trouble shooting Assesing State of Battery help?

[Wycowboy]

Hi all,
I’m new here and just feeling my way around and moving to upgrade to a DIY lithium battery build.
Anyhow currently have a 48 V system with 8 6 v lead acid 200 ah batteries. We installed the system a little over three years ago and I really noticing some performance drop. We have done some testing and with pretty much everything is shut down at night except for a few timers on some appliances we lose about 7% charge our system usually shuts off with our batteries reading right around 68% we have been through our charge controller parameters and they all seem to be set fine we have been pretty careful and it really never run our batteries past 50% I was wondering what other tests I might be able to run for the batteries or are they just getting too old. When they were installed the solar company we used suggested we would get 6 to 8 years out of batteries. This seems drastically short are batteries also never read 100% state of charge anymore. We would love to get another one or two years out of this before I switched li-on but that doesn’t seem likely I truly appreciate if anyone has any ideas of tests we can use to diagnose problems or ways to get some more performance. Thank you so very much
J

 

[snoobler]

Where is the % number coming from?

FLA or AGM?

Absorption voltage?

 

[MisterSandals]

When they were installed the solar company we used suggested we would get 6 to 8 years out of batteries.

This sounds reasonable for AGM batteries. Any details on the batteries?

If not AGM, have you checked the fluid levels? (and maintained that regularly?)

 

[Hedges]

Check voltages - entire bank, each 6V battery. If individual 2V cells are accessible, check them.
Perhaps it hasn't been reaching correct full charge periodically.
Perhaps some cells are out of balance, and equalization would help.


How deeply have you normally cycled them each day? That affects the number of years lifespan.

 

[Wycowboy]

Hi everybody, thanks so much for responding! Very much appreciated.

The percent numbers are coming from the state of charge in the battery management settings on our midnight controller remote.

I can’t remember what the absorption voltage is right now and I’m not at the house, since we have very poor Internet there and I’m traveling dealing with some family health emergencies But I reviewed it with a technician a few months ago and seem to be set appropriately

The batteries are sealed, pretty sure they’re AMG. So can’t check the fluid levels and shouldn’t need to.

also don’t think I can check each cell individually

so 6 to 8 years for AMG sounds pretty reasonable?

I checked the voltage of each battery individually. And they were all within 1/10 of a blot of each other again can’t remember the exact numbers but I think it was appropriate for the cells does 6.9 6.8 sound right?

Ran equalizing cycle about two months ago didn’t seem to change anything.

we really try not to cycle too deeply every day if we have sun, the lowest we get usually is high 80s by morning, but when it is cloudy, which is very rare for us, we can get into the 70s, when we get down to 75 or lower we usually switch to generator power or just shut everything off since the whole system usually shuts down at 68% or so
really appreciate all your thoughts and help!
Jeff

 

[Hedges]

If battery voltages are similar, no equalization needed.
Possibly equalization settings are same voltage as absorption (mine are) and don't actually equalize. If required you'd have to drive voltage higher.
6.8 x 2 = 13.6V, sounds good for float. Voltage will be higher during absorption, lower during discharge.

6 to 8 years life is reasonable for shallow discharge around 20% or 25%.
1.5 years if 100% DoD.

DoD vs. SoC is just measuring from opposite ends.
You say system shuts down at 68% SoC, 32% DoD?
Most nights only gets down to high 80's, say 87% SoC (13% DoD)?
That should give long life. Occasional deeper is OK.

Have a manual for your model of battery?
I suggest changing shut-off setting from 68% SoC to 30% SoC. That way you use the battery more heavily if you have a cloudy day, keep running longer. (more or less depending on what manual says)
If you have a generator it could kick in at some depth of discharge, reducing demand on battery.

It could be the voltage for 68% SoC is different with an aged battery.
But anyway, before replacing with new batteries I suggest just drawing them down deeper, see how it works.

Here's a link for my SunXtender AGM. Last page gives cycle life vs. DoD.
Some batteries are half this life. Marine "Deep Cycle" are much lower. Some batteries are 2x as long.
My system disconnects house loads at 70% DoD, keeps critical loads going to 80% DoD.

From about 5% DoD to 80% or 90% DoD, the total number of Ah drawn in the battery's lifetime is pretty much unchanged. 50% DoD gives half as many cycles as 25% DoD, same product Ah drawn x cycles.

http://sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

 

[Wycowboy]

Im not familiar with DoD and how to tweek it.. ,
If we can get our system to give us usable lights and well pump down to 50-60% that would be much more usable we are happy shutting off most aplances when we get below 82%

 

[snoobler]

AGM = no checking cells
AGM = no equalizing. Equalization of AGM batteries will likely damage them

6-8 years is not a realistic expectation from AGM when people talk about 50% depths of discharge.

AGM typically have cycle lives on the order of 1000-1200 w/50% depth of discharge where FLA are typically around 2000, but FLA requires proper maintenance to get there.

Each cycle degrades capacity a small amount. A 100Ah discharge is 50% on a brand new battery, but on a battery with 1000 cycles on it, it will be 60% of the total capacity, thus is a discharge down to 40% - which further accelerates wear, reducing cycle life.

I suspect that after 3 years, your entire bank is degraded by 20%-ish. The battery monitor is programmed for 200Ah where your batteries now have about 160Ah. If your cut-offs are voltage based, then you're hitting that low voltage cut off at a higher reported SoC because the battery doesn't know your battery is a 160Ah battery. It still thinks it's a 200Ah battery.

Alternatively, one or more of the 6V are going bad at a faster rate. That's what @Hedges was getting at with checking the individual 6V batteries. This would be most effective at low states of charge, under load before solar charging starts.

DoD is depth of discharge - how much of the battery you're using.

Discharge down to 68% is a DoD of 32%.

 

[Wycowboy]

Dod Makes sence

If we can get our system to give us usable lights and well pump down to 50-60% that would be much more usable we are happy shutting off most appliances when we get below 82% If what you say our batteris are 20% deplated. so 160 ah batters . is that likly why we nolonger get to 100% charge and only 99% ?

be great to some more life out of these cells if we can

 

[snoobler]

It's all just guessing at this point.

First, need to check all 8 6V batteries when at a lower SoC. You may find that one or more has a failed cell. If this is the case, it needs to be replaced. Let us know the results.

If they are all pretty uniform, it's just degradation, and you can only reduce power usage. The battery monitor should be re-programmed for a lower capacity. Determining that capacity without direct testing is just guessing, but you can get a decent approximation based on voltage.

It may also be that your well pump is a big load, and it hits the battery with enough current to pull the voltage down below the voltage limit even though the battery may not be down to 50%. Any high current device can cause this.

While the battery is under a light load - around 10A, and at a lower state of charge, it would be useful to record total voltage, SoC and current.

 

[Wycowboy]

This is so helpful! I wont be able to test for a while but I am getting my plan together while I have internet =) I did not know I need to test at a low SoC . The well pump is a big load 1 hp,soft start varible speed 220V , we usaly shut it down when we get below 82% any way,

 

[Hedges]

Im not familiar with DoD and how to tweek it.. ,
If we can get our system to give us usable lights and well pump down to 50-60% that would be much more usable we are happy shutting off most aplances when we get below 82%

It sounds like your inverter has a low voltage cutout. Some have a more sophisticated method of considering current draw at the moment, or counting amp-hours. (voltage for a given state of charge depends on current draw or charge rate.)

See if there is some sort of low-battery disconnect setting. If voltage, look up the voltage it is using on a battery discharge graph, see what state of charge it represents. Set a lower voltage more like 30% or 50% state of charge (70% or 50% depth of discharge), see if you can get more usage from the battery.

In that state will also be a chance to check batteries while low.

But I think after a bank has been charged, if it is left to sit for a couple hours, a single bad cell will drop low and that 6V battery will read low.
Typically I've dealt with 12V car batteries, and after charging they drop low (like 10V or 11V) due to a bad cell.