What is the optimal SOC range for cycling lead acid batteries?

Re: What is the optimal SOC range for cycling lead acid batteries?

Just my opinion...

I think batteries like to be used, and if shallow cycled and brought to full charge every day is now great for them. That said, normal use should also not include periods of more than a day or 2 when they don't reach absorption stage of charging (roughly 80-85% full).

In the normal use of an off grid system, my batteries will find time of deeper discharge, I've been running an A/C over night and since I've been away most days and won't be around when the batteries reach float, I've run my water heater for an hour or so in the morning. I worry about the charging being fooled by the heavy load while charging the batteries. by loading while the batteries are still in bulk I prevent the load during timed cycles and likely drop the battery down to 60% DOD but also at a time when the batteries will be recharged soon so as not to leave them in a lower state of charge.

In the winter and spring cloudy weather will bring the battery down to similar levels most weeks. I don't use a generator, so the need for the batteries to reach fully charged once or twice a week does require some thinking as to weather and what state of charge the batteries are in. I also don't normally use a battery monitor, but can kinda keep a running idea of the state of charge the batteries are in. I can get some confirmation of that from the system voltage with my known loads.

I'm a firm believer in doing a maintenance equalizing cycle once a month with flooded batteries, now that I have a forklift battery, it is simply required.

Re: What is the optimal SOC range for cycling lead acid batteries?

They say it takes between 20-50 cycles ( define cycle ) depending on the batteries. Now that could have changed some as Manufacturers pre-form plates and different chemistry ( antinomy, calcium, selenium alloy ) would react differently. New batteries can be sulfated some and that depends on how they were commissioned. You also have the dip and recovery syndrome to consider, it depends on the use before the voltage reading was taken..

Re: What is the optimal SOC range for cycling lead acid batteries?

northerner wrote: »

So, is that something you practice, preventing your system from going into absorb most days? If so, how do you accomplish this? There are 2 methods that come to mind. One is through system design, having slightly less solar (wind), and slightly more battery capacity. The other is to increase your loads, perhaps using waste not on those days.

I'm also going to skip absorptions most of the days. I believe bubbling and heating is harmful to batteries and doing this only when absolutely necessary is beneficial. I don't have big loads as Chris does, so I simply going to force them into float if they're not discharged enough to justify an absorption. They're still going to get some charge during float, so you may end up your day with 90-95% charged batteries. We'll see. It was a thread here (can't find it) where Sunny Boys where used on an island following the same strategy.

It is a certain risk in this, of course. You don't fully charge today, and you're going to hit with real bad weather tomorrow. You wish you would've charged yesterday, but it's too late. However, I believe the impact of this is minimal.

Re: What is the optimal SOC range for cycling lead acid batteries?

vtmaps wrote: »

It is at odds with the advice over at gbbattery.com:



--vtMaps

Thats because they are protecting their 1500 cycle warranty. A much larger Battery Co than GB , Enersys Industrial Battery says something a little different.

9. FAST CHARGING AND
OPPORTUNITY CHARGING
If a single battery is being used in a lift truck for multiple shifts or is
partially recharged during breaks, lunches and other idle periods, it
may be in a fast charge or opportunity charge mode of operation.
Opportunity charging can be used to keep the battery’s state of
charge above 30% depth of discharge during the daily discharge
cycle thereby reducing or even eliminating the need to change out
spent batteries in a heavy single shift or multi shift operation. The
total accumulated discharged ampere-hours should not exceed
120% of the batteries designed capacity rating per day. Discharge of
more than 120% of the batteries designed capacity rating in a
24-hour period will shorten battery life. If engaging in opportunity
charging, the battery must be returned to nameplate specific gravity
at least once per week. However, more frequent recharges to
nameplate specific gravity are desirable. Charge rates during this, of course, refers to the actual temperature of the cell and not the ambient temperature. Thus a battery may be operated
in quite low ambient temperatures for short periods without the actual battery temperature falling to a point where the capacity is seriously curtailed. For example, batteries used in cold storage
plants or similar locations will deliver close to normal capacity if they
are moved into warmer areas for charging and whenever not in
actual use.

http://www.enersysmp.com/documents/28.00-InstructionsforFloodedBatteries_000.pdf

Re: What is the optimal SOC range for cycling lead acid batteries?

Typically, the voltage will hang just below absorb for hours so the batteries get sort of "float charged".

That's interesting Chris. I started doing something similar with the Waste Not function and hot water heating this Spring. This was after concluding like others here that I didn't need/want a near-daily full absorption cycle.

As long as the typical daily loads are fairly constant (and large enough), simply changing the voltage offset on the Waste Not Hi function does a pretty good job of letting me determine how often the batteries are going to complete the absorb cycle per week for a given season of year (it's still subject to weather variations within that season, of course).

It's a nice added "perk" of the already great Waste Not Hi + SSR system, and further increases capacity utilization of the solar panels.

Re: What is the optimal SOC range for cycling lead acid batteries?

ChrisOlson wrote: »

As 'coot mentioned in his post, you still have to have the generating capacity to meet your daily loads.

I have designed our system to use "opportunity" loading on the inverter with electric water heating. I used to use Classic controller on the solar with Waste Not mode. I am now using XW-MPPT60-150's on solar with diversion mode, in cooperation with wind power on Classic 150's using Waste Not mode. As voltage approaches absorb the controllers bring AC SSR's online that turn on AC power from the inverter to the water heaters. This adds enough load to the system so it struggles to make absorb and sends a good chunk of energy to the water heaters every day.

Chris

I have basically the same "Waste Not" design in my system.

I use several control factors to optimize the Solar power output in a mainly automatic manner. What you optimize for can be power efficiency to battery life by varying several control points that I adjusted to power for testing.

E = efficiency (0-100%)
PVE = current solar array power vs max power from the solar array
CCE = current load (battery+inverter) power on the output of the charge controller vs the input of the charge controller
USE = current load (battery+inverter) power USAGE vs max power from the solar array (can be greater that 100%)
A battery health (weight factor, the bigger the better) factor that combines several measurements or conditions into a single number that gives the health of a battery like Ah capacity, charge cycles, depth of discharge, internal resistance, etc ...

From the PVE I also generate a PQ (for power quality) two day running tally of daily energy collection so the software can decide using all these factors if TODAY is a good day to try diverting power to loads or to waste energy fully charging batteries.
If the software decides it's a good day it chooses between one or more of two methods of power diversion.

1: A PWM channel that sends the PV DC output before the charge controller to a DC (water) heater load controlled by making the CCE factor as close to 100% as possible during absorption or float.
2: A AUX power relay that switches AC power from the grid to inverter power (via a 120VAC isolation transformer) on several circuits with non-sensitive loads (mainly lights) or are connected to a 'online' UPS that only keeps it's internal battery charged from the incoming AC power.

Re: What is the optimal SOC range for cycling lead acid batteries?

Over the past few years have gone through periods where purposely avoided daily full recharge, primarily to force discharges to a lower SOC, and to reduce the heat dumped into the batteries by the Absorption stage. Initially would do this by just switching the PV input breaker to OFF. This worked fine, although on unattended systems variable loads like A/C added uncertainty to just how many days of no charging would equate to a certain, approximate SOC.

More recently, beginning about three months ago, have changed to avoiding daily full recharges, by setting the CC's Absorb voltage to Vfloat +0.1 V, and then manually resetting the Absorb V to the correct value for the individual bank every 5-7 days. This was primarily done to reduce battery heating in HOT weather. It works quite well for this , of course, on the full recharge days there is still battery heating, but the average battery temp is quite a bit lower than would result from daily full recharges.

Regarding the minimum DOD, usually, for FLA batteries the number of discharges vs DOD start at 90% SOC. And it has been specifically stated by a number of battery manufacturers that a "Cycle" should be to a point of 90% SOC or lower. The personal strategy here is to frequently discharge banks to about 75-80% SOC, and less frequently discharge to about 55% SOC, or so.

Believe that Chris might have been making an indirect reference ("plate erosion") to the SUrrette Bulletin #614, where it states that a battery can be discharged to 50% SOC, and on occasion considerably low (20% SOC), and this is GOOD for the battery, as it forces electrolyte deeper into the pores of the plates, which maintains Capacity, and improves Charge Acceptance, and so on:

http://www.altestore.com/mmsolar/others/Charging_and_Discharging_BU-RS-614.pdf

There has been about three versions of that Bulletin over a period of a year or two.
While this Bulletin was on the Surrette site, some of their Tech folks, would mention that this bulletin was "controversial" within Surrette (Engineering).

Have gotten the feeling from speaking with Surrette Tech, that every cycle (at least for series 5000 batts) really should be to 50% SOC ... the old thing of "Our batteries are designed to be worked".

However, deep discharges usually require large RE charge resources, or the willingness to burn fuel for recharges. Everything is a balance.

Am with vtmaps, generally, efficiency, to me, is not a huge thing, although heating the battery is not a good thing, and should be avoided to the extent possible.

My opinions, Vic

Re: What is the optimal SOC range for cycling lead acid batteries?

I find this thread interesting, albeit paradigm altering. We spend top dollar on a charge controller that is supposed to know how to look after our batterys, weve programmed the exact right ending amps to get that perfect charge. And now thats all kind of tossed out

Here our system is designed to operate without a generator, hence theres tons of spare capacity generally. This winter there was only a dozen days when it didnt float. An average day sees the bank in float by 9am. Cycle depth i would say averages 5%. So these batterys spend almost all of their time at or near full charge.

I do have a water heater ready, but dont have the inverter capacity, nor dc elements yet. Its on a list. But what about this for an idea. They are agms, so i dont eq. I could set absorb=float, and set eq=absorb, at say 7 day intervals. Would that work?

On a semi-related matter, the classic says that its charge voltage setting defaults err on the low side, (better to under then overcharge). Turns out midnite was wrong. We have 2v agms, and the classic defaults for agm are 28.8V/ 27.4V. My initial look at the battery specs said the classics voltages would be about right. However after recently talking to the battery manufacturer again they recomended using voltages quite a bit lower 27.9/26.9. Thats a whole volt different. Since lowering them there is now no (feint) squealing during absorb, and no drop in rest voltage.

Re: What is the optimal SOC range for cycling lead acid batteries?

vtmaps wrote: »

Stephendv has written about how the sunny island will not do a daily absorb unless the batteries are discharged far enough. I don't think I can easily mimic that with my controller, but I am hopeful that lower absorb voltage with longer absorb time will have much the same beneficial effect.
--vtMaps

Another useful feature for controllers would be predicting the weather. If that was possible, they could alter their charging and usage patterns based on the weather predicted for the next day or so. For example, if the controller knows that the next day is going to be sunny, it could make fuller use of the battery capacity the day before, perhaps with opportunity or waste not modes. It could also signal to hold back on generator charging if the battery state was getting close to 50 % or so. And if the next day is predicted to have heavy cloud, the controller could charge more aggressively the day before, and hold off on using the opportunity modes.

Also, would it be possible to monitor the SG of a battery cell, and feed that info to the controller? Ideally, it would be good to know the SG at various levels within the cell, one would think, in case the electrolyte has not recently been mixed. Perhaps then the controller could learn and figure out what the battery needs?