Battery:Expected Cycle Life vs. Depth of Discharge

[David Sanner]

How accurate is the 50% battery discharge rule of thumb?

I've been looking at this graph from US Battery... (see attached)
[attachimg=#]

Perhaps I'm missing something here but the slope of the curve
between 40% discharge and 90%  discharges makes me question
the 50% rule for battery discharge. (I know it's a semi-log graph)

The graph shows that if you run your batteries down 40%
(60% left) you should expect 1475 cycles in it's life.  

However if you run it down twice that, using 80% of the battery
capacity, you should only expect 675 cycles.

But here's the catch that I'm wondering about...
Since 80% DoD is twice amps the 40% DoD one would
only have to cycle half as often.

So doing the math the life cycles for a similar total
energy usage would be: 1475 vs. 1350 (2 X 675)  

That's less than 10% difference... not bad considering
how much better the acceptance is on a 80% discharged
battery and hence less overall time running the engine.  

And not bad since you would only have to by half the
battery bank to get the same number of amps between
charges.

Surely I'm missing something considering how many times
I've heard the 50% rule but it sure looks like if you
really want to get life out of your batteries don't drain
them much below 20% ... and once you get get below
30 or 40% (and the curve straightens out) taking them down
to 80% might not be as harmful as you may think, especially
if it's only for that week long cruise now and again.

Here's a link to the original chart:
http://www.usbattery.com/usb_images/cycle_life.xls.pdf

Then again perhaps this chart is not be believed as I've
also read that number of cycles to 80% will be less than
half the number of cycles of one cycled to 50%.

One would think US Battery didn't just make those
numbers up.

[jfssail]

David, I don't know how you take the US Battery curve and come up with an expected battery life, since each boat has a different operating mode for their batteries. In my case, I installed 4 US Battery U2200 batteries in 2004. During the first month of operation these batteries were drained to zero, 11.45 volts
( charger off with the frig on during the week)
Since then, my batteries remained on charge (Truecharge 40) 24/7 while at the dock. When cruising for 8 weeks during the summer I seldom drop the batteries below 40% charge. All four batteries are still at 6.3 V when fully charged, sp gravity of the acid in all cells is the same, and I usually equalize the banks twice a year.

To say the least, I have been very happy with these batteries. Their capacity appears to be the same as when they were new, and will be starting their 7th season this year. I can go 3 days without charging the banks and keep the drawdown at 40%.

Before starting out for the Door Peninsular in late June, I will give the banks a load check at dock to check the drawdown after 24 hours under normal loads like frig on, running cabin lights, anchor lights and other normal acessories.

Jack F Stewart
1993 C36 #1233 "Windancer"
Port Clinton, OH