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Old 13-10-2018, 18:17   #1
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Power equation calculations

We have a new, to us, boat and we are trying to figure out how everything works. Right now it is the turn of the 12V electrics. We have just installed 4 x 120A AGM batteries and just have to figure out how best to keep them charged.

We have a battery charger and generator on the boat and we are trying to understand the relationship between them. The battery charger is a CTEK M300 battery charger and we have emailed CTEK a few times but do not understand their replies. This is no doubt due to ignorance on our part but we are lost and we are hoping someone can point us in the right direction.

This was the email we sent CTEK:

"Dear Sirs, I have just bought a yacht which is fitted with a CTEK M300 Battery Charger. I have 4 X 120A AGM batteries on the yacht and I also have a Honda EM 1000F portable generator that I would like to use to charge the batteries when I am sailing. I have the manual for the CTEK M300 Battery Charger and I am trying to do the power equation calculations to see if the Honda generator will be adequate for the battery charger. The Honda generator has a maximum output of 850W, a continuous output of 750W, and the voltage is 240VAC. I am not sure whether to use the 240VAC or the charging voltage of 14.4VDC figures for the calculations as they produce very different results. Using the 240VAC figure the power equation is 240V X 2.9A rms = 696Watts. Using the 14.4VDC figure the calculation is 14.4V X 25A = 360Watts. I must admit that this is all new to me so if my calculations are incorrect please accept my apologies. Please can you let me know which calculation is correct. Please also advise if I should be including a Power Factor. "

This was their reply:

"M300 take about 400W from the main. Because of cos fi is 0.5 current and voltage in relation to each other over time, we recommend to have a generator about 800W.
So your Honda generator will work fine."

I did not understand their reply and sent them this email:

"Thank you for your email and for confirming that my Honda generator will be adequate for my M300 battery charger. Just so that I can properly understand how everything works, please can you tell me how you calculate the 400W draw from mains. My mains are 240V."

To which I received this reply:

"Max output power 350W and efficiency about 80% = is about 400W what it takes from main."


The CTEK battery charger specifications are as follows:

Charging voltage - 14.4V
Charging current - 25A max
Current, mains - 2.9A rms (at full charging current)

We would be grateful if someone could help us understand the power equation calculations.

Peter
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Old 13-10-2018, 18:51   #2
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Re: Power equation calculations

Not going to go to deep in this but the Power requirement is Volts x Amps. the Power from the mains is Volts x Amps x Power Factor. They gave you a estimate of PowerFactor and they have answered your question.
If you are confused by Power Factor, well its because there is lots of windings that are inductive ie the Voltage wave form is not in sync with the Load wave form.



There you go. Clear as mud.
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Old 13-10-2018, 19:02   #3
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Re: Power equation calculations

14.4V * 25A so 360W **output**, inefficiencies so 400W **actually consumed**

Being conservative, output of upstream power source likely exaggerated, they recommend 800W available for input .

Clear as a bell.
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Old 13-10-2018, 19:03   #4
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Re: Power equation calculations

I don't understand either of your e mails. What are you trying to figure out? If the generator will charge the batteries?
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Old 13-10-2018, 19:03   #5
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Re: Power equation calculations

Watts are watts no matter the volts.

Amps change with volts, so forget them in situation where voltage is not constant.
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Old 13-10-2018, 19:28   #6
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Re: Power equation calculations

> Using the 240VAC figure the power equation is 240V X 2.9A rms = 696Watts. Using the 14.4VDC figure the calculation is 14.4V X 25A = 360Watts.

Simplest way to look at it is: 696W is the maximum draw from the mains, but because of inefficiencies, 360W is the maximum that you will be putting into the batteries.
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