When To Charge?

Last week I installed new AGM house batteries and a battery monitor. The electrician that helped me (I suppose technically I actually helped him) recommended I recharge at 12v and 50%. Another person said 12.2v and about 50-60%.

Chapman's recommends 12.2v and 50%, which I'm inclined to follow.

Question - Do any of you true electrical gurus have a strong opinion one way or another? Or, are these numbers so close that it really doesn't matter?

This is the charging recommendation for my Lifeline AGM batteries. 50% SOC is a good number not to go below.


Battery Charging:

WARNING: All batteries must be adequately vented during charging to avoid accumulation of explosive hydrogen gasses. Never install or charge in a sealed container or room.

The following charging voltages are recommended for maximum battery life for all Lifeline models.

Charging Phase 12 Volt Battery
Bulk/Absorb 14.2v - 14.4v
Float 13.20v - 13.40v

Nsail said:

Last week I installed new AGM house batteries and a battery monitor. The electrician that helped me (I suppose technically I actually helped him) recommended I recharge at 12v and 50%. Another person said 12.2v and about 50-60%.

Chapman's recommends 12.2v and 50%, which I'm inclined to follow.

Question - Do any of you true electrical gurus have a strong opinion one way or another? Or, are these numbers so close that it really doesn't matter?

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For more years than I willing to admit been using a volt meter 12.2 volts

Maybe this will help

Al, can you povide similar data for 24-volt batteries? May I presume your data relates to "deep draw" (as in home) batteries, in contrast to "starters"?

The very best way to determine your battery state is to use a battery monitor. They cost less than $200 and count amp hours in and amp hours out.

markpierce said:

Al, can you povide similar data for 24-volt batteries? May I presume your data relates to "deep draw" (as in home) batteries, in contrast to "starters"?

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Just double the voltages for 24V systems. Different battery types have different voltage levels.



I agree with ksanders. A battery monitor is a great tool at a reasonable price. It's as easy to read as a fuel gauge.

My battery state was 99+% this afternoon. The week before it had dropped to 65% because I hadn't flipped the right switch while hooked up to shore power.

As to "when to charge" as soon as you can is always Da Book answer , although not realistic while cruising.

Most folks that do spend most of the time on the hook and less motoring that have an independiant style boat (no noisemaker for cooking) will use the SOC meter to decide when to charge.

As few will run a noisemaker for the time required for 100% charge , the usual cruiser will cycle between 85% down to 5o% .

This loss of useable capacity MUST be taken into account when sizing/installing the house batt set.

Battery life is based on age and duty cycles. The less depth of discharge the longer your batteries will live. AGM's discharged to 50%, should get ~370-500 duty cycles. Discharge to 25% and you should get ~925-1200 duty cycles.

Larry M said:

Battery life is based on age and duty cycles. The less depth of discharge the longer your batteries will live. AGM's discharged to 50%, should get ~370-500 duty cycles. Discharge to 25% and you should get ~925-1200 duty cycles.

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That's always been my understanding.

Thanks everyone! And, FlyWright, thanks for that chart. Guess I won't be using Chapman's recommendation very often.

And ksanders, I already installed a battery monitor per post #1

FF said:

As few will run a noisemaker for the time required for 100% charge , the usual cruiser will cycle between 85% down to 5o% .

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When hooked to shore power and with the circuits on the "charge" mode, the Coot's house battery capacity drops to 93% before recharging to 99+%. (Power is being used for the refrigerator and monitoring systems.)

Nsail, sorry...my post was for lead acid, not AGM. I just reread your original post.

Here's an AGM link that supports 12.2V as being an AGM 50% level. It appears that Chapman's is correct if they were referrring to AGM batteries.

Lifeline Batteries - Marine & RV Deep Cycle Batteries

Sorry I mislead with lead-acid data.

FlyWright, I was somewhat wondering if it was specific to lead acid only, but after I read LarryM's post about number of cycles, I came to the conclusion that it would still be probably best to follow your graph.

Do you agree?

What I have found is following the battery manufacturer's recommendations is best. I recharge on the hook long before I even get CLOSE to 12.1 (I have L-A batts). No harm done recharging your AGMs at 12.2V or even higher. Your battery monitor, if set up correctly for your banks, should give you accurate % levels based upon Amp-Hrs coming and going.

I find that I run the generator when it's most convenient...even if it's a bit early for the recharge schedule. If I'm sitting at 65% and am planning to leave for a few hours, I'll fire up the generator and and recharge. But that's just me. For the record, I'm using a Honda eu2000i, not a marine genset.

Nsail said:

Thanks everyone! And, FlyWright, thanks for that chart. Guess I won't be using Chapman's recommendation very often.

And ksanders, I already installed a battery monitor per post #1

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Something is not making sense here.

You have a battery monitor, that from your post tells you the percent of battery remaining.

Why even think about the voltage then? The voltage becomes irrelevant (except as a failsafe for the battery percentage)

The percent of battery remaining tells you when to recharge.

The "recharge on this voltage" tables presented indicate a typical AT REST VOLTAGE with NO LOAD APPLIED to the batteries for 2 hours. While this is interesting information it is not relevant to recharging since your batteries on a boat (at least not any of the boats I've owned) are never at rest with no load when disconnected from shore power.

Here's the table from a previous post, and it is about as misleading as it can get for real life use on a boat.




The part at the top of the "table" tells it all. to quote "READINGS TAKEN VIA VOLTMETER AFTER RESTING OFF CHARGE OR LOAD FOR 2 HOURS PLUS"

Batteries on a boat not plugged into shore power are not resting with no load in real life. Not ever. They are either being discharged by the normal loads on a boat or they are being charged. They do not sit with no load or charger.

So, based on your clairifing your first post you should ignore the voltage and recharge based on the percentage remaining that you feel most comfortable with, given that the less you discharge a cell the more discharge cycles you'll get from the bank before you start seeing it degrade.

When your bank gets near the end of life what you'll see is that the battery monitor percentage will not be jiving with your voltages. It'll be easily noticable. If for example your battery monitor says you have 50% charge left in your batteries but they are running at 10.5 volts under moderate loads thats a great indication that you'll need to replace your house bank soon.

My theory and practice is charge at every opportunity. Keeping the batteries topped up is a good thing. AGM 50% state of charge is 12.2 volts at rest. A battery under load will read less voltage. Your SOC meter reads amperage in and out, and keeps count with a few adjustment factors calculated. When it reads 50% if you have set it up correctly it is 50% SOC.

Why wait to get to that percentage to charge. Keeping your battery as close to full charge as possible is a good thing. It kinda goes with my philosophy of never passing up a chance to pee, and never trusting a fart.

ksanders and Moonstruck,

Thanks for your explanations. I only brought up voltage and percentage because that's what the 2 people I talked to brought up and Chapman's did as well.

Going by percentage makes sense to me.

Nsail said:

ksanders and Moonstruck,

Thanks for your explanations. I only brought up voltage and percentage because that's what the 2 people I talked to brought up and Chapman's did as well.

Going by percentage makes sense to me.

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I understand, and the nice person posting the voltage table is just taking information he found in good faith and sharing it.

Something to think about.

The state of charge meters utilize current readings from the shunt and apply them to a time base, just like a AC kilowatt hour meter.

In DC we express battery capacity in amp hours, but we coud also express it in Watt hours, or kilowatt hours. So many amps over so much time equals so many amp hours. Apply voltage to the equation and we get watt hours. Divide that by 1000 and we have kilowatt hours.

Moonstruck said:

It kinda goes with my philosophy of never passing up a chance to pee, and never trusting a fart.

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There's a third one, but I guess in some cases it no longer applies.

ksanders, I agree with you. If you have a batt monitor, the voltage is usually irrelevant and often misleading. On my boat at anchor, I have very little load on the batts throughout the night ...normally just a small GPS and an LED anchor light. For years, all I had to refer to was a voltmeter. Even now, my first action when getting out of bed is to compare house voltage with % SOC. If I saw a great divergence from the table (which I have printed and laminated at my lower helm), I'd know something is going south. My second action is to start the coffee on the inverter!!

I posted the chart to help answer his original question and give an idea of the % SOC voltages at rest. I missed his AGM comment and posted the lead acid voltages which are incorrect for AGM batts.

FlyWright said:

There's a third one, but I guess in some cases it no longer applies.

ksanders, I agree with you. If you have a batt monitor, the voltage is usually irrelevant and often misleading. On my boat at anchor, I have very little load on the batts throughout the night ...normally just a small GPS and an LED anchor light. For years, all I had to refer to was a voltmeter. Even now, my first action when getting out of bed is to compare house voltage with % SOC. If I saw a great divergence from the table (which I have printed and laminated at my lower helm), I'd know something is going south. My second action is to start the coffee on the inverter!!

I posted the chart to help answer his original question and give an idea of the % SOC voltages at rest. I missed his AGM comment and posted the lead acid voltages which are incorrect for AGM batts.

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You did wonderfully, you just posted what you'd been exposed to.

At most of my jobs over the years I tested high capacity battery banks in Telephone CO's and places like that as part of our Preventative Maintenance. I didn't do it every day, but did do it several times a year over a period of decades. The banks were 48 volt systems. We applied a steady load of generally 100 amps then read the voltages every x number of minutes creating plots of the system. What we found is that under load batteries very quickly (generally within 15 minutes or so) went to exactly 2.0 volts per cell, then stayed there for the duration of our test, which generally ran for an hour, sometimes two.

The big things we checked for were low voltage cells, and high intercell millivolt readings which pointed to high resistance intercell connections (which could lead to fires).