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Old 01-12-2019, 03:07   #1
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Mastervolt AGM Absorption Voltage

Our bank of 510ah Lifeline AGMs had to be replaced and the best we could get in our location was Mastervolt. So we now have 675ah of Mastervolt AGM.

The boat has solor, wind, and Cristec chargers. I haven't looked at the wind or Cristec's yet but the solor chargers are set up with an absorption voltage of 14.4. I believe Mastervolt calls for 14.25 but I'm not sure.

Is this going to make a difference? Should I change it?
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Old 01-12-2019, 04:46   #2
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Re: Mastervolt AGM Absorption Voltage

First, learn for sure what the Mastervolt's want... for both bulk/absorption and float.

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Old 01-12-2019, 05:11   #3
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Re: Mastervolt AGM Absorption Voltage

Yes, Mastervolt recommend that their gel and AGM batteries be charged with a voltage of 14.25 V for 12 V systems.
I doubt that 14.4V would do any harm.

The absorption phase is followed by the float phase, in which the voltage is reduced to 13.8 V.
A rule of thumb, for gel and AGM batteries, states that the minimum charging current should be 15 to 25 % of the battery capacity. During charging, you usually continue to supply power to connected devices, and this power consumption should be added to the 15-25 %. The maximum charging current is 50 % for a gel battery, and 30 % for an AGM battery.
These figures assume a temperature of 25 C.
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Old 01-12-2019, 05:21   #4
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Re: Mastervolt AGM Absorption Voltage

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Originally Posted by GordMay View Post
Yes, Mastervolt gel and AGM batteries should be charged with a voltage of 14.25 V for 12 V systems.
The absorption phase is followed by the float phase, in which the voltage is reduced to 13.8 V.
A rule of thumb, for gel and AGM batteries, states that the minimum charging current should be 15 to 25 % of the battery capacity. During charging, you usually continue to supply power to connected devices, and this power consumption should be added to the 15-25 %. The maximum charging current is 50 % for a gel battery, and 30 % for an AGM battery.
These figures assume a temperature of 25 C.
Thanks.

I set up the two Victron mppt solar controllers to 14.25 and 13.8 volts but the shore/generator chargers use 14.4 as the AGM setting and it doesn't look like I can change them.

I also have a Victron Smart charger that I use when I'm at a dock with 230v (my boat is US spec) and it also is hard set to 14.4.

Are these chargers going to cause issues?
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Old 01-12-2019, 05:24   #5
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Re: Mastervolt AGM Absorption Voltage

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... Are these (14.4V) chargers going to cause issues?
I doubt it, but cannot be absolutely certain.
For instance, Rolls specifies a charge voltage of 14.7V @ 25C (77F) (lower temperature allows higher voltage).
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Old 01-12-2019, 07:52   #6
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Re: Mastervolt AGM Absorption Voltage

I would contact Mastervolt to be sure that 14.4 is without problems. My concern is that 14.4v is where gassing begins for lead acid batteries.
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Old 01-12-2019, 09:45   #7
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Re: Mastervolt AGM Absorption Voltage

Most vendors spec a voltage range.

Mastervolt gives a single-point spec only, of 14.25V.

I'd follow that if possible.

An example of the advantage of charge sources allowing user-custom setpoints.

https://files.mastervolt.com/images/...ristiek-nl.jpg
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Old 01-12-2019, 09:55   #8
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Re: Mastervolt AGM Absorption Voltage

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Originally Posted by GordMay View Post
(...)

A rule of thumb, for gel and AGM batteries, states that the minimum charging current should be 15 to 25 % of the battery capacity.
Addressing the: "Myth of a Minimum."

I've had two 100 AH Lifeline and two 100 AH Odyssey batteries in service for nine years. Underway, they are recharged by solar and a towed impeller generator that together produce a maximum of 20 amps (10% of capacity) and average 10 amps (5% capacity) during the day.

The batteries have well over 1,000 cycles, never below 50% state of charge. I capacity test each battery in the bank separately at least every 3 months at a 5 amp discharge rate expecting to see a resting voltage of 12.2 volts (50% state of charge) after 10 hours. I finally replaced the oldest Odyssey after nine years of service. It's capacity dropped to 90%. ("End of service life" is defined as 80% rated capacity, but I was impatient.)

The "minimum charge current" specification appeared to be a myth, based on my experience. So I called Odyssey Tech Support and challenged their specification. They told me that the spec was based on deep cycling use to a low state of charge, and that they wanted users to recharge completely discharged batteries promptly to prevent sulfation.

My experience is: if you are discharging to no less than 50% state of charge, any charge current that can recharge the battery to 100% state of charge in less than 24 hours is sufficient. For a 100 AH AGM, that's 3 amps (accounting for charge inefficiency). A PV array that's capable of producing 6 amps average of charge current (surplus above consumption current), based on a 12 hour production cycle, is adequate.

In sub-fractional storage battery operations (not deep cycle), the belief that there is a minimum charge current is a persistent myth. That myth has dissuaded some users who have only solar charging sources to believe AGMs can't receive the "minimum" charge rate, and therefore that solar can't be used with AGMs.

Don't worry. It'll work just fine. You may someday decide to replace your AGMs with lithium batteries that have a higher charge efficiency and complete immunity to sulfation. Because of the higher charge efficiency, that change will have the same effect as increasing your PV array output by 30-50%. My plan was to wait until my AGMs aged-out to do that, but it's taking a lot of patience on my part because they just won't die after nearly a decade in service.


I'm retasking my "old" Odyssey AGM to an uninterruptable power supply in my home. Here in Northern California, having backup power is becoming a necessity. This fall, I spent several extra days on my boat because I had no power at home. "Living off the grid while connected to the grid" is becoming a new normal here.
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Old 01-12-2019, 10:01   #9
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Re: Mastervolt AGM Absorption Voltage

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Originally Posted by john61ct View Post
Most vendors spec a voltage range.

Mastervolt gives a single-point spec only, of 14.25V.

I'd follow that if possible.

An example of the advantage of charge sources allowing user-custom setpoints.

https://files.mastervolt.com/images/...ristiek-nl.jpg
I've set the solar chargers to 14.25 and 13.8(for float) but I can't change the hard coded 14.4 AGM setting in the Cristec and Victron AC chargers.
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Old 01-12-2019, 10:06   #10
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Re: Mastervolt AGM Absorption Voltage

As stated "likely" no harm, especially critical is not too long holding Absorb,

but best to ask them.
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