How to calculate current draw from a step up transformer

[Guest]

Right no current leakage discussions this time!

I just need help from the electrical engineers in how to calculate something.

I need to top up the charge on a 24v mobility scooter battery overnight from a 12v car battery or leisure battery.

The 24v battery to be charged consists of 2 x 22AH 12v lead acid batteries wired in series.

I would use this step up transformer/charger http://www.ebay.co.uk/itm/PRIDE-GO-GO-MOBILITY-SCOOTER-IN-CAR-CHARGER-CHARGE-WHILST-DRIVING-/272447401916?hash=item3f6f216fbc:g:-O8AAOSwzaJYB-So

So output from the battery charger to the 24v battery is 2A

But how do I calculate how much current the battery charger will be drawing from the 12v battery?

Until I figure this out I won't know how long it will take to flatten the 12v battery (hence how many nights can it charge for), nor what AH leisure battery I will need, or indeed could I just run it off a car battery?

Thanks

[Mr Fro]

I thought you had an A-level in physics.

This site might be helpful.

[RantMachine]

I've got an A-level in psychology if that helps. Well, half of one.

Let's approach the issue more directly by discussing the deep-seated parental issues that are clearly behind your desperate need to "top up your battery". For the purpose of this session I'm assuming that "battery" is what kids these days are calling opiates.

[JRH]

Without knowing the efficiency of the unit, how long is a piece of string.

But.......

the out put of the unit in watts (power) is

watts = V x A

w = 24 x 2 = 48 watts.

so ignoring efficiency and other losses the 12 volt battery need to supply 48 watts. using the same formula

so 48 = 12 x A

48 / 12 = Amps = 4.

ball park figure depending on efficiency say 5 Amps best guess.

HTH

[Joeman]

My honors degree in experimental physics from a prestigious London university made me too underqualified to reply...

But P=IV is GCSE level stuff..

that charger has a three pronged outlet suggesting it might actually be charging each battery individually at 12v rather than in series at 24v which might open up other possibilities for charging from a battery...

[JRH]

I have a mains version of the charger in the shed. keep meaning to disassemble it and see if it can be utilised as a 12v charger.

But [mention]Joeman[/mention] why does P=4

[Westbeef]

I couldn't have even began to attempt this, but I'm mechanical so that's my excuse

[Guest]

Thanks JRH, I knew it couldn't be that simple, is there any way to tell? Is the efficiency usually published in manufacturers specs?

1 or 2 amps matters when you are estimating drain on a battery that only has a limited AH.

If there's no way to tell and your guesstimate is 5 amps, I'll go with that.

[Guest]

My honors degree in experimental physics from a prestigious London university made me too underqualified to reply...

But P=IV is GCSE level stuff..

 

Shame your P=IV is so oversimplified that it leaves you 25 ish percent off the actual number

[JRH]

[mention]Baloo[/mention] on many items the input and the output ratings are shown on the devices.

But see here

http://www.mobilitysmart.cc/parts-spares-c-106/battery-chargers.html

second item.

(i rest my case)

[Joeman]

That wasnt supposed to be the answer! I don't meet your requirements to provide an actual answer I was mearly pointing you in the direction of the Basic GCSE level physics formula you need to calculate the actual answer.

Clearly there are two values for I and two values for V but as you love to quote Einstein energy cannot be created or destroyed, so the power (P) on each side of the transformer must be the same (ignoring losses)

So without actually providing an answer it's pretty obvious that:

I(primary) * V(primary) = I(secondary) * V(secondary)

Does it really need an explanation to someone with an A-level in physics??

[Guest]

That wasnt supposed to be the answer! I don't meet your requirements to provide an actual answer

Does it really need an explanation to someone with an A-level in physics??

 

No P=IV doesn't need an explanation to somebody that has A level physics, however the thread is asking for the right answer.

It's not your qualifications that don't meet requirements, it's your fondness for post sarcastic unhelpful shit.

[JRH]

My fault.

P=IV = 4

4 in roman numerals =IV. but my humour is obviously not appreciated

[Guest]

@Baloo on many items the input and the output ratings are shown on the devices.

But see here

http://www.mobilitysmart.cc/parts-spares-c-106/battery-chargers.html

second item.

(i rest my case)

 

Great cheers.

That was another doubt that was niggling in my head though. You can pay £35 quid for the one my ebay link, or £60 quid for the one in your link, or some places are charging near £100 for another charger that is still rated at 2 amps. Is the difference between those chargers the efficiency?

I googled for a users manual for the £35 quid charger, but I couldn't find one.

[Joeman]

That wasnt supposed to be the answer! I don't meet your requirements to provide an actual answer

Does it really need an explanation to someone with an A-level in physics??

 

No P=IV doesn't need an explanation to somebody that has A level physics, however the thread is asking for the right answer.

It's not your qualifications that don't meet requirements, it's your fondness for post sarcastic unhelpful shit.

 

It was helpful had you understood the BASIC GCSE level physics!!

Read this - the BBC have written a page especially for you!!

http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/keeping_things_moving/transformers/revision/4/

P=IV is all that needed to be said...

[JRH]

Possible or the quality of the components so they last longer.

[JRH]

Last word on the subject before the MODS close us down.

Transformers do not work with DC only AC.

Just saying

[Guest]

That wasnt supposed to be the answer! I don't meet your requirements to provide an actual answer

Does it really need an explanation to someone with an A-level in physics??

 

No P=IV doesn't need an explanation to somebody that has A level physics, however the thread is asking for the right answer.

It's not your qualifications that don't meet requirements, it's your fondness for post sarcastic unhelpful shit.

 

It was helpful had you understood the BASIC GCSE level physics!!

Read this - the BBC have written a page especially for you!!

http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/keeping_things_moving/transformers/revision/4/

P=IV is all that needed to be said...

 

4 is not the correct answer. It's not that simple. Not helpful.

[RantMachine]

[Guest]

Possible or the quality of the components so they last longer.

 

OK, before buying one I'm going to ask the place I rent the mobility scooter from if they have a 12v charger I can borrow, at least then I can get multimeter on it. From what you've posted though it's going to be somewhere between 5 and 6 amps depending on the quality of the charger, so that's been really helpful, thanks.

I've wanted to get a leisure battery for ages anyway and set up a split charge system in the car. Then I can run a cable into the tent for laptop, phone charging, LED lighting and my electric coolbox. The electric coolbox draws 3.6 amps so a decent size leisure battery will only run it for a day or two, so I'd need a solar panel to pop on the roof of the car to top up the battery during the day.

Edited July 13, 2017 by Anonymous

[Mr Fro]

I couldn't have even began to attempt this, but I'm mechanical so that's my excuse

 

Bollocks Beefy, this is year 1 stuff!

[JRH]

I couldn't have even began to attempt this, but I'm mechanical so that's my excuse

 

When i was doing my ONC many years ago we had to do electrical and mechanical engineering.

I got higher marks in the first year mechanical exams than the electrical (and i am supposed to be an electrical engineer)

[RantMachine]

Relevant .'. not shitposting

[Joeman]

No P=IV doesn't need an explanation to somebody that has A level physics, however the thread is asking for the right answer.

It's not your qualifications that don't meet requirements, it's your fondness for post sarcastic unhelpful shit.

 

It was helpful had you understood the BASIC GCSE level physics!!

Read this - the BBC have written a page especially for you!!

http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/keeping_things_moving/transformers/revision/4/

P=IV is all that needed to be said...

 

4 is not the correct answer. It's not that simple. Not helpful.

 

I never said 4!!

Did you get your English A-level from the same place as your Physics A-level?? Cuz clearly your can't read or do physics haha

[Mr Fro]

I never said 4!!

Did you get your English A-level from the same place as your Physics A-level?? Cuz clearly your can't read or do physics haha

 

In all fairness, I don't think he said he passed the exam.

What did you get [mention]Baloo[/mention]? I got a "B".