Voltage drop under load

I’ve got 2- 12v 115AH FLA batteries in parallel (12V 230AH) connected to a 3000W inverter.
Just performed first test, and these are the results:
-1200W microwave operating for 30 seconds
-battery voltage prior to test = 12.84v
-voltage dropped to a low of 11.93v during test.
-once microwave stopped, voltage read 12.72v
-5 mins later, voltage stabilized at 12.91v

my question is in relation to the low reading of 11.93v during discharge. How does this low voltage reading relate to the %depth of discharge? I’ve read starter batteries drop to 9-10v during starting.
Will recommends a low voltage cutoff of 11.5v as the setting on my all-in-one (separate system) even though a battery shouldn’t be discharged below ~12.1v, and I guess now, I see why, as I’ve been confused about that.
Where can I find a good read regarding safe levels of voltage drops under load, in relation to depth of discharge?

it just means your battery is perhaps a bit small or some wires are too thin.
12V x 230V is less than 3000W, so you inverter is able to pull an heavy load on your batteries.
the fact you get the issue while pulling only 1200w would show there is a problem with cabling.
For the wires, it is easy to check , they get hot.
if you do not experiment the inverter or BMS (if present) to trigger because low voltage, all is fine.
if you want to double check, just put a third battery in parallel and check again.

Liam M said:

I’ve got 2- 12v 115AH FLA batteries in parallel (12V 230AH) connected to a 3000W inverter.
Just performed first test, and these are the results:
-1200W microwave operating for 30 seconds
-battery voltage prior to test = 12.84v
-voltage dropped to a low of 11.93v during test.
-once microwave stopped, voltage read 12.72v
-5 mins later, voltage stabilized at 12.91v

my question is in relation to the low reading of 11.93v during discharge. How does this low voltage reading relate to the %depth of discharge? I’ve read starter batteries drop to 9-10v during starting.
Will recommends a low voltage cutoff of 11.5v as the setting on my all-in-one (separate system) even though a battery shouldn’t be discharged below ~12.1v, and I guess now, I see why, as I’ve been confused about that.
Where can I find a good read regarding safe levels of voltage drops under load, in relation to depth of discharge?

Click to expand...

The voltage you see when a battery is under (heavy) load has no relation to the voltage vs state of charge curve. In the latter, the voltage refers to the "resting" voltage - no charging or discharging for a few hours. It is perfectly normal/acceptable for the battery voltage to sag by 0.5 to 0.7 V under heavy load. As you observed, a good battery will recover in voltage when allowed to rest. The fact a battery voltage recovers is why applying Peukert's correction is not very useful (you didn't ask this question, but I wanted to use this opportunity to say it).

To properly manage your batteries, you need a good, reliable battery monitor, which measures the charge flow in/out of the batteries and calculates the remaining capacity.

Thanks.
#2awg 1 ft cables connect the batteries in parallel.
#2/0awg 3 ft cables connect to inverter from battery bank.
Any problems there?

So that I may understand it then, increased capacity would lessen the voltage drop?

In this particular instance, (and for other situations) what would be considered a safe voltage drop under load?

there is no "safe" value. It is just it should not avoid you to use the system by triggering a low level alarm or break.
for example you could try running the microwave for 1 or 2 minutes and see if the drop increase.

fat_old_sun said:

The voltage you see when a battery is under (heavy) load has no relation to the voltage vs state of charge curve. In the latter, the voltage refers to the "resting" voltage - no charging or discharging for a few hours. It is perfectly normal/acceptable for the battery voltage to sag by 0.5 to 0.7 V under heavy load. As you observed, a good battery will recover in voltage when allowed to rest. The fact a battery voltage recovers is why applying Peukert's correction is not very useful (you didn't ask this question, but I wanted to use this opportunity to say it).

To properly manage your batteries, you need a good, reliable battery monitor, which measures the charge flow in/out of the batteries and calculates the remaining capacity.

Click to expand...

Thanks.
I have a battery monitor coming today, just performing some tests using a voltmeter.
Not familiar with Peukert’s law, but thanks for the reference, I’ll read up on it. What other battery info should I dive into?

nosys70 said:

there is no "safe" value. It is just it should not avoid you to use the system by triggering a low level alarm or break.
for example you could try running the microwave for 1 or 2 minutes and see if the drop increase.

Click to expand...

I checked the inverter specs for a low voltage cutoff number, haven’t found it yet, will keep looking.
I’ve seen mentioned before, a “sag” with lead acid batteries under load, I take it this is what is meant by that?
What I mean by a safe low voltage cutoff is, is there a “standard” setting for fla batteries?
The other time this has come into question for me, Will recommends a low voltage cutoff for my all in one at 11.5v.
My lack of understanding regarding this, resulted in me setting the low voltage cutoff at ~12v to prevent the lower voltage during discharge, but is the reason for his 11.5v setting, due to this “sag”, and is not representative of the actual depth of discharge?

FLA chemistry is 2V, and is very sensitive to deep discharge, so a cell should not be allowed to go under 1.9V.
but it can go high when charging, (about 2.4V)
but this is not when under load, so if a few seconds after load is cut your battery recovers to 2.0V it is ok.
for a 12V battery you multiply all numbers by 6.
so 2V cell x6 is 12V and is considered as flat.
max charge is 2.4Vx 6=14.4V
1.9x6= 11.4V, you need to to an emergency charge asap

Liam M said:

Thanks.
...What other battery info should I dive into?

Click to expand...

Spend some time on battery university, lead acid related wikipedia articles, etc and learn the basics of electro-chemical batteries. Since you have flooded cell type batteries, get a hydrometer & learn how to 'safely' measure the sp. gravity of the electrolyte (tip: mix the electrolyte well in each cell & then take the sample for measurement).

fat_old_sun said:

Spend some time on battery university, lead acid related wikipedia articles, etc and learn the basics of electro-chemical batteries. Since you have flooded cell type batteries, get a hydrometer & learn how to 'safely' measure the sp. gravity of the electrolyte (tip: mix the electrolyte well in each cell & then take the sample for measurement).

Click to expand...

Thanks, I just found battery university earlier today, looks like a wealth of info I can dive into, as well as the others you've mentioned. I've only had lead-acid as car starting batteries before, and never paid them much attention. I'm looking forward to doing the maintenance on them, as part of the learning process, and have a goal of trying to make them last as long as possible.
The flooded batteries I have, I bought the 1st initially, as a portable power source for a 12v fridge I bought for camping. That went well, just threw a 100W panel on it every 2 days to charge it. I bought a 2nd along with a 3000W inverter, thinking I could run an electric kettle for hot water in the morning, and maybe a few other things. For years I've used a camping percolator and propane for my coffee. I realized this trip, how horribly inefficient that is lol.
I've got another 2 battery 12v bank of AGM 100AH on an MPP Solar all in one. The AGM is certainly an improvement in technology, although I've read flooded cells have some advantages as well.
I'm building as I learn, which means buying some of the wrong things, and spending too much on products, at times. I'm starting small, so that as I learn more, I can expand and dial things in, properly.
I'm pretty sure I've learned also, that in expanding, I will transition to lithium, but enjoying what I have thus far.

Very simple problem. You need more battery. My microwave uses 150 amps on high. 12 volts through a 2000W inverter. I would not do that with less than 4 GC2 batteries. 460 ah with 12 volts.

Someone said your wires might be to thin, if your voltage reading at the battery differs from your voltage reading at the inverter, yes you need to look at your wire size. This is a good test for line loses.