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We have a device that runs off of a AC adapter, or a pack of 8 AA batteries. Since in normal use we use both, it would seem to make sense to have the battery pack recharge while using the AC adapter - instead of throwing the rechargeable batteries into a separate charge each time they're depleted.

What are the basics of designing/modifying the existing battery pack and AC adapter to implement that kind of charging?

A quick observation shows that the battery pack looks to be just the cells in series, with a capacitor (perhaps for protection from voltage spikes, it's powering a motor).

My thoughts are that I'll need to make sure the AC adapter can provide enough current to power the device and charge the batteries. The pack will have to somehow be modified to charge the cells in parallel (right?) while powering the device in series.

Other than those vague ideas, I don't know where to start/how possible this is. So what are the basics I need to know when it comes to building this kind of charger?

Slight Update: I know I need a circuit to control the recharge, I'm not just looking for a circuit to build, but the basic purposes of that kind of circuit. How does it monitor the charge (and determine when charging is needed)? What are the basic components involved and their purposes?

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    \$\begingroup\$ @Electric_90: It is nice to fix up questions that need improvement. Trivial edits like making links out of URLs just pops old questions onto the front page, distracting from newer questions. There is plenty of editing to be done on new questions. You don't need to dig up ancient history. \$\endgroup\$
    – JRE
    May 11, 2019 at 14:30

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The pack will have to somehow be modified to charge the cells in parallel (right?) while powering the device in series.

Yep. A relay is an easy way to do this.

See http://www.batteryuniversity.com/ for the specific type of battery you're using.

To recharge lead-acid batteries, for instance, you start out with a trickle charge to make sure the battery doesn't have any shorted cells, etc.

When it's up to a certain threshold, you do a constant-current charge. The maximum current is specified by the battery manufacturer. (You measure the current with a small current-sensing resistor in series with the battery. A 0.1 ohm resistor with 100 mV across it has a current of 1 A going through it, for instance.)

After the constant current stage, you apply a large constant voltage, and then when that stops taking current, you drop back down to a float charge.

Here are instructions for nickel batteries.

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    \$\begingroup\$ @Electric_90 STOP HIDING LINKS. Even if they were not destructive, your pointless edits to ancient questions are unjustified. But they are actively destructive, because it's useful for users to see clearly what sort of resource and site is being linked, especially in the case of very old links which may in some cases no longer work, but where the original link address gives a clue to re-finding the resource. \$\endgroup\$
    – Chris Stratton
    May 11, 2019 at 17:08
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    \$\begingroup\$ @ChrisStratton So it just not me then. I was thinking about asking on Meta how to handle his vast numer of edits on old posts. He has good intentions but ends up bumping age-old questions. \$\endgroup\$
    – winny
    May 11, 2019 at 17:40
  • \$\begingroup\$ That was definitely a bad edit and should not have been approved by @JRE \$\endgroup\$
    – endolith
    May 13, 2019 at 20:55
  • \$\begingroup\$ or by @RoyC ........... \$\endgroup\$
    – endolith
    May 13, 2019 at 20:55
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You can't just connect an AC adapter to a bunch of rechargeable batteries directly. Instead you need to have some sort of supervisory circuit to shut off (or taper) the charge when the batteries are fully charged. These days that is usually done with a specialized IC doing most of the work.

Here is a charging circuit that supposedly works with up to 8 AA NiCad cells.

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  • \$\begingroup\$ Right, I knew I needed some kind of charging control circuit - thanks for the link. \$\endgroup\$
    – Tim Lytle
    Oct 6, 2010 at 12:50
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Batteries can be very temperamental, depending on chemistry, which you don't mention. If you get the voltages wrong even by a little bit it can mean the difference between long life and short life.

Lithium-ion is really several different types of chemistries but all are rather fickle for charging.

This isn't perhaps the best answer but if you really depend on having your batteries last longer, don't do it yourself -- use the manufacturer's charger.

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  • \$\begingroup\$ It's just regular 'ol AA rechargeables. So they're not hard to replace, and in this case the convenience of having the pack charging (or charged) when the AC adapter is used outweighs any battery lifespan (at least for me). \$\endgroup\$
    – Tim Lytle
    Oct 7, 2010 at 12:42
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This Maxim appnote talks about similar setups (especially Li).

http://www.maxim-ic.com/app-notes/index.mvp/id/3241

You could also see whether you can do constant voltage charging, but it's usually not ideal. It all does depend on the batteries and usage patterns.

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I believe it's based on opamps. An opamp is a simple device that when wired together with others can serve as an indicator of how full a battery is. Think of them as different height holes in a vertical tank. You know where the level of the water in the tank is depending on the height of the hole spilling water.

These get wired into a small board such as battery management systems (bms boards) for charging series- parallel batteries in a pack. Otherwise if you just want to recharge a single battery there are simpler lipo charging modules but I assume you're talking series of batteries since you mentioned battery pack. And this is for lipo batteries which are more commonly used in these projects I think. .

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