# Why does it take so long for a battery to charge the last few percent? **URL:** \ **Category:** Factual Questions\ **Created:** [November 24, 2016, 3:29am UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870 "2016-11-24T03:29:33Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![obbn](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@obbn](https://boards.straightdope.com/u/obbn)\ **Post date:** [November 24, 2016, 3:29am UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/1 "2016-11-24T03:29:33Z")
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? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 24, 2016, 3:43am UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/2 "2016-11-24T03:43:07Z")
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. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [November 24, 2016, 11:40am UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/3 "2016-11-24T11:40:06Z")
> [@engineer\_comp\_geek](#): > > … until the battery reaches its peak voltage. What is the difference between “peak voltage” and “fully charged”? > [@engineer\_comp\_geek](#): > > … which is why that part seems to take so much longer. Does it merely seem to take longer, or maybe it really does take longer? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 24, 2016, 12:08pm UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/4 "2016-11-24T12:08:37Z")
> [@Keeve](#): > > What is the difference between “peak voltage” and “fully charged”? 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. > [@Keeve](#): > > Does it merely seem to take longer, or maybe it really does take longer? 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. ---
**Author:** ![obbn](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@obbn](https://boards.straightdope.com/u/obbn)\ **Post date:** [November 24, 2016, 5:36pm UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/5 "2016-11-24T17:36:50Z")
Thanks for the info! ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [November 24, 2016, 7:31pm UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/6 "2016-11-24T19:31:29Z")
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. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 24, 2016, 7:52pm UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/7 "2016-11-24T19:52:03Z")
> [@HoneyBadgerDC](#): > > 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. Ok, but how does HoneyBadgerAC work? ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [November 24, 2016, 8:00pm UTC](https://boards.straightdope.com/t/why-does-it-take-so-long-for-a-battery-to-charge-the-last-few-percent/772870/8 "2016-11-24T20:00:04Z")
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%.