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Topic Title: Battery Chargers
Topic Summary: Transformer losses
Created On: 01 August 2006 04:38 PM
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 01 August 2006 04:38 PM
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burn

Posts: 249
Joined: 06 June 2003

I have to install mains supplies to an 8 hour charger with a 24v 375Ah battery. I assume 375/8h = 46.88A at 24V = 1125VA. This then equates to 4.89A at 230V.
Am I correct and how much extra should I allow for losses in the transformer and regulating gear?

Burn
 01 August 2006 05:17 PM
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Arnot

Posts: 346
Joined: 25 April 2005

Your calculation is OK but you cannot assume that the charger is constant current, it is more likely to be constant voltage with a peak current ceiling or a multi stage charger that varies between the two systems as the charge level goes up.

Also you need to know if the charger is the older transformer/resistor type or a more modern switch mode type. The efficiency can vary significantly.

Either way it's best to find out from the charger manufacturer what their take is, they will have already travelled this road. The ID plate on the charger may even tell you.

If this is not possible, to be fairly safe, I suggest 20A, double your calculation for a 2:1 peak to average current ratio and double it again for a minimum of 50% efficiency.

Hope this helps....

Regards

Arnot
 01 August 2006 05:22 PM
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John Peckham

Posts: 8791
Joined: 23 April 2005

Single Phase I assume? I asume a fork lift truch charger? They have big chunky low efficency transformers that take a big inrush current to magnetise the transformer. You will need a type C or maybe Type D breaker and you may have to increase the conductor size to acheive current and Zs requirements. Have a word with the manufacturer to see what the input characteristics are for their chargers if there is not a data sheet or handbook.

John Peckham

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 01 August 2006 07:27 PM
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deleted_johnwoods

Posts: 997
Joined: 05 September 2005

Just moved two of these, as one was tripping breaker as too many on single circuit. Best to look at the plate for the input current as this can vary a lot and seems to bear no relation to any calculations you can do.

One was 80A 48V output so you would think input would be about 17A but it was 47A. It was on a 50A type C breaker but still tripped it, so now on a 63A type B breaker with appropriate cable size.
The other one was a 40A 48V charger which you would assume would be about 9A but was rated 17-18.5A (depending on input voltage). Now on a 32A RCBO as in a covered outside area with a 32A plug to replace the 16A one it had which I suspect was not original.

So rule of thumb would be work out the input current and double it, then go up a breaker if you can.
True about the inrush current hence the larger breaker and conductor size suggestion.

-------------------------
You must be the change you want to see in the world - Mahatma Gandhi
 14 November 2017 05:39 PM
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broadgage

Posts: 2435
Joined: 07 August 2007

Originally posted by: burn

I have to install mains supplies to an 8 hour charger with a 24v 375Ah battery. I assume 375/8h = 46.88A at 24V = 1125VA. This then equates to 4.89A at 230V.

Am I correct and how much extra should I allow for losses in the transformer and regulating gear?



Burn


I would allow a lot more than that.

Firstly, to charge a battery requires about 10% more AH than can be obtained on discharge, and secondly to charge the battery in 8 hours probably requires an initial charge rate of more than one eighth of the capacity. Allowing for both these factors, I would presume an initial charge rate of one fifth of the battery capacity and not one eighth, say about 75 amps.
Also, on charge the battery voltage will be about 28 volts, not 24 volts.
75 amps at 28 volts is 2.1 KW. Allow at least 10% more for losses if switched mode, and at least 30% if copper/iron.
That suggests a running load of about 10 amps.

I would install a 16 amp circuit, from a C16 MCB, in at least 2.5mm cable, with final connection by a 16 amp ceeform plug and socket.

Try to get Ze low enough to permit of retrofitting a D16 MCB if the C type trips on inrush current.

For my own use, or for a test, a 13 amp plug might suffice but I would not stake my reputation on this for a paying customer.
Too much risk of the plug overheating or the fuse blowing IMHO.
I would also consider it poor practice to add a 10 amp or so and long hour load to a general purpose circuit needed for other purposes.
 14 November 2017 07:40 PM
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davezawadi

Posts: 3905
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There are more problems too. Assuming the charger output is simply bridge rectified AC, the the charge current is the average across the cycle. But as the battery will only take charge when the applied voltage is above the cell voltage, real charging is by a series of pulses corresponding to the peak area of the input waveform. So the RMS input current may well be reasonable, but the peak input current will be several times the average (possibly 10 times) so the circuit losses will be higher than you expect, so needs larger cables to minimise losses. This is why transformer/rectifier chargers are so inefficient as cited below, the peak powers require much larger components, particularly the transformer, otherwise they will get very hot! The supply volt drop during the peak will also be much larger than expected. Such supplies also have a fairly poor power factor because the transformer leakage inductance is the major current limiting component by design, so a kVAr meter will charge for a lot of unused energy!

Modern chargers use switch mode supplies, which take a current closely related to the input waveform, have a unity power factor, and are much more efficient, probably 90%, so pay for themselves quite quickly.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 15 November 2017 11:21 AM
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AJJewsbury

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Joined: 13 August 2003

Hopefully the OP's sorted now - since the thread was started over 11 years ago!
- Andy.
 15 November 2017 02:05 PM
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burn

Posts: 249
Joined: 06 June 2003

Yes - by the manufacturer. I can't even remember which job it was for. burn

Edited: 15 November 2017 at 02:19 PM by burn
 15 November 2017 02:17 PM
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potential

Posts: 1641
Joined: 01 February 2007

Originally posted by: burn

The question wasn't posed by me.
Seems we have a poltergeist.
burn

or a phoenix perhaps?
 15 November 2017 03:37 PM
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broadgage

Posts: 2435
Joined: 07 August 2007

As we all know, adding a new post to an old thread brings it to the top of the board.
If the new post is found to be spam and is deleted by a moderator, then on this forum, the thread remains at the top rather than dropping back to its former obscurity.

Foolish persons than read the old discussion and add to it. If the old thread was of a general nature, perhaps about the merits or otherwise of different types of equipment, or different installation methods, then adding to the discussion can be interesting and still relevant.
If however the old thread was a specific enquiry that has long since been resolved then little is gained by adding to it.

Other forums may vary, it depends on the software. Sometimes a post added to an old thread and then deleted moves the thread back to where it came from, in other cases, like here, it remains at the top.
 15 November 2017 04:48 PM
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potential

Posts: 1641
Joined: 01 February 2007

Originally posted by: broadgage

As we all know, adding a new post to an old thread brings it to the top of the board.

If the new post is found to be spam and is deleted by a moderator, then on this forum, the thread remains at the top rather than dropping back to its former obscurity.



Foolish persons than read the old discussion and add to it. If the old thread was of a general nature, perhaps about the merits or otherwise of different types of equipment, or different installation methods, then adding to the discussion can be interesting and still relevant.

If however the old thread was a specific enquiry that has long since been resolved then little is gained by adding to it.



Other forums may vary, it depends on the software. Sometimes a post added to an old thread and then deleted moves the thread back to where it came from, in other cases, like here, it remains at the top.

I'm puzzled.
It was you who recently replied to the original post dated 01 August 2006 wasn't it?
 15 November 2017 04:56 PM
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burn

Posts: 249
Joined: 06 June 2003

yes I think so - before I lost my memory

burn
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