At Pure Watercraft, we spend a lot of time thinking about how to get the most out of batteries for electric boats. Along the way, we’ve learned a lot that can also be applied to smaller batteries used in everyday life. All of us have lithium-ion batteries controlling an increasing share of what we do. They power our cellphones, laptops, tablets, fitness bands, and bluetooth headsets. For some, they power lawn mowers, drones, cars, and boats. Yet most people know very little about how to care for the batteries to make them perform their best. Have you ever wondered why your phone doesn’t last a full day any more? Or had to replace a non-replaceable battery in a cellphone or tablet? Caring properly for lithium-ion batteries can maintain their capacity, and extend their lives by 2-3 times.

Here are the top six ways to make them perform their best:

1. Mostly full is a LOT better than fully full

If you only fill a battery to 86%, you double the number of battery cycles. What does that mean? It means you get double the energy out of the battery over its lifetime if you only fill it to that level. (Note: a “cycle” means discharging and charging a battery its full capacity one time, not necessarily in one shot, so if you charge and discharge it by 50% of its capacity twice, that counts as one “cycle”.) Stopping at 90% is better than 100%, and 80% is better than 90%.

2. Don’t leave them full

Keeping a battery fully charged seems like the right thing to do. You’re always ready to go. But sitting in a fully charged state is very costly to a battery’s lifetime. Unintended chemical reactions occur more often in a fully charged battery, reducing its useful life. While it’s not good for the battery to fill it 100% at all, it’s much worse to keep it at 100% for a long period. As an example, if you store a battery 100% charged for one year at at 77 degrees F (25C), you’ll permanently lose about 20% of its capacity, while if it’s stored under the same conditions at only 40% filled, you’ll only lose 4% of its capacity.

Most studies show that 40% is the optimal charge level for long-term battery storage, but you’re pretty safe up to about 80%, and much better off at 90% than at 100%.

3. Don’t empty them all the way

Batteries don’t like to be empty. It reduces their capacity permanently if you discharge them to 0%. It’s much better to use the battery from 80% down to 30% twice than to discharge it from 100% to 0% once. It’s a little less harmful to the battery than overcharging, but both are harmful. An additional problem from over-discharged batteries is that the protection circuits that manage the battery for you don’t have the power to operate. To get the most from your battery, keep it near the middle of its charge most of the time.

4. Don’t get them too hot

Batteries are worn out by many uses, or by sitting around for years, but both of these are worse when the battery is hot. Getting a battery very hot will shorten its life, because the bad chemical reactions happen more at high temperatures. And while storing a battery at high temperatures is costly, using (charging or discharging) it at high temperatures is even worse. As an example, if you if you store a battery at a healthy 40% charge for a year at 77 degrees, you’ll permanently lose 4% of its capacity, but if you store the same battery at 104 F (40 C), then you’ll lose 15%.

So, don’t keep your cellphone on a hot dashboard.

5. Don’t get them too cold

Getting cold isn’t as bad for a lithium-ion battery as getting hot, but it reduces the energy you can get out of it. If a battery is cold, it will empty more quickly. So a “cycle” is less useful to you at low temperatures. One reason this is less of a concern than hot temperatures is that a battery will heat itself somewhat when discharging or charging, but you still have the problem until it heats up, and you lose capacity during that cold period.

What’s the ideal temperature? A good guideline is that batteries like the same temperatures that people do. 70-75 degrees F is a great temperature range.

6. Don’t charge or discharge them too quickly

Most of the guidelines above apply similarly to the different flavors of lithium-ion batteries, but the rate at which you can safely and efficiently charge/discharge your battery depends on which one you use. Some general principles apply, though: the faster you charge or discharge the battery, the more it degrades the battery’s lifetime, and the heat generated during charge/discharge can make the problem a lot worse if not actively cooled. Most batteries are unhappy if you discharge them at a rate that would take them from full to empty in less than 30 minutes, or if you charge them at a rate that would take them from empty to full in less than 60 minutes, but the slower the charge/discharge, the better. There is a LOT more detail on this topic (specific behavior of different chemistries, nano technology, etc.), but these simple principles always apply. If you need high discharge/charge rates, then choose a chemistry/battery type that can handle them (often called “power optimized” battery types).

They’re like people

A good way to think of batteries is that they’re like people. We are happiest in 70-75 degree weather. We like to eat when we’re just a little hungry (not starving), and we live a lot longer if we don’t overeat. And stress shortens our lifetimes. If you treat batteries like you want to be treated yourself, then they’ll respond by being happy and productive for a long time.

Practicalities

Practically, managing your own battery is not very easy. Most chargers charge until a battery is full, and you have no control over where it stops. And when you disconnect the phone to avoid over-charging it, you start a discharge cycle that also affects your battery life. But in some products (most notably EVs, especially those from Tesla Motors and BMW) you have good control over how your battery is treated. Being familiar with these 6 simple principles will help you get the most out of the batteries that power your life.

For more information on the care of lithium-ion batteries, see Battery University.