DOD life cycles example and how BD Batteries fixed it.
Johnny bought a used sailboat from a guy. The boat was great, had new batteries, and is fun at the river. Johnny uses his microwave to make dinner every night on the catamaran sailboat in harbor. The ship had a pair of flooded 80 AH, high quality, deep cycle batteries installed when we got to it. The system worked great, and was able to run the 1000 watt microwave for 15 minutes to cook dinner, every day for 10 days. In addition, he runs nearly 4 kilowatts through his system each day, and the generator only kicks on 8 times a daily. When he arrived at BD Batteries, he couldn't get his batteries to last more than a few weeks at a time. He loved the gas savings and runtime on the generators, but couldn't figure out why the batteries wouldn't last. Let's look at his system, and see why Depth of Discharge was killing his pocket book.
2000 watt hours / 12 volts = 166.67ah = more than the batteries in the boat.
4000 watt hours is more than double that; 333.33 amp hours per day.
8 runs of the generator could make each battery storage event then 84 amps hours, and the batteries would roughly discharge to 50%.
WRONG :) This would be the right math in a perfect world, but we don't live there.
Those batteries were flooded not AGM, so figure 72% efficient. When Johnny was buying cheap batteries, things were even worse. Figure the wires are really good, at 90% efficient. The system efficiency is then 65% (.9 * .72 = .65). With batteries at 160 AH, the power you can get is only 65% of the total, or 103ah (160ah * .65 = 103.68 ah). So every day, he cycled his batteries not to 50% DOD, but to 80% (80ah used/103.68ah available = .8). In 30 days, over 8 cycles a day, he would accrue over 320 cycles on those batteries at 80% Depth of Discharge, wringing them completely out.
To fix the problem Johnny faces, we had to do two things, increase the amount of power in the tank, and the efficiency of those batteries that make up the power tank. By switching to AGM batteries, we increase both the efficiency, and the power that fits in the same space (80ah -> 100ah). The calculations looked like this. Those batteries are back to being OEM spec, AGM, deep cycle batteries. Battery efficiency goes from 72% to 87%. The wires stay the same, at 90% efficient. The system efficiency is then 78% or 13-14% better (.9 * .87 = .78). With the AGM batteries, there is 40 AH more power too. So 200 AH with the better efficiency, results in 157ah, or 50% more power than the 103ah he came in with (200ah * .72 = 156.65ah). Now every day, he cycles these batteries to the prescribed 50% DOD, not to 80% any more (80ah used/157.68ah available = .49). In 30 days, over 8 cycles a day, he would accrue over 320 cycles on those batteries at 50% Depth of Discharge. The batteries now last him 1000 cycles, and he goes all summer on one set of batteries.
Bonus: Upgrading the power tanks saves you much more.
The following year, Johnny came back, and said "WOW, it worked!" and then he asked if he could make it better. We said "OF COURSE!" and here is how. Since we know Johnny's usage, we could size his batteries to run for 1 or 2 years with the current configuration, or upsize it to run for up to 5. The trick is the depth of discharge. When we doubled the battery capacity, he went from 200 ah, to 400ah capacity. This took his 8 cycles, now down to 15-20% DOD and he ran this for two years, for nearly 2000 cycles.
Amazed, Johnny then returned and asked for the 5 year answer.
We made him take four of the batteries to put nearly 1000 ah on his boat. He hasn't been back, but Johnny sends us postcards from around the United States, and has folks he meets tell us where he is while they order.
We thank him for letting us share his story.
We are working on the permission for the photos.
He declined the first request.
He said tell the story, but let him be, please.
Even cordial when declining :)
We want to take a minute to thank him again though. (yes we know this is the department of redundancy department), but he earned the accolades, even a paragraph apart. Word of mouth advertising like this is what keeps BD Batteries alive. We thank him for his continued efforts to spread the word, book mark us, Digg our site, tell friends, and send customers who need their own problems fixed with the knowledge gained in places like this one.