Why do battery chargers (the type that charges AA and AAA battery’s) require 2 battery’s before actually charging them?

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Why do battery chargers (the type that charges AA and AAA battery’s) require 2 battery’s before actually charging them?

In: Technology

2 Answers

Anonymous 0 Comments

Because they’re low quality. A high quality smart charger caters to each battery individually to give it the perfect charge, and maximize its life. The funny thing is, the highest quality charger on the market only adds about $6 to the cost of these AA Eneloop NiMH batteries:

* [AmazonSmile: Panasonic K-KJ17MCA4BA Advanced Individual Cell Battery Charger Pack with 4 AA eneloop 2100 Cycle Rechargeable Batteries: PANASONIC: Electronics](https://smile.amazon.com/dp/B00JHKSMJU/)
* [HKJ Review of Charger Panasonic BQ-CC17 – lygte-info.dk](https://lygte-info.dk/review/Review%20Charger%20Panasonic%20BQ-CC17%20UK.html)

> A nice -dv/dt termination with only a small temperature increase and no trickle charge, this looks very good. […] The charger is very good at filling the batteries.

You need that charger to get the full life out of Eneloops. If you take care of them, Eneloops will last at least a decade, maybe longer. The charger also works with other NiMH batteries. If you don’t have good NiMH batteries yet, get the ones I linked to above, and make sure your battery specifications match the packaging in this photo *(there are fakes out there):

* [New battery day! 24 AA Eneloop NiMH batteries in 16 and 8 cell packs. : r/AAMasterRace](https://www.reddit.com/r/AAMasterRace/comments/bo0wtt/new_battery_day_24_aa_eneloop_nimh_batteries_in/)

Anonymous 0 Comments

Because they charge both batteries in series, at the same time. While you put the batteries in end-to-end, the negative end of one battery is connected to the positive of the other, and the charger provides ~2.4 volts across the pair of cells, charging them to 1.2 each.

This is sort-of OK with NiMh or NiCd cells, because you can gently overcharge them without causing damage. But these chargers have to charge them slowly, and it isn’t really good for them. If you are charging Lithium cells, though, you have to monitor each cell individually, and have some balancing step to charge all of them and not overcharge any.