an electrical circuit with an analogy of a dairy and a milk float...want to hear it?

Here is an idea, ask questions, one at a time, on here and "we" will help educate you on the subject. Not quite as good as a face to face, one to one, with a T25 present, but should help you understand it.

I will give an opener for you = if you can remember the following, it will help.
Take 1 standard headlight bulb, it is rated at 55 Watts, it runs of 12 Volts, if you divide the 55 Watts by the 12 Volts you get 4.6, that 4.6 is the Amps it uses from the battery.

So:-
Dividing Watts by Volts = Amps
and:-
Multiplying Amps by Volts = Watts

So know you know how to convert one to another. If say you had a cool box that had 12 Volts and 8 Amps written on it, then 12 x 8 = 96 (the 96 is the Watts) or if it said 12 Volts and 96 Watts on it, then 96 divided by 12 = 8 Amps.

You also now can "compare" these power consumptions with summat you already have a ball park figure of, ie your headlights. 2 headlight bulbs would be 110 Watts (2x55) and you know that leaving them on will flatten the battery pretty quickly, say ~20 minutes. You now know that they are 110Watts which = 9 Amps (110 divided by 12), so you can now compare power usage of other stuff with that (like cool box = ~8 Amps)

This all helps with deciding what fuses and wire size to use. You want the wire to easily handle the current (or call it "power") and you want the fuse to be just above the power rating of the device, so an 8 Amp cool box should be fed with wire that can easily take 8 Amps, say 15 Amp wire, and it's fuse should be say 10 Amps (next size above 8 Amps)

Aside = Volts, Amps and Watts all start with a capitol letter because they are all named after a person.

Your stereo is a bit more difficult to work out, but the good news is you don't need to. It may say "50 Watts per channel" on it, and you know it has 4 channels (ie 4 x speakers) so you might think that it uses a huge 200 Watts (50 x 4) but it doesn't.

Reason = if, hypothetically, you played a sound from your stereo at absolute full volume, and that sound was just a constant hiss, then yes it might use 200 Watts. But we don't ever do that. We play music that has lots of ups and downs in it, and also we rarely play it at full volume. So I would estimate that listening to say Radio 4 loud enough to hear whilst parked up actually only uses about 4 Watts. Listening to a music track at a good volume for inside the van might = ~25 Watts. Listening to a music track loud, in a "party" situation, with the van doors open might = ~80 Watts.

Just rough estimates of course and note the figures are without the use of any extra power amps as used by some.

ghost123uk wrote: So:-
Dividing Watts by Volts = Amps
and:-
Multiplying Amps by Volts = Watts

So know you know how to convert one to another. If say you had a cool box that had 12 Volts and 8 Amps written on it, then 12 x 8 = 96 (the 96 is the Watts) or if it said 12 Volts and 96 Amps on it, then 96 divided by 12 = 8 Amps.

Easy method is to use the following triangle:



P = Power (Watts)
I = Current (Amps)
V = Voltage (Volts)

P = I x V

I = P / V

V = P / I

The second example should say 96 watts rather than amps shouldn't it? 96 watts divided by 12 volts equals 8 amps.

RobH wrote:The second example should say 96 watts rather than amps shouldn't it? 96 watts divided by 12 volts equals 8 amps.

Correct, bit of a typo on my part ! - Now edited - Thanks for "proof reading" it for me

Ghost, I didnt want to be asking you all questions that have been asked a million times!

But waht you have wrote helps a lot!

So just to check as an example a 70A battery will let me play my music using ~24 Watts for (25/12) ~2 hours, using the full battery. However my CD player cuts off at 7 Volts, So will it only play for about an hour?

ianders, why P & I rather then W & A? because surely that would have made more sense?

Smosh wrote:Ghost, I didnt want to be asking you all questions that have been asked a million times!

But waht you have wrote helps a lot!

So just to check as an example a 70A battery will let me play my music using ~24 Watts for (25/12) ~2 hours, using the full battery. However my CD player cuts off at 7 Volts, So will it only play for about an hour?

ianders, why P & I rather then W & A? because surely that would have made more sense?

I teach motor vehicle in a College and I find the best way to 'get your head around the concept' is to think of electricity as 'water' and the way you know water behaves.
So:
Volts = pressure (how hard the water pushes...out of a hose for instance)
Amps = quantity/amount
OHMS = the amount of restriction for the electricity to 'flow' (like someone standing on a hose pipe when you are washing the car)
Watts = combination of the above

So with that concept in mind...a battery is a 'sink full of water.....large sink = large battery, small sink = small battery......if the battery is fully charged that equates to the sink being full....flat is the sink being empty.
The alternator therefore, refills the sink and varying rates slowing as the sink fills.

Hope this helps in some small way.

Martin

the water analogy is a very good one but it misses the concept that a circuit has to be made for electricity to flow.
The dairy/ milk float analogy below explains that well I think

So in the days when milk was delivered you have large bottles at home and at school you had small bottles
The larger the bottle the higher the voltage
The number of bottles is the current
So power is Volume of milk
Lots of small bottles is the same volume as fewer large bottles
more amps at low voltage is the same power as less amps at high voltage
Empty milk bottles are collected and returned to the dairy to be refilled, if you don't return the bottles the whole process stops

I explained this to somebody on a whiteboard once here is a pic

Smosh wrote:
So just to check as an example a 70A battery will let me play my music using ~24 Watts for (25/12) ~2 hours, using the full battery. However my CD player cuts off at 7 Volts, So will it only play for about an hour?

Nope - missed a step. 24W divided by 12V = 2A (not 2 hours) (That's from I = P/V)

If you run that unit at that volume for 3 hours, you'll use 2A x 3H = 6AH.

Your '70A' battery is actually a 70AH (Amps x Hours) battery, so if it was able to deliver all of those 70 Amp.Hours you'd be able to run your CD player for 35 hours (70AH/2A = 35H). But a lead acid battery likes to be recharged when it's given up roughly half it's stated capacity (at which point it'll measure around 12V), so you'll get about 17-18 hours before you should recharge.

'why P & I rather then W & A? because surely that would have made more sense?' - P is the symbol for power and W is the units. Similarly, I is the symbol for current and A is the units. At a simple level you can interchange them.

CD's water analogy is a good one. I'd only add that you can think of amps as the current in a river (easier to visualise than flow, to me anyway).

what would you do with a relay? it is a kind of remote controlled switch, usually used when you want to switch a powerful load. It has an electromagnetic coil in that takes a small current to operate- so you can have a low current (cheap) switch on the dash for say lights. Operate switch on dash energises the relay coil which switches over some meaty contacts in relay which are connected to your lights. Got a bit wordy that.

Another basic is that you need a bigger cable to carry a higher current, cable will overheat if it carries too much current. There are tables to tell you how much current each size will take.

If in doubt, ask, I never cease to be amazed by the help and damn good advice that is available on here (my own excepted). Bri

Smosh wrote:ianders, why P & I rather then W & A? because surely that would have made more sense?

To answer that, you'd probably need to go back a few hundred years and ask the physicists of the past.
I think it was because current used to be referred to as intensity

P stands for Power which is measured in Watts.