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Old 05-05-2024, 07:58   #1
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AGM battery seasonal storage

I am storing my boat on the hard for 6 months over hurricane season in Martinique. I have four 140w solar panels connected to four Victron MPPT 75/15 controllers, and 4 Mastervolt 8D AGM 225 AH batteries routed through a Magnum MS2812 Inverter/Charger. I am still trying to determine the optimum storage practice for this 12v system.

Six months appears to be too long to leave the batteries unattended in 30 degree heat. In the past I just left the system unchanged which I think resulted in overcharging. Absorption charge for these batteries is listed as 14.1v and float charge 13.1v at these temps.

The Victron controllers go to absorption charge each morning which I think overcharged the batteries. Would it be reasonable to unplug all but one panel and set its controller absorption and float charge to 13.1v? That would have the batteries constantly on a float charge. I have the boat checked and aired out every 2 weeks.

All suggestions appreciated.
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Old 05-05-2024, 08:05   #2
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Re: AGM battery seasonal storage

When sitting is there ANY load on the batteries at all?

Do not get too clever with the voltage settings to compensate for the temperature, the Victron controllers should be already adjusting that if they were configured properly, and were correctly located so they experience a temperature similar to the batteries.
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Old 05-05-2024, 08:20   #3
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Re: AGM battery seasonal storage

When sitting there would be no load. The temperature compensation is only 0.1v so not really an issue.
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Old 10-05-2024, 17:01   #4
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Re: AGM battery seasonal storage

Is it OK to keep an AGM battery on float for 6 months?
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Old 10-05-2024, 17:06   #5
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Re: AGM battery seasonal storage

Why not just bring them up to full charge and then completely disconnect them? This is how I store my AGMs over the winter here.
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Old 10-05-2024, 17:12   #6
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Re: AGM battery seasonal storage

If you're ending absorb based on time instead of amps, either turn the absorb voltage down to match float or shorten the absorb time as much as possible.
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Old 10-05-2024, 18:58   #7
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Re: AGM battery seasonal storage

Quote:
Originally Posted by zippy View Post
I am storing my boat on the hard for 6 months over hurricane season in Martinique. I have four 140w solar panels connected to four Victron MPPT 75/15 controllers, and 4 Mastervolt 8D AGM 225 AH batteries routed through a Magnum MS2812 Inverter/Charger. I am still trying to determine the optimum storage practice for this 12v system.

Six months appears to be too long to leave the batteries unattended in 30 degree heat. In the past I just left the system unchanged which I think resulted in overcharging. Absorption charge for these batteries is listed as 14.1v and float charge 13.1v at these temps.

The Victron controllers go to absorption charge each morning which I think overcharged the batteries. Would it be reasonable to unplug all but one panel and set its controller absorption and float charge to 13.1v? That would have the batteries constantly on a float charge. I have the boat checked and aired out every 2 weeks.

All suggestions appreciated.
1. Follow the manual (as best as you can)
2. Mastervolt manual suggests storage self discharge is 3% per month at 20C and recommends recharging intervals of 6 months (20 - 30 C) or 3 months (30 - 40 C). Google tells me the average hurricane season temp will be in the high twenties C. (daily air temp between 25 & 31 and sea temp 29). This puts you close to needing a recharge every 6 months rather than every 3 months.
3. Manual suggests a float voltage of 13.8 at 25C, temperature compensated –30mV/oC.
4. Solar only charges during daylight so it is impossible to float 24/7.

I think you only have 2 choices.

A. leave batteries fully charged and then completely disconnected for 6 months
B. Program Victron controllers for absolute minimum absorb time and a float of 13.7V.

Me - I'd do B.


Is there any parasitic drain via the Magnum MS2812 Inverter/Charger?
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Old 13-05-2024, 07:43   #8
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Re: AGM battery seasonal storage

Is there any parasitic drain via the Magnum MS2812 Inverter/Charger?

I power off the inverter off during the season so no draw from there. The only input to the batteries will be solar. There should be no parasitic draw. The temperatures are at the max for leaving disconnected for 6 months so I thought that the best solution is to lower the absorption level to match the float level and leave it on. The question then is to leave all for 140 watt panels on or unplug 3 and leave one on.
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Old 18-05-2024, 07:54   #9
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Re: AGM battery seasonal storage

If the voltage is set to 13.7 v on the MMPT solar controllers to not overcharge the AGM battery, does the battery only accept as many amps as it requires? One active panel would make ~10 amps available while 4 panels would make ~40 amps available. Does it make a difference to a fully charged battery?
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Old 18-05-2024, 15:23   #10
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Re: AGM battery seasonal storage

Quote:
Originally Posted by zippy View Post
If the voltage is set to 13.7 v on the MMPT solar controllers to not overcharge the AGM battery, does the battery only accept as many amps as it requires? One active panel would make ~10 amps available while 4 panels would make ~40 amps available. Does it make a difference to a fully charged battery?
Yes.
No.

You are correct in saying the battery will only accept as many amps as it's chemistry allows. So if the float current is say 1.5 amps, then it makes no difference if you have a supply of 10 or 40 or 100 amps, the float current remains at 1.5 amps.

Disconnecting several panels is only doing unnecessary work.
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