Buster
Active member
About 15 years ago I worked on a medical device powered by lithium batteries, so I had to deep dive into the technology as part of product design and regulatory approval. The charging information of different cells was, as Ray Clark says, all over the place, but storage at full charge was never desirable. Storing for long periods at full charge will damage the cells. If you have a newer phone, computer or other device, you may notice they don't fully charge right away if you plug them in over night. They'll charge to 75-85% and then stop, giving a final boost to full charge around the time you normally start using them in the morning. The laptop I'm writing this on is almost always plugged in and as such holds its charge at 80%. If I need a full charge for a day or two off the charger, I have to tell it to fully charge. Likewise, if I wake up at 3AM my phone won't be fully charged, even if I dropped it on the charger many hours ago. But at 5AM when I normally pick it up it's charged to full. Whether 30%, 50%, or 85%, anything below full charge is better. As CRS says, the voltage check for lithium isn't reliable without highly sophisticated circuitry, so the MinnKota press to check feature is pretty worthless. That's a feature of the discharge curve, which for lithium is very flat (compared to lead acid which has a steady and easily monitored decline.) Monitoring Ah used is the far superior way, thus something like a SmartShunt or similar battery monitor (I mention SmartShunt only because I use one and can vouch for it--I'm sure others are good.) If I fish a few hours twice a week, I leave the batteries in whatever state of charge they were in after fishing, unless they're below about 40%. Then I plug in the night before I head out.
Another thing to watch is charge rate. Generally, lower charge rates are better, but of course that sucks if you've used 50Ah and want to charge overnight with a 3A charger. 1/20th to 1/10th C is a good range, but don't worry about anything under 1/2C (That's a ratio of charge rate over total capacity, so a 5A charger on a 100Ah battery is 1/20C. A 10A charger on a 20Ah battery is 1/2C, and so on.)
With good quality modern batteries, the degradation from "abuse" is not fast. Don't get overly concerned if you charge to full today expecting to go out tomorrow and then work or weather keeps you off the water for a week. But it is worth worrying about when storing a battery over the winter, or even if you're a Saturday fisherman and always charge to full after use. Charging the night before use would be better.
Unlike a cell phone, you probably aren't charging every day, 365 days a year. So if you abuse the snot out of your trolling battery and only get 600 cycles out of it instead of 1000, that's still decades of use for a weekend fisherman in Minnesota getting out 20 times a year, but it cuts my cell phone from about 3 years to 2 years. So it's relative. You don't have to be overly concerned. If you're the kinda person that geeks out about this stuff, a good battery monitor is helpful. I you don't care, do the best you can and the battery will still likely outlive your boat ownership until you sell it for something better or something cheaper, depending on your needs.
Another thing to watch is charge rate. Generally, lower charge rates are better, but of course that sucks if you've used 50Ah and want to charge overnight with a 3A charger. 1/20th to 1/10th C is a good range, but don't worry about anything under 1/2C (That's a ratio of charge rate over total capacity, so a 5A charger on a 100Ah battery is 1/20C. A 10A charger on a 20Ah battery is 1/2C, and so on.)
With good quality modern batteries, the degradation from "abuse" is not fast. Don't get overly concerned if you charge to full today expecting to go out tomorrow and then work or weather keeps you off the water for a week. But it is worth worrying about when storing a battery over the winter, or even if you're a Saturday fisherman and always charge to full after use. Charging the night before use would be better.
Unlike a cell phone, you probably aren't charging every day, 365 days a year. So if you abuse the snot out of your trolling battery and only get 600 cycles out of it instead of 1000, that's still decades of use for a weekend fisherman in Minnesota getting out 20 times a year, but it cuts my cell phone from about 3 years to 2 years. So it's relative. You don't have to be overly concerned. If you're the kinda person that geeks out about this stuff, a good battery monitor is helpful. I you don't care, do the best you can and the battery will still likely outlive your boat ownership until you sell it for something better or something cheaper, depending on your needs.
