LED, batteries, and current

[phil on a fazer]

Afternoon peeps

I'm interested in buying a 10W LED floodlight (12v/24v) to run off a car/deep cycle battery.           
http://www.onsolar.co.uk/12V-10W-LED-Floodlight.htm

I currently have a 60W spot light. Its like the ones you see on the roof of utility vans or land rovers - its round with a plastic mesh on it.

So if its 60W and runs from 12V does that mean it draws 5ah? the LED floodlight draws 900ma at 12V or 450ma at 24V.

If i linked to 12v batteries together in series it would give me 24V so the light would draw 450ma. So is that 225ma from each battery?

I know your only supposed to put batteries in series if they are the same age, condition and type. I have two leisure batteries that are the same but not quite in the same state. One charges to 13.24V and the other will fully charge to 12.82v. (which my charger class's as "weak" ??)

so.....i take it i shouldn't wire those two batteries in series? 

Cheers

[rustyrider]

It doesn't matter if you are connecting them in series other than the fact that if one goes flat before the other then you'll get no volts at all.  They only have to be matched if connected in parallel as one would go flat first and the other would try to charge it.

You're sums are right though, so drawing 225mA per battery means they will last longer than running off one 12V battery.  Assume the batteries are 60Ah they will last for 60/0.225 or 267 hours in theory although I doubt you'll get that in practice if the batteries aren't perfect.  You'll still get a long time though.

[limax2]

I'm no expert but I'll give an opinion anyway and maybe get corrected.
The  60w from 12v would be drawing approx 5amps. (Not 5ah, as in ampere hours, if that's what you meant).
If you linked the two 12v batteries together in series you would have the equivalent of a 24v battery as you say. However as I see it the draw from each battery would be 450ma. (I'm in disagreement with rustyrider here).
 Only if you linked two matched batteries in parallel would you divide the current between them. In this case it would still be 12 volts so each still taking 450ma.
You could just run the light off one 12v battery and at 900ma you would have a good number of hours between charges with a normal leasure battery.

[BBROWN1664]

60w from 12v is 5 amps. From 24 volts it would be 2.5 amps.

sticking with the 12v calculation, 5amps for 1 hour is 5ah so a 60ah battery would ladt 12 hours. A 24 volt battety would last 24 hours in theory.

[BIG MAC]

Batteties which ladt 12 hours available at glood sloptists

[MurphySG]

Worked out most of what you were writing BM - but what is a sloptist 
(ps - eeurgh - this thread reminds me of O Level physics lessons)

[phil on a fazer]

i thought that a stand alone battery (as in not connected to an alternator/chargering system) the ah that is used shouldn't be more than 10x greater than the battery. So a 40ah battery is best at 4ah or less.
 
I put my 12v light onto a car battery the other day that was fully charged to 12.8V. 45mins later the light was dimming and the battery was on...10.5v(ish)...(cant quite remember  ) Hence me wanting a LED light with a lower current draw.
 
If you have a battery connected to a charger and then you connect a 12v light to the battery at the same time...it's o.k isn't it? (only like what happens in your car/bike?) As for it drawing more power than the battery charger is putting in...that will be down to the charging pattern of the charger wont it?
 
These might seem like odd questions but just bare with me. 
 
 

[rustyrider]

If you linked the two 12v batteries together in series you would have the equivalent of a 24v battery as you say. However as I see it the draw from each battery would be 450ma. (I'm in disagreement with rustyrider here).

Why?  You've got 2 batteries connected together in series to give 24V and a load which is drawing 450mA.  If you were to draw 450mA from each battery, you would have 900mA of current flowing.  The lamp draws 450mA so where is the other 450mA going?

If you used a pair of 12V batteries in parallel, the draw at 12V would be 900mA, that would be 450mA per battery but by increasing the voltage you decrease the current required.

[Fazerider]

If you linked the two 12v batteries together in series you would have the equivalent of a 24v battery as you say. However as I see it the draw from each battery would be 450ma. (I'm in disagreement with rustyrider here).

Why?  You've got 2 batteries connected together in series to give 24V and a load which is drawing 450mA.  If you were to draw 450mA from each battery, you would have 900mA of current flowing.  The lamp draws 450mA so where is the other 450mA going?

If you used a pair of 12V batteries in parallel, the draw at 12V would be 900mA, that would be 450mA per battery but by increasing the voltage you decrease the current required.

If you connect batteries in series the voltages add, the current is the same through each ... so both are supplying 450mA.
Think of the batteries as pumps pushing water uphill: in parallel a pair would give twice the current, in series they could pump the same current as one pump... but twice as high (they're pumping the same current because it's the same water).


As for the original problem of which is better in this instance, it makes little real difference. A low voltage cell in a battery will get worse as the battery discharges (in fact it will become reverse charged), but it's in series with the other cells anyway so if that battery is doing anything useful the sick cell will get worse whether the batteries are in series or parallel.
I'd have a slight preference for series connection, at least that way the healthier battery isn't wasting power by trying to charge the sicker one... though it would make charging more fiddly if you only have a 12V charger.

[pointer2null]

Fazerider is correct.

The current will be the same thoughout the whole series circuit. So if it's 450mA through the first battery it must be 450mA through the second.

If you're not sure why, imagine a single track road where cars represent the current (cars per hour = amp hours), if they cannot pass each other, and connot be removed by a side road, then the number of cars passing point A but be the same as the point after it, and the same at point B.

Same with batteries in series (or even cells in a battery), if they are not matched then one discharges faster and recarges slower, so each charge/discharge cycle increases the difference between batteries until one becomes reverse charged. The more batteries in series the worse the problem. (Those older electric bicycles had three 12V batteries in series to power them and the charge was wired so the batteries charged individually - effectivly 3 chargers in one)

[rustyrider]

The current flow will be the same but the original question was the current drawn from each battery.  In other words, how quickly will it go flat?  Assuming a pair of matched batteries in series and they will both go flat at the same rate as they are both having the same amount of current drawn from them, half of the total current required.  Using the old water pump analogy, volts is pump pressure, amps is the quantity of water so the same quantity of water will be emptied from each battery and each will supply half of the amount needed by the load.

[pointer2null]

From your description the LED torch uses 450mA on 24V (we can assume there is some sort of internal regulator) so the currently in each battery is 450mA. If you run the light for an hour each battery will supply 450mAh (0.45Ah), so the answer to you question depends on the capacity of the battery. I.e. if your batteries are 45Ah type then 0.45A load will take 100 hours, a 22Ah pair will last 50 hours.

[Fazerider]

The current flow will be the same but the original question was the current drawn from each battery.  In other words, how quickly will it go flat?  Assuming a pair of matched batteries in series and they will both go flat at the same rate as they are both having the same amount of current drawn from them, half of the total current required.  Using the old water pump analogy, volts is pump pressure, amps is the quantity of water so the same quantity of water will be emptied from each battery and each will supply half of the amount needed by the load.

No, both are supplying all the current (but half the power, since that is volts x amps).
In the water analogy Amps is represented by volume of water per second.

[pointer2null]

Power  = Volts X Amps (joules per coulomb x coulombs per second = joules per second = watts)
12V 0.9A = 10.8W
24V 0.45A = 10.8W
2 batteries, equally matched in series will supply half the power each - 5.4W
2 batteries, equally matched in parrallel will supply half the power each as well.
1 Battery will suppy all the power.
2 batteries lasr twice as long as one as you'd expect.
How long for one or two batteries depends on the capacity.
 
 

[phil on a fazer]

If you have a battery connected to a charger and then you connect a 12v light to the battery at the same time...it's o.k isn't it? (only like what happens in your car/bike?) As for it drawing more power than the battery charger is putting in...that will be down to the charging pattern of the charger wont it?
   

Sorry to quote my self but has any one have any idea about my above question?
 
Volts and current....and stuff: i understood you can double your volts, but not your current. (thats badly explained)
 
2 x 12v 40ah batteries in series = 24V 40ah
2 X 12v 40ah batteries in parallel = 12v 80ah
 
SO.....it will take 450ah PER battery...???
 
As i said in my first post my two marine batteries at fully charged are not exactly the same voltage...so does that mean there miss-matched? or does it only matter when it comes to charging? (i could charge  seem separately?)

[pointer2null]

"each battery will supply 450mAh [if you run the light for one hour]" 

[alan sherman]

Someone lend him an extension lead to put us all out of our misery!

[pointer2null]

 
 
Ignore all the math, a 10W light will last a long time on one or two bike batteries

[limax2]

If you have a battery connected to a charger and then you connect a 12v light to the battery at the same time...it's o.k isn't it? (only like what happens in your car/bike?) As for it drawing more power than the battery charger is putting in...that will be down to the charging pattern of the charger wont it?
 
These might seem like odd questions but just bare with me. 

It depends on the type of battery charger I guess but basically I can't see a problem with that. The voltage at the bulb will be a bit higher than just the battery on its own, but as you say no different to what the light gets on a car or motor bike. If you use a fancy battery charger that monitors the state of the battery I'm thinking it will indicate a duff battery. This is because the bulb will be drawing current and making it seem like the battery is discharging itself.