Battery Acceptance

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Stu Jackson

We've had some discussions about battery acceptance (search on "acceptance") in the past.  I thought it might be helpful to describe just what happens, step-by-step when charging after, say, a simple day sail.

The battery acceptance drops off the ability to recharge, to an incredibly low rate of charge as the bank gets fuller and nearest to full. Many times, now based on my Link 2000 experiences, it takes almost overnight to fully recharge a bank.

What goes OUT of your house bank becomes easy to learn from the Energy Budget previously discussed and posted. It's what goes back IN that most people are missing, and relates to the state of charge of your house bank in a very direct manner.

What I've seen, for example is this, from our Link 2000:

--- Return from a day sail, bank is down 15 amp hours, plug into shore power with our 75 amp charger, batteries will only ACCEPT 15 amps (360 AH house bank, they're pretty fully charged if only 15 ah down)

--- Most people will then assume that it will take an hour to replace the 15 amp hours at 15 amps of charge

--- NOT!

--- As the batteries absorb that 15 amp charge, they get fuller

--- The charge RATE goes down to 10 amps after 15 or 20 minutes and the bank is now down to 10 amp hours down

--- Now, it should only take another hour to charge those minus 10 amp hours at 10 amps -- wrong assumption again; we're now up to an hour and twenty minutes to fully charge, right?

--- NOT again, because this keeps recurring

That's why a full charge takes a LOT longer that most people expect.

What goes OUT is easy, what goes BACK takes MUCH, MUCH longer.  That last 15% to 20% takes a long time because of the REALITY of battery acceptance.

Please believe me, folks, because I see this all the time now that I have the Link 2000 installed and working.

Please note that this true with alternators as well as shorepower chargers.  Battery acceptance usually "rules" the charging regimen, so even with 100A alternators or shorepower chargers in excess of 40 A, the battery acceptance will determine the amount of charge that can go back into a house bank, usually around a max of 50A for a 400 ah bank.  And it "tapers off" as the bank gets fuller as noted above.

This is why people say the BEST investment for un-knowledgeable and knowledgeable skippers alike is to buy a battery monitor FIRST. Wish I did, because I'd been chronically UNDERCHARGING my house bank, even after all the spouting I do here and on other forums! :cry4` :shock: 8) :roll: :nail

Please, believe it!

For a parallel discussion of how much comes OUT, please see the Energy Budget topic, here:,3976.0.html


For everyone installing a battery monitor:  The "Gotcha Algorithm" thread, a "MUST READ",4922.0.html

DEFAULTS are factory settings that are made to be modified to suit your setup.
Stu Jackson, C34 IA Secretary, #224 1986, "Aquavite"  Cowichan Bay, BC  Maple Bay Marina  SR/FK, M25, Rocna 10 (22#) (NZ model)

"There is no problem so great that it can't be solved."