Minimum rate of discharge

larcal · October 2021

Here's an issue that I can find little mention of anywhere and battery suppliers seem unfamiliar with.

If, for example, one is drawing 70 amp/hrs per day out of a 1270 amp/hr deep cycle LA fluid battery, thus a daily discharge of only 5.5%, does this over time negatively affect the capacity of battery? Rumor around about this but maybe just comes from grid tie batteries that are in constant float, don't even come down 5%.

If is some validity here is 10% appreciably better or what is the minimum rate and is there any studies/data on this? Links?

Otherwise, the charts all show only increasing cycle life as discharge percentage decreases, so one is tempted to buy big in order to get longer lifespan. Plus of course, more room for future consumption expansion but which may never happen.

Doesn't seem like sulfation would be involved in such a hypothetical loss of capacity so it's unclear how you could detect or measure it.

Kinda going nuts over this choice and not sure what is real or who to believe. Manufacturers don't mention which is par for the course but still wonder why not

Thanks

Vic · October 2021

Hi larcal,

IMO, the question that you are asking, is, What is the minimum DOD of a FLA battery, per charge cycle(?).

Using that as a search term, or similar will probably be more helpful.

Here is a starter-Discussion, start on the first page of this, if the link presents a page further into the Discussion:
https://forum.solar-electric.com/discussion/19562/what-is-the-optimal-soc-range-for-cycling-lead-acid-batteries

For off grid systems, the best CCs provide a Skip Day function, that only tries to fully charge batteries after a settable number of days of skipping a full charge. The MidNite Classic CCs provide this function. The ideal number of Skipped days may vary a bit from day-to-day, week-to-week (etc).

Edit, to add: Have not re-read the above for years, But, personally, try to avoid recharging the FLAs here, until their DOD is greater than 10% (ie, less than 90% SOC),

And usually only look at Ah Removed from the battery, to make that calculation. The greater the number of days between recharges, will have a negative impact on how meaningful is that number, as discharge rate, relative Float voltage, etc, affects the accuracy of the Net Ah. More Skip days, the less accurate these numbers, also, because most devices that measure the Net Ah go to 0, on the transition from Absorb, to Float, and so on.

FWIW, Vic

RCinFLA · October 2021

Perhaps understanding some of the degradation causes will explain what different use cases do to to battery longevity.

There are several variants to lead acid batteries. Floaded. electrolyte starved (AGM, Gel). Within flooded type, there is lead-calcium alloy and lead-antimony alloy (deep discharge) plate structures. AGM's and Gel are pure flexible lead plates supported by physical binding compression within their glass mats.

It is pretty well known that leaving a lead acid battery partially discharged for some time will allow any discharged lead-sulfate to crystalize which is resistant to recharging. How long this takes is more debatible. My personal experience and opinion is about a month.

Lead acid have relatively high self discharge rate. Lead-antimony is worse, lead-calcium is less at about 10% per month. Pure lead AGM and Gel are lowest self discharge rate.

Float charging voltage is a compromise between what is good for positive plates to minimize support grid oxidation and what is good for negative plates to keep sulfate discharge build up. Optimum for pos plates is about 13.2v, neg plates need more at about 13.8v for six cell 12v battery.

During charging, when six cell battery gets above about 13.5v, excess energy will split water in electrolyte. This bathes positive plate in oxygen gas and negative plate in hydrogen gas. At higher charging voltage the gas creation gets more vigorous. It is the bathing of the positive plate with oxygen that is damaging, accelerating the grid support structure corrosion.

Constant floating charge without any discharge will cause build up of lead oxide coating on positive plate which will increase cell impedance and increase positive grid corrosion. Particularly for golf cart batteries, it is good to periodically give them a 10% depth of discharge to burn off excess lead oxide build up on positive plates. It is good to once every month or two to give a charge to 14.2v to ensure negative plates do not build up sulfate that then hardens.

All battery chemistries have a overpotential versus cell current. This is terminal voltage drop under discharge and terminal voltage rise under charging. It is the overhead power consumed to run the ion engine within the battery. It is the creating of ions and their transport to support the external current demand. The general rule is the more complex the chemistry the greater the overpotential needed. Lead acid battery is fair complex chemistry (compared to Li-Ion). Lead acid requires molecules of sulfuric acid to be broken into water and sulphur and lead to be converted to lead sulfate, and back again for charging.

There is a depth of penetration in plate and local electrolyte access that impacts the ability to access the needed chemical reactions that is dependent on cell current demand which requires greater rate of ion creation and movement to support the demanded current. The surface of plates and electrolyte nearest plate surface is accessed the easiest first. For fast discharge the lead sulfate can build up on the surface and block access to material deeper in the plate. It also takes some time for electrolyte locally depleted to water at plate surface to diffuse and bring the needed sulfuric acid close to the plate surface to create more ions need for demanded current. This requires higher overpotential terminal voltage drop to drive the process at higher discharge current rate. Flooded cells have a greater path distance for ions to flow through, requiring higher overpotential drive.

Slow discharge rate allows deeper penetration within plates as does slow charge rates. If you have a slow discharge rate with a fast charge rate you may not achieve full state of charge due to lead sulfate deep in plate will be insulated by recharged surface layer of lead at surface of plate.

Vic · October 2021

Perhaps I did not understand the OP's question, and prattled-on about things that were not asked ...

Thank you RCinFLA, for your detailed description of battery charge/discharge, and Rate tradeoffs. Always appreciate your take on the fine detail of our power systems.

Vic

larcal · October 2021

Thanks Vic. Very helpful link, much to study. Like your idea posted there #22, and above, of delaying charging until dod is at least 10%. And now see a few others echoing this % if not your method.

If you were buying today and didn't have the ability to keep up with this type of regular attention would you say get a battery small enough so that ones daily consumption used up 10 %? Maybe even 15%? More? This is a good rule of thumb? This for a set it and forget it system aiming at the maximum battery life possible without too much care. Efficiency and cost not as important.

Not just Rolls, either, right? which are hard to acquire now. Gather you think any deep cycle Fla needs their gym time.

larcal · October 2021

Heh Vic, haha, Don't know why you think that. You understood me great and very helpful. and yes, been out of touch long time and was stupidly searching Duck duck

larcal · October 2021

RCinFLa--Hello and Thanks. Very erudite and will print out.

This lead oxide coating that gums things up when in float too long.-- You say to aim for a 10% discharge "periodically"

On a 2 volt lead antimony cell how periodic would you guess this needs to be? If several times a week would you recomend getting a cell small enough so that one's daily draw accomplished this?

mainly tho, I'd love to ask both you and Vic---How does one measure or detect this loss of capacity causing by not discharging enough. Measure amp/hr consumption against volt drop or is easier way? maybe volt drop under load but thinking that's to hard to quantify accurately.

Learned from that link Vic dropped that Rolls has discussed this in some of their bulletins and so must do that when rains start but right now trying to do quick catch up cause batteries failed.

BB. · October 2021

Why do you want a 10% daily discharge bank? Are you trying for more storage for more days of poor sun/bad weather?

More or less--Looking at the cycle life vs depths of discharge, the actual Amp*Hours of discharge (i.e., 100 AH * 1,000 cycles vs 200 AH * 500 cycles, etc.) seems to be pretty constant... I.e., a battery bank that is discharged to 75% SoC @ 3,000 cycles will last on 50% SoC @ 1,500 cycles (very roughly.

So, a 2x more expensive bank lasts ~2x more cycles--All things being equal.

Of course, we will "bump into" the aging life of the bank (something like 3-5 years for "cheap" batteries; 5-7 years for better batteries... And upwards of 10-15+ years for industrial/fork lift batteries).

On average, a battery bank that supports 2 days of poor sun, and 50% max planned discharge (or 4x daily loads). This seems to be a good "optimum" for FLA battery bank design. For FLA batteries, a 75% SoC cycling allows the "average" hours of sun per day (a 50% discharge typically needs "2 days of sun" to fully recharge). Also a 75% SoC bank has good surge capabilities and seems to be a "good mate" with a typical AC inverter Wattage rating (and 2x surge rating).

Charging to 90% SoC typically--Less gassing, less plate erosion, less water usage, less heat in battery bank... Aka not as hard on the battery bank.

A 90% SoC cycle bank--At this level, you are generating more gas (hydrogen+oxygen)--And charging is less efficient (80% SoC and below, charging is much closer to 90+% efficient (no gassing, little heat generated).

And other issues... If "something happens" to the bank (somebody leaves loads on during bad weather, problems charging, etc.)--A larger bank costs more money to replace.

-Bill

larcal · October 2021

Hi Bill, thanks for coming by. I'll just quote you and leave comment between paragraphs marked by **

"Why do you want a 10% daily discharge bank? Are you trying for more storage for more days of poor sun/bad weather?"

**-Partially. More storage is good in emergencies if when Gennie is needed but fails to perform. Also, it gives you ability to expand your consumption in future if needed or desired.

"More or less--Looking at the cycle life vs depths of discharge, the actual Amp*Hours of discharge (i.e., 100 AH * 1,000 cycles vs 200 AH * 500 cycles, etc.) seems to be pretty constant... I.e., a battery bank that is discharged to 75% SoC @ 3,000 cycles will last on 50% SoC @ 1,500 cycles (very roughly.

So, a 2x more expensive bank lasts ~2x more cycles--All things being equal."

**-Well exactly, I understand that. In fact, the MAIN reason to get a bigger bank, (back to your first point), big enough so only draining it daily by 5-10% is to get more lifespan out of batteries. If 75 is better then 50 then 90-95% soc better then 75%. MAYBE. That's the foundation query of this thread.

Sure a bigger bank is more expensive although not sure it's as bad as the expense doubling when cycles double, but in a few years when the smaller, cheaper bank wears out the fast diminishing dollar will mean a new bank is impossible to afford. Then you die. Plus, in off grid it's a huge job to haul in and install heavy 2 volt cells, especially as you get older, so perhaps is best for some pilgrims to get something that will last as long as they do.

"Of course, we will "bump into" the aging life of the bank (something like 3-5 years for "cheap" batteries; 5-7 years for better batteries... And upwards of 10-15+ years for industrial/fork lift batteries)."

**-Fascinating, Bill. Please elaborate on this. Why will such premium batteries die in 10-15 years if you keep them unsulfated? Sure, I guess plates wil dissolve after too many cycles but what if lightly cycled (as in 10%) and clean heavy plates?

"On average, a battery bank that supports 2 days of poor sun, and 50% max planned discharge (or 4x daily loads). This seems to be a good "optimum" for FLA battery bank design. For FLA batteries, a 75% SoC cycling allows the "average" hours of sun per day (a 50% discharge typically needs "2 days of sun" to fully recharge). Also a 75% SoC bank has good surge capabilities and seems to be a "good mate" with a typical AC inverter Wattage rating (and 2x surge rating).

Charging to 90% SoC typically--Less gassing, less plate erosion, less water usage, less heat in battery bank... Aka not as hard on the battery bank.

A 90% SoC cycle bank--At this level, you are generating more gas (hydrogen+oxygen)--And charging is less efficient (80% SoC and below, charging is much closer to 90+% efficient (no gassing, little heat generated)."

**-Please explain this Bill. Unclear to me what you're saying. You believe in not fully charging battery? not filling above 90% soc? that can't be right interpretation.

"And other issues... If "something happens" to the bank (somebody leaves loads on during bad weather, problems charging, etc.)--A larger bank costs more money to replace."

**-Anyway, look forward to your comments. also hope you will give your opinion or guess on the ideas expressed earlier in thread, which don't have energy to repeat. But you know, the theory that too shallow a cycling like under 10 or even under 20% causes a loss of capacity different then the capacity loss resulting from sulfation and/or plate deterioration and that will counteract any gain you might get in cycles from the lower discharge. So, as you say, at 75% get more cycles then 50% but at 90% soc get FEWER cycles then 75%

As you know, deep cycle batteries, unlike car batteries, die under constant float when stored as in maybe a grid tie setup but how far do you have to get from float to avoid this diminishment is the query and is a very avoided topic it seems

Seen any evidence of this, either yeah or ney or whadyatink?

Bye

Minimum rate of discharge

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