Charging time and charger output - SailNet Community
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Charging time and charger output

It would appear that battery charging times are a function of the square root of the ratio of the charger output. In other words doubling the output does not cut charging time in half. It only cuts the time by multiplying by 0.707 or the inverse of the square root of two. If you want to cut charging time in half you have to increase your charger output by four times. I'm sure that I am not the first to notice this. Is my arithmetic correct? Thanks. Steve S.
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Old 03-03-2013
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Re: Charging time and charger output

RE charge time doubling or halving. I have never heard this before . Interesting - where did you come across this?

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Re: Charging time and charger output

I was looking at the Noco Genius charger website at various chargers. They list the time it takes to charge a battery. A charger with twice the output does not cut the charging time in half. It appears to be a function of the square root of the charger ratios. Thanks. Steve S.
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Re: Charging time and charger output

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Originally Posted by sstuller View Post
It would appear that battery charging times are a function of the square root of the ratio of the charger output. In other words doubling the output does not cut charging time in half. It only cuts the time by multiplying by 0.707 or the inverse of the square root of two. If you want to cut charging time in half you have to increase your charger output by four times. I'm sure that I am not the first to notice this. Is my arithmetic correct? Thanks. Steve S.
It does not really work like that with lead acid batteries. Lead acid batteries can only take/accept so much current so at a point, even in bulk, you hit a ceiling of accepted charge current. Some AGM batteries such as Odyssey can take an in-rush of 3C or 3X Ah capacity. This in-rush however does not last very long and the accepted current quickly diminishes. AGM batteries can take roughly 30-50% of "C" in charge current during "bulk" with initial short in-rush periods. Deep cycle flooded LA batteries can take about 25% in bulk mode and GEL about 25% - 40%....

These acceptance rates are only in bulk mode before the bank has reached the absorption voltage limit. Once you hit absorption voltage the current the battery accepts declines rapidly. This is why most cruisers cycle banks between 50% and 80-85% SOC. Trying to charge much above 80-85% SOC, due to acceptance limiting, is often a huge waste of energy unless you are doing it with solar or wind.

Once lead acid batteries hit about 80% SOC they come up to "absorption" voltage. This is with a charger properly sized for the bank to be in the 10-40% of "C" range depending upon chemistry. Once at absorption voltage the accepted current quickly diminishes and the time it takes to "finish" charging has nothing at all to do with the size of the charger provide it was sized correctly to begin with.. The last 20% can take as little as 5 hours and as long as 15+ hours depending upon the battery being charged..

The only way to truly "charge faster" and quickly to 100% SOC is to buy a technology that does not "limit acceptance" which means one of the lithium technologies.

Today, the fastest charging lead acid batteries today are Odyssey AGM but they are grossly expensive, about \$400.00 per 100Ah... Lifeline is probably the second fastest, then other AGM's like Deka or Full River etc.. After that comes GEL and the slowest to charge are deep cycle flooded LA batts..

It should be noted that Odyssey wants to see a charge source sized to a MINIMUM of 40% of "C" and Lifeline a minimum of 20% of "C"....

I would strongly urge you to look at true, programmable heavy duty marine chargers if you want the fastest charge times possible. Be wary of companies like NOCO that claim "smart" but really are far from that...

The claims on the NOCO site of charging a 25Ah battery in 1.6 hours are totally misleading. What is "charging"? Is that back to 100% SOC, as most would assume? In bulk that battery only takes about 7.5Ah's between 50% and 80% SOC. Once that 7.5Ah has been replaced the bank will be at about 80% SOC and begin declining accepted current rapidly for the remaining 6 or so Ah's that need to go back in. It can take a loooooong time to reach 100% SOC when your battery is only accepting less than 1% of "C" or about 0.25Ah per hour towards the upper 90's....... Just getting the last 5% into the battery can take 5+ hours many, many hours......

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Last edited by Maine Sail; 03-03-2013 at 11:48 AM.
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Re: Charging time and charger output

As usual, MaineSail has it exactly right!

The information cited from the NOCO site is totally wrong.

Example:

- you have a flooded golf-cart battery bank totaling 450AH (4 T-105s in series/parallel).

- this bank is at 50% state-of-charge

- your 120 amp alternator with external smart regulator is putting out 112 amps @ 14.8 volts (about 25% of the AH capacity of the house battery bank)

- you wave your magic wand and switch to an alternator having a 1,000 amp capacity

- this huge alternator is now putting out 112 amps @ 14.8 volts....same as the smaller one

Why?

Because, as Maine said and anyone who's experimented a bit with charging voltages and currents can tell you, the batteries will only accept so much charging current at a given voltage and state-of-charge.

It doesn't matter a whit how large your charging capacity is....the batteries will themselves limit the charging current.

It takes hours...sometimes many hours....to reach a full charge.

And, in this example, they won't reach full charge a minute faster with a 1,000 amp alternator than they will with a 120 amp alternator, given the same charging voltages.

Bill
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Re: Charging time and charger output

I don't see a conflict here. From Noco's website their 3.6 amp charger will take 17.5 hours to fully charge a 100 AH 12v battery @ 50% DOD. Their 7.2 amp charger takes 12.5 hours. The charger with twice the output only reduces the charging time by .707 times 17.5 hours to ~ 12.5 hours (.707 is the inverse of the square root of two). So Noco is agreeing with you by saying there is a practical limit to increasing charger output. I'm just trying to quantify that limit. Thanks. Steve S.

Last edited by sstuller; 03-03-2013 at 06:16 PM. Reason: Bad English
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Old 03-03-2013
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Re: Charging time and charger output

Here's one of the commandments on SN:

Thou shalt not argue with Maine Sail or btrayfors on matters marine electrical!

From both posts above it should be clear the batteries will not allow the math you cite to rule....

Ron

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".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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Re: Charging time and charger output

Maine Sail + btrayors. On related question
? Is it better to run the high electrical users ( frig/AC etc.) when the high charger ( high output alternator) is running or does it impact on battery life if recharge is via the sun panels and D400s.
Generally turn things off and check house battery bank time to time if using high utiliers . If it falls too much leave things off until it comes back but have no clear idea what's too much.

s/v Hippocampus
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Re: Charging time and charger output

realized I expressed myself poorly
?Assuming you start and stop at the same levels of discharge- does it matter how rapidly you discharge and charge the batteries as regards life of the batteries?

s/v Hippocampus
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Last edited by outbound; 03-03-2013 at 07:32 PM. Reason: spelling
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Re: Charging time and charger output

Good question.

I don't have any objectively verifiable evidence, but from "feeling" and from comments made by battery manufacturers who have collaborated in some of my research I believe there is reason to believe that charging the batteries as fast as they will accept current is good for them.

This may or may not be due to internal bubbling which could help avoid stratification of the electrolyte and, possibly, speed up the process of dislodging loose PbSO4 deposits on the plates.

Maybe MaineSail has a view on this.

Bill
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