Tuesday, June 12, 2012


It's a widespread misconception that 50% discharge is a magic number, and going past that will cause damage. The truth is that there's a nearly linear relationship between how far you discharge a battery and how many charge/discharge cycles (total battery life span) you can expect to get. This is best shown by the chart found partway down this page of Eureka! Live:
(Footnote: the chart is based on one I found on Concorde's website, and the original referred to their AGM batteries. Nevertheless, although the exact numbers may vary, the curve shown holds true for just about any lead-acid battery--flooded-cell, gel, or AGM.)
Looking at the chart, you can see that if you only discharge your batteries 10% (not that that would be practical), you can get 5,000 cycles out of them. If you go to 50%, you're looking at about 1,000 cycles. At 80% discharge, you're down to about 500 cycles. And so on.
The exact numbers may vary for different types of lead-acid batteries, but the point is that the relationship between discharge level and lifespan is a linear one: the lower you drain them, the fewer cycles they're good for. There's no "knee" in the curve, so there's no magic number (e.g., 50%) above which everything is fine and below which you cause permanent damage. It's a continuum, and it's up to you how you want to use your batteries.
Some folks choose to push their batteries hard in order to support the lifestyle they prefer, accepting that they will probably have to be replaced more frequently. That's a perfectly valid choice.
I tend to go in the other direction, rarely discharging my batteries below 80% charge (20% discharge) and almost always charging them up to 100% at least once every day. I can get away with that because I have 520 amp-hours worth of batteries, so I can pull 100 Ah out of them and still only have discharged them 20%.
But again, that's a personal choice I make, and it isn't the right choice for everyone. If you have the factory-standard 225 Ah batteries, limiting yourself to a 20% discharge (45 Ah) would probably put unreasonable limits on your lifestyle.
The moral is: understand what your batteries can do, and what to expect of them. Then use them as best suits your needs. They can always be replaced--and in the long run, they will have to be sooner or later; it's just a question of when.

From the Trojan web site:
1. Shallow discharges will result in a longer battery life. What they should add is "you'll get more cycles, but not more total amp hours over the life of the battery".
2. 50% (or less) discharges are recommended. Reasonable suggestion when you have no idea how well people are monitoring their batteries.
3. 80% discharge is the maximum safe discharge. I think this is sensible, as it corresponds with what I know about deep cycle lead acid batteries but requires more careful monitoring.
4. Do not fully discharge flooded batteries (80% or more). This will damage (or kill) the battery.
5. Many experts recommend operating batteries only between the 50% to 85% of full charge range. A periodic equalization charge is a must when using this practice. This is usually about charging efficiency, rather than battery life. It's how I do it when we use the genset to charge, because the last 15% of charge is done at a low rate and requires excessive genset running.
6. Do not leave batteries deeply discharged for any length of time. This is a real killer, worse than deep discharges followed by immediate charging.
/If you are using AGM batteries, be sure to check the manufacturer's data sheets. My experience with deep cycle AGM's is they are more tolerant of deep discharges than flooded batteries.

Buying new Trojan batteries:
- Each battery should have a voltage between about 6.2V to 6.4V.  This would be not connected and having been at rest for a while.
- Check the date code. All batteries have a manufacture date code on them. Trojan uses two codes: one on the positive and one on the negative terminals. One tells you the date of mfg and the other tells you when electrolyte was added and it shipped from the factory. They use letters and numbers. A-L represents the months Jan-Dec. On date of mfg they use two digits for year. On date shipped they use a single digit, so L3 would mean it shipped Dec. 2013.
- Be sure to follow proper, safe procedures: read the Lazy Daze Companion article on how to disconnect batteries.

Contributors: Andy Baird, Eric Greenwell, WxToad, Jim C.

Date revised: 16 Jan 2014

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