Question about my mppt app statistics

bgu1982

I have a question. On my Bluetooth app for my mppt controller ,for instance, the SOC says 75% but the battery voltage says 12.7v. I thought a reading of 12.7v for FLA batteries is a full charge. Sorry if this is a stupid question but I'm concerned because I don't want to go below %50 dod. I'm using chargepro 2.0 app. Other than that everything seems to be working great.

Photowhit

I guess I never reprinted my battery charging info, see if this helps;

The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

The morning voltage may reflect a load present that is effecting the voltage level. With sealed batteries, you would want to disconnect the battery from the system and allow it to 'rest' for a while to get an accurate idea of it's SOC (State Of Charge) from the voltage

bgu1982

Oh I think I see what your saying. At night time when there's no power coming in from the panels the batteries need to sit awhile with no load to get a accurate SOC reading. So what I'm seeing under load at night is more of an approximation.

Photowhit

bgu1982 said:

Oh I think I see what your saying. At night time when there's no power coming in from the panels the batteries need to sit awhile with no load to get a accurate SOC reading. So what I'm seeing under load at night is more of an approximation.

Yes, using the 'system voltage' is always iffy. You can get a feel for it, light loads won't change a lot on your system voltage, dumb guess say 1/100th of capacity (remember to add some for the inverter!) but loads getting close to the 1/20 capacity will change the system voltage and above that may be drastic.

I'm lazy and working this out for 6 - 8 volt 5 year old batteries that I've replaced my 12 year old forklift battery with, hoping I can see reduce price on LiFePo4 batteries that are UL1741 for home use as competition increases. I paid about 60% more than scrap for them but I knew the previous owner and they were well maintained and lightly cycled. They are also Trojan which is a premium battery.

I've not yet done a commissioning equalizing, it's been too hot. I'll check the SG and record after I do. So, understand roughly what I've pulled from the battery bank and knowing the rough capacity if new (170 ah x 2 strings for 340 ah at 24 volts or about 8,000 watt hours). I can rough out my energy use, about 1000 watt hours for my 20 year old fridge and around 2000 watt hours from my air conditioner running 3-4 hours as and another couple hours through the night. Add another 500 watt hours for laptop, lights, fan and inverter use. I should be around 50% or just above.

So, checking the battery in the morning before I get effective charging in the morning and finding the battery at 24.6 I was quite happy. representing about a 70% full battery with loads equaling the charging both minimal <100 watts. The batteries are warm, this is damaging but also adds some capacity. It reminded me to move my battery temperature sensor over the new/used battery bank.

It's certainly a touchy-feely way to figure battery capacity.  Your charge controller might be more or less accurate... It's using voltage too! and information you have given it. Unless you have a Midnite Classic with a WhizBang Jr to measure input and output of amps into the battery bank, I think Schneider has a similar system and Outback reads the use of everything and input to system shouldn't be as accurate. They guys who designed the Outback system abandoned it when they made the Midnite classic.

Clear as mud?