- How big is the greatest load?
- How much depth of discharge are we willing to tolerate?
The next concept to grasp is the like conventional batteries, Lithium Ion batteries that are discharged more deeply have a shorter lifespan. In this table below you can see the variation of depth of discharge with cycle life.
Discharging a battery to 50% makes it last 5000 cycles but discharging a battery to 80% makes it only last 2500 cycles, This may end up being the most important attribute to selecting a battery size. If you have a bigger battery bank it costs more up front but the lifetime cost is less. If you have a smaller bank it costs less up front but wont last as long and will cost more per cycle of use.
3 Comments
Cory Wikel
2500 cycles (one daily) is more like 6 3/4 years, not 9. There is also the unknown of newer technology and battery cost coming down over a period of time. Let's say in 3 years the price of the batteries is 1/2 the original cost or newer technology is introduced that doubles the capacity at the same original cost. That leans more towards a smaller more often used bank for technology refresh sooner but... that's a guess at the future vs. what's here now. I lean more towards building a system that will "run" over an extended period and building a bank a bit bigger for a 70% duty cycle vs. 80%, assuming that space is available. A 10% increase in battery bank = 18% increase in battery longevity. Of course all this is per manufacturer spec with the best possible conditions and lacking real world use reality.
Peter Kennedy
I think your points are all valid. We don't know how this will all look in the future. I am not expecting a major drop in price on these batteries, the raw material used to make them is in such demand that a giant price drop seems unlikely. But will they drop in price a bit over time? Quite likely. Will there be improved technology in the future? Almost certainly. The main point though is that these batteries are such an improvement over what went before that they are starting to be become very widely used even now, despite their cost. The fact that they also last a long time is an additional bonus
Joe Payne
I have a 1400AH, 10-year old AGM battery bank on my PH trawler in Puget Sound (mild weather makes for pretty good battery life). Looking to replace these with a Lithium system and was enjoying this article when it seemed to stop before answering the question. We anchor out a lot and don't like running the genset in a nice quite cove being shared by others (we hate all-electric galleys!). Having added a 2300W solar system (actually two 1155W with two charge controllers), we now only use our genset on very cloudy days. Our AGM bank rarely drops below 12.0V, but has on occasion, so I think we got the bank sized to a 50% DoD. If that's true, we use 700AH, so if we convert to Lithium and are OK with an 80% DoD, that would require bank size of 875AH. Am I correct with these assumptions? Next concern is how to integrate these with multiple charging systems (alternator, solar, inverter/charger) and three AGM banks (starting and thrusters).
Peter Kennedy
Your assumptions are correct, your AGM bank has a useable capacity of 700 amp hours and to get the same capacity with lithium and using an 80% DOD that means a bank of 875 amp hours. That would be three of our Victron Lithium 300 amp hour batteries Victron Energy BAT512130620 LiFePO4 Battery 12.8V/300Ah NG We can help you design a system. I would just need a list of the equipment you have on board, chargers, solar, alternator(s), inverters and any other pertinent information. Please get back to us using our Support.pkys.com site where you can include attachments if you wish.
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