Q: I may buy an iPod touch and I’d like to know how it works in regards to the charger. I currently have a 20GB iRiver that connects to the stereo to listen to music and that I connect to the provided AC adapter. The battery, when charged, is not in use when the electricity runs through it…which is ideal when listening to 2-3 hours a day of music in the living room. I was therefore wondering how the iPod and charger works. Is the battery cycle being used or not when the battery is full and the iPod working with the AC adapter/electricity?
Further, if I disconnect and reconnect the iPod in the charger/power adapter a few times over a period of several hours (ie, to switch between home sound system and personal/headphone use), how does this affect my battery life?
A: There are a number of misunderstandings about how rechargeable batteries work in most modern electronic devices, most likely caused by some significant advances in battery technology over the years.
The first and most important consideration is that the majority of modern electronic devices with rechargeable batteries now use “Lithium Ion” batteries (Li-ion). Unlike earlier generations of rechargeable batteries which were based on Nickel Cadmium (NiCad) or Nickel Metal Hydride (NiMH), Lithium Ion batteries do not suffer from the “memory effect” when it comes to recharging. Previous Nickel-based rechargeable batteries, particularly NiCad batteries, would lose their maximum charge capacity if they were not fully discharged prior to charging them up.
Li-ion batteries, on the other hand, actually prefer to be topped up, and should never be completely discharged, as this will actually destroy the battery. Most electronic devices, including the iPod, have a cut-off circuit that turns the device off when the battery drops critically low in order to prevent this from happening, thereby leaving a small charge in the battery, so this is rarely a practical concern for an end user.
Likewise, while lithium-ion batteries can technically experience problems from overcharging (continuing to charge the device after the battery is fully charged), any properly-designed electronic device will incorporate a “cut-off” circuit to stop charging the battery once it’s reached its full charge level in order to prevent this.
Another point to note is that there is no requirement for a lithium-ion battery to be “primed” before use. Recommendations that a new iPod be plugged in and charged overnight before using it for the first-time are based on older nickel-based battery technology. For a Li-ion battery, there is no difference between the first charge, the tenth charge, or the 100th charge.
The life expectancy of a lithium ion battery in terms of how many charges it will take is measured in “charge cycles” which refers to the number of complete discharge and recharge cycles, not simply to the number of times the battery is “topped up.” Therefore, if you drain your battery by 25% and recharge it fully four times, this will count as a single charge cycle. Again, there is rarely a reason for the average iPod user to be worry about watching the charge level or being concerned about reaching a certain level before recharging—simply dropping the iPod into the charger whenever necessary is fine.
In fact, the only reason in the case of an iPod for doing a complete discharge and recharge of an iPod is to re-calibrate the battery meter itself (ie, the iPod’s display of how much power is remaining). This has no impact on the actual battery life, but will help the iPod provide a better estimation of the remaining life for the user by ensuring the battery gauge is accurate.
In terms of how the charging process itself works, Li-ion batteries charge in two stages: If the battery is below approximately 70-75% charge level, the first stage involves applying full charging power to get the battery charge up to that level. Once the battery reaches the 70-75% level, a “topping” charge is applied, whereby the current to the battery is gradually decreased as the battery capacity reaches 100%. This is done in order to avoid overcharging, and is sometimes referred to as a “trickle” charge, although this is technically incorrect by definition, since a “trickle charge” refers to continuous power being applied to a battery once it has reached full charge, which Li-ion batteries do not do, again in order to avoid the risk of overcharging.
With a Li-ion battery, once full charge has been reached, the charging circuit will shut off completely and stop providing any charge to the battery. At this point, as long as the device remains connected to external power, the battery goes dormant and the device simply runs from the external power source. What this means for the iPod is that as soon as you see the “Plug” icon on the battery indicator, charging power has been cut-off and the device is simply running from the external power source.
If you leave the iPod connected to an external power source for long enough, the battery will drop slightly in power just from normal energy loss (in the same way that it would if it were simply sitting on a shelf turned off). Once the battery falls below a certain level, a topping charge will be reapplied to bring it back to full, but this normally happens very infrequently as long as the device remains connected to external power—possibly as rarely as only once every three to four weeks.
Disconnecting the iPod from external power and reconnecting it will re-initiate the charge circuit, since the battery level needs to be re-checked, and a small topping charge may need to be applied to get back to 100%, but unless the device has been used on battery, the iPod should return to the plug icon within a few minutes, once again indicating that the battery is fully charged and the device is running from AC power. In many cases no topping charge is applied, and this delay is just the time it takes for the iPod to resolve that the battery is, in fact, fully charged.
The net effect of disconnecting and reconnecting your iPod from its dock should be basically neutral in terms of battery life. The topping charge is applied only to bring the battery back to 100%, so this is a fraction of a battery cycle in the same way as any other charge.
Unfortunately, many of the now-outdated issues with nickel-based batteries have become urban myths for modern electronic devices, and can cause many iPod owners much completely unwarranted angst about their device and their battery life. In reality it is almost never necessary for a typical iPod user to worry much about the iPod battery… The simple rule of thumb is to use the device as you normally would, charge it when it needs it, and don’t worry too much about leaving it on the charger for reasonable periods of time.
In fact, just about the only issue to be aware of is for those rare users who use their iPod from external power all the time. In this case, since the Li-ion battery is not being used (the device is running from external power), the battery itself doesn’t get properly “exercised” and this can decrease the battery life over time. This is only an issue for users who almost never run their iPod from the battery. Apple’s own support site simply suggests that the battery be put through at least one complete charge cycle per month. Considering that even twenty 5% “top-up” charges still counts as a charge cycle, the reality is that for the vast majority of iPod users, normal everyday use will easily take care of this.