Unexplained Voltage drop in my system under no load, need some advice.

[Calsa]

Hi Everyone, I just got my system up and running two days ago so I have been monitoring things rather closely. At night I am noticing the following problem. My Battery is over the course of 3 hours dropping almost an entire volt from 13.6v+ down to right now about 12.8v. However, absolutely nothing is plugged in or on. In fact I disconnected my power inverter from my battery this morning wondering if the inverter was drawing voltage when turned off. I have been monitoring my voltages tonight and it had the exact same voltage drop with the inverter still disconnected. Can someone explain whats going on?

My Setup:
1x 48volt 250watt solar panel into an Tristar MPPT 60 Charge Controller down to 1x 12 volt 170 (200) amp hour AGM battery, then through a 250amp fuse to a 2000watt Xantrex Puresinewave Inverter. Wiring is all 4AWG Out of the inverter through the rest of the system.

I know my inverter is way overkill right now, I plan on upgrading batts and panels in the future.

Questions?
1) Any ideas as to what I am seeing?
2) Does the Tristar MPPT serve as basically blocking diode to prevent back current draw from the battery to the panels? If not is the loss I am seeing the panels drawing power @ night?
3) What can I do to limit / stop this drop as that is a massive drop for a battery to take every night over a few hour period with no load in the system.

Thanks for any help or advice you can offer.

[axis11]

My Battery is over the course of 3 hours dropping almost an entire volt from 13.6v+ down to right now about 12.8v.

12.73 is 100% SOC.The higher voltage you see is a surface charge. If your voltage drops below 12.6 without load, start worrying.

[russ]

Like Axis alluded to - the battery has to sit for a few hours before you can really check the voltage.

[Sunking]

. My Battery is over the course of 3 hours dropping almost an entire volt from 13.6v+ down to right now about 12.8v. However, absolutely nothing is plugged in or on.

Nothing is wrong, absolutely what should happen. 12.8 volts on a rested battery indicates 100% charged up.

[Calsa]

Thanks, good to know

Thats a relief, I spent most of the night debating what to try and isolate next. Thanks for the help. Out of curiosity, at what voltage should I consider my battery empty and stop using it to prevent long term damage from over drainage?

[axis11]

Do not wait till the battery is empty. Voltage wont tell you much. You need a good hygrometer to test the SG of the electrolyte for accurate SOC. Or just limit your use to 20% of battery capacity. In your case, thats 40AH or 480 watts. You can go as low as 50% occasionally or even lower but it will shorten the life of the battery.

[Calsa]

Any way of building a Hydrometer or any other way of testing things on the cheap (yes I know solar is expensive to begin with). I am volunteering in Madagascar right now and getting the C.C. and inverter here was a nightmare, Not to mention took forever to save up for on the $200 a month living stippend I get for being here. I hate to see how much a hydrometer will cost.

Right now I have a 65 watt hour laptop I am powering and that is about it, so even with loosing power due to the 12v -> 120volt(inverter) -> 18 volt conversion (laptop power brick) I should have 8 hours of use a night. Is this math correct?

One days worth of usage (my panel is overkill for one battery so I am assuming a full charge every day)

12x170 = 2040 total battery watts

2040 * 0.3 (It is not good to go lower than %30 full charge) = 816

816 * 0.7 (inverter loss) = 571 usable hours

571/65 (peak usage of laptop power brick) = just over 8 hours

Does this make sense?

Edit: Also my battery is AGM how would a hydrometer hook up as there is no liquid electrolyte to test?

[russ]

No way to build one but they are cheap - one of the few things in solar that is cheap.

Sunking has often provided a catalog page showing one he says is quite good for the application.

[Sunking]

Edit: Also my battery is AGM how would a hydrometer hook up as there is no liquid electrolyte to test?

Do not under any circumstances go below 12.3 volts or 50% DOD. With the stated load you mentioned of 60 watts x 8 hours = 480 Watt hours will require a 12 volt 200 AH battery.

[Calsa]

Do not under any circumstances go below 12.3 volts or 50% DOD. With the stated load you mentioned of 60 watts x 8 hours = 480 Watt hours will require a 12 volt 200 AH battery.

Ok, thanks for confirming my math and the heads up.

Out of curiosity is there a way to hook up a hydrometer to an AGM or is there any other tool that functions in the same way when the electrolyte is suspended and therefore not liquid? Is monitoring voltage the best bet when dealing with AGM? I have a decent voltage readout on my inverter and can change the low voltage alarm setting on my Charge Controller to go off at 12.35 or their about, but if there is a better way to do it I would be willing to save up and implement it. I need this system to last a while.

[Sunking]

Out of curiosity is there a way to hook up a hydrometer to an AGM or is there any other tool that functions in the same way when the electrolyte is suspended and therefore not liquid?

Nope just a good DVM to keep track of voltage, or a very expensive Mho DLRO Meter aka Conductance meter to measure the internal resistance. Those will set you back some $5000

[Calsa]

Nope just a good DVM to keep track of voltage, or a very expensive Mho DLRO Meter aka Conductance meter to measure the internal resistance. Those will set you back some $5000

Yup, not doing that one on a Peace Corps living stipend. I'll just stick to monitoring voltages.

By the way thanks for all the help, you always seem to be around when I have questions. It is much appreciated.

Thanks to everyone else too!

[Sunking]

You are welcome.

[inetdog]

Nope just a good DVM to keep track of voltage, or a very expensive Mho DLRO Meter aka Conductance meter to measure the internal resistance. Those will set you back some $5000

For someone with the electronics hobbyist knowledge to put together a reasonable AC current source, in conjunction with a sensitive AC voltmeter, that would seem to be an inexpensive, but not "off the shelf" way to determine internal resistance. The AC internal resistance (at a fairly low frequency) should be a good enough guide to the DC internal resistance, and would not require high power equipment.
The battery would have to be disconnected from CC and inverter for this test.

[Sunking]

A DLRO or Battery conductance meter is a 4-Point Bridge type of meter. Two points inject 10 amps of DC current, the other two points measure the voltage. The puter then uses that to read out resistance. One can be MacGyver made with a constant current charger, volt meter, and calculator. Think about it for a moment and it will come to you. Keep it to yourself. Not DIY friendly.