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Old 11-06-2011, 18:44   #1
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AGM Battery Bank Charging Options

AGM battery bank charging options
I seeking advice on charging options for my AGM house batteries.

I am a liveaboard and my boat has a 534Ah battery bank consisting of six new Trojan No.27/89amp AGM batteries. I'm confused by the technical charging options offered by Trojan and seek a simplified recommendation.

My current option is to keep my 240VAC isolation transformer permanently connected to shore power when in the Marina. The isolation trasnsformer puts out 12VDC to a VictronEnergy combined inverter/battery charger unit which in turn then places the battery bank on permanent trickle charge of 0.9Ah at +/- 13.97VDC.

Question: Am I on the right track and is this the preferred option??

Alternatively would it be preferable to disconnect shore power completely and run power requirements solely from the battery bank (240VAC power being provided by the inverter). When the battery bank capacity falls to 80%, only then reconnect to shore power for the time necessary to recharge the battery bank until the capacity is restored to 100%.

Thoughts from the forum are appreciated.

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Old 11-06-2011, 20:24   #2
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Re: AGM battery bank charging options

First of all your isolation transformer does not put out 12 VDC. Your battery charger uses AC from the isolation transformer, converts it to DC to keep the batteries charged.

It wont hurt AGM batteries to keep them fully charged.

Your main concern will be keeping them charged when you are away from the dock.

I haven't checked the voltage that Trojan specifies so I'm not sure that what you have quoted is the correct float voltage, but keeping them fully charged wont hurt them.

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Old 12-06-2011, 03:49   #3
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Re: AGM battery bank charging options

Thank you DeepFrz. I've checked the data sheet for the isolation transformer and you're absolutely correct. The in-put and out-put voltages are the same. In this case 240VAC.

Status update. Battery bank capacity is currently registering 100%. The battery charger has switched to "float" mode is registering a steady trickle charge of 0.1A at 13.15Vdc.
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Old 13-06-2011, 13:00   #4
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Re: AGM Battery Bank Charging Options

With the information you have given us, I agree with DeepFrz that you should remain plugged in to shore power and keep the batteries fully charged. Repeatedly discharging/charging them will needlessly shorten their life.

What you will probably observe at dock when the refrig, lights, pumps etc. come on is that the Victron will increase its current output to offset the demand, thus serving as DC power supply.

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Old 17-06-2011, 14:18   #5
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Re: AGM Battery Bank Charging Options

If you are at the dock, stay on shore power.

One aspect of how long a battery will last is how many times it's been discharged. Using it when shore power is available makes no sense.
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Old 21-06-2011, 14:15   #6
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Re: AGM Battery Bank Charging Options


Trojan recommends limiting voltage to 2.35 to 2.40V per cell @ 25C/77F during the absorption phase, so that would be 14.1-14.4Vdc. Current should be limited to C20/5 (534S/5), or about 107A max, which should taper off to C20/200 (534A/200), or 2.67A, at which point the bank is fully charged and the voltage should then float at 13.5V.

Your Victron charger should either have a preset very close to this, or custom setting that you can enter. Floating at almost 14V is very high and could damage the bank. The charger should also have a temperature probe, which should be placed on a + terminal near the middle of the bank, not one of the ends...
Cap'n Jon (KB1HTW)
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"If it's gonna happen, it's gonna happen out there." - Captain Ron
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Old 21-06-2011, 16:04   #7
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Re: AGM Battery Bank Charging Options


I have six group 31 AGM batteries, but in my case they are for propulsion purposes (connected in series, as my system is 72V). I charge them with a QuickCharge MB1210X6 (manufacturer's website: HOME), which allows for each battery to be charged independently. It has settings for two AGM charging profiles (plus several other profiles for other type of batteries). After consulting with the manufacturer which setting to use, the bank has been working flawlessly for over a year.

There is plenty of information available online about charging multi-battery banks and the effect of the different loads they are subject to due to the resistance imposed by the wiring (I assume you have the six batteries in parallel and the charger would charge them all concurrently).

In my system (again, series), when the batteries were new, they all reached absorption and float mode at the same time. One year later the charger shows that the “more positive” batteries reach each stage faster than the “more negative” ones (it happens in order, 1 to 6, and with the same timing difference between contiguous batteries, which I read as all batteries being OK).

As expected, the “more negative” batteries are working harder, as they are “fighting” the resistance of the ones closer to the positive end. This charging time difference is of a couple of minutes at the most, so for the moment I will not make any changes, but I think that in the near future I will switch batteries 1 and 6 (the most positive with the most negative), and later 2 and 5.

With this, my suggestion is to check if there are significant differences in wiring lengths between your six batteries and keep an eye on the charge condition of the less and most “burdened” batteries, it may be worth switch their position in a couple of years.

BTW, I keep the charger always connected with the batteries receiving float charge all the time (same with the house bank).


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