setting low voltage cutoff point

[Jim Burrow]

I have got a question or puzzle if you will. I bought two 12v 220ah AGM (lead-acid) batteries yesterday because I am a poor boy and can’t afford to go Lithium at this time. – The batteries are connected in series for 24v

My 2424lv (MTTP controller/inverter) option 29 (low voltage cutoff) is set to 21.0v, but in option 05, I have AGM battery type selected.

Here is the issue, running a test load of 900 watts, my 2424lv controller reached 22.3v or and of course the buzzer goes off as it is getting close to my battery cutoff voltage point.

But at the same time as the MTTP (2424lv) display is reading 22.3v, the voltage at the battery posts is reading 23.9 volts. – I do not have a cheap meter!

With respect to my house, the most it would draw in the evening is around 500 watts to 600 watts. The 900 watts above is just for testing and what I happen to have available at the time.

So, it seems to me that I should lower my battery low voltage cutoff point to the minimum for my controller which is 20.0 volts.

The question is, to set the low voltage cutoff point, should I go by the voltage reading at the battery posts which is showing (23.9v) or what the MTTP controller is reading of 22.3v? Hence, lower the cutoff point down to the minimum of 20.0v – note that there are no datasheets for these batteries, I looked all over the internet.

I may have to change option #5 to User-defined (presently set to AGM) and control the battery voltage levels manually.

What’s everyone’s opinion on this issue? Note PV basically no power due to no sun and the Utility to the MTTP is turned off for this testing.

 

[Jim Burrow]

during my testing, As I reduced load power required, I noticed that the battery post voltage and the MTTP controller battery voltage readout became more in line. After several hours of battery discharge at 4 amps both the controller battery voltage reading and the voltage reading at the battery post were the same. - 22.1 volts and drawing 5 amps.

I ran across an article on low cutoff voltage for lead-acid, AGM batteries that basically answered my question above.

in the attachment, I attached this article as it might be of interest to others as they set up their solar system. - for us newbies.

 

[pollenface]

I would set to 12v, as regularly going below 12v will shorten the life of the AGM.

 

[Hedges]

I would set to 12v, as regularly going below 12v will shorten the life of the AGM.

According to this charge, 12.0V could be 65% DoD at 120 hour rate or 35% DoD at 1 hour rate.

 

[HRTKD]

It sounds like there is a significant voltage drop between your 2424lv and the battery. What size wires and what lengths are we talking about here?

 

[Jim Burrow]

It sounds like there is a significant voltage drop between your 2424lv and the battery. What size wires and what lengths are we talking about here?

The difference in voltage was under a 900-watt load. Under lighter loads like 200 watts, the voltage was about the same. I'm using a car battery jumper cable of around 000 AWG and is about 6 feet long. but to the controller is a 1 foot 6 AWG wire for both positive and negative connections.

Right now I'm charging the batteries (2-12v in series for 24v) at 13 amps and the voltages at the MTTP battery input and the battery post voltage are the same.

I just set up my step utility to help charge the batteries when the solar panels aren't producing like with no sun available. So with no real PV due to it, raining outside, and the battery is charging at 13 amps, the Utility is only drawing 2.6 amps. So it looks like I can pump up the amperage so the battery can charge at a higher rate. But will need to monitor the wire heat when I do that.

These batteries are Marin batteries rated at 220ah lead-acid AGM. The true capacity at the operating power of the house throughout the day (when I connect it to the house) is unknown. More testing is required.

I don't want to tear into the Utility power lines and install my system until I know the system will handle the loads and charge the batteries without a 24 hour day.

 

[HRTKD]

With the caveat that I'm no Electrical Engineer, my understanding is that the more amps you throw at something, the more likely you are to see a voltage drop if the infrastructure is insufficient.

 

[Jim Burrow]

With the caveat that I'm no Electrical Engineer, my understanding is that the more amps you throw at something, the more likely you are to see a voltage drop if the infrastructure is insufficient.

very true. My experience with Lead-acid car batteries is that with a couple of 100 watts, the battery post voltage will drop a good 1 volt from the get-go even with a fully charged battery. So when I tried to draw a load of 900 watts, the battery voltage dropped down close to the low-voltage cut-off point set in my MTTP controller. So that was my concern about where to set the low voltage cut-off point.

The other thing I don't know is how long have these two batteries been setting on the store shelves and if they were fully charged. The batteries are being charged now via utility and solar. I do have a battery charger, but don't think it charges much more than 8 to 10 amps. Plus I what to see how long it takes my mttp to charge the batteries at 13 amps.

I don't plan on installing the system into the house power until this May, so just playing around testing one thing or another.