My New Battery Bank Seems to Sag in Voltage

[Sunking]

Thanks so much all for this brilliant education. Your effort to explain this to a beginner is much appreciated. The Math work really helps a lot. Being a visual learner who learns thru asking questions, I can much more easily visualize the pro and cons of my little system now. Or more specifically , the Do's and Don'ts of what I have and it's limitations.

You are welcome.

[Matrix]

...So despite the battery being fully charged up, when you hit it with a 60 amp load, the voltage will sag to 24 volts. Your Inverter sees that and trips off-line from under voltage on a perfectly healthy fully charged battery...

So one more question if I may ... I am indeed be slipping below 24 volts on heavier loads for the duration of the larger load (usually 1 -10 minutes). My max Amp loads when heavy are about 50 amps. The battery is definitely sagging as noted in your math levels indicating the battery bank is too small. And as you noted, "the voltage will sag to 24 volts. Your Inverter sees that and trips off-line from under voltage on a perfectly healthy fully charged battery".

So with the battery being at about 20-25% discharged (not fully charged - the SG was reading +/- 80%) ... is it bad for the battery to sag say to 23.6v for 1-10 minutes while the heaviest load (the well pump) runs? When the pump cuts off the battery rebounds back to 24.4v no problem. I can set the LBCO in the inverter to 23.5v (or lower) with a 30 sec or more cut off delay so that the inverter will not kick off, but I don't want to do that if I am Killing the battery.

My Goal here is simply not to kill the battery bank (4 New Trojan L16H-AC 435Ah in Series 24v).

I am still debating if I want to purchase 4 more batteries ... OR just remove the well pump from the off-grid side. If I bought 4 more batteries I would remove my existing 4 from the system, and only run the newest 4 for the same amount of time that the older 4 (actually only 12 days old now) ran before connecting them all together in 2 series strings in parallel. My thinking there would be to not mix old and new batteries and give the new ones a chance to "catch up" in use to the old ones before paralleling them together.

Thanks again all for the help and patience.

[sensij]

I am still debating if I want to purchase 4 more batteries ... OR just remove the well pump from the off-grid side. If I bought 4 more batteries I would remove my existing 4 from the system, and only run the newest 4 for the same amount of time that the older 4 (actually only 12 days old now) ran before connecting them all together in 2 series strings in parallel. My thinking there would be to not mix old and new batteries and give the new ones a chance to "catch up" in use to the old ones before paralleling them together.

Thanks again all for the help and patience.

Is there a third option? It sounds like you have a big enough PV array to fully power the well pump "directly", without relying on energy discharged from the batteries except for startup surge. Do you have enough water storage that you could get by with running the well pump only in the middle of the day, when PV can supply most or all of the power?

[Matrix]

Is there a third option? It sounds like you have a big enough PV array to fully power the well pump "directly", without relying on energy discharged from the batteries except for startup surge. Do you have enough water storage that you could get by with running the well pump only in the middle of the day, when PV can supply most or all of the power?

Yes I actually do have a big enough Array and that is an excellent option. And may very well be what we do. Right now in full sun we produce 1.9 KWs and with everything running (except surge loads) including well and fridge we are using about 1800 watts. So That does indeed work. But as to storage, we only have a minimal storage. Enough to flush a few times or get drinking water as needed. It could work though, but not enough to shower more than once.

[Sunking]

So one more question if I may ... I am indeed be slipping below 24 volts on heavier loads for the duration of the larger load (usually 1 -10 minutes). My max Amp loads when heavy are about 50 amps.

So with the battery being at about 20-25% discharged (not fully charged - the SG was reading +/- 80%) ... is it bad for the battery to sag say to 23.6v for 1-10 minutes while the heaviest load (the well pump) runs?

My Goal here is simply not to kill the battery bank (4 New Trojan L16H-AC 435Ah in Series 24v).

No you cannot hurt the battery. Well you could IF you have loose connections on the battery term post in which case you could melt them off. However you just gave me a few pieces of info that is raising a Red Flag.

1. Trojan L16H-AC 435 AH
2. Max load 50-Amps
3. Rebound Voltage 24.4 volts

OK those are excellent batteries and can easily handle 50 amps no problem. You will get a little sag, but it should be 2% or less. 2% of 24.2 = .02 x 24.4 = .492, lets call it .5 volt sag. That would be roughly 23.9 volts and you are seeing 24 volts. No problem.

OK what is getting my attention is 24.4 volts. Why is it so low? Red Flag. If you are doing this during the day with bright sun you should be up around 29 volts in Bulk/Absorb mode, or if fully charged and the controller is in Float Mode I expect you to say about 27 volts. At sunset with a full charge, the battery should be up around 25.4 volts.

So why is the battery voltage so low? 24.4 volts resting is roughly 50 to 60% SOC. Something is not right. I am missing vitaL INFO.

[Matrix]

No you cannot hurt the battery. Well you could IF you have loose connections on the battery term post in which case you could melt them off. However you just gave me a few pieces of info that is raising a Red Flag.

1. Trojan L16H-AC 435 AH
2. Max load 50-Amps
3. Rebound Voltage 24.4 volts

OK those are excellent batteries and can easily handle 50 amps no problem. You will get a little sag, but it should be 2% or less. 2% of 24.2 = .02 x 24.4 = .492, lets call it .5 volt sag. That would be roughly 23.9 volts and you are seeing 24 volts. No problem.

OK what is getting my attention is 24.4 volts. Why is it so low? Red Flag. If you are doing this during the day with bright sun you should be up around 29 volts in Bulk/Absorb mode, or if fully charged and the controller is in Float Mode I expect you to say about 27 volts. At sunset with a full charge, the battery should be up around 25.4 volts.

So why is the battery voltage so low? 24.4 volts resting is roughly 50 to 60% SOC. Something is not right. I am missing vitaL INFO.

No this was not during charging ... this was at the end of the discharge this morning 7:30 am before the sun came up. But this is what has been bothering me all along. The Voltage reading of SOC does not seem to be correlating with what I am getting from the Hydro readings. I am reading 1.240 on the Hydro which Trojan says is 80% but my voltage is more like 60% at 24.4 ish. There is a small 200 watt or less load on them at that point so perhaps that is what is accounting for the lower voltage ... not truly resting.

I am just trying to not end up as a Noob Statistic killing my first set of batteries in a year.

[Sunking]

Are you using a Temperature Compensating Hydrometer?

In any event, the voltage sag is normal and judging from what you have said the Sag is within specs. So the Sag is a Non-Issue.1/2 volt Sag is what he should see with 50 amp load. No if the sun is out and the panels producing power, then the voltage sag will be very small and almost undetectable.

[Sunking]

There is a small 200 watt or less load on them at that point so perhaps that is what is accounting for the lower voltage ... not truly resting.

Just caught this. YES it will affect voltage. Remember:

Battery Voltage Under Load = OCV -(Ri x Amps)

OK at 50 amps you incur 0.5 volts Sag right? Well 200 watts pulls 9 amps. So that would account for .1 volts.

Again, Are you measuring directly on the Battery Term Post? Any other place is invalid.

[Matrix]

Are you using a Temperature Compensating Hydrometer?

In any event, the voltage sag is normal and judging from what you have said the Sag is within specs. So the Sag is a Non-Issue.1/2 volt Sag is what he should see with 50 amp load. No if the sun is out and the panels producing power, then the voltage sag will be very small and almost undetectable.

Well that is pretty good news then. I have ran the Fridge and Well Pump and lights and just lived life like I was pretty much on AC Line Power the past 2 days, and it seems from all indications (temp compensated Hydro, etc) the most I can draw the batteries down from solar charge to solar charge in a nite (about 11-12 hrs) is about 22% leaving 78% in the battery. I am recharging from the 1.9 (actual) Kw array in full sun in just over 4 hours before going int float. The Array produced 4.8 KWs today, So guess that is OK and things are pretty much doing what I want.

Still doing some tweaking with the Absorb volts and End Amp settings in the CC, But I think I am pretty fully charge. But I broke the float in the glass Hyrdro yesterday ... Grrr. I ordered a Midnite Solar Hydro Volt that should be here Saturday, so I can get back to testing.

[Matrix]

Just caught this. YES it will affect voltage. Remember:

Battery Voltage Under Load = OCV -(Ri x Amps)

OK at 50 amps you incur 0.5 volts Sag right? Well 200 watts pulls 9 amps. So that would account for .1 volts.

Again, Are you measuring directly on the Battery Term Post? Any other place is invalid.

Yes ... I feel pretty lame about that one. Calling it Resting with a 200 watt load is not resting at all. I am not thinking clearly.

And Yes I have tested from the posts. But They always seem to match up with what the CC voltage is reading.

[Matrix]

I probably worry too much. I have just read so much about the First battery bank being Sacrificial ... and I don't want to do that if possible. So if it means buying 4 more batteries, I want to do that now before the age of this first 4 gets to the point I cannot add more. I would think 14 days would not be to that point yet, especially if I run the new ones for a few week first before connecting the all together in parallel.

[Sunking]

And Yes I have tested from the posts. But They always seem to match up with what the CC voltage is reading.

Trust me, I know what I am doing.

[Matrix]

Trust me, I know what I am doing.

I do. Post tests it is. ;D

[Matrix]

So I'm thinking of going up in battery bank size from 435 Ah to 870 Ah. But I read a few threads on charging tonight that seem to indicate I MAY NOT have enough watts in the panels to charge in the C/8 - C/12. Right now I have a STC rated array that is 285 x 9 = 2565 watts. The NMOT rating is 214 x 9 = 1926 watts - which is what I am seeing on a sunny day in October.

24v (system voltage) x 72.5 (C/12 Charge rate on 870 Ah Battery) = 1740 watt Array Size ?? Is this the Correct formula to configure the needed Array Size?

But after reading some of SunKing's posts about charging etc, I am not certain this equation is the correct formula for figuring out Array Size. (Although that link is for a 48v system)

Do I have a large enough Array at 1.9 KWs to charge a larger battery bank of 870 Ah's at C/10 - C13 and at the same time provide about 200-300 watts during the day to the home? Or would I need to get a larger Array? Which would also mean getting a 2nd Classic 150 Charge Controller because I do not think a single 150 can handle a larger array than what I already have.