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Old 03-04-2019, 08:26   #1
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How to calculate charge rate?

How do I calculate the length of time it should take to charge a 12 volt battery bank with a battery charger connected to 110 shore power, based on: Size in amps of battery bank, charger amps output, and % of battery bank discharge?

Is it as simple as... % of battery bank discharge MULTIPLIED BY total battery bank amps DIVIDED BY amp output of charger?

e.g. Battery Bank Total Amps = 500 amps, 50% Discharged, 50 Amp Charger Output. So... 500 amps X 50% = 250 amps ÷ 50 amp charger, would take 5 hours to get back to 100% SOC. Is that correct?
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Old 03-04-2019, 08:41   #2
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Re: How to calculate charge rate?

That isn't correct because as you get closer and closer to being charged the battery acceptance rate will decrease. In your example it probably would take 4-5 hours to get fully charged once the batteries were at 90%. So it would probably take your 50-amp charger about 5 hours to put in the first 200 amp-hours and another 4-5 hours to put in the last 50 amp-hours. So that's 9-10 hours or so.

and it's amp-hours not amps
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Old 03-04-2019, 09:27   #3
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Re: How to calculate charge rate?

Battery Acceptance by Stu Battery Acceptance
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Old 03-04-2019, 09:36   #4
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Re: How to calculate charge rate?

My rule of thumb is 7hr from 50% for a reasonably sized charger. For an oversized charger you could shave maybe 1/2hr off. You can up the voltage to shave this down more but seriously compromise the total life of the battery.

Using your example above the charger will put out its max rate of 50amps for 2-2.5hr. This is called bulk charging. Charging voltage will rise slowly from 12.0v to something around 14v.
At the point that the battery reaches the absorption voltage then the charger will start decreasing amperage to keep the charging voltage from exceeding target absorption voltage. Target voltage depends on whether you have FLA, AGM or Gel batteries.

Different chargers use different means of deciding when the battery is at 100%. Some you a simple timer that starts when the battery gets to absorption voltage. Some chargers use charging amperage to decide when 100% is reached. I believe battery makers usually indicate that when charging amps gets to 0.5-1.0% of capacity (C). Using your example above when charging current drops to 2.5-5.0amps then the battery is pretty much full.

When a charger decides by whatever means that the battery is full then it usually switches to a float voltage of about 13.5v. This counteracts the self discharge of all lead-acid batteries including Gel and AGM.

The exact voltages for absorption and float depend on which type of battery you have. In setting up a charger you will need to read the manual to figure out how to set it for the battery type you are using. Don’t mix battery types.
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Old 03-04-2019, 09:41   #5
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Re: How to calculate charge rate?

If you are anchored out and use a combination of engine alternator/generator and solar then you want to use the engine or generator first thing in the morning when the battery will accept the higher output of that source.
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Old 03-04-2019, 13:14   #6
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Re: How to calculate charge rate?

No matter how many amps the charger, with lead batts figure 5+ hours, often 7+, if it starts over 20-30% depleted, and if your goal is to get to 100% Full.

Which should be reached at least a few cycles per week for bank longevity.
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Old 03-04-2019, 14:34   #7
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Re: How to calculate charge rate?

Quote:
Originally Posted by BozSail View Post
How do I calculate the length of time it should take to charge a 12 volt battery bank with a battery charger connected to 110 shore power, based on: Size in amps of battery bank, charger amps output, and % of battery bank discharge?

Is it as simple as... % of battery bank discharge MULTIPLIED BY total battery bank amps DIVIDED BY amp output of charger?

e.g. Battery Bank Total Amps = 500 amps, 50% Discharged, 50 Amp Charger Output. So... 500 amps X 50% = 250 amps ÷ 50 amp charger, would take 5 hours to get back to 100% SOC. Is that correct?
a major variable it specific battery chemistry.

The example you gave would be real close for lifepo4 batteties
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Old 04-04-2019, 19:27   #8
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Re: How to calculate charge rate?

50a charger is small for a 500ah battery bank. probably like 7-8 from 50%. it also depends what is running. because anything running takes away from the amps that can charge. IE if you have 10a of lights and fridge on, now you only have 40a to charge.
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