Why battery capacity is 50% of rating

[gpounder]

A Case study

Setup
6 - GC2 6 Volt batteries rated 230 AH ea - total set 690 AH (4 mo old)
DC Clamp & volt meter
Victron battery monitor
Magnum ME2000 charger/inverter (Charger max is 100 amp)
Liquid specific gravity battery tester
40' DP with residential fridge and LED lighting in driveway with 50 amp shore power available.
200 watt solar panel (disconnected for test)

History
Batteries have been moderately (20%) discharged about 6 times and heavily (80%) discharged 2 times.

Manufactures generally say a new battery will provide about 80% of ratting until cycled 50-100 times. I could not find any further definitions to what cycled means 10%, 80%??

Draw down
Turned on the fridge (was warm) and all the led lights to create a DC load of 27 amps.
15.5 hours later with the battery still under load @ 11.97 volts. Specific gravity was in the red.
The Victron reported 355 Amp Hours used or 48.5% remaining.

Observation
By the numbers the battery set was below 10% and still supplying 27 amps of current at nearly 12 volts. A 27 amp load is just 9 amps per string, not a high discharge rate.
In my opinion discharging further would damage the battery.

Recharge
Loads were removed so nearly all of the 12 volt charger capacity could charge the battery.
The Magnum was set for a 90% charge rate which is 90 Amps. (this thing get hot so I do not run it at maximum, opened the bay door)
Shore power applied - Charger cycled to bulk charge at 89 amps on the meter and the batteries got 85 amps measured on the clamp tester. Charge voltage is 13.3 volts initially on the magnum display. After an hour the batteries were getting 75 amps and were at 13.6 volts measure at the battery. (Special note, expect some voltage reading difference between the magnum and the battery. At a high current charge I had a .3 volt difference)
The 85 amps fairly distributed to the 3 strings of batteries at 29 amps.
I also checked each battery voltage to make sure they were close, if not close that would indicate a problem battery
Manufacture note - batteries should be charged not more that 10% of AH rating (230ah/10= 23 amps) some manufactures say 15% is okay.
Note - If I was using 12 volt devices then the charge to the batteries would be less as the charger was operating at it's set capacity.

Three stage charger
This is the first stage whereby the charger pushes the set current to raise the voltage is called a bulk charge. So you have a fairly steady amp charge and the voltage rises as the battery charges. At about 14.6 volts the second stage called absorption starts. Now the charger will hold the voltage at 14.6 volts and the current tapers off as the battery charges. The Magnum has a adjustable timer for the absorption stage. The book recommends about 2 hours for my setup. When the times ends the float mode starts and the battery voltage is reduced to 13.2 volts.

Charge time
Now you wait and do the math. So lets use the 75 amp bulk rate charge. It would take 4 hours to get to 300 amps hours, note the discharge was 355 amp hours. The book says the absorption charge will last 2 hours but the rate would be significantly less. If I guess it might be about 30 amps or about another 60 amp hours. You might think the battery should be charged as I am at 360AH for a 355AH discharge. Wrong, there is not a 1 to 1 rate, there are losses, maybe 20%. After just over 6 hours my batteries showed specific gravity in the middle of the white band on the tester so they were not fully charged.

Need extra charge
Like I said 6 hours later my battery set was not fully charged so I decided to use the equalize feature on the charger. It is activated by pressing the charger button for 5 seconds (a Magnum feature).
You must fully monitor this as you can over charge the batteries. The equalize brought the voltage to 15.6 volts and holds it there. This results in a high current charge. You can hear the battery gassing so open doors for good ventilation. I checked the specific gravity every 30 minutes and after about 1.5 hours the reading was in the green. My static no load voltage was 13.0 volts, a bit high but I do believe my battery set is now fully charged. One cycle competed!

Generator
In my case I plugged it and it still took almost 8 hours to charge the batteries. Running the generator for a couple of hours would not due and the risk are that leaving a battery substantially under charged will shorten the life. When camp I will run the AC late in the day and it will charge the batteries nicely. I also get the side advantage as that may be when I am watching TV, cooking etc, not loading the charger.

Charge while driving
Your alternator should supply about 13.8 to 14 volts to the battery set so for that long drive you get a nice top off. This is above the float voltage. I would still check with a specif gravity tester.

Chassis battery
After all this charging it was nice to see my chassis battery at 13.5 volts, that tells me it got charged too and the systems are working. That may also explain the difference in charger output and current measure at the battery.

Final observation
I made a point to draw down the battery set. In real life the batteries would last very well overnight and most of the day and not be so deeply discharged. Add my 200 watt solar to the equation and I would bet in moderate temperatures the battery set would last 2 days with light loads.
Plan to operate the generator for many hours.
Buy yourself the tools to measure and monitor the batteries, volt meters are not enough.
My battery set is not seasoned, I may never fully cycle the batteries 50 times as I do not camp that often.
If I had a fully seasoned battery set (excess of 100 cycles) my results would be better but by how much?

GOING BEYOND 50% PROBABLY IS A BAD IDEA.

Disclaimer - Your system may be different and the numbers may differ too but the process and idea should be relevant to all RV uses.

[Solo_RV_Guy]

Great write up, thanks for this!

[Milford 47]

Very well written, thanks for taking the time

[TQ60]

Cabin batteries can be classified similar to "cycle service" in that they are cycling instead of floating.

Float service are where primary power is commercial and they provide backup.

9 amps per string should be under label rating which is usually 8 hours but some use 20 hours.

Full discharge is 1.75 VPC or 10.5 volts.

Voltage curve may not be linear and if you want to have realistic chart then float charge the batteries for a week, not exceeding manufacturer specified voltage then repeat your test and chart it to 10.5 to get full actual capacity and voltages along the way

[gpounder]

There certainly was more power left in the battery and yes a dead battery is 10.5 volts. It is interesting that the inverter low battery cut off default is 10 volts. The magnum inverter operating range starts 10.8 volts.

My test was not to determine the battery capacity, rather determining a practical capacity that would not hurt the battery. Provide power for an extended day and not need the generator to run all night when you return from a day trip.

There is also a usability factor that as you draw more current from the battery the voltage drops and as the battery gets weaker the voltage drop becomes bigger. So say you tun on a hair dryer at 1000 watts, it takes about 83 amps @ 12 volts but the battery is so low it dips below the operating voltage of the inverter and shuts off. But your computer charger needs only 100 watts and it draws 8.3 amps, the battery may support that device.

As the battery set ages capacity diminishes too. Something to consider when it comes to service life.

[twinboat]

Your measurement of battery voltage under load is not how capacity is read.

The battery has to be resting for a number of hours with no load on it for accurate SOC readings.

[puttin]

Just my thoughts:


I'd watch that Magnum charger like a bully in a school yard. I'd lower the amperage rate below 50a. My bank of 2 group 27's has lasted over 7 years with the o.e. single stage charger...WFCO, I think. I don't boondock much and so no solar either, but- it would all be roughly the same otherwise. The thought of gassing to the point of a possible explosion isn't very good to think about- at least your bay is open. Anyway, just my thoughts. Good experiment!

[justinpirie]

Originally Posted by twinboat

Your measurement of battery voltage under load is not how capacity is read.

The battery has to be resting for a number of hours with no load on it for accurate SOC readings.

Only if you’re just using a voltmeter, not if you’re measuring amps in and out like with the Victron battery monitor which uses a shunt. That gives accurate SoC regardless of the load

That said, the other thing not mentioned here is the temperature when the voltages were measured as lead acid varies SoC voltage based on temperature.

[chuckbear]

You said,"My test was not to determine the battery capacity, rather determining a practical capacity that would not hurt the battery. Provide power for an extended day and not need the generator to run all night when you return from a day trip."...

The entire reason for not discharging batteries beyond 50% is not how they will work today. Every time you do this you shorten the useful life of the batteries. A pair of 6-volt batteries typically last us 8 years with regular use. Continually discharging below 50% will cause those batteries to fail much sooner depending on how often and how much you continue to discharge below 50%. Can you do it? Yes. Should you do it? No. Will it hurt to do it once in a while? Maybe. Will your appliances and equipment be harmed being run on low voltage? A good chance. Anyone reading your case study needs to consider this and so should you. Chuck

[Gary RVRoamer]

chuckbear is right on target. The deeper the discharge and the more often it occurs, the shorter the battery life. "Life" refers to the number of discharge cycles before it has no useful capacity at all. A quality deep cycle battery can do many hundreds of minor discharge cycles, say 10-20%, but relatively few deep discharges (50-80%).


There is nothing magic about the 50% level - it's just a convenient rule of thumb. A little bit of useful life is lost on each cycle, and the deeper the discharge, the greater the loss.

[twinboat]

Its not " many " light discharges compared to " relatively few " deep discharges.

The Depth of Discharge or DOD verses cycle life is a published number on many batteries.

Here is a Trojan Battery chart.

Notice the lower amount of discharges with the 12 volt Gp24, Gp27 and Gp 30 batteries compared to the 6 volt T105s and others.

If you discharge your battery bank down to 20% capacity remaining, instead of 50%, you will take 1 day off its life. Not a game changer.

If you have the room and $, set it up to only use the top 20%. Of course that's the slowest recharge range.

[gpounder]

This is all good conversation for anyone watching.

The idea of 50% provides a balance of what is possible to what can be expected and for most owners following the battery condition may be more than what they want to do. It puts any guessing in the safe zone. There is a lot of people that ask "can I power the residential fridge". Do I need a bigger battery bank, what size to I buy. I hope this helps.

It is clear that the life of the battery does depend on how deeply discharged and the number of discharges. Batteries also do not like high rates of discharge or charge. That heats the battery, causes excessive gassing and you add an explosion risk if the battery water gets too low. The battery ventilation may not be real great in some rigs. A battery will provide more power in totality when slowly discharged.

What I do know for sure is you cannot go wrong with a larger battery set, coming up short is well, short!

[Gary RVRoamer]

If you discharge your battery bank down to 20% capacity remaining, instead of 50%, you will take 1 day off its life. Not a game changer.

How did you arrive at that 1 day number? I see about a 15% loss of the total available charge cycles between 50% and 80% depth. If a battery had a 84 month expected life from those 3000 cycles, that would potentially reduce it by 13 months (assuming the charge cycles are evenly distributed over time, of course). [84 months x 15% = 12.6 months]


In any case, that chart (and other similar ones) shows exactly what I was explaining: the deeper the depth of discharge, the greater the loss of future charge cycles (battery life). 80% is worse than 50% and 50% is worse than 25%. It's a tradeoff of life cycles vs capacity. If you have more AH of capacity for a given load, you can lower the discharge percentage and extend the life. The cost is space, weight and upfront cost (but probably not long term costs).

[Mark_K5LXP]

Originally Posted by gpounder

A Case study

A lot of thought and effort went into this and on that level is to be commended. A couple technical warts but measured data is always good.

After all the data was presented the conclusion at the end is, going beyond 50% is probably a bad idea, but no reason why. One could read into it one or more possibilities but none expressly stated. Wondering what the connection is between the data and the conclusion.

Mark B.
Albuquerque, NM