Originally Posted by a64pilot
My example is straight out of Lifeline's battery manual, and your trying to use total amp hours drawn out, which wasn't the point, number of cycles is.
Now I assume those numbers are essentially un-obtainable outside of a lab, but they do illustrate the point.
Regularly discharge to 50% SOC and you are doing good to get much over two years out of a bank, shallower discharges will mean the bank lasts longer. Plus it's tougher to recharge a smaller bank to full capacity as the acceptance rate is lower.
I'm new and just getting a handle on this myself, but if you discharge a bank to 50% on a cruising boat not on shore power, how are you going to
fully re-charge it?
The point is if you only allow 10% discharge, you need a battery bank that is 5 times the size.
So if we calculated 8 batteries will give us a 500 usable amp-hrs at 50% discharge, we would need 40 of the same batteries to limit the discharge to 10%. There are a number of problems with that:
- That is a huge volume and weight to be putting on a typical 35-45' cruising boat.
- While I've heard stories of longer lives, I've never seen a battery that lasts more than 7-10yrs. 5000cycles (assuming 1 cycle/day) is a little shy of 14yrs. Odds are they die anyway. Anything goes wrong (say a dead cell not caught while you are away from the boat) and you could trash the system far earlier.
- 1000cycles is a little shy of 3yrs. You can have a brand new bank every 3yrs and pocket any unused money
if you sell before 14yrs.
- If you only discharge 10%, that means you are always working in the hard to replace last 10%. Where as if you take it down to 50% most of your charging
is in the easy part where you can dump lots of amps in so it charges quickly.
- Theoretically, you have to buy the same number of batteries over 14yrs but with the smaller bank, you can earn interest on the money
from the batteries you didn't buy.
Reality is I lean more towards the 30% range and if you look at the example I gave, I suggested 50% only if you have a multiday storage
which means it would only cycle down by maybe 25-30% most days. with rare cycles down to 50% (probably will happen a handful of days per year).
Now if you talk about something crazy like 90% discharge, you may only get 100cycles, so the theoretical 100amp-hr battery will only have a life cycle of 10,000amp-hrs which is drastically less over the life of the battery compared to the other two.