Hi Steve,
All batteries lose capacity as they age and LIPO's are no exception. Recommending cycling down to 5% is problematic over time. Within 5 years it is almost guaranteed that one of your cells will lose at least 5% of their capacity. So if the BMS is set to alarm or disconnect loads or charging sources at 5%, then there is a good chance that sooner or later you will be fully discharging the LIPO battery. Once fully discharged, a LIPO battery can short their cells if current continues to flow through the discharged cell - especially if you leave the batteries in a discharged state for any length of time. I have even heard that when they are fully discharged, they can invert - and positive can become negative - though I have no experience with this.
It would be far better to keep a large margin of error and set your lowest discharge point at more like 20% - for safety over time reasons.
The NSB can be set at 20% as its discharge floor which is the same as my recommendation for LIPO.
The other thing that everyone should think of when they install any battery on any boat is how gracefully will the battery fail at the end of the battery's life. To be safe, any installation should be engineered to be safe through a battery failure. Batteries are one of the only components on a boat that are guaranteed to fail over time, so precautions and warning systems should be in place for this guaranteed event.
The only way to guarantee that a boat will be safe through a battery failure is through monitoring of each individual battery for voltage, and each battery or battery enclosure for temperature. This can be done manually, but far better and in real time, if done automatically.
Batteries are load tested each time you use a high power setting on an electric boat. Operators, or their monitoring system, should be aware and log system voltage at a certain current - say 50 or 100 amps. If after some time going by, the voltage under a 100 amp load starts to drop, then you know that you have lost capacity of the battery pack, or one of the cells or batteries has failed.
I have yet to come across a battery meter that changes capacity of the battery over time. This is a big issue because almost all battery meters will show 100% full, even on an aged battery. This means that at some point in time, the boat could leave the dock with an empty battery and the unknowdegeable could be going to sea with empty batteries.
Like it or not, all batteries are maintenance items in that they need to be monitored to insure the safety of the vessel. Monitoring should include voltage and temperature of each thermal enclosure. With Lead Acid, a thermal enclosure is the battery itself. With Lithium, more than one cell can be installed into one thermal enclosure.
James
James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
James and all.
I've run the Northstar batteries in a string of 12 NSB's for 210AH and 144V. As pretty much as everyone has said they were GREAT batteries for the time I had them, about 7 years, until I switched to LiFePo4's. I will disagree with you James on the LiFePo4's. They are fantastic batteries. You concern with the BMS failure is not justified IF you use a good system. I'm using the Orion BMS which in all cases of failure will shut down the charging systems I have protecting the batteries from over and under charge conditions.
The 4's are 1/3rd the weight. 550lbs vs 1800lbs
The 4's will discharge to 5% with no damage vs 40%
The 4's have a life expectancy twice of the AGM's (Based on your mileage of course.
My NSB's would have cost about $8,000 to replace, the LiFePo4's cost $13,000 but if you look at the life expectancy they are cheaper than the NSB's.
0 loss of charge if they sit for 5 months over the winter (my experience) with no load.
A bunch of people are using the LiFePo4's for house batteries.
I couldn't be happier with the LiFePo4's.
Steve in Solomons MD
Lagoon 410 S2E
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