mike-s wrote:
Whether to apply the "don't discharge more than 50%" rule of thumb depends on how much you'll use the battery.
If you're less than a full-time boondocker, the difference may simply not matter. Only 4% of campers do over 30 days per year. The majority camp less than 15 nights. And even fewer if only considering boondocking.
You can find pessimistic estimates for generic deep cycle lead acid of 200 cycles @ 100% discharge. Then look at a premium battery - Trojan Battery's literature for their SPRE 06 255 solar battery (same size as a T-105) shows a life expectancy of about 1900 cycles @ 50% discharge, and about 1200 @ 80%.
The inexpensive Interstate GC-2's sold by Costco are supposedly 650@80.
So, take the extreme high end of 30 full battery cycles per year, and use the pessimistic expectation of only 200 cycles of lifetime, and that's still almost 7 years. Not many RV batteries last longer than that regardless of how well they're treated (now someone will describe their 20 year old battery bank).
But let's say, age and cycles combined, the battery lasts 6 years instead of 7. We did that by limiting how much battery capacity we used - 50% instead of 80%. So, over those 6 years, we avoided using 30% of the available capacity to save (7/6=1.17) 17% in battery cost. It's still a false economy.
Use more realistic figures, and things only get better. Use the Interstate 650@80 numbers, and that's over 21 years. Cycles simply aren't a significant factor at that point. Even a battery on constant float charge isn't going to last that long.
I had a pair of T-125's that I consistently discharged to 25% SOC around 40 times/year, then when home charged them fully. kept them watered. After 10years the SG was still near new. Then they went to daughters in-laws for another 4 years of abuse. Last year they finally were getting pretty worn out, SG on a few cells was dropping, but they still were lasting a weekend. But total number of cycles to 75% DOD was around 500, close to what Trojan claims for life.
And I asked trojan what determines EOL. They said when capacity is 75% of new. So even the cycle life they claim really isn't taking them down to dead and gone, just 75% of initial AH.
So, yes for most campers, you CAN deeply discharge GC batteries and have reasonable cycle life. What that means is that when comparing AH, if you use 50%DOD for 12V, vs 75% DOD for GC, GC has a significant advantage in useable AH. And even trojan 12V deep cycle won't give near the cycles at the same DOD as the GC does.
Now, there are tradeoffs, as BFL mentions, and as I have experienced, GC are really weak when it comes to heavy discharge currents, the long life design sacrifices internal resistance. coupled with not sharing current between parallel batteries, and GC are at a signficant disadvantage when used with high inverter loads like a microwave. A pair of GC has a hard time with a 750W inverter load below about 80% SOC, where a pair of 12V won't grunt till 50% or less.
we boondock a lot, and like to occasionally use the microwave on the inverter. and not have to be anal about battery capacity, so to solve both problems I have 4GC. with that I CAN run the microwave with the batteries down near 50%. with a pair I was pressing my luck at anything below 80%SOC.
My 4GC with an 550AH won't give the CCA that a single 12V 75AH starting battery will give.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!
