I plan on making 2 battery packs from rechargeable AA batteries. Each pack will have 15 AA batteries which will put out 20 Volts to power a meter.

Since I plan on soldering tabs on the batteries, I can not use a standard charger that charges individually. I will need to make or get a charger that will charge the 20 Volt packs.

How do I go about this? How many Volts and Amps should the charger put out to charge 15 AA's in series?

And is Over-Charging an issue? Should there be a controller that cuts power when they are charged?


  • \$\begingroup\$ Old NiCd batteries were quite tolerant to overcharging and could withstand a dumb charging method (resistor, current limiter...). NiMH batteries are more fragile and still prone to memory effects and should be charged with a proper device. There are too many elements in series, you would have to test and balance elements. Can't you use a 24V lead battery (which is easy to charge and low maintenance) ? \$\endgroup\$
    – TEMLIB
    Jan 22, 2015 at 1:39
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    \$\begingroup\$ Popular wisdom is that it's unwise to solder tabs to batteries- spot welding is the way to do it (no current goes through the battery). Over charging is a serious issue with NiMH - it liberates hydrogen gas and permanently damages the battery. \$\endgroup\$ Jan 22, 2015 at 2:02
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    \$\begingroup\$ Are you planning on soldering tabs onto the batteries, or soldering to existing tabs to connect the cells in series? If it's the first, you will be in trouble. \$\endgroup\$ Jan 22, 2015 at 2:59
  • \$\begingroup\$ So basically, if the cells are NiCd, you can apply C/10 for 12 hours or something (look it up). If the cells are NiMH, then you need to look into charge termination detection. You can terminate on slope of the voltage or, if you have a temperature sensor, you can terminate on temperature change vs time. Search for dV/dt and dT/dt charge termination. One last thing. Make sure the batteries are at an equal state of charge when you put the pack together. I am not sure how good of an idea it is to have a 15 cell stack. Maybe look into a boost regulator? \$\endgroup\$
    – user57037
    Jan 22, 2015 at 4:19
  • \$\begingroup\$ if you don't have a spot welder to put tabs on the batteries find someone who does, like a battery psck refurb place. for charging you;ll probbly want to do the thermistor thing to detect end-of-charge. althogh C/40 is supposed to be ok indefinately, (that's about 40mA for 2 days or more IIRC) \$\endgroup\$ Jan 22, 2015 at 10:32

1 Answer 1


A battery's charge current is not determined by the basic battery size (eg: AA). The battery's capacity specification (in AH, mah) needs to be considered. A simple AA battery can have a wide range of capacity values, some are low cost low capacity while other are high cost high capacity. The best way to start with a charger design is to consult the manufacturer's specifications, most quality battery manufacturers provide this. In the specifications the required charge current, voltage, safety, and other charging parameters are often given.

In regard to building up a 20v assembly: Did you consider that battery packs in this range already exist? For example manufacturers such as Black & Decker have battery packs available for many of their portable products. 20v units are quite common. If you were to use one of these packs the charger could also be a standard Black & Decker part. If the large interface connector is a problem for your project you may be able to remove/modify it as needed. If you start with a known matched up pair of battery and charger you can save yourself a lot of time and effort. Not to mention you would know they work well together.

Note that many of the newer B&D battery products use Li battery types, though some other brands still use NiMH battery types. Some commercial battery packs are actually made up from stacks of several AA batteries.

Even if you choose not to use a commercial battery pack and charger for your project, looking at the manufacturer's specifications (and other hacks) of such a systems can give you a lot of good ideas on making your own.

  • \$\begingroup\$ @user1542685 That's good advice. I did something precisely like that, in fact! I've used a Makita Li-Ion pack, and simply got the connector from an online parts website. powerhousetoolparts.com will have parts and diagrams for most cordless tools, and they have the requisite part diagrams. You can then purchase a given part number there, on eBay and elsewhere. You can often purchase broken cordless tools cheaply on Craigslist, too - they are a good source of connectors for you to reuse. \$\endgroup\$ Jan 22, 2015 at 16:15

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