Lead Acid 50% DOD Limit; Rule or Myth

[luthj]

Greetings. I have a fair amount of experience with lead acid deep cycle batteries in vehicle and PV systems (fork lifts, golf carts, small EVs, etc). I have seen/heard the following truism for as long as I can remember.

"Don't discharge your lead acid batteries below 50% ever"


I believe this statement is not true. It is a useful guide, but there is no mechanism in the LA chemistry that drives this specific number.


My take; A true deep cycle battery will provide roughly comparable total AHs delivered for any cycle depth (DOD) between 30 and 80%. This assumes a correct charge profile with sufficient power to recover the bank in reasonable time.

My Data; Here are a few % DOD vs Cycle life plots.

On this plot lets compare 30% DOD vs 80% DOD for a 100AH battery. At 30% total AH delivered is 61.5kAH, at 80% DOD it drops to 54kAH. A reduction of 12%.

For lifeline 30% is about 60kAH and 80% is 44kAH, a reduction of 20%.

It is had to tell due to the logarithmic scale, but the curve is fairly linear between 30-80% or so. I argue that for a weight/size limited mobile application, designing for 50% max DOD for most use cases, and allowing up to 80% DOD in fringe use is an acceptable compromise between bank cost, weight, and other aging factors besides routine DOD.

For off grid or stationary energy storage situations sticking to the top 10-20% makes a lot more sense. The banks size/weight is not an important concern. Instead, minimizing the cost per kWhr delivered is key, so sticking to the steeper part of the curve pays off.

In conclusion, deeper average DOD will result in fewer cycles, but the effect on total AH delivered is not severe. Occasional excursions to 70-80% DOD will not destroy a quality battery with sufficient charging.

Furthermore, I suggest that installations in vehicles where weight and bank size are limiting factors, should consider their batteries to be a partial expendable. Sticking with a smaller, quality flooded bank, and cycling to deeper DOD when needed will provide the best combination of cost per AH/cycle, and weight/size.

This is backed up by my work in the electric forklift and golf cart applications. These vehicles are limited in weight/space for battery storage, and as working vehicles they are run hard. Excursions to 80% DOD are common. This provides an workable trade off, and the banks provide acceptable service lives. Forklift banks operated in modest temps will sometimes exceed the MFGs cycle life by 10-20% if kept charged correctly.

 

[john61ct]

It is not in fact a rule, each batt has its own curve between avg DoD and cycle lifetime.

No hard B&W line, all continuous greyscale gradation.

But it IS true that lifespan is **radically** reduced if you "more than occasionally" discharge deeply, compared to more shallow usage.

As in 200 cycles compared to 1400. True for all chemistries.

So get the graph for your candidate batts and make your own choice.

Fully informed, way to go.

But for those not looking to analyse and research, it is a **great** rule of thumb.

 

[DiploStrat]

...

In conclusion, deeper average DOD will result in fewer cycles, but the effect on total AH delivered is not severe. Occasional excursions to 870-80% DOD will not destroy a quality battery with sufficient charging.
...
Regular excursions to 80% DOD are common. This provides an workable trade off, and the banks provide acceptable service lives. Forklift banks operated in modest temps will sometimes exceed the MFGs cycle life by 10-20% if kept charged correctly.

Because of my Quixotesqe search for battery powered air conditioning, I have had this discussion several times with my Lifeline rep. His points:

-- You trade depth of discharge for duty cycles.

-- As long as you do not discharge the battery to greater than 100%, you are unlikely to do permeant damage.

-- Most batteries fail, not because they are over discharged, but because they are never fully recharged.

So, I would say than Jon is exactly right:

-- Design a lead acid system for a maximum 50% discharge, but, even more important,

-- Make sure that your design/usage pattern allows you to FULLY recharge your batteries at least once a week.

For most of us, this means a good shore charger and a healthy solar kit. My rule of thumb is 100w of solar panel for every 100Ah of battery capacity.

 

[luthj]

But it IS true that lifespan is **radically** reduced if you "more than occasionally" discharge deeply, compared to more shallow usage.

As in 200 cycles compared to 1400. True for all chemistries.

I am not seeing evidence of discharges to 80% DOD causing this "radical" of a reduction. 500 cycles vs 2000 cycles may seem like a big drop, but the total AHs delivered doesn't drop hugely as mentioned above.

My point is that the cost per AH delivered can be lower.

For example here is a usage case.

Daily max usage is 160AH. For a 50% discharge design, a bank of 320AH is required. That would be 2x 902, 4x GC2 etc for $300+. Assuming about 800 cycles, that is about $2.5/kAH+. With irregular usage, the bank may age out before it reaches 800 cycles, so the cost may be even higher.

For a 80% DOD design a 200AH bank is needed. 2 GC2 batteries provide a 210AH bank for 2x$94=$188. Assuming 450 cycles that comes to about $2.6/kAH.

So there is some argument for a smaller bank, and expecting a more frequent replacement.

So, I would say than Jon is exactly right:

-- Design a lead acid system for a maximum 50% discharge, but, even more important,

-- Make sure that your design/usage pattern allows you to FULLY recharge your batteries at least once a week.

For most of us, this means a good shore charger and a healthy solar kit. My rule of thumb is 100w of solar panel for every 100Ah of battery capacity.

Those are my thoughts and experience. Failure to fully recharge can cause significant degradation over a fairly short period of time (a few dozen partial cycles over a month or so).

 

[john61ct]

With irregular usage, the bank may age out before it reaches 800 cycles

Totally separate issue.

As long as properly cared for, reasonable storage temps, calendar lifespan is 20+ years for quality batts.

For cycle lifetimes, let's just work with 200+ cycles per year.

> For a 80% DOD design a 200AH bank is needed. 2 GC2 batteries provide a 210AH bank for 2x$94=$188. Assuming 450 cycles

I don't think any but the most expensive batts would survive over even 2-300 at 80% avg DoD in real life.

If you wanted to go with 60-65% and did not mind the inconvenience of replacing more frequently, I'd say yes maybe makes sense. But 70% or over I could only imagine if the extra weight was a critical issue.

In which case should move up to LFP.

 

[john61ct]

Big bank of Nicad hundreds of AH?

Never heard of that being done, out of 100000+ threads all around the world, must be a reason.

Fire risk? Lifespan?

 

[luthj]

Charging nicad is tricky, they need a voltage differential change to trigger charge termination. The actual charge voltage will vary wildly depending on memory effect. They also need to be fully discharged before recharging, otherwise they get rock content, and capacity loss.

 

[luthj]

Safety as a standby power source, and a deep cycling application are a bit different. That said, they are too expensive for anything not weight critical like aerospace. Assuming you can get a cycling recovery machine on them, you can break up the rock content and they should return to original capacity.

 

[DLTooley]

Very good point. Real world usage patterns also go into those numbers. I've designed my system for approximately 20% discharge, under normal conditions.

If there were two weather days with zero power generation I'd be just below 50%. The margin of error on that measurement is a factor, but if that point is rarely reached it isn't so much of a concern - especially if you get back to 100% on those 'normal' days. A 20% DoD design makes that point easier to reach. I like to drive into town on the third day, using the alternator to charge the battery back up a ways. Power generation is also rarely zero.

The debate on how much panel wattage to have shouldn't be driven primarily by battery bank size, actual consumption should drive this selection, in a dynamic context with the above.

 

[luthj]

I find that the rules of thumb for solar watts vs battery size are much to vague, as insolation varies dramatically with the seaons and regions.

The PVwatts calculator combined with some conservative estimates makes all the difference.

https://pvwatts.nrel.gov/

 

[Rando]

This is a very good point. When I had lead-acid batteries I used 70 - 80% DOD as my design criteria. Particularly for us weekend warriors, the 500 - 600 cycles this would yield equals a whole lot of years worth of weekends. As others have pointed out, the batteries are more likely to die from other forms of abuse.

The decrease in lifetime is certainly not 'radical' - one discharge to 80% DOD is the equivalent of ~ 1.8 discharges to 50% DOD, and in terms of available AH over the lifetime, is only a 12% decrease.

 

[dwh]

The "50% rule" comes from fixed-base solar, where recurring total daily loads are known and more or less fixed.

There is a sweet spot in most lead-acid DOD vs. lifecycle charts where 50% usually results in around 1000 cycles.

So when designing a system to support say, a house, the 50% rule results in around 3 years of daily cycling before having to replace the batteries.

Since a battery bank large enough to feed a house is a major investment, there is a tradeoff that has to be considered; bigger bank = lower daily DoD = less frequent battery replacement....but costs more initially.

Years of solar designers/installers/estimators playing with the numbers, including TCO over time (important for a FBO solar installation) has distilled down into the "50% rule" as the standard rule of thumb to start with as a default battery bank size when estimating, because going below 1000 cycles for a battery bank in a FBO ends up costing more over time - just as going oversize to get a lower daily DoD can cost more over time.


Years of Internet parroting of poorly understood principles of solar sytem design has resulted in a meme that 50% is a hard and fast rule.

It isn't. It never was. It's a rule of thumb used by estimators.


Besides, designing for vehicles is different than designing for FBOs.

 

[plh]

we have had 4 hours (solar charge worthy) of useable sun in the past 6 days.

 

[Joe917]

One other factor usually overlooked in house bank sizing is the maximum amp draw. 30% for AGM, 25% for flooded. ie, your microwave draws 150 amps so you should have 600 ah flooded or 500ah AGM (this is the only real advantage of AGM over flooded)

 

[BigSwede]

It's a guideline, not a hard and fast rule. It's not like your battery becomes permanently dead at 49% of full charge. But many people aren't even aware that batteries can be damaged by drawing them way down, so at least it gets them thinking about it.