Discharge batteries

AGM and flooded both showed similar patterns of small changes of total amp hours over battery life. Lithium (unconfirmed data) showed a big deterioration with depth of discharge.

Here is the Lifeline chart everyone quotes that shows more cycles at 50% than 80%.



Here is the data from the same chart calculated to show total amp hours out of the battery over it's entire life at the various depths of discharge.



It gives a totally different, and IMO, much more relevant picture of what is really going on

Are they agm or flooded? Agm goes way deeper more often without giving up. The deeper you go the less they will last, but there is a lot of variation on what deeper and less mean.

Voltage is a very poor way to determine your state of charge, as to be even close to accurate, the batteries have to be sitting with no load for something like 4 hours. If any load is on at the time you check, it gets even worse, or even is if you just had a large load on. I you have solar, it's voltage will mess up your reading. A battery monitor will actually be able to accurately tell you how much state of charge you have at any given time. They are not all that expensive and work very well.

IMO, the 50% rule is severely overrated, and in many ways does not reflect the real world use in an RV. What folks are comparing are the number of recharge cycles at the various discharge percents, when what really matters is how much power can the battery store, and give up over it's entire lifespan, either in amp hours or watts hours. Sure, you get a lot more cycles at 50% down, but you also get a lot less power out per cycle, so not apples and apples. If you look at the total lifetime amp hours of a battery, the difference between 50% discharge and 80% is closer to 10% difference, not the 50% that you often hear quoted. Most will only use the deeper discharges occasionally, and since discharge depths average over all of them, any particular deep discharge is further reduced in influence on life. Having deep discharges now and then does not make the other shallower discharges cause more wear, which is also commonly stated. By using more of the available capacity, many folks can reduce the size of the battery bank, when compared to holding hard and fast to the 50% rule, saving a lot of precious space and weight. I think it is highly likely that almost everyone will do more damage to their batteries by using the rather poor quality charging systems (both shore and driving) in class b's than they would do by not adhering to the 50% rule. Almost all the chargers use timers and algorithms to determine charging, with no measurement of actual state of charge, so they are near certain to over or undercharge the batteries almost all the time. The engine charging is totally uncontrolled in most RVs, so overcharging on long drives is extremely common. A battery monitor will very quickly show this to be true for both sources.

There have been a couple of recent discussions about this on the class b forum, and they include reference data, charts and graphs for the different battery types. It is a very eye opening set of data for those of us that have always heard about how the 50% is set in stone.

We won't hesitate to take the batteries to 80% to get an extra day or two in a nice spot without having to go for a long drive of find shore power. Those discharges would have a minimal effect on battery life (plus we have good charging to get accurately recovered), and we aren't dragging around lots of extra batteries (even though we do have a lot of capacity.

RoyB is right on my target. We mostly boondock and battery management is therefore critical. I use the same chart he displayed, my inverter shows the input voltage and I carry a voltmeter. Also, I have a digital voltmeter mounted in the battery area along with my solar controller. And RoyB's chart laminated and stuck in there as well.

12 volts is the bottom. Done that on 3 Bs and 2 Cs. Works for me and I don't have to fuss with all the techno stuff. Also, like RoyB, I get many years of service from my batteries.

Paul

12 volts. Prefer 12.2 or so but have seen 11.9
What is your charging source? How many volts?

me me em I can do a SOC chart...

I just picked 12.0VDC to my absolute bottom number. This is my 50% charge state number... If my batteries drop to 12.0VDC then I will shut them down if I can't start re-charging right away...

here's Johnny...


This has been my camping style since early 2009 and I am just now starting to have failing 12VDc Interstate batteries... Time to get new batteries this season

Roy Ken

Around here we generally stick to no more than 50% discharge, meaning around 12.2vdc. I'm sure someone will post an SOC chart soon.

It depends on the specific brand and type of battery.....and how much potential loss of lifespan you are willing to risk.

Contact the maker of your batteries for the best answer.