# Q: What are the calculations to figure out how long to charge battery

Discussion in 'General Electronics Chat' started by foolios, Jan 27, 2015.

1. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
I have a 75AmpHour AGM battery. I want to use a 15Amp quickcharger to charge it. The quickcharger's comments state that the output on the DC side is 280 watts. But my calculation shows 180watts(15A x 12V). How did they come up with that wattage? Is my calculation wrong?

I'm also trying to figure out how many amps draw that will be on the AC side. If it is indeed 280 watts on the DC side, wouldn't it be 280watts/12V = 23 Amps of DC current.
But then the conversion from 12V to 120V is a factor of 10 so 23Amps/10 = 2.3Amps.
Does that seem right?
Hmm... the product does have a comment about 3Amp draw. So... hmm... 2.3A x 120V = 276 watts.
Which must be where they are getting their 280 watts.

How do I calculate how long it takes to charge a 75AH battery with the quickcharger set on 15Amps?
Is that 15 amps an hour? 75AmpHrs/15Amps = 5 Hrs? Taking 75 divided by 15 and crossing out the Amps leaving me with hours.

Something like that, right?

2. ### MrChips Moderator

Oct 2, 2009
14,296
4,198
Something like that.
But watch out that there is the danger of over charging the battery.
When the battery voltage reaches 14.5V you want to reduce the charging voltage to 13.8V.

3. ### #12 Expert

Nov 30, 2010
17,842
9,178
So much fudge (the numbers)!
You already figured the 280 watts is the input power, not the ability of the charger.
15 amps might be the capability of the output, but your battery will not accept 15 amps unless it is horribly discharged, and batteries don't like that.
Also, the inefficiency of the battery shows up as increased charge time.

So...go with the voltage readings. A clock is just not the right way to charge a battery.

4. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
Thanks for the voltage reminders.

When #12 wrote: battery will not accept 15 amps;
How do you figure out how many amps max a battery can handle incoming?
I mean max inrushing amps. Err, something like that...
I have a 35AH and a 75AH.

Thanks for the responses.

5. ### #12 Expert

Nov 30, 2010
17,842
9,178
Plug it into the charger and measure the current.

Every cheap charger that lies about its ratings has a different voltage capability. Every battery has a different ability to accept charging current, and this changes as the battery gets older. Just measure it if you want to know.

6. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
Ahhh, ok, so the battery will resist taking in the 15 amps even if the charger tries to dump 15 into it. The voltage will rise, then the charger(if it's smart), will just shut off and more than likely the battery won't even be close to charged. hmmm.... I'll have to look for something on the net somewhere that can kind of give a clue to how many amps a battery rating can handle incoming. Since I don't own the device yet and I'm trying to find the right quick charger for my batteries.

7. ### #12 Expert

Nov 30, 2010
17,842
9,178
Not likely you can find that information because the battery manufacturers can not tell the ratings of your charger. Besides that, the acceptance ability of a battery changes with time and temperature. There is no such thing as a battery and charger combination that starts charging at 15 amps and finishes charging at 15 amps.

You could design one and build it, but you can't buy one (that I know about).

For instance, I designed and built a 50 amp charger (200 amp surge) and I have never seen a dead car battery that would accept more than 35 amps from it. I even over-wound the transformers! Then I tried to charge a different car battery and it was not the same as the battery I used to design the charger. It looked the same, but it didn't act the same.

Last edited: Jan 27, 2015

Feb 4, 2009
160
1
9. ### #12 Expert

Nov 30, 2010
17,842
9,178
That certainly looks better than your average \$29.95 cheapo.

Notice: Normally \$180, on sale for \$40.
People, in general, will not pay over \$100 for a battery charger that they might use 3 times before it gets lost or broken. I only have \$40 in the 50 amp charger I built because half of it was in a surplus store and half of it was laying around in the shed. A dead car battery just isn't that common that these things get a lot of use.

You have a different purpose, but you will not find an exact match for sale because there is no profit in building a \$200 charger for the needs that drive the marketplace right now.

10. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
They usually like between .1C and .3C max. So if I were you I would buy one sized for the 35 ah battery. The bigger one will still charge but will take longer.

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