Yet another battery questions thread

[PaulBates]

I have a new to me 2020 Flagstaff Hardside Popup - T21FSHW. It came with a double battery box with one battery in it.. this one Aqua Edge Tournament Series Deep Cycle Marine Battery Part 24DC100. I read the recommended battery primer links where it mentioned marine batteries can be kind of deep cycle and kind of not. So, here goes:

Story: (see pic of the graph)

• I used it recently at a nat forest campground sans plug in... eg, on the battery. On arrival at camp I switched the 3 way fridge from 12v to gas. Turned the healthy gas heater on and watched on a battery meter decline due to 12v fan usage... better than I expected. Turned the tstat down at bed time to high 50s. The next day hit 50% and my battery monitor, which the software was designed for a car, turned red as expected. But no operational problems.
• Sticker on the battery says 12/20, rotate 7/21. The previous owner indicated it was stored outside somewhere.
• There's a WFCO converter that the battery meter proves is faithful to managing the 3 usage modes per the manual
• We're not chronic boondockers.. sometimes a night at a rest stop or at most 2 nights in the woods like the trip above, followed by a drive home or several days on plugin power
• Someday I'll investigate the wiring and put in 2 matched real deep cycles in the box if its ready

Questions

• Based on the story and the graph, I'm looking for experienced opinions on what's likely to happen if I use this battery beyond 50% as I'm looking forward to a dual battery electric upgrade.
• Not sure I'm in a hurry for solar as it feels like a lot of expense and work for something that maybe delays running out a little... and we tend to prefer shady campsites

I realize I'm responsible for the results.

 

[Boat Bum]

The main purpose of a marine deep cycle battery is that it can take repeated discharges of <50% with proper recharging w/o the detriments of regular cranking batteries doing this.

 

[Boat Bum]

BTW, that doesn't look like a marine deep cycle battery, it looks to be a group 24 starting battery. I realize it says deep cycle, it is the smallest size, group 31 is where you want to be, or there abouts.

 

[Mark_K5LXP]

Deep cycle batteries have well documented depth of discharge vs cycle life graphs. Sometimes you can find data for "RV/Marine" batteries. Absent any specific data you can only guess, but "half" of a deep cycle is a fair guess. So nominally 300 cycles to 100% DOD, 600 cycles to 50%.

From there it comes down to how deep you go, and how often. Using the worst case figure of 300 cycles, and your stated "non chronic" use, how many days per year do you anticipate boondocking? Take that number and divide by 300, and that gives you the number of years of cycle life you might expect. If it's more cycles than 5 years' worth or so, then your degree of use is of no consequence, the battery will age out or fail for other reasons first.

Your voltage graph shows about 12V under load, which represents about a 25-30% DOD. At that depth your cycle life is likely approaching 800 or maybe more, so you would be hard pressed to wear this battery out through casual use. That would be one cycle a day for over two years straight (800/365).

Others will offer that RV/Marine batteries are not robust enough to withstand any degree of cyclic service but again, absent any manufacturer cycle life data "you get what you get" until proven otherwise. Given that this battery is "paid for" you have nothing to lose by using it until it gives up. Since you have a battery monitor there will be little question when you're approaching that point, and from there you can decide what upgrade path to choose.

Mark B.
Albuquerque, NM

 

[TonyL]

I agree with boat bum, a size 24 battery isn't really big enough for anything other than a pop up or small camper. A size 31 is the way to go if the battery box is big enough.

 

[Gary RV_Wizard]

I agree with boat bum, a size 24 battery isn't really big enough for anything other than a pop up or small camper. A size 31 is the way to go if the battery box is big enough.

But PaulBates does have a pop-up, and says he doesn't plan to boondock all the time. In my opinion a second Group 24 marine deep cycle will be entirely adequate for his needs. Not everybody needs 230+ AH of battery capacity. And marine vs true deep cycle or lithium is just a cost tradeoff.

Mark explained the cycles, so I won't repeat that.

 

[Gary RV_Wizard]

BTW, that doesn't look like a marine deep cycle battery, it looks to be a group 24 starting battery. I realize it says deep cycle, it is the smallest size, group 31 is where you want to be, or there abouts.

Marine deep cycles are by definition combined starting and deep cycle, i.e. a hybrid design. They are suitable for cranking smaller engines (that's the "marine" part) or moderate auxiliary loads (the "deep cycle" part). Group 24 or Group 27 are the typical sizes for marine deep cycles, which are most likely to be used where space and weight are limited.

 

[TonyL]

But PaulBates does have a pop-up, and says he doesn't plan to boondock all the time. In my opinion a second Group 24 marine deep cycle will be entirely adequate for his needs. Not everybody needs 230+ AH of battery capacity. And marine vs true deep cycle or lithium is just a cost tradeoff.

Mark explained the cycles, so I won't repeat that.

Sorry Gary, I missed that vital bit of information. I should have read his post more thoroughly.

 

[Gary RV_Wizard]

Sorry Gary, I missed that vital bit of information.

Hah! Wish I had a dollar for every time I overlooked some vital piece of info like that.

 

[PaulBates]

I appreciate everybody’s experienced feedback, leading to a conclusion of “push the battery harder”. I think if I went to an indicated 30% I could get 3 nights out of it, depending on use of the heater’s fan.

Sorry Gary, I missed that vital bit of information. I should have read his post more thoroughly.

It’s all good Tony. I error on the side of supplying as much info I think might be relevant, which leads to a longer post, but less asking for additional info. I greatly appreciate all of your feedback.

Hah! Wish I had a dollar for every time I overlooked some vital piece of info like that.

… me too!!

 

[Babe2201]

I was considering upgrading my 2 batteries earlier this year and I was going to use the Interstate SRM-24. I wanted to use the SRM-31 but battery box was not big enough. I would consider those. The price was reasonable. After looking at our reservations I realized we had hookups at all our sites so I decided not to. We have recently sold that camper so will not need them now.

I think if you got the box for 2 it might be worth it for you to get 2.

 

[PaulBates]

...I think if you got the box for 2 it might be worth it for you to get 2.

Thanks.. It can fit 2.. And the one in there has a piece of foam to keep it from bouncing around front to back.. it can take a bigger one too, the question is how big.

After reading all the comments, I don't think I'm going to need anything bigger than a second of what I already have.. And I think I'm going to try pushing what I have a little further before adding a second one.

 

[Babe2201]

Thanks.. It can fit 2.. And the one in there has a piece of foam to keep it from bouncing around front to back.. it can take a bigger one too, the question is how big.

After reading all the comments, I don't think I'm going to need anything bigger than a second of what I already have.. And I think I'm going to try pushing what I have a little further before adding a second one.

I would agree that would be enough. That is what we had with a good-sized TT and our fridge did not have an option for propane so that took some juice. I would have to use a generator to charge up at least once during a weekend but we were not careful with usage knowing we had that.

 

[Kirk]

I think if I went to an indicated 30% I could get 3 nights out of it, depending on use of the heater’s fan.

I think that you will be dissappointed because taking your battery below the 50% level will have a much great impact on the battery life.

 

[Mark_K5LXP]

taking your battery below the 50% level will have a much great impact on the battery life.

Depends on what you call "life". The lifetime supplied Ah is pretty flat across the range of DOD from 20% to 100%. Calendar life is dictated by numerous factors, mostly environment. Cycle life is also fairly linear, the deeper you discharge the fewer cycles you get. But the power output goes up commensurately, so the net Ah delivery is nearly the same. So it becomes an application decision just which one you decide is most important and ensuring the use profile promotes that. For the "average" camper that uses their equipment 21 days a year, with proper care the battery will age out long before it would wear out. For off-grid solar you ride the line so cycle life hits about the same point as calendar life. For maximum Ah per $ you run 'em deep and often, ensuring you get all the Ah you paid for. Charge efficiency is rarely considered in RV's but frequent shallow cycles take more energy than fewer deep ones. If you're charging with a generator you're running it more often and longer than you likely need to. If one follows the "50% Rule" you are almost assured you are leaving unused capacity on the table. Even for "frequent" cycling - a few times a week - it's still very difficult to wear a deep cycle battery out before it expires for other reasons. Picking a DoD point is useful for making sure you have a power reserve you want but for battery longevity is a strategy that I would say rarely if ever applies to RV service.

Mark B.
Albuquerque, NM

 

[Contributor]

I don't like to go down lower than 50% but have. The issue I've had is with old batteries that show voltage but have no real capacity to deliver amps. Yes, you can turn lights on and the refrigerator works on propane. But my water pump will draw 7A when running and will pump water fine but when I shut the faucet off, the pump can't pressurize the lines and turn itself off it keeps running so I shut it off with the switch. The other one is the furnace, the current draw dips the battery voltage down below what the furnace likes to see.

That is the big advantage I see with LiFePO batteries, steady voltage under load down to 10% or so.

 

[Mark_K5LXP]

If the battery is "old" and can't "deliver amps" then it's probably shot. It *could* be caused by repeated deep cycling but if the battery is spec'd for it (and true deep cycles are) then it will perform to rated spec/cycle life for the DOD you use. For "real" deep cycle batteries like the T105 this is around 600 cycles to 100% DOD. That's running it to 0% every other day for over three years. You'd be hard pressed to find anyone that comes close to doing that in the real world much less in an RV. Odds are much higher the root cause of batteries giving up are some other trauma like letting electrolyte get low, leaving it sit for weeks or months half charged, jamming too much charge current into it or any one of a dozen different things that usually kills batteries but DOD >50% gets blamed for.

Mark B.
Albuquerque, NM

 

[Contributor]

Yes, my "old" batteries were cooked by my battery tender. I used to just store them on my work bench and charged them as needed and they lasted. Then I fell into the "battery tender is easy" trap and killed my previous set.

I no longer keep them on a tender, I don't trust the tenders on GC2 batteries. I buy nothing but deep cycle 6 volt batteries and I've had them last 8 years before the tender trap.

Now I just charge them, store them, then every month or so top them off.

My point is that it's not easy to know the condition of a battery until your out camping (boon docking in my case as that is all I do) then you have the experience I had even though the batteries seemed fine.

But for those that don't have enough capacity (single size 24) even with a new battery, going from 50% to not having usable capacity happens fast.

 

[John From Detroit]

The main purpose of a marine deep cycle battery is that it can take repeated discharges of <50% with proper recharging w/o the detriments of regular cranking batteries doing this.

DEEP CYCLE batteries can go to 50% before "Rapid aging" as I call it sets in
Starting batteries like to be 80% full or better
MARINE/deep cycle (note the way I typed that) are primarly STARTING (MARINE) with lip service to deep cycle.. say 70-70% minimum.
What I Can not see in the photo is the DEPTH (Or height if you like) of the battery box.
A GC-2 Golf car battery is about the same length, and width (You said it's a dual battery box)
So here are some numbers
One Group 24 = about 75 AH (Give or take by brand) 2 = 150 AH and see abobe about using only about 1/4 of that safely.


One pair of GC2 (They are 6 volt sitting on the shelf) wired like this
-{Batt 1}+==-{Batt 2}+ to get 12 volts

220 amp hours give or take a bit half usable
I'd go with GC2s.

 

[Mark_K5LXP]

the "battery tender is easy" trap and killed my previous set.

Interesting, I wonder what the specifics of that are. I've got 3 different tender-type mantainers and to date they've all done a good job. In days of yore it was the "trickle" chargers that wrecked batteries (where'd all the electrolyte go?) but since using one of the first tenders on a motorcycle battery in the 1990's through today on my cars and RV they've been a check the box solution for me.

it's not easy to know the condition of a battery until your out camping

I check capacity twice a year, when I de-winterize and winterize. Batteries of good merit will not degrade quickly so basically I know they're "good" at the beginning of the camping season, and I'll see how much if any they've degraded during active use. During the winter I let a tender keep watch.

for those that don't have enough capacity (single size 24) even with a new battery, going from 50% to not having usable capacity happens fast

Can't blame the battery or the DOD for that though, if it's under-spec'd then there will be other issues besides cycle life. If it's too small then only running it to 50% only makes it effectively smaller, and pretty inconvenient to use. So matching capacity to the application is the key to the balance of run time, $ per Ah and service life.

Mark B.
Albuquerque, NM