# Amps and watts and stuff



## Mike63 (Nov 18, 2009)

Hi...in my 'van I have a 75 amp leisure battery.
If I only use the one light with 2 x 8 watt florescent tubes, how many hours will the battery safely last with this light burning non stop, without electric hook up ?

...thanks...Mike


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## trevorf (May 16, 2005)

*Oops*

Post withdrawn due to extreme embarrassment

:roll: :roll: :roll:


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## wilse (Aug 10, 2007)

Mike

See my thread if you want to see cold batteries in action!

here it is

I was surprised to say the least how the cold has effected these little suckers.

Trevs' suggestions are good.

Keep them warm!

w


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## clodhopper2006 (Aug 13, 2006)

trevorf said:


> You should not let your battery discharge more than 50% so from fully charged you effectively have 37.5 Ah available.
> 2X 8W lamps = 16W
> 37.5/16 = 2.34 hours
> Maybe less in this cold weather at present. Frightening isn't it !!!!!!
> ...


It is 16W as you say but this means it draws a current of 16/12 = 1.33A
So to use up your 37.5A/h you can run it for 37.5/1.33 which is 28.13 hours


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## teifiprt (Oct 2, 2007)

Hi Trevor,
I think the maths look something like this.
2 x 8 = 16 watts
watts = volts x amps
amps = watts/volts
amps = 16/12 = 1.33
hours = 37.5aH/1.33 = 28.2
think thats right,
peter.


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## Broom (Oct 8, 2007)

Hi All

Peter you get the tick, sorry Trevor on this occasion your sat in the corner

Best Regards
Broom


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## trevorf (May 16, 2005)

My apologies, I deleted my original reply as I realised my maths was totally wrong, yours is correct Peter. 
I will sit in the corner with my Dunce hat on    


Trevor


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## clodhopper2006 (Aug 13, 2006)

trevorf said:


> My apologies, I deleted my original reply as I realised my maths was totally wrong, yours is correct Peter.
> I will sit in the corner with my Dunce hat on
> 
> Trevor


Oiy! Mine was correct too :lol:


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## trevorf (May 16, 2005)

Sorry, yes, 5 stars to you both, none to me, having a bad day :lol: :lol:


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## CliveMott (Mar 10, 2008)

More brain testers here!
http://www.motts.org/HOOKUPS.htm


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## Mike63 (Nov 18, 2009)

Thanks all...that's not too bad at all.

....Mike


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## dovtrams (Aug 18, 2009)

This is all very interesting, but why 37.5/amps and not 75 as in the battery?

dave


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## Mick757 (Nov 16, 2009)

dovtrams said:


> This is all very interesting, but why 37.5/amps and not 75 as in the battery?
> 
> dave


Because its advised not to go below 50% charge.


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## SidT (May 9, 2005)

Hi Trevor. I see from todays paper that sitting in the corner is banned in England as it is against peoples human rights. :roll: :roll: :roll: 
Cheers Sid


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## airstream (Jan 7, 2009)

*A/H what A/H?*

Hi all,
A 75 a/h battery or any battery is rated at a temperature of 28c using one of several discharge standards, most take a 12v battery voltage down to 10.5 volts - which will destroy the battery if used in this way

Most commonly used standard is "C20" and is specific to a new battery in lab conditions 
Some manufacturers use a more flattering "C100" - ie Elecsol whose 110 a/h battery is comparable to a "C20" rating of 95 a/h

Also to be considered is that most motorhome chargers and alternators are not designed to fully charge a battery so you will only have say 90% of your 75a/h 
You then lose x% for each degree below 28c, less y% for age/use/abuse etc and then, you can only use 50% of what is left
Bottom line is from your theoretical 75a/h you have not a lot to play with on a freezing cold day
Regards Ray


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## nicholsong (May 26, 2009)

Thanks Clive Mott for your link

All good 'O' Level physics stuff for those who 'bunked off' from those lessons.

Clive, do you fancy a go at a tutorial on Fleming's Left-hand Rule andor Induced Currents? 

No maybe not!

Geoff


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