Originally Posted by djmarchand
It all depends on how long you want your batteries to last. Here is a chart of number of discharge/recharge cycles you will get versus state of charge. It is from memory and really rough, but it illustrates the point:
Pct OC V Cycles
80% 12.5 2000
60% 12.2 1000
40% 12.0 500
But also recognize that you get three times the amphours by discharging to 40% versus 80% but your battery life is about 1/4. So it may work out for you to discharge to 40%. You just have to replace your batteries more often but you will need fewer batteries to deliver the same daily amphour usage. So the cost difference isn't that much.
Also note that it is difficult to recharge beyond 85% or so. The current accepted drops dramatically as the battery gets beyond 85%. Solar panels can help. If you recharge your batteries in the morning to 85% with a genset or propulsion engine alternator then your solar panels will top them off during the day.
Personally, I try to operate the batteries between 40% and 85%, with a boost to almost 100% during sunny days using solar panels.
These are valid points. Does one want "battery reserve and lifespan", or alternator powered charging
I was going for 100% solar charging
during the 90% of the time we are at anchor
. During that other 10% of the time spent at sea, our consumption
has doubled, so we run the engine about an hour, at 6:00 AM, when the batteries are lowest and can accept the most amps. Then the solar does the rest, to 100%, over the day.
We normally (at anchor) cycle our batteries down to no more than about 88 or 90% charged, and top them off every day with the solar panels, (usually fully charged by noon). This gives us a battery lifespan of over 10 years (on our 6v X2 @ 340 a/h, "wet" Trojans), and also gives us huge cushion for cloudy days.
On normally "medium dark" and overcast days, we can still top off our batteries, as we have enough solar for our normal a/h needs... X 2.
On "totally dark" overcast days, so dark that you can hardly read, we start accumulating an energy deficit, (like 10 a/h per day), but are still good for a week of this much overcast sky, before we have to crank the engine.
Also, since we have batteries that are mostly full, the v is high and things run better. This has been our magic combination for 15 years, but we are a super energy efficient trimaran
, and have room for that much solar, relative to the need. (285 w of solar)
Admittedly, our formula doesn't work out or make sense for a lot of folks, but we have found that the money
saved on diesel fuel
over the years, more than paid for the slightly more expensive, (lower energy consuming) accouterments, as well as the solar panels.
It is also more pleasant.
PS... You need a smart monitor like a "link 10 or Link 20". Voltage tells you very little about a batteries state of charge.