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post #4 of Old 03-03-2013
Maine Sail
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Re: Charging time and charger output

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......

-Maine Sail / CS-36T

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Last edited by Maine Sail; 03-03-2013 at 10:48 AM.
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