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Old 05-09-2015, 10:12   #16
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Re: AGM Battery Sudden Drop

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Originally Posted by Stu Jackson View Post
The repeated comments about not using the battery switch when the engine is running are meaningless UNLESS you know how it is wired. If the alternator output (AO) goes to the C post of the battery switch, then it is prudent to not move the switch. If the AO goes to the house bank, you can turn the damned switch off when the engine is running and there will be no harm because the AO is connected to a battery.
Sure, you need to know how it is wired. However:

My alternator, and everything else on the boat except the bilge pump, is wired to my Blue Seas 1-Both-2-Off switch. For 17 years and 6,500 engine hours, I've been switching batteries with the engine running with no problem.

Since it is "make before break", as is just about any reasonably modern battery switch, you can switch between 1, Both, and 2 without offending the alternator diodes - so long as you don't go through Off.

Some switches, such as this one



are designed such that this is easy to do without accidentally going through Off.

Others, such as this one



make it riskier, as it's relatively easy to flip all the way to Off when you're intending to go from 2 or All to 1.
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Old 05-09-2015, 11:23   #17
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Re: AGM Battery Sudden Drop

Wolfgang-
You will need to see the spec for your batteries. Some starting batteries are ruined by as few as 4-6 total discharges. Most deep discharge batteries would be rated to provide more like 50-100 or more total discharges. Provided they were in all other ways healthy, and varying with how long they were left discharged.
If yours were and are otherwise healthy, two complete discharges should not require you to worry overmuch.
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Old 05-09-2015, 13:54   #18
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Re: AGM Battery Sudden Drop

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Originally Posted by Snore View Post
Also the SOC readings on the SmartGauge are inconsistent with the voltages. Any thoughts?

Thanks for the anticipated assist.
The Smartgauge works very well wben the battery is discharging, but not that well when it is charging, eg when the panel outweighs the loads.


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Old 05-09-2015, 14:04   #19
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Re: AGM Battery life?

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Originally Posted by WolfgangSeaLife View Post
I have 4 Deka, 3 years old, previously 4 Lifeline, each lasted 4 years.
Both 1100 CCA at 0 F or 1595 at 68 F, with 210 Ah each. That should total 840 Ah. When sailing I use 120 Ah a day, so I figured to 50 % I could sail for 3 days without recharging.
Your math is based on two assumptions that are flawed.
First, the capacity of your batteries is not 840Ah anymore. Only a capacity test will tell you the capacity of those batteries, but given the way you maintain them you can expect them to be shot. Post 13 is spot on. Partial State of Charge Cycling kills AGMs.

Second, you do not start at 100 per cent. Trust Maine Sail when he says that you cannot trust the "synchronization" formula in your battery monitor unless you tweak it quite a bit. For Lifeline AGMs you know they are at 100 per cent when, while held at 14.3 V at the terminals, they only take 0.005 amps per Ah of capacity. That would be about 4 amps for your bank.


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Old 05-09-2015, 14:25   #20
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Re: AGM Battery life?

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Originally Posted by WolfgangSeaLife View Post
After a day to full charge to 13.4 V the charge monitor said "synchronizing" when I stopped.
If the "synchronization" fomula in the BATTERY MONITOR(which guesses when the batteries are full) kicked in at 13.4V then it is configured incorrectly. There are two settings: voltage and current (as % of capacity). Voltage should be equal to absorption voltage as per the Deka manual ( over 14 volts), not the wimpy default value of 13 volts and a bit, which is useless. Current should be as stated in the Deka manual, which will be close to Lifeline's 0.5% rather than the 4% default in many monitors.
Another story is that your AC-powerded CHARGER knows nothing about all this and it is most likely that it switches to float not when the batteries are full (see above) but based on a very unsmart algorithm that cut off to soon when the battery bank is large as in your case.

Therefore in krder 100% once or twice a week as required for AGMs you may nee

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