Let 12V battery drop to 50 pct, or top it off daily?

[profdant139]

(I apologize in advance if this topic has already been asked and answered -- my search of the archives came up empty -- possibly using the wrong keywords?)

Anyway, here is the question: is it at all beneficial to discharge a deep cycle 12 volt battery to around 12.1 volts (50 pct state of charge) repeatedly, or is it better to keep it essentially topped off? (These are group 31s, allegedly "marine" batteries, rated at 110 amp hours.) I am, of course, trying to extend the useful life of the batteries. I am guessing it is better to top off than to let it run down even a little, but that is just a guess.

The reason I ask is that in the old days, before I got a 120 watt portable solar panel, I would let the voltage go down to 12.2 or so after three days of use, and then swap out the used battery for my fresh spare.

But now with the solar panel, my battery almost never drops below 12.4 -- the solar panel tops it off every day, and one night's usage never causes it to fall below 12.5 or 12.4. (We are very careful when it comes to energy usage -- LED lights, minimal furnace use, etc.)

If it is somehow beneficial to let the battery run down a little more, I would skip a day with the solar panel, now and then. Otherwise, I will continue to top off every day.

I should also add that when we are not camping, the batteries are hooked up to a float charger.

Thanks in advance for your advice!

[Almot]

It's not just about depth of discharge. You should also bring them to 95-99% on most days.

With solar you might still have to run a generator if solar alone can't bring batts to over 90% for more than a few days. Then you run a genny for an hour and let solar complete the charge. On weekend trips I wouldn't worry about charging them full 99% if it stays on shore charger with 14V hump rest of the week.

Shallow cycling under 5% - led acid battery is not AA NiCd, they have no memory effect. There is "some" evidence of damaging effects of shallow cycling in wet batteries, but it seems that frequent discharging below 50% kills them much faster.

[Naio]

I keep my meds tucked away under the bed, away from heat sources like fridge and microwave. They stay cool there even when the rest of the rig is hot.

Mex (and others), is it really best to NEVER deeply cycle batteries? Like, over years?

I had the impression that it was better for them to get down to 12ish at least once in a while -- with a 100% recharge right after -- and that only using the top 5% caused a loss of capacity over time.

[profdant139]

Hmmm. I did not think of the medications in the sun. We are of the age where we travel with a medicine chest (actually a small dufflebag) full of prescriptions.

I wonder how backpackers cope with this issue? My guess is that anyone healthy enough for real backpacking does not need too many prescription meds.

[MEXICOWANDERER]

Ask the "wuuuuuunderful" gas absorbsion refrigerator if it"minds" bread baking temperature full sunlight parking. Extended really high ambient temperature slices the expected service life of food, YOUR MEDICATIONS*, and electronics, like a radio playing for patio operation.

*They don't insist on 30c 86F temp maximum, for the hell of it.

[profdant139]

Smkettner, we don't hang out during the day, but we do return to the trailer during the late afternoon for drinks and snacks and so forth. If the trailer has been baking in the sun, that can be uncomfortable. Also, we put our ice chest into the trailer during the day (there are no bear boxes for boondockers!), so a hot trailer causes the ice to melt faster.

And here is the real key: DW is a tree hugger. She grew up in dry hot dusty urban LA (the Valley!) and just craves trees and shade and greenery. So you know the rule: If She isn't happy, nobody's happy. (We've been together for 42 years, so I know that rule by heart.)

[time2roll]

Do you really hang out in the RV during the heat of the day?
Put some panels on the roof and park in the sun, plenty of shade at night.

[mlts22]

I would argue that perhaps both would be best, and have at least two charge controllers, one for each set. One CC would be for the onboard panels, and the other would be for portables. Even if the onboard panel is just 100 or so watts, it is better than nothing, and with the battery switched off, would definitely help keep it topped off, even when stored for a long while.

The portable panels would definitely help out as well. I have seen some interesting solar setups, one of which used three 20 foot long flexible panels staked into the ground, which (when combined with a stout battery bank) seemed to do a good job at providing most electrical services, with the exception of A/C.

[profdant139]

That graph is pretty persuasive -- if you do an eyeball average of those curves, the pattern is clear. The less frequently and less deeply that the battery is discharged, the more cycles you will get out of the battery.

So the lesson is to keep it topped off and never use it! 😉

No, seriously. There is a hidden message, I think -- this is a big argument in favor of solar. Instead of running the generator every so often, keep the juice flowing all the time, as much as possible, to extend the life of the battery.

It is even an argument in favor of permanent panels, instead of portable -- my portables only perform when deployed, not while we are on the road or stopped for lunch or whatever. But I still prefer portable solar because it allows me to park in the shade and still get some power, by moving my panel into the sun.

Life is a series of compromises.

[red31]

In the posted graph is depth of discharge based on the original capacity or as capacity is lost is the ah discharged adjusted to the same %?

At what %capacity of original is the battery replaced?

[time2roll]

Deleted

[MEXICOWANDERER]

Whewee Trojan's AGM and Concorde's AGM have "slightly" (?) different life cycle estimates.

[NinerBikes]

Shallow discharges, and as frequently as you can top charge it are preferred. See Mexicowander's graph.

[MEXICOWANDERER]

A good question deserves good answers...:)

[RoyB]

I have found topping off is great if you have a battery bank at 90% charge already.

In my case when my batteries run down to the 50% charge state I have to start charging right away and due to the time restriction I usually only get to the 90% charge in a three hour time frame. If I had solar then the batteries will eventually get to the 100% charge state.

The problem with going all the way down to the 50% charge state first your batteries have put them selves in danger. In my case I start losing battery bank performance if I continually drain them down to the 50% charge state and then bring them back up to their 90% charge state if I don't do a full 100% charge state after 12 or more of these 50% to 90% charge state runs.

I guess crud builds up on the plates of the batteries that only a full charge will burn it off...

I don't think I have ever had a problem with a good full time trickle charge on the batteries if they were at least at the 90% charge state to start off with.

Guess all of this makes sense haha...

Roy Ken