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Old 08-08-2014, 08:30   #1
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State of Charge

So, being told that we should not drop our state of charge below 50% (and trojan actually recommends 80% for best long life) I am wondering under what conditions that voltage is measured.

For example, when our fridge is running our voltage drops significantly but recovers significantly when it stops running. Does that 50% rule apply during the amp draw-down or without the amp draw-down?

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Old 08-08-2014, 08:55   #2
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Re: State of Charge

Measure with things off. The deeper the battery cycles (ie: lower voltage) the more material the plates give off with each draw down/recharge. It's the nature of the beast. So over time the plates deteriorate. Deep cycle batteries are really just thicker plate batteries so they last longer with "deep cycling". Most my cruising boats were regularly deep cycled, often to the 40% range. Batteries seem to last about 5-6 years that way. That's enough for me.
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Old 08-08-2014, 09:14   #3
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Re: State of Charge

I do know that you only get a real reading if the batteries have been untouched for 24 hours or so but that is not feasible on a boat with refrigeration.

Per your advice, I'll use the reading when the fridge is not on, which seems to be more accurate than otherwise.
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Old 08-08-2014, 09:27   #4
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Re: State of Charge

Yeah, you don't need to wait to check for this type of thing. No big deal. The waiting is more important if you have just charged in my experience.
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Old 08-08-2014, 09:51   #5
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Re: State of Charge

Yes. If you are trying to determine the state of charge of a battery by voltage (generally a very inexact method) it should be done with all loads switched off.

Agree also that sitting to measure voltage is more critical after charging. You don't have to let them sit a long time while using the batteries before measuring voltage but a few minutes doesn't hurt.
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Old 08-08-2014, 10:03   #6
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Re: State of Charge

Hey, that was my 1000th post - which is just as boring as my 1st post!
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Old 08-08-2014, 10:11   #7
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Re: State of Charge

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Hey, that was my 1000th post - which is just as boring as my 1st post!
I think there is a lot to be said for consistency.
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Old 08-08-2014, 14:32   #8
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Re: State of Charge

I check battery soc with a digital voltmeter. Very late at night when the solar panel isn't charging I turn off the fridge and wait ten minutes. Nothing else is running at that late hour. This has worked for me for ten years; no fancy amp usage measuring devices. Last set of trojan t105s lasted six years. Current set of sams club golf carts are over three years old and still strong.
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Old 08-08-2014, 14:42   #9
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Re: State of Charge

Mainesail said he was impressed with the Balmar SmartGauge but it seems to me that it would suffer from the same issue...
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Old 09-08-2014, 00:16   #10
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Re: State of Charge30-30-30 Rule

THE ONLY VIABLE way to check true state of charge
is after a 24 hr rest period, discharge the Surface charge, after 30 seconds disconnect all parallel connections, then measure voltage with a calibrated DVM, then draw a SPG on each cell, compare the Voltage, to the SPG.

For a working bank...I use the 30-30-30 Rule.

Discharge at 30% of the 20 hr rate for thirty seconds, then wait 30 seconds. Then wait 30 seconds, measure the voltage with a calibrated DVM.

This is probably as close as you need for a bank working between 50% SOC, and 80% SOC.

This test is only accurate for a healthy bat bank, operating in the 70-80 F range.

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I check battery soc with a digital voltmeter. Very late at night when the solar panel isn't charging I turn off the fridge and wait ten minutes. Nothing else is running at that late hour. This has worked for me for ten years; no fancy amp usage measuring devices. Last set of trojan t105s lasted six years. Current set of sams club golf carts are over three years old and still strong.
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Old 09-08-2014, 09:23   #11
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Re: State of Charge

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Originally Posted by onestepcsy37 View Post
I check battery soc with a digital voltmeter. Very late at night when the solar panel isn't charging I turn off the fridge and wait ten minutes. Nothing else is running at that late hour. This has worked for me for ten years; no fancy amp usage measuring devices. Last set of trojan t105s lasted six years. Current set of sams club golf carts are over three years old and still strong.
Yep, this isn't rocket science if you are just determining how much or when to charge daily. All the complicated formulas, procedures and test equiptment will not recharge your batteries.
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Old 09-08-2014, 10:16   #12
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Re: State of Charge

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Yep, this isn't rocket science if you are just determining how much or when to charge daily. All the complicated formulas, procedures and test equiptment will not recharge your batteries.
Yes, you can get carried away with all kinds of fancy instruments but I've found that I can sometimes "feel" that the batteries need charging by how bright the interior lights are at night. When we're anchored in the same place for awhile I also know when to recharge because our daily draw is pretty much the same day to day.
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Old 11-08-2014, 23:51   #13
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Re: State of Charge

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Mainesail said he was impressed with the Balmar SmartGauge but it seems to me that it would suffer from the same issue...
It will remain accurate, more so as it learns your system. Amp counting monitors - Xantrex, Victron, etc - will lose accuracy over time as the battery capacity diminishes.
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Old 12-08-2014, 03:49   #14
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Re: State of Charge

[QUOTE=onestepcsy37;1602377] I've found that I can sometimes "feel" that the batteries need charging by how bright the interior lights are at night. ./QUOTE]

Yeah, if the lights are dimming it is probably time to recharge the batteries

other than that I haven't noticed a difference in my interior lights brightness between 14.4V under charge and 11.5V when running the coffee maker
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Old 12-08-2014, 04:07   #15
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Re: State of Charge

Nobody else is wondering why the fridge makes a significant draw down on the battery voltage?
Fridge is 5 or so amps?
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