Lead Acid 50% DOD Limit; Rule or Myth

[john61ct]

There is no general statement than can be made about avg DoD vs lifetime.

Not in absolute # of cycles, not in ratio format.

The relationship curve **varies by brand and model**. And I would not buy from a mfg that didn't make those charts readily available. But they are based on LAB tests, not real life, and given that mobile banks are usually subject to lots of unintended abuse, I personally want to control all those factors I can.

I assume those discussing such topics are spending many hundreds if not thousands on a large quality bank. Or have mission-critical needs in primitive / remote conditions, where "no surprises" predictability is key.

If not, then it really doesn't make much difference, just do what feels right for you.

 

[Bayou Boy]

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)

Other "real" advantages of AGMs are the higher ruggedness concerning rough handling, not having the add water, the ability to install them in enclosed spaces, and the ability to install them in orientations other than top-up. All of those are significant with the limited installation locations in most trucks and SUVs.

For instance, I have my aux battery installed alongside the frame rail under the truck. If I had to add water to a normal battery, I would have to remove the entire bracket and drop the battery to do it. Not going to happen. A lot of guys have to mount their aux batteries inside the cabin in SUVs. Not a wise choice with a normal lead acid battery but a non issue with an AGM.

 

[john61ct]

But if you don't need to install sideways, or the higher CAR,

FLA batts are much more robust, last much longer, especially in adverse charging conditions, less fiddly requirements and much cheaper for top quality.

Well worth the tiny trouble of watering. Plus can use a hydrometer, and equalize / condition.

 

[rkj__]

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.

Good point Rando. I camp no more than 20-25 trips a year typically, and most trips only bring my battery down to 40-50% DOD, but even if I did get into the the 500 cycle range, at 25 cycles per year, that is still 20 years. I think practical experience has showed most of us that you should not expect a battery to last 20 years. So, maybe the data is too idealized / controlled to apply to real world abuse? I don't know.

What I've taken away from this discussion, is that I don't need to have a panic attack, and shut down all power draw, if my DOD dips below 50% occasionally. Some comments I've read on the internet seem to suggest that going below 50%, ever, will immediately "permanently damage" your battery. I think we can agree that is not exactly true.

 

[dwh]

Some comments I've read on the internet seem to suggest that going below 50%, ever, will immediately "permanently damage" your battery. I think we can agree that is not exactly true.

Technically...going below 100%, ever, will permanently damage your battery. As will age/time. It's just a question of how much damage you can live with.

For FBO installations battery bank size / DoD is just one of a dozen variables that has to be tweaked to achieve a goal such as "lowest cost over the life of the system", or "maximize grid backfeed to reduce average annual grid power cost to zero".

For vehicles, lowest cost over time or a zero electric bill aren't usually the highest priorities. Cold beer is usually more important.

 

[john61ct]

I don't need to have a panic attack, and shut down all power draw, if my DOD dips below 50% occasionally.

Absolutely.

Same as eating high-fat desserts, no hard black and white rule, all grayscale percentages and interrelated with other factors.

But big-picture statistically the causation is undeniable.

 

[FlipperFla]

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.

A battery is discharged at 10.5V running it any lower will drastically reduce life

 

[rayra]

When 'rule of thumb' and Pedantry / OCD collide, is what this topic is. Having a guideline of 50% helps protect your battery when you inadvertantly go beyond that. At least then you aren't zeroing your battery.
And if you use that as a guideline you arne't over-stressing your batteries and are somewhat overbuilding your system, all of which contribute to greater longevity in use.
If you are trying to calculate the exact point where you will overtax and break your stuff, I leave you to that fruitless exercise.