Gradual battery drain from charge controller?

[K1ngN0thing]

At 100%, my AGM battery reads a voltage of 13.7. Once the sun's down, it begins to drop, and falls to around 13.2 before the sun's back up. My I've disconnected everything from the battery apart from the charge controller, which is an MPPT Tracer1210RN.

 

[WheelEstate USA]

I'd say a 12V battery that holds a 13.2 over night is doing just fine. The voltage always drops some after sundown just because no solar is being fed into it.
I can't explain the math and all that but sounds to me like you have a healthy situation.

 

[jimindenver]

The charge controller should use a tiny amount of energy. What you are seeing is the charge voltage vs the resting voltage. At some point in time the battery will even self discharge slowly. Luckily with AGM's, it is very slowly.

 

[SternWake]

Your solar controller has the AGM at 13.7v float voltage.  This does not mean it is 100% charged, only that the solar controller has decided to hold it at 13.7v.

100% charged on an AGM can ONLY be determined when the battery can only accept 0.5% of its capacity at the manufacturer recommended absorption voltage, at 77F.



A fully charged, fully rested healthy AGM can be anywhere from 13.23 volts to 12.80 volts.  A Very wide range, depending on the manufacturer.

A healthy  fully charged AGM can hold surface voltage for a LONG time,  A lifeline GPL-31XT I had for a friends project maintained 13.16 over a week after a full recharge, AND i had drawn 6AH from it in that time.  Impressive

My Northstar AGM holds 13.06v for weeks disconnected from all loads

If the AGM sees little or no loading, expect it to hold voltages in the mid thirteens .

All lead Acid batteries self discharge, and at a higher rate at higher temperarues.

AGMs have much less self dischargew as flooded batteries
Healthy batteries have much less self discharge than older used batteries

Some AGMS have much less self discharge than other AGM batteries

Voltage is not like the gas gauge in your vehicle.  It does not relate directly to state of charge, unless on a well rested battery, and not always then either.

Voltage is a trend.  Voltage can be used to get a general Idea of the state of charge of the battery, but it CANNOT definitively ascertain the health of, nor state of charge on a battery in use. It can only indicate a general range that a human can decipher

Voltage is electrical pressure.  The battery can only hold so much pressure on its own.  Charging sources must have higher pressure to cause electrical flow into the battery.  Absorption voltage is the maximum pressure allowed in all but rare occassions.  When the charging source is trying to bring a battery to 14.4 to 14.8v, this is referred to as bulk stage, or bulk charging.  It takes time at a  large enough pressure difference for the battery to achieve absorption voltage.  From then on the amount of amperage to maintain Absorption voltage decreases.

When the amperage neded to maintain a single 100AH AGM falls to 0.5 amps at 14.4v, only then can it be considered fully charged.

Do not hope your solar controller is just automatically going to meet these threshholds

Do not think a solar controller dropping to float voltage indicates the battery is fully charged.

Blinking green lights are mocking you, unless you program those blinking green lights to flash only when the specific battery's parameters have been met for the level of discharge it had endured.

 

[WheelEstate USA]

Thanks SternWake... I knew this one would draw you in
We were quite impressed with your system when we were there and learned some very helpful hints from you.
I know now that we don't need as much solar to have a fridge as we first thought as long as its done right.
Thanks again.

BtW...next time I won't hand you that 6th beer
Did you get that surfboard fixed??
(Yes, everyone, there is an inside joke there)

 

[SternWake]

Thanks SternWake... I knew this one would draw you in
We were quite impressed with your system when we were there and learned some very helpful hints from you.
I know now that we don't need as much solar to have a fridge as we first thought as long as its done right.
Thanks again.

BtW...next time I won't hand you that 6th beer
Did you get that surfboard fixed??
(Yes, everyone, there is an inside joke there)

Yep 5 beers and I can still surf OK, but add that 6th on a ledging sandbar and the success rate plummets.

I've not yet fixed that board.  i did strip the wax off for a full inspection.  It has a few areas which need repair.  

[font=Verdana, Arial, sans-serif]If the AGM sees little or no loading, expect it to hold voltages in the mid thirteens .[/font]

[font=Verdana, Arial, sans-serif]Since I cannot edit this, it should read that if the agm sees very little or no loading( discharging) after an  actual full charge, then one can expect it to retain voltages in the low to mid 13's for perhaps a week.  This is still surface charge.  AGMS really like to hold on to theirs.[/font]


[font=Verdana, Arial, sans-serif]This feature does not instill the AGM with superbattery status, it is just a characteristic of them.  The overall energy they can store is the same as an equal capacity wet/flooded battery.[/font]

[font=Verdana, Arial, sans-serif]AGMS when recharged properly and promptly each discharge can be a super battery, but if they are not, then they cost twice as much for half the cycle life of a true deep cycle battery like trojan t-105's, or the same lifespan as a 12v marine battery.[/font]



[font=Verdana, Arial, sans-serif]Surface charge fools many people into believing their battery is at a higher state of charge than it actually is, Especially with alternator recharging.[/font]

[font=Verdana, Arial, sans-serif]http://www.pbase.com/mainecruising/battery_state_of_charge[/font][/SIZE]

 

[K1ngN0thing]

Thanks, that cleared things up.