Solar charging voltage question

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Not disagreeing that it is a flooded wet cell battery but it is designed to be maintenance free - That is it is not vented and you should no remove the cap.  It does have a vent for when the battery does get overcharged and it will vent.

And not trying to be argumentative but these batteries (and I have used many) will last 10 years if properly maintained with <30% discharging.

I agree 100% that if used as a "deep cycle" ~50% + discharges you can expect to buy new ones every 3-5.  On boats where you can have an 8 battery bank, battery life is a really hot topic. Boat guys freak out when they have to spend >$1,300+ for a new set of Trojan t105s.

But again - I am learning the expectations of the RV world are a lot different.

Gary RV_Wizard said:
Not an AGM.  An AGM is totally sealed and would not have the removable caps on top.  That is a basic Marine/RV flooded cell, which is a slightly modified car starting battery design.    In RV Use, it has a useful life of about 3 years if lovingly cared for and as little as 16-18 months if not.
 

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Ex-Calif said:
And not trying to be argumentative but these batteries (and I have used many) will last 10 years if properly maintained with <30% discharging.

Right, but you only get to use 30% of the capacity you paid for at a time.  So you paid for and are hauling around 3 times as much battery as you're using.  There's no overhead associated with that? 

Calendar time isn't the only measure of life.  If the goal was to get them to last as long as possible you'd put them in a cool, dark place and never use them at all.  Batteries run your stuff.  If you run them twice as hard they last half as long but you still got everything you paid for.  Unless I missed it you don't get a prize for limping these things along for the better part of a decade.  Given the typical use model most RV's have you'd be doing really well to cycle them hard enough to actually use them up before they expired.  You can get anything to "last longer" by not using it.  Why would I do that?  I buy batteries to run my stuff, and that use costs me money.  It's hard to picture a use scenario with an RV (or a boat) where a bank replacement is but a drop in the bucket in overall acquisition, operation and maintenance costs.  I don't see any discussions about maintaining a certain speed to optimize oil or tire life.  Batteries are no different in terms of a consumable.  I *want* to have a bank sized so I wear it out, if it's any bigger than that all I've done is paid more for it at the front end and lost out on use in the back end.  The DOD and cycle life discussion would be different if this was a large off grid storage bank powering a house.  RV's aren't that.

Mark B.
Albuquerque, NM
 
Life cycle cost can get complicated but here is the basics.

Let's say one consumes 50 amps a day. One buys a 100 ah battery and cycles it to 50%. It lasts 3 years so over 9 years you buy 3 batteries.

Another buys 2 batteries and cycles it to 30%. It lasts 9 years and you bought 2 batteries. You also have 100 extra ah in case of a dry spell. One could argue about the up front investment cost but opportunity cost these days is like 2%...

One issue is do you have room on an RV to double the battery space?

The bigger issue is how do you use the RV. If you are plugged in 100% of the time, you should just have an AC powered RV. So if you are plugged in 90% of the time even 1 battery is probably enough. The more one decides to boondock the more important the plan becomes.

I am already thinking to add a second layer of batteries to my rack (4 total & plenty of space) and 400w of solar on the roof.  With this setup I can basically boondock full time as long as propane holds out.

I know I have only been here a nano-second but I am pretty shocked at all the solar and battery discussions. RV folks may think batteries lasting 3 years is great but it's really not in my opinion.

Mark_K5LXP said:
Right, but you only get to use 30% of the capacity you paid for at a time.  So you paid for and are hauling around 3 times as much battery as you're using.  There's no overhead associated with that? 
 
Part of the problem with the RV application is the wildly varying and disparate use of batteries and chargers.  Many have grown to accept that their batteries will expire quickly because the way they're used, that's all you can reasonably expect.  Even if the use model is carefully understood it's usually not practical to optimally size a bank for a given user.  So even with proper care and charging you can end up with a wide range of mild to severe cycling under one owner.  Compounding the problem is the idle time.  These things can sit for months at a time and there are all manner of hazards that cause early battery death, from inadvertent draw through parasitic loads to improper float voltages to stratification damage.  While I wouldn't consider the application itself severe there are plenty of opportunities for problems that a lot of folks aren't aware of and don't have the experience or knowledge to mitigate.  Compound the problem with lowest common denominator charging and monitoring equipment most RV's come with and it's a train wreck in the making.  I'm guessing few spend weeks on end studying battery data sheets and charge profiles, then deploy a temperature compensated battery management system to cover all the bases.  Most folks buy an RV to travel or go camping, not to become an electrochemical engineer.  The usual progression of events is you run the thing, the batteries crap out for some inexplicable reason, you buy new batteries and move on.  If it weren't for my past life working with these things I'd probably be right there too.

Your idea of doubling the Ah and extending the life works only when the cycling is that neat and predictable, and even then the benefit is slim.  Most days of the year the DOD is zero, some number 20%, some number run all the way down and maybe left that way for a while.  Often recharges are just bulk from day to day, then there might be a period where they're floated for an extended time.  On and on, the variables are far too mixed to forecast any kind of lifespan and when compounded with the storage issues the chances of a battery surviving more than a few years of that isn't likely.  So my premise is to use them up before the other cumulative trauma takes it's toll.  Which isn't as easy as it sounds, you mentioned a 50% duty cycle in your example lasting only 3 years. That's three years straight of daily cycling - who does that?  Guessing that even full timers would plug in for some percentage of that.  While I believe a storage battery's potential life is greater than 3 or 4 years in my experience they're showing decline at 6 and by 7 they're pretty much done.  That's under controlled conditions, which would be tough to pull off that long in an RV.  Then we're back to service life.  The service life - delivered Ah - between 80% DOD and 50% DOD is only a small percentage different.  So it becomes a race between using up the battery before age and all the other factors does it in.  So doubling the Ah is sound in terms of operating margin but it's my position you'll never have that bank around long enough to recoup the small percentage service life advantage that configuration offers.  Folks are welcome to try but all it takes is a few operational insults over a span of a few years to negate that incremental efficiency margin. 

Mark B.
Albuquerque, NM
 
I agree fully that the duty cycle for an RV can be very difficult to predict. I agree that "most" RV owners just want to "use" their RV for their particular purpose. I agree there are hazards to the duty cycle inherent in how an RV is used.

The area where we diverge is the relative harm of a 50% vs. 30% discharge - By most all measures and data battery cycles are halved compared to 30% duty cycle.

So for the RV owner who just wants to "use" his RV, peace.  They can live with whatever they get.

I am of the opinion that a diligent owner who wants to learn a bit and mitigate the hazards, and yes probably add some better battery monitoring equipment can have significant savings over someone who does not. For the vast majority who travel from site to site plugging in every day?  You don't even need solar. But something as simple as a SoC device would alert the owner to run the generator for a while if SoC approaches 40%...

At the other end the more one plans to boondock, the more one can save by planning the entire DC system better.

I spent many years off shore racing, sometimes up to 10 days at a time. I spent a lot of time "vacationing" in my own boat "on the hook" - DC self sufficiency was/is critical for off-shore boats but admittedly with 800ah of batteries, 400w+ of solar and a 150 amp daily energy budget it's a whole nuther financial model.

So if someone here asks, "Why do my batteries only last 3 years?" I feel compelled to lay out at least the major factors.
 

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Ex-Calif said:
The area where we diverge is the relative harm of a 50% vs. 30% discharge - By most all measures and data battery cycles are halved compared to 30% duty cycle.

Define "harm".  Batteries are "harmed" by any degree of use, including non-use.  The more you use them the quicker they die, just like anything else.  Like tires they have a "best by" date that no matter how many miles, or "cycles", they will no longer be serviceable.  If you wear a tire out before it ages out, you got all the service you could ever get from that tire.  Same exact thing can be said for batteries.

The measure of a battery's service life isn't cycles.  Cycles are a measure to quantify the effects of depth of discharge to be sure, but the measure of performance is how many Ah it delivers.  That is a function of cycles and DOD, how many amps it delivered to the load.  When you create a graph that factors Ah delivered then the cycles becomes much less significant.  Depending on the battery and how it's used it usually shows that 30% DOD is no better in terms of delivered Ah than 80% DOD.  There is usually a small "bump" in efficiency somewhere between 40 and 60% DOD, about 5 to 7%.  So all this whizzing about the 50% rule is all to gain a single digit percentage net in delivered Ah.  That would be under ideal conditions.  Throw in the above vagaries of storage and use and I would offer that target would be difficult or impossible to achieve.

Going back to the cycle life graph we should look at the 80% DOD point.  I recall with T105's that number is around 600 cycles.  Most folks will never come close to 600 cycles on their batteries, ever.  3 weeks a year plus some weekends is only a couple hundred at most before the clock runs out.  If you're full time then you would probably want a bit more operating margin and only go to 50% on a new pack since as it ages you will be drawing into it deeper and deeper and at some point you'll be hitting the 80%DOD capacity limit.  But this is still many hundreds of cycles which represents years and years of use, all at 50% DOD or greater.  If you don't get years and years it's not DOD, it's some other operational factor.

So if someone here asks, "Why do my batteries only last 3 years?" I feel compelled to lay out at least the major factors.

There are many but way down on the list is DOD.  Way high on the list is improper charging, storing and maintenance.  These effects are cumulative and folks just don't get that the ongoing subtle operational insults to the battery affect longevity.  People treat them as an "energy bucket" they empty and fill back up which works for a while but if you really want them to last you have to pay attention to temperature, absorption time and voltage, and float voltage.  Plus electrolyte if they're flooded.  Batteries don't complain and operating parameters complicated to interpret, so they will work to some extent under marginal conditions until they don't.  With so many offering the advice "don't run your batteries below 50%, you'll kill them very quickly" the first assumption when batteries crap out is yep, I ran them low last year, that must've done it.  When more likely their converter is marginal or they were run dead in storage, left partially charged for an extended time, or who knows what.  DOD is not the enemy, not by a long shot and there should be no anxiety running them right down to 10.5V if that's what the situation demands.  They're made to do that or the manufacturers wouldn't spec them for it.

Mark B.
Albuquerque, NM
 
Peace brother... I think you can have you opinions and they?ll work for you but you can?t have different science. I have have a lot of experience with batteries lasting 8-10 years. There?s a ton of battery science out there.  3 years or less is a terrible life for a battery....
 

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