# Why don't they charge small batteries in parallel?November 16, 2015 10:06 PM   Subscribe

I'm sure this is a stupid question, but I wondered. Why don't cell phones and other chargeable electronics charge multiple small batteries in parallel? The analogy is like this: * I have one cell phone with a large battery that takes 3 hours to charge. * I have three cell phones with small batteries that take 1 hour to charge. Why can't cell phones just have 3 smaller batteries and charge them all at the same time, and therefore make the charging faster?
posted by jonclegg to Technology (8 answers total) 3 users marked this as a favorite

Best answer: Because the maximum safe rate of charge is closely related to the capacity of the battery. So if you take a battery with capacity of 3Ah and can charge it at a rate of 1A, you could alternately take three small batteries with a capacity of 1Ah and safely charge them at 1/3A. The net result is the same.

Also, faster charging might be limited by the current supplied by your charger.

Many battery packs already are multiple cells in series and/or parallel (e.g. laptop batteries).
posted by ssg at 10:12 PM on November 16, 2015 [2 favorites]

Why can't cell phones just have 3 smaller batteries and charge them all at the same time, and therefore make the charging faster?

Cell phones will already charge as fast as current USB chargers will allow (right now, that's generally ~2A, which is already above USB specification). Splitting that between multiple batteries would just make each battery charge slower, for no net decrease in charge time.
posted by saeculorum at 10:13 PM on November 16, 2015

No matter how many batteries you have, you're limited by the amount of current available. To charge three batteries at 2A, you'll need 6A of current.

Think of the current as marbles, the batteries as boxes and the charging cable as a tube. You have 100 marbles available per second coming down a tube. If you direct those marbles into a box, it will fill at 100 marbles per second. Split the output of that tube into three boxes, and they'll only fill at 33 marbles per second. Either way, you end up with the same amount of marbles over the same amount of time.

You could decrease the charging time (ie, the time it takes the boxes to fill) by having more current (more marbles) or by reducing the Ah of the batteries (make the boxes smaller). Current battery specifications mean that there's a limit to how much current they can accept. New tech does seem to be emerging, though.
posted by Solomon at 11:26 PM on November 16, 2015 [3 favorites]

Another consideration: using multiple batteries is less space efficient than having just one big one. That is a big consideration when trying to make a small phone.
posted by rongorongo at 1:48 AM on November 17, 2015

@rongorongo, it may help to not think about a battery as a single thing. The name is a collective noun, short for a battery of cells. Each cell, and how those cells are wired gives us the combo of voltage and current we have available.

This question touches on the how of wiring cells, and not about the physical size of the battery, which is the same size regardless of how the cells are wired. Making small batteries is about engineering devices that use less energy, with a long term effort in engineering cells with greater energy density. But you have to design with the cells and density you have today.

Not to mention that many phones already have two batteries. One main one, and one on board super capacitor or cell that can be used to stash system stuff that does not depend on the main battery pack.
posted by clvrmnky at 5:59 AM on November 17, 2015

If you had a removable battery which had its own charger then you could probably get it charged much quicker, but they don't make phones like that because the trade-offs aren't perceived to be worth it. The new USB spec and quick-charge battery technology mean that if you buy a new phone next year (or certain phones like the Nexus 6P right now) it will only take a few minutes to get your battery mostly charged.
posted by any portmanteau in a storm at 9:49 AM on November 17, 2015

Expect Faster Charging, Not Longer Lasting, Phone Batteries just ran today and addresses your question somewhat.
posted by phearlez at 10:21 AM on November 17, 2015

Response by poster: Thanks @ssg, that makes sense.
posted by jonclegg at 11:22 AM on November 17, 2015

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