Too-fast AGM Drawdown?

[DenverGuy]

I know very little about solar, so if I am asking some dumb stuff here, that's why.

I have a 12V system that was installed 10 months ago for a detached garage. I have a Windy Nation P30L charge controller, two 100Ah AGM batteries, one 100W PV, and a 1500W PSW inverter. So if I draw down to 12.2 volts (50%), I should have 1200 watt hours. (I am adding two more batteries and three 100W PVs next weekend).

When the system is fully charged and I plug in a 390 watt appliance, I can watch the voltage drop. In 30 seconds it's down to 12.2 with the PV generating 2-3 amps, and then it (battery voltage) recovered and bobbled between 12.2 and 12.3A.

That seems pretty quick. I thought with 1200 watt hours I could run it for 3 hours! In addition, the battery health indicator goes from 5 windows being filled it to 4. (See photo. I'm not sure what the relationship is between battery voltage and battery health).

Before I bought the second battery last year I accidentally discharged the first one below 12 volts. I hope I didn't wreck it.
When I added a second battery I didn't notice any change in performance.
It is set for balanced charging.

Some questions:
1. Is that quick drawdown normal. and will it stabilize at 12.2V?
2. Did I damage a battery by letting it drop that one time?
3. What is 50%? Is that 12.2?
4. Low-voltage shutdown is set to 10.7. Isn't that way too low?
5. Is it safe to drop to 50%, or should it be only top 60% (12.3 - 12.4V)?

Thanks.

 

[time2roll]

What is the charging voltage? How long does the controller hold absorption?

I would expect amps charging should taper to 1 amp at maybe 14.40 volts to declare 100% charged. Easy to undercharge.

 

[DenverGuy]

What is the charging voltage? How long does the controller hold absorption?

I would expect amps charging should taper to 1 amp at maybe 14.40 volts to declare 100% charged. Easy to undercharge.

I have to say I don't understand what you're asking. I have never heard of controller absorption. It charges at 12V. When the batteries are fully charged the controller reads 14.2 volts.

 

[time2roll]

Absorption is the time held at 14.2 volts. Normally 1 to 4 hours. Maybe more. This time should extend until current tapers down to 1 amp.
14.2 seems low.

 

[DenverGuy]

It is at 14.2 and 1 amp. Does that change anything? Lots of sun on the PV right now.

 

[time2roll]

Yes if getting down to 1 amp indicates very close to full charge.
If the voltage is adjustable I would go 14.4 to 14.6 volts depending on specification.

Discharging at 390 watts is fine except this is faster than the rating. 100AH is usually at the 20 hour rate or 5 amps per battery. Even with 4 batteries the draw would need to be under 20 amps to get the rating. The 390w is probably closer to 35-40 amps.

At the 10 hour rate may lose 15%, At the 5 hour rate may lose 30%.

 

[Bud Martin]

BTW, if you have 390W load on the AC output of the inverter, the power on the input side of the inverter will be about 390W/0.85 = 459W draw from your batteries due to conversion loss.

 

[sunshine_eggo]

12.2V is only 50% when the batteries have been completely at rest with no charge or discharge for at least a couple hours.

Get a battery monitor that can actually compute SoC.

 

[DenverGuy]

Can anyone recommend a device that show accurate information about the state of charge, PV output, etc., besides the
display on the controller? Do is a battery monitor for each battery the way to go? Or is there a wall-mounted unit that does it all?

 

[sunshine_eggo]

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

 

[DenverGuy]

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

Thanks. I am looking at the Bayite. Do you need one for each battery? Ideally I could find a device for all four batteries that I can
monitor via Bluetooth by phone. Is there such a thing?

 

[mikefitz]

The Bayite will, not be as accurate as the Victron as there is no way to set charge efficiency and Peukerts value. Also the Bayite shunt is not very robust. The Victron smart shunt communicates via Bluetooth and phone app.

A single shunt is connected between the battery bank negative, the parallel connection of the all the batteries negative posts, and all other negative wires.
The monitor displays the status of the battery bank.

I guess you are undercharged the AGM batteries.

There is some discussions of battery ratings. The most common specification is the 20 hour rate. A 100 Ah battery will deliver 5 amps for 20 hours. The discharge rate is not linear, Peukerts law, the more current the the less the effective battery capacity.

With a single charged 100Ah AGM battery consider the maximum short term load of 50 amps and a constant load of 20 amps.

Ideally you need to replace the simple low cost PWM controller. It's not possible to set a correct charge profile with such a unit. AGM batteries will not give a long service life unless charged correctly.
You need a MPPT solar charge controller. This will have preset charge voltages for AGM and user settings.
In addition a MPPT controller will convert all the panel power to useful charge power, expect upto 30% more solar power into the batteries.

your questions,
1)the volt drop as the battery discharges is normal, the higher the load current the greater the drop.
2)no
3)50% discharges will be around 12v or just over, depends of battery age and how well its looked after. Under load the voltage will be lower, see chart.
4)yes, 11.5 volts is better as a lower limit.
5)50% is a useful compromise between using power and battery life. In general the less taken from the battery the longer the life, see graph.


Mike

 

[DenverGuy]

Thanks, Mike. I ordered a Epever 30A MPPT unit. That will work for my four 100Ah batteries.

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