Stupid Me - AKA "negligent discharge"

morpho · April 2020

Okay, I am hanging my head in off-grid shame.
A load was left on the system by accident and the system drained down well past anything I would ever have it go to.
I woke to the SOC saying 30% (if you can ever trust those things) The SG readings were all in the 1.150 range.

So I have been charging the system all day with just a little bit more than 10% of the soc (which would be normally be 23 Amps). I turned it up to 30 Amps this morning. The readings are now more or less 1.195 after about 3 hours of bulk. It has kicked over into Absorb the SOC says 92% and the absorb cycle is set for 4.5 hours.

Any advice? (besides not being a dumb arse)

Thanks

morpho · April 2020

An almost brand new bank I might add!

UPDATE:
The absorb cycle is almost complete and the SG is now - more or less - at 1.225.
So not charged.
Do I throw an EQ on it with the remaining daylight?
Let it sit until tomorrow and see what it does? and then throw an EQ at it?
Do I up the Absorb voltage? Currently at 58.3 V
The Eq is set at 61.3

706jim · April 2020

Keep charging it tomorrow. It may take more current than you think it should but hopefully will recover in time.

petertearai · April 2020

Keep an eye on the battery temps . You don't want thermal runaway .

BB. · April 2020

You probably are looking at 6+ hours of absorb setpoint to fully recharge a lead acid battery bank.

Watch the charging current... The batteries will be "full" when the charge tapers down to 1-2% (or less) of battery bank capacity (say 300 AH, then 1% would be down to ~3 Amps of 58.x volt charging current).

-Bill

mike95490 · April 2020

Keep loads light, and run longer absorb - the 6 hour suggestion sounds like a happy point. EQ is tough on the battery and this is not a case where it would correct anything - at least not till the batteries are all full, and you test.

morpho · April 2020

Thanks folks.
I will see what happens today. Ugh!

Marc Kurth · April 2020

If I understand your post, the battery bank reached something like 30% SOC overnight? That is not a big deal whatsoever, it was simply one deep cycle on a deep cycle battery made to do this. I do not see any cause for angst, just charge them fully as you have been advised.

Am I missing something?

Marc

morpho · April 2020

Well by 30% SOC I mean I had inadvertently used up 70% of the total capacity. Normally I only use 15 to 20 % of the capacity through the evening - even in the dead of winter. I know getting below 50% is not advised.

UPDATE: I ran the cycle today and it tapered down to more or less 2 amps by the end of 6 hour absorb cycle.
The temps were pretty steady at 25-26 degrees Celsius.
The SG readings are a bit wonky between batteries and even between cells in a single battery. 1.275 is about the high end and 1.250 on the low end.

What next? Let it cycle a few times to see if it settles out or run an EQ?

Marc Kurth · April 2020

morpho said:

Well by 30% SOC I mean I had inadvertently used up 70% of the total capacity. Normally I only use 15 to 20 % of the capacity through the evening - even in the dead of winter. I know getting below 50% is not advised.

Yes, Sir - I understand that. A 30% SOC is 70% DOD and that will not cause immediate damage to a deep cycle battery.

The key is how often you discharge, how deep. It's like eating and drinking for your body. Within reason, you can overdo it on occasion and not do any serious damage. The "never go below 50% thing" is internet lore. The "hard limit" is 42v on a 48v battery bank. Going below that is getting into damage territory.

The reality is that 50% is simply a general (not so great) guideline that somehow became a hard limit on the web.

Every deep cycle battery made has ratings for "How deep, How many times." They vary per manufacturer and battery type, but generally, the number of cycles you get out of a battery doubles between 25% and 50% SOC, then doubles again between 50% and 75%. These numbers are based on average use - NOT an occasional deep dive to 10% or 20% SOC.

Marc

morpho · April 2020

I like your style Marc! Optimistic. I generally fall on the: "If the batteries can die an early death brought on by my own stupidity...they will"
Lets just say I have learned a lesson and will ALWAYS set an alarm to remind me I have a load on that I need to turn off. I am getting a bit older and the brain is a bit more forgetful.

Thanks.

Marc Kurth · April 2020

Not being optimistic, just sharing what I know to be factual!

Set your inverter low voltage disconnect (LVD) for something above the "hard limit" of 42v (I like 44v to 44.8v) as an emergency cutout point. If you forget, no harm done. Many of my customers use a higher number because they know that they will hit the cutout point often for a variety of reasons. Remember that there is a big difference between daily, monthly and quarterly deep discharges.

Additional anecdotal info on the opposite end: I have industrial customers who choose to deeply discharge their batteries to 10%-20% SOC and simply replace them every year or two.

Photowhit · April 2020

morpho said:

The SG readings are a bit wonky between batteries and even between cells in a single battery. 1.275 is about the high end and 1.250 on the low end.

What next? Let it cycle a few times to see if it settles out or run an EQ?

@M@"Marc Kurth" is the expert at least for AGM batteries, sells and helps clients maintain them, so I would defer to him, but for me...

The difference between cells is enough, that once you reach float tomorrow, I'd do an equalizing charge and watch the SG of the cells each hour. Once you've gone 2 hours without an increase in SG you will be close to maximum amount of sulfites back in solution. The goal being narrowing the gap between cells.

You say a near new battery bank, did you do a commissioning equalizing and record SG?

I ask as some come as low as 1.260 to start, most are 1.275-1.30 these days.

morpho · April 2020

Marc, yes, I am on the LVD mission next. I recently switched to the Classics and "Assumed" the LVD had been set when it was installed.
Once again, my bad.
It's funny, I really don't see much difference from month to month and season to season. Summer sees me use up about 10% and winter about 15%. Year after year. (I did almost quadruple the array this winter. Completely eliminated the generator use Love it.)
Thanks for the reassurance that I may have only slightly stressed the batteries.

Photowhit, I'll EQ it manana.
The batteries started life with SG readings at 1.260 and two that were at 1.250. I was told to let it cycle for a while. The two batteries at 1.250 came up to 1.260 and the others all settled into 1.275. I ran an EQ but the low-end ones really didn't seem to move up too much but the others started moving up. Today is cycle 28 since launch.

Thanks, guys.

BB. · April 2020

Sounds good... Typical "time to EQ" runs from once per month for some Mfgs. To a maximum "spread" from low to high cell SG of 0.015 to 0.030 SG units (0.015 may be a bit "too close" together).

In any case, charge to "Full", then run the EQ cycle. Measure all cells (SG) and stop when all the cell SG stops rising for all cells. Log the cell "high" SG (temperature compensated, knock any bubbles from the float, etc.) in your book for each cell. That is your "new" 100% State of Charge.

If you have a wide spread (and some Rolls/Surrette industrial type batteries). Sometimes they need a higher EQ voltage (i.e., upwards of 16 volts for 12 volt bank) and more hours (sometimes several days) to fully EQ (and even multiple discharge/charge cycles).

Monitor battery bank temperature--Stop EQ if the battery bank gets too HOT (see mfg. specs). Don't over EQ Voltage (too much EQ Current)--Don't boil the cells (and don't overheat the cells).... I think a good range for EQ is around 2.5 to 5% of battery bank AH capacity (20 hour capacity) as a start.

And, as always, refer to the Mfg. Manual.

-Bill

Marc Kurth · April 2020

Photowhit I am not an expert in anything aside from sampling IPAs

I was really just trying to explain that a rare discharge to 30% SOC is not going to damage his battery bank as he feared. The internet megaphone has people believing that 50% is some sort of cliff where terrible damage occurs.

You are of course correct that a large imbalance between cells should be addressed with EQ, regardless of a single discharge event to 30% SOC.

Stay well!

Marc

Estragon · April 2020

FWIW - A 30% SOC could be a "cliff of terrible damage" for those of us living in colder climates (supposed to be ~ -20°C here tonight).

Sounds like OP is warmer, but for the benefit of later readers - a discharged battery can freeze at much higher temps than a fully charged one. If frozen solid, the case should be checked carefully for burst damage and the battery warmed up before attempting to charge.

I dodged a bullet in my banks first winter... caught at low SOC but they were just slushy, not frozen solid. No apparent permanent damage after ~6years.

BB. · April 2020

Morpho is probably somewhere around Calgary, Alberta, Canada.

So, a "sub freezing battery bank" is certainly a possibility during winter.

-Bill

Marc Kurth · April 2020

Estragon said:

FWIW - A 30% SOC could be a "cliff of terrible damage" for those of us living in colder climates (supposed to be ~ -20°C here tonight).

Sounds like OP is warmer, but for the benefit of later readers - a discharged battery can freeze at much higher temps than a fully charged one. If frozen solid, the case should be checked carefully for burst damage and the battery warmed up before attempting to charge.

I dodged a bullet in my banks first winter... caught at low SOC but they were just slushy, not frozen solid. No apparent permanent damage after ~6years.

Good point, but hopefully your batteries are in a location to avoid reaching a core temperature that low. I would estimate that 30% SOC would begin to freeze/solidify at roughly -15C.

morpho · April 2020

I'm about 4 hours north of Calgary, so yes, cold. It was -26 a couple of days ago. (I've heard that spring has arrived in most places, I've still got 3 feet of snow on the ground) The batteries live in a heated space so not a lot of chance they will freeze.

So the bank is in pretty good shape now, thanks, everyone.

Stupid Me - AKA "negligent discharge"

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