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Old 25-01-2016, 07:53   #1
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How long will it take to charge my batteries?

I have two domestic batteries, "Varta Blue ProMotive K8 12V 140Ah 800 A (EN) 640 400"

My charger is a Dolphin 12v 25 Amp Battery Charger REYA code: 299735

Assuming I have discharged my battery bank (280AH) to 50% how long would it take me charger to charge them to full again?

This questions comes from the fact that I have a Honda EU20i Generator, and I am wondering how long I will have to run it to charge the batteries.

I am a complete novice in the electrical department so any hint or tips are appreciated.
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Old 25-01-2016, 08:12   #2
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Re: How long will it take to charge my batteries?

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Originally Posted by AFKASAP View Post
I have two domestic batteries, "Varta Blue ProMotive K8 12V 140Ah 800 A (EN) 640 400"

My charger is a Dolphin 12v 25 Amp Battery Charger – REYA code: 299735

Assuming I have discharged my battery bank (280AH) to 50% how long would it take me charger to charge them to full again?

This questions comes from the fact that I have a Honda EU20i Generator, and I am wondering how long I will have to run it to charge the batteries.

I am a complete novice in the electrical department so any hint or tips are appreciated.
Just look at it as a simple math problem.

280 AH used 1/2 way is 140 AH.

Your charger will charge at 25 AMPS / hour. So 140 / 25 = ~6 hours.

The issue is that if the charger is a smart charger, then it does a bulk charge at max A / H, then at some point cuts down to Absorption charge which is a lower A / H.

https://www.batterystuff.com/blog/3-...-chargers.html

So figure that the getting to 80% will be about 3 hours. However you should look at the charging curves from your charger manual to get the exact info on how long it holds bulk charge, then absorption charge.

The bottom line is that to reduce charging times, you want higher AMPS available for the bulk and absorption periods. The time to full charge is directly limited by the 25 AH max charge current. If the rate were 50 AMPS then the times would be roughly 1/2. Within limits of course. A million amp charger would be no faster than a 100K amp charger.

Float is meaningless in your case since you are never going to use that.
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Old 25-01-2016, 08:22   #3
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Re: How long will it take to charge my batteries?

Due to battery acceptance and charger curves you will soon find that getting to 100% charge is a PIA when on a generator. Going from 75% to 100% charge will easily take twice as long as going from 50% to 75% battery charge. The more the battery is charged the less amps of charging current it will accept. When we charge by generator we normally don't charge over 85% because of this. Takes a lot of generator run time to get batteries all the way to 100%.
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Old 25-01-2016, 08:26   #4
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Re: How long will it take to charge my batteries?

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Originally Posted by jwcolby54 View Post
Just look at it as a simple math problem.

280 AH used 1/2 way is 140 AH.

Your charger will charge at 25 AMPS / hour. So 140 / 25 = ~6 hours.

The issue is that if the charger is a smart charger, then it does a bulk charge at max A / H, then at some point cuts down to Absorption charge which is a lower A / H.

https://www.batterystuff.com/blog/3-...-chargers.html

So figure that the getting to 80% will be about 3 hours. However you should look at the charging curves from your charger manual to get the exact info on how long it holds bulk charge, then absorption charge.

The bottom line is that to reduce charging times, you want higher AMPS available for the bulk and absorption periods. The time to full charge is directly limited by the 25 AH max charge current. If the rate were 50 AMPS then the times would be roughly 1/2. Within limits of course. A million amp charger would be no faster than a 100K amp charger.

Float is meaningless in your case since you are never going to use that.
Thank you for your reply. That makes sense. It seems quite a long time to charge the batteries. Is my charger particularly small?
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Old 25-01-2016, 08:38   #5
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Re: How long will it take to charge my batteries?

My control panel has an lcd display that shows the voltage of the batteries. I normally see both the service and starter battery are on around 13.7v. How do I know when my batteries are at 60% or 50% or 40% etc?

How low can I discharge my batteries before I risk damaging them?
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Old 25-01-2016, 08:50   #6
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Re: How long will it take to charge my batteries?

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Originally Posted by Dennis.G View Post
Due to battery acceptance and charger curves you will soon find that getting to 100% charge is a PIA when on a generator. Going from 75% to 100% charge will easily take twice as long as going from 50% to 75% battery charge. The more the battery is charged the less amps of charging current it will accept. When we charge by generator we normally don't charge over 85% because of this. Takes a lot of generator run time to get batteries all the way to 100%.
And this is why a solar panel is so handy. It will continue to push current into the batteries as long as the sun shines.
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Old 25-01-2016, 08:54   #7
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Re: How long will it take to charge my batteries?

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Originally Posted by AFKASAP View Post
My control panel has an lcd display that shows the voltage of the batteries. I normally see both the service and starter battery are on around 13.7v. How do I know when my batteries are at 60% or 50% or 40% etc?

How low can I discharge my batteries before I risk damaging them?
OK, so there are as many opinions on this as there are sailors.

This web site is pretty darned good. Look down the page for charge states by voltage.

Measuring State-of-charge - Battery University

~12.25 is 50%.
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Old 25-01-2016, 09:03   #8
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Re: How long will it take to charge my batteries?

count on longer than your math says. one d 8, or its equivalent, will take a week on shore power at 6 amps per hour input. 24/7.
when you are using your boat's alternator, count on an extra hour every time. and solar--divide by half the bragged about rates. they aint happening.
also make sure when you charge that your outgo is less than your input. i have met many males who cannot understand this concept.
good luck.
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Old 25-01-2016, 09:22   #9
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Re: How long will it take to charge my batteries?

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count on longer than your math says. one d 8, or its equivalent, will take a week on shore power at 6 amps per hour input. 24/7. good luck.
Lead Acid battery charging is a pretty complicated subject, made even worse by AGM and gel forms. Smart chargers run the show and they decide when to switch from one charge state to the next based on... whatever their own algorithm is.

In the end the charger holds a bulk charge voltage (that the charger manufacturer thinks is correct) and watches the charge current. When the charge RATE drops below a specific value (that the charger manufacturer thinks is correct) then they drop to the Absorption charge voltage (that the charger manufacturer thinks is correct) and watches the charge current. When the charge RATE drops to an even (much smaller) value then the charger switches to float voltage (that the charger manufacturer thinks is correct).

As you can see, it all really depends on (in no particular order):

1) The max current the charger can supply
2) The charger itself and the algorithms that the manufacturer uses to switch the voltages.
3) The cell chemistry (open lead acid / AGM / Gel)
4) The battery itself (brand / size)

So there will be no "one size fits all" answer to the question.

Quote:
also make sure when you charge that your outgo is less than your input. i have met many males who cannot understand this concept.
LOL, don't quite know where to go with this one.

I do have to laugh however. I bought a boat with a charger already in place. It provided two independent circuits of 5 amps (max). As is normal i guess, I have a big red battery selector switch and I was selecting A as my house battery, saving B for my starter battery.

I was running a electric ice box from walmart which draws 6 amps. Obvious (after thinking about it) my charge on the house battery was dropping. Once I moved the switch to A+B I was drawing current from BOTH batteries but more importantly I was drawing current from BOTH legs of the charger. Now my house battery voltage wasn't dropping any more.

Let me say that 5 amps per leg is pretty darned wimpy. The previous owner did not live aboard!!!
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Old 25-01-2016, 09:32   #10
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Re: How long will it take to charge my batteries?

Thank you for all your replies. They are very helpful. And the voltage table in the link is great too. I used to have one of these dual battery monitors in my 4X4. It was very handy to easily see the battery charge state. National Luna
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Old 25-01-2016, 10:08   #11
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Re: How long will it take to charge my batteries?

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Originally Posted by jwcolby54 View Post
Just look at it as a simple math problem.

280 AH used 1/2 way is 140 AH.

Your charger will charge at 25 AMPS / hour. So 140 / 25 = ~6 hours.
On a previous boat of mine, I used 2 separate (inexpensive, marine) chargers...one for each battery. Cut charge time in half. Which was important because I was stealing power from the marina and didn't want to get caught.
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Old 25-01-2016, 10:16   #12
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Re: How long will it take to charge my batteries?

The reply from Dennis G is exactly correct and a very important issue to consider.

To emphasize, the higher the state of charge of the battery, the slower more charge will go into the battery.

Think of it like trying to pump water into a rubber bladder. If the bladder is half empty then it is easy to pump in more water. As the bladder gets full then the water pressure in the bladder increases and it is harder to pump in more water to completely fill the bladder to capacity. Batteries work exactly like this.

Regarding voltage, you have to disconnect any charging source, like generators, shore power, solar panels, etc AND disconnect any loads pulling power from the battery THEN let the batteries sit for a few hours to get a perfectly accurate voltage reading. Measured under these conditions a completely charged, lead acid battery will be around 12.6-12.7V. If you are reading 13.5 V or so you are seeing either the charger voltage or the residual voltage from the charger on the lead plates that will give an inaccurate reading.

Measured under the same conditions a battery that is 50% charged will be +/- 12.0V.

Because of these issues, many cruisers that are not connected to a shore power for charging cycle their batteries between 50% and 80%. 50% minimum because less than that over time will damage the batteries. 80% maximum because it takes too long with a generator to get from 80% to 90% and just forget about going from 90% t0 100%.

This is where solar panels and wind generators will help.
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Old 25-01-2016, 10:31   #13
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Re: How long will it take to charge my batteries?

:-0) Zeehag's comment clearly doesn't apply to the young man from Ghent :-)!

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Old 25-01-2016, 10:33   #14
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Re: How long will it take to charge my batteries?

Best answer I can give you is to hook up to shorepower for a couple of days, after awhile the battery charger amps will hit an equilibrium, that will be when your batteries are fully charged, remember that number.
It is unlikely you will see that number often as you will get tired of running that generator all day long.

So most people end up doing as Skipmac says, cycling between 50% and 80%, and yes that is bad on the batteries and does shorten their life, but in the long run maybe buying a new bank from Sams club in the form of golf cart batteries every two years is cheaper than installing the systems necessary to get to 100% or close to it a few times a week, and the bank lasting say five years.
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Old 25-01-2016, 10:40   #15
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Re: How long will it take to charge my batteries?

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Originally Posted by AFKASAP View Post

I am a complete novice in the electrical department so any hint or tips are appreciated.
Battery acceptance:

Battery Acceptance by Stu Battery Acceptance

This comes from my

Electrical Systems 101 Electrical Systems 101

Read away.

Good luck.
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