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Originally Posted by Ghoda
Bottom line - if you've got the time a trickle charge at low amperage is always better than a quick charge at higher amperage.
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Originally Posted by Solicitous
I always thought with mA rating was the maximum the charger can put out, not what it does put out consistently. What I mean by this is if a wall charger is rated 5v 1A and the device charges at 5v 750mA, then all the device will pull from the charger will be 5v at 750mA. Same goes if the device is say 5v 1.5A, because the charger has a maximum output of 1A, that is all the device will receive.
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Solicitous is correct in the sense that a circuit will only draw the amount of current it is needs, at a given voltage. However, Ghoda is correct that low-current charging is better for the battery in every way, except perhaps charge time (At lower current, we'll get to 90% faster. At higher current, we'll reach 100% faster).
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Generally speaking, batteries live longer if treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions will have a negative effect and shorten the battery life.
Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help. Figure 5 shows the cycle life as a function of charge and discharge rates. Observe the improved laboratory performance on a charge and discharge rate of 1C compared to 2 and 3C.
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I also found a couple other points that seem to agree with Ghoda:
- The time at which the battery stays at 4.20/cell (100% charge) should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures. Spinel is less sensitive to high voltage.
- The charge current of Li-ion should be moderate (0.5C for cobalt-based lithium-ion). The lower charge current reduces the time in which the cell resides at 4.20V. A 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage into voltage limit prematurely.
- Increasing the charge current does not shorten the charge time by much. Although the voltage peak is reached quicker with higher charge current, the topping charge will take longer.
I really don't know what the 200mA difference in current between a USB 2.0 port (500mA) and the Kobo max (700mA) actually quantifies into, and I'm not much in the mood to to pull out my Engineering textbooks to find a bunch of equations right now. If someone really wants to know, I might look into it later.