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RV Batteries: Depth of Discharge vs Cycle Count
Photographer and cyclist and Mac expert and software engineer Lloyd Chambers is available for consulting on general Sprinter considerations at his usual consulting rates via phone, or in person in the Palo Alto, CA area. Save yourself hours and mistakes by discussing issues up-front. More about Lloyd....
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There is a not very helpful way of thinking out there that discharging lead acid batteries more than about 50% is a bad idea versus, say, 80%.
The “50% rule” is actually very poor advice for most users (assuming high-grade AGM batteries), for practical and technical and even cost reasons, a false choice for so many usage scenarios. The actual usage scenario carries great weight in whether the 50% recommendation makes any sense at all, or whether it actually has many negative consequences.
For example, in order to follow the 50% rule, one might buy heavier and larger and more expensive batteries. More money is spent upfront, the vehicle is heavier, and interior space is reduced. By the time the batteries are at the end of their service life, alternatives such as lithium batteries may have declined in price. Moreover a smaller/cheaper initial set could have been replaced if need be, after 5 years or whatever. Furthermore, most users (summer campers) will never run into the cycle life limit before the batteries are getting old anyway.
If one assumes an 80% discharge, even for lead acid batteries, there will be fewer cycles, but one has to charge them less often. And if they expire under heavy use in 4-5 years, the money saved upfront on smaller/lighter/cheaper batteries goes to their replacement, possibly with far superior new technology (lithium or fuel cell).
Usage and enjoyment matter: discharging to 80% with fewer cycles of lifetime versus discharging to 50% or 75% with more cycles is really saying that one is willing to compromise on power usage, because that deeper discharge means enjoying more power and/or recharging less frequently. The loss in cycle lifetime is offset by greater power enjoyment—there is no real loss in “watts enjoyed” when looked at that way, in terms of power consumption enjoyed. Indeed, having to constantly monitor the batteries, and to stop using them in order to recharge surely has a huge negative “enjoyment ROI”.
In general, don’t fret over discharging to 80% versus 50%. The batteries should get out of the way and do their job for one’s enjoyment.
Excepting those living in their vans every day, most users are likely to never approach the cycle count before end-of-life for the batteries anyway. That’s assuming a high quality battery of course (AGM batteries if lead acid).
Lithium (Lithionics) battery longevity
Here’s where real value comes in vs lead acid batteries and the cheap Chinese-made cells-in-a-steel-box-in-case-of-explosion lithium alternatives (which many companies do sell!)
I look at this pretty simply: If I spend 3 months (91 days) a year in the Sprinter van and I deplete the battery by 90% every single one of those days (the chance of this is about nil), then I “use up” 91 battery cycles. If I do that for 7 years, I use up 637 cycles. At that rate, it would be two decades before it would become an issue, but normal battery aging will mean that year 10 is the real realistic cutoff, year 12 if lucky and some loss of power is acceptable (yes).
Suppose one spends 365 days a year using the battery, discharging it by 90% twice a day—that’s not even realistic, since charging it takes hours—it would have to be shore power, so the battery would not be discharging at all. But just for giggles, figuring one discharge per day by 90% at 365.5 days a year for 7 years = 2558 cycles—and the battery should still be yielding 80% of its capacity.
Anyone who realistically needs that much power every day 365 X 7 years should discharge less deeply, or get two batteries (in parallel) and discharge to 50%, thus getting 5800 charge cycles = ~15 years at once per day.
DoD = Depth of Discharge e.g., 90% DoD means 10% charge left
Lead acid battery longevity
Therefore, for lead acid AGM batteries 80% discharge makes a lot of sense for most users, because a cycle count of 600 or more still means a full service life for at least five years (600 cycles means 120 discharges per year, which is plenty for my usage and for most users). In effect, there is no downside over that 5-year time frame, because I won’t use that many cycles anyway.
Those living in a camper permanently should size up the capacity and/or just plan on replacing batteries a bit more often and/or not fretting over somewhat lower capacity towards the end of the service life. It’s just not worth worrying about 50% versus 80% discharge since van build-out and operating costs will dwarf the cost of replacing the batteries 5 years in. Moreover, lithium batteries make even more sense in order to recharge less often and for a very long cycle count and more real power.