Driving down the State of Charge for healthy bank

Freewilley · September 2014

I have never focused too much on getting my bank low in SOC. But in researching my Surrette issues, the attached document has intrigued me as it stresses that it is important to get the SG VERY low once every 3 months, but also that you should not start the genny until the bank is discharged to 50%. Lets take the latter direction first.
I have only fixated on getting the SOC UP previously and have been anal about checking the SG against the Voltage reading as we know that when charging we see high voltages compared to true SG. But it appears the reverse is also true???

Today is a cloudy day with chance of thunder so I thought I would just shut down the PV and let the SOC go down.
A couple of things are apparent, so far.
1. When my CC or Mate read 12volts even after no charging happening for several hours, this is not nearly the SOC. I have had .4KWH of power today as I shut down the PV at 11am, the SG is reading 1.185 at 3:30pm and the volts are showing 11.8. This is very interesting as over the years I have figured that if my volts were under 12.1 on the Mate, it was time to kick on the genny. But in fact, I may well have been WAY early on this reading...eg. after 4.5 hours of use (and we have not been holding back, in fact I turned things up to use up some juice) we still are about 45%SOC. From 11 to 3:30 the SG moved 1.205 to 1.185, or approx 60% to 45%SOC.

2. My thinking is that I have a LOT more capacity in my bank than I have been using. And that running the generator is going to become much rarer, once the bank is cycling properly.

3. If I am starting my genny at 50% SOC I should not be looking at the Voltage readings on the CC or Mate, I should be testing SG and waiting until I see 1.120. I have never let the bank get even close to that level.

On the issue of dropping SOC to 11volts (I think Surrette does not currently subscribe to this theory, but as you folks know, this flooded battery stuff seems to be more of an art than a science and Surrette's opinions vary based on the leadership's experience. Steve, the chief tech was not aware of this advice, and in my discussion did not advise to do it...but it still was formulated as a tech direction 6 or more years ago, and never reversed so who is to say it is wrong?) and I am not sure that I will do this, or even HOW to do it. The Hydrometer does not go that low...and based on my experience above, it would not be OK to rely on the voltage reading.

Love your thoughts and experiences. Am I off base on this?

zoneblue · September 2014

Re: Driving down the State of Charge for healthy bank

Managing any kind of lead chemistry battery is an excersise in balancing numerous competing factors as you have found

Failing to cylce a battery amounts to not using something you bought and paid for. Most packs have an optimum life cycle cost when cycled 25-50% DOD. Yet offgrid 'capacity anxiety' (akin to range anxiety on electric vehicles), creates an impetus to keep the SOC up. If you dont have it you cant use it. Yes a genset helps some there, but if it was that simple you wouldnt have solar or batterys at all just an big old genset.

Few charge controllers are smart enough to help much with this. Midnites days between bulk is a start, and i they have promised to enhance this to trigger bulk only when a certain SOC is reached. Then you can go further and hook up a microcontroller to look up the weather forecast for the next 2 days and use this to help make decisions about the capacity you are likely to need, and thus how low you can discharge the pack.

Vic · September 2014

Re: Driving down the State of Charge for healthy bank

Hi again Willey,

As mentioned before, I would NOT take my Surrette batteries down to 11V per 12 V equivalent. Have never done it, and it is not planned ...

AND, one thing that was not mentioned before, it is essential when you discharge a battery into the 50% DOD and lower range, that the recharge begin immediately, and ideally finish with a full Absorption stage, to 100% SOC (or very close to it).

It has seemed to me, that you do NOT have sufficient PV for the Capacity of your battery bank, and this is underscored by the fact that you seem to have had problems with your batteries. Have not reviewed all of your posts here, and this might have been touched on previously by others.

Yes, Bulletin 614 is not on the Surrette Battery site now, and probably was removed four or five years ago. Therefore, it is not consistent with the current thinking at Surrette (as you mentioned).

In conversations with Surrette, when 614 had first been issued, was told by one of their chief tech folks, that this Bulletin was written as guidance for RE customers that did have small PV arrays, and were concerned about poor Charge Acceptance, and long generator run-times. You DO seem to have a smallish array, and you may might benefit from deeper-cycling your batteries (perhaps to about 50% SOC, if you are not now doing that.

Managing rechargeable batteries is always a balance, plus guessing, changing a few things, and observing results. It is a very good things that Flooded batteries are quite forgiving, and will tolerate a reasonable level of "abuse".
My opinions. Good Luck, Vic

Blackcherry04 · September 2014

Re: Driving down the State of Charge for healthy bank

I cut this off a Surrette Charging Document, I have posted it several times, you can believe it on not. I'd still have a question on the loaded voltage on how much the load in amps is and what voltage sag one system over another could have. Based on counting amps out on the FNdc on my system about 11.8 V is 50%.

My batteries are doing great now, it just took time and cycles for them to get that way. Here is a charge scenario from last week. I watered them and then used them for 3 days and kept them in the 50-80% range. I did a full charge @ 14.8 volts and 30 ending amps. Every week I have been raising the ending amps. The absorb took about 3 hrs. The next morning I checked the SG level, 1.230 I am like what heck. I have been reaching 1.265 every week without issue. So, I am thinking, surely the electrolyte has mixed in the added water after 3 hrs in absorb, NOT SO. I put them in EQ and after 30 minutes they had climbed up to the 1.265. Lesson learned. When you add water, make sure the reading is not from stratification.

Attachment not found.

Freewilley · September 2014

Re: Driving down the State of Charge for healthy bank

I printed off a similar chart from Surrette, but the values are radically different.
We know that we only discharge to 50% of total, so some charts (and this is one of them) views 50% as totally discharged.

But the fact is, the values disagree....just using 12V, yours says 100% is 12.6 and mine says 12.65
Yours says 75% is 12.06 and mine says 12.24
Yours says 50% is 11.58 and mine says 11.89 is discharged

Anybody know which one I should be using? I really like to switch to BC's

But nobody has commented much on the whole voltage discrepancy thing....here I am at 9pm and the Mate says 11.5 with very minor load (discharged by any measure) and the SG is reading 1.150 or still 25% SOC.
Is this normal?

Blackcherry04 · September 2014

Re: Driving down the State of Charge for healthy bank

Here is where the table came from. Again it's loaded voltage.

http://www.rolls-battery.com/content/technical-downloads?q=node/51

Freewilley · September 2014

Re: Driving down the State of Charge for healthy bank

I googled and found a 3rd chart.

Blackcherry04 · September 2014

Re: Driving down the State of Charge for healthy bank

Freewilley wrote: »

I googled and found a 3rd chart.

Not really surprised, I guess it depends on who's in charge. Trojan is about the same.

Vic · September 2014

Re: Driving down the State of Charge for healthy bank

Freewilley wrote: »

I printed off a similar chart from Surrette, but the values are radically different.
We know that we only discharge to 50% of total, so some charts (and this is one of them) views 50% as totally discharged.

But the fact is, the values disagree....just using 12V, yours says 100% is 12.6 and mine says 12.65
Yours says 75% is 12.06 and mine says 12.24
Yours says 50% is 11.58 and mine says 11.89 is discharged

Anybody know which one I should be using? I really like to switch to BC's

Willey,

This just shows why trying to use battery voltage as a good indication of SOC works SO poorly.
Once one knows the nature of their system, using battery V with a KNOWN load may be OK. Until you are at that point, with stable, good SGs, please try to use measured SGs, IMO.

I DO have an LVD set on the inverters here -- 46.4 V. This will shut off the inverters if at or below this value for 15 minutes. Works fine here, but, when cycling the bank to about 50% SOC, run the risk of hitting this value, so have to watch it, although the 15 minute qualifier helps avoid the shut down.

YMMV, opinions, and comments on the "whole voltage discrepancy thing-- do not use voltage, except for a broad, general, fairly inaccurate indication of SOC. Vic

Freewilley · September 2014

Re: Driving down the State of Charge for healthy bank

Thanks Vic, wise words I am sure.

What I have discovered through this little exercise is very much in line with not going by voltage. I got up this morning and the system had rested overnight. The SG had got down to 50%, actually a tad below, with the voltage and SG relating at 11.6 and 1.100. If I had been relying on voltage, I would have kicked on the genny at noon yesterday when I was at 1.200 or only halfway thru my capacity. OUCH.
So for years I have been anal about not letting the voltage get below 12.0, thinking that was accurate and the right number. OFTEN starting up the genny to get up in the the 12+ range. Now I know to test SG even at the low end of voltage and it very well could be that I am only at 25% discharge, not approaching 50%.
And the memo I attached in my first post is telling me to relax and let the bank drop to much lower before starting the genny, if at all.

I guess my question now is, where can I linger for a day and not be concerned about over sulphation? As long as I am taking it back up to, say, 80% in a day or so, and 100% every week or so, can I start using that lower 50% of my range, close to 1.120? Is it at 75% discharge (below 1.190) that I do not want to sit for a day or two? Or is that just fine for a few days?
This gets more complicated by having to measure SG than read a Mate, but it also means that I have a LOT more capacity than I have been getting by with....and that is a really good thing for living off the grid!
It appears that I have been staying in the 60-80% SOC (to be clear, 1.220 to 1.250) range all summer with this new bank. I can easily move to the range 1.150-1.265 and be much more comfortable and use less genny gas!

I am in the 7th year of managing batteries and continue to learn very important fundamentals! And this board has been a lifesaver.

Freewilley · September 2014

Re: Driving down the State of Charge for healthy bank

Ok, I am just getting punchy looking at various terms and percentages....can someone review the basic...

What SG is the level of 50% discharge that I should not go below. Is it 1.200 (which by the chart on my wall is approx 12.25 volts, open circuit, and far above what I thought was 50%) or is it 1.120 which my reference chart says is 11.89 open circuit....and that is where I thought the 50% hazard line was...

I am now confusing myself.

Cariboocoot · September 2014

Re: Driving down the State of Charge for healthy bank

Voltage and Specific Gravity do not actually correlate. You will drive yourself crazy trying to make them do so.

Resting Voltage as an SOC indicator presumes the battery is capable of 100% of its capacity.
Specific Gravity shows the state of the electrolyte; lower values indicating not only lower SOC but also advancing (irreversible) sulphation.

Driving down the State of Charge for healthy bank

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