Lithium-Ion Battery Care Guide – Part Four

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Lithium-ion batteries are the most common battery in consumer electronics. They are used in everything from cell phones to power tools to electric cars and more. However, they have well defined characteristics that cause them to wear out, and understanding these characteristics can help you to double — or more — the life of your batteries. This is especially useful for products that do not have replaceable batteries.

Battery wear is loss of capacity and/or increased internal resistance. The latter is not a well known concept, but over time the battery is able to put out less amperage as the battery ages and eventually the battery is unable to generate power quickly enough to operate the appliance at all even though the battery is not empty.

The standard disclaimers apply, all advice is for informational purposes only, CleanTechnica is not responsible for any damages caused by inaccurate information or following any advice provided. Also, new technology may change the characteristics spoken about, making them less or more relevant in the future or even rendering them obsolete.


Lithium-Ion Batteries Age From The Following Factors:

  • Time – Part One
  • Cycles – Part One
  • Storage/operating temperature – Part Two
  • Charging – Part Two
  • Discharging – Part Three
  • Depth of charge – In this article
  • Time spent at near full/empty – Part Four (upcoming)
  • Depth of discharge – Part Five (upcoming)
  • End Of Life – Part Five (upcoming)
  • Summary – Part Six (upcoming)

Depth Of Charge

Unlike most other battery types (especially lead acid), lithium-ion batteries do not like being stored at high charge levels. Charging and then storing above about 80% hastens capacity loss. Most EVs allow you to choose a percentage to charge to via software. Select 75-80% if that will get you through the day/week. Charging above 80% is not a big problem if you intend to draw it down quickly. If you have a long driving day planned, then setting the software to charge to full by morning (not storing the vehicle overnight at full) and driving until you are below 80% rather quickly will not cause much extra wear to your batteries. In general it’s the storage time above 75-80% that causes most of the extra high charge wear.

Elon Musk has said 90% is okay for the Model 3 battery chemistry of the time, but didn’t provide any data.

Many people plug in their cellphone before going to sleep and wake up to a fully charged phone. Most phone chargers will fully charge the device in 1-3 hours and then it spends up to 8 hours or more at full charge. This is one of the big reasons phone batteries are toast after a year or two — it has spent more than 1/3 of its life at 100% charge. There are several ways to work around this problem. One is to charge to 80% and then unplug from the charger. This way it is not spending any time at 100%. Or even better, charge it in the morning (so it does not spend overnight at 80%). Plug it in after you wake up and by the time you are ready to leave for work (or WFH) it should be charged to whatever percentage you’re aiming for. This allows the battery to spend much more time at a lower charge level.

Software such as Accubattery (for Android) has the ability to beep when it hits a selected charge level (such as 80% or whatever you choose). It also measures the current battery capacity and capacity loss. There may be an iDevice equivalent app. Some iDevices measure remaining full battery capacity and report it somewhere in the settings. However, they typically have no native 80% charge beep/cutoff feature. If you can get through your day on 80% charge then you’re good to go. If you anticipate a long day (or you have heavy phone use days everyday) where you can’t get to a plug beforehand, you can charge to above 80% while drawing it down below 80% rather quickly hence minimizing the time spent at high charge. Also consider a USB Powerbank — units that can fully charge a phone are quite cheap these days, though bear in mind the full charge wear issue will occur with them also, but they are cheap enough to replace more often than your phone.

Some phones (such as some Samsung models) have an ability to select a max charge percentage. They may not be ideal (such as being 85%) but they are another available option.

For non-phone/EV devices, if it has a charge meter then you can go by that. Some devices do, but some do not. Also some have a rough charge indicator, such as five lights that indicate how full it is. But it is hard to know where the threshold is — for example 4 out of 5 lights could mean 50% to 90%. You can try to figure this out by determining a full charge to empty run time then dividing that by 5 and seeing if the first light goes out around 1/5 of that runtime and then seeing how much longer you get till 2 lights go out (or 2/5 runtime). You may be surprised to find that the lights are not as accurate as you would think. You can also try timed charging, if it takes 2 hours to charge from empty (and you can’t avoid emptying it in regular use) you can measure the runtime from full to empty, try charging for an hour and compare the runtime and experiment to time the charge from empty to about 80%. Adjust timings until you get a good value and use that as a charge time using a timer set on your phone. 80% of the empty to full charge time will not work as a proxy for 80% charged because the charger will switch from constant current to constant voltage and you will unwittingly end up above 80% charge if you try this. Finally some devices have charge meters that are not calibrated to tell you how much charge the battery has, but how much charge it needs for a given task. For example, some Powerbanks designed to boost a car battery are calibrated so that it tells the user how much juice is has for the recharge task (not what is left in the battery). For example it may have 90% charge left, but claims 60% as it has 60% of what it was designed to put into the car battery. This is a royal headache for determining storage charge and can cause premature failure when you think you are storing your battery properly, but are actually not. Also some crummy charge indicators don’t take into account parasitic/self-drain (See Part Three), so the charge level may be lower than what is indicated, making the battery look more worn out than it really is.

Laptops are often very hard on their batteries (wearing them out quickly as a result). They charge to 100% and maintain that charge, they charge the batteries very rapidly, and they often subject batteries to high temperatures.

If a device has a removable battery, you can also try using a multimeter to determine its current level of charge. This can be far more accurate than relying on their built-in gauges. You would need to know how many batteries it has in series, then divide the charged voltage by that number to determine how charged the batteries are. You can often Google what cells it uses and then try to Google its voltage/percent charge curve. Or contact the manufacturer and ask what model/brand cells are in the product if it is using commodity cells (14500, 18650, 21700, 26650, etc). In general, 3.95-4V is 80% charged, but this is not an absolute. If you can determine what voltage equates to 80% charge you can use your charger to only charge to this voltage. You will get a feel for how long it takes to get to this voltage if your device uses a rather regular duty cycle.

For items such as power tools and yard equipment that are not used for long periods (or over the winter), you want to try to charge to 50-60% and then store it indoors in a cool location. A basement works well, as it’s often cooler than living areas but not freezing. 40% is the ideal charge voltage for storage, but many devices have the self-discharge spoken about in the discharge characteristics (See Part Two), so you want extra power to account for it. Ideally you should check the charge level after a month and again after 2-3 months to determine its rate of self discharge and to make sure the battery does not drain itself before you start to use it again.

Stay tuned for Part Five, Depth of discharge, Miscellaneous Battery Information and End Of Life


A summary of the terminology used in the battery world:

Charging algorithm = Battery is charged at Constant Current, then near full charge (typically over 80%) the charger switches to Constant Voltage. The charging rate slows until the battery reaches 100% charge. Many EVs modify this algorithm.

C = Capacity of the battery

  • Battery ability to output power is measured in 1/C. 1C means the battery drained in one hour, 2C means 30 minutes (1/2 hour), 3C means empty in 20 minutes (1/3 of an hour) and so forth.
  • Charging can also be measured in C, 1C means charged in 1 hour, 0.5C charged in 2 hours, 2C charged in 30 minutes and so forth.
    Charge rates are not typically linear, the battery is typically charged more rapidly until it reaches the Constant Voltage stage.

Series = Multiple batteries linked in a chain to increase the total voltage of the pack.

Parallel = Multiple batteries linked side by side to increase amperage instead of voltage.

(x)S(x)P configuration = explains how multiple batteries are linked. 4S2P for example means 8 cells, four in Series and two Parallel rows

Volts (V) = Electric potential. Power outlets are measured in volts.

Amps (A)= Number of Coulombs of electrons carrying those volts.

Watts (W)= Volts x Amps. Energy/Power usage is often measured in watts. A kilowatt is 1000 watts. kWh is Kilowatts per hour.

Energy is measured in Joules and is convertible to Watts/second if you have a time component.

Power = Energy over time. Typically measured in Watts. One Joule per second is 1 watt. The same number of Joules or Watts in half the time is twice the power.

Nominal voltage = Voltage used to calculate Watts of a battery.

Battery capacity = How many Ah of power the battery can output (when new).

Load = Device that uses the power from the battery.

Internal resistance of a battery affects its Power output. Increased internal resistance is the reduction in rate of Power output the battery can deliver. Energy output is affected somewhat by increased internal resistance.


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