Why does it take so long for a battery to charge the last few percent?

Factual Questions
1

Hello Everyone,

I have a Samsung Note 3 and I Orchard extra batteries and a stand alone battery charger. It sends that the batteries charge to 90-95% fairly quickly, but the last 5 or 10% seem to take much longer. Why is this?

2

That has a lot to do with the chemistry of a lithium-ion battery.

Charging algorithms vary a bit, but a typical charging algorithm is to charge the battery with a constant current until the battery reaches its peak voltage. Once the battery reaches its peak voltage, a constant voltage is then applied, and the charging current slowly drops off. When the charging current drops to close to zero, the battery is fully charged.

The battery charges fairly quickly during the constant current part of the charging cycle. The charging slows down quite a bit during the constant voltage part of the cycle, which is why that part seems to take so much longer.

3

What is the difference between “peak voltage” and “fully charged”?

Does it merely seem to take longer, or maybe it really does take longer?

4

A lithium-ion battery reaches its peak voltage before it is fully charged. During the constant voltage phase of charging, you add capacity to the battery but the battery voltage does not increase.

This is different than something like a lead-acid battery or a nickel-cadmium battery where the battery’s voltage continues to increase as the battery capacity increases.

It really does take longer, but it’s charging it in a different way, so it’s kind of an apples to oranges comparison. During the constant current phase, the battery’s capacity increases pretty much constantly with time. During the constant voltage phase, the battery’s capacity increase slows down significantly and gets slower and slower until the minimum current is reached and the charger shuts off.

5

Thanks for the info!

6

I always think of electrical current like a liquid. If you were to remove a dam the water would flow very fast until it got toward the end, eventually it would just be a trickle.

7

Ok, but how does HoneyBadgerAC work?

8

It’s also worth noting that just because your phone says it’s 70% charged, doesn’t mean it’s actually 70% charged.
About the only two readings you can really count on are 0 and 100%.