The best power source choice for an Arduino based small robocar project?

I'm building a project based on something like one of those Elegoo Arduino based smart robocar kits.

These kits usually come with LiIon 18650 battery holders but I don't have such batteries and I'm not sure if I want to mess with LiIon batteries, considering that they require special care when using and charging and how many fakes there are on the market.

So, I'm considering my options to power my hobby project based on what I currently have on hands:

  • Anker Astro3 10000mAh USB External Battery Pack and Charger (Power Bank) - I have it for a year or so and I have been using it when traveling by train and watching movies on my smartphone. I guess, it's also LiIon but at least I know it's good quality. Still, it's somewhat bulky and might be too heavy for the small-ish robocar. I'll have to modify the kit to make the powerbank fit firmly and still be removable when I need to use it.

  • I have 8 of NiMh AA batteries and I can buy more. They are so versatile. Also I have Duracell CEF22 NiMh Multi-Charger which works great. The only thing I don't have is a battery case to make a pack of 6 batteries to make 7.2 volts. But I've heard that it would be a good idea to power the motors from a different source to prevent Arduino from brownouts when the motors are under stress while fighting against some obstacle. Still, it seems NiMh AA is great to mix and match as I wish, but I just have to buy appropriate cases and think how to fit them on the chassis of the car.

  • I have also an old pack of 5 NiCd batteries wrapped in a pack and with just two wires going out. I could use them alone, but I think 5 might not be enough for both motors and Arduino, so again - I might mix them with NiMh AA batteries.

But one more huge problem with the last option - I don't have a charger for this 5 NiCd pack! How do I charge them? Can I use my Duracell Multicharger also for NiCd battery pack?

I know, this is NiMh charger and it might not be appropriate for NiCd, but at least I hope they won't explode? And how do I attach them to the charger? I don't want to disassemble the pack because the batteries are soldered together in series. Can I charge the entire pack by attaching the wires to a single battery slot on the charger (observing the polarity, of course) or it might be bad idea for the charger or the batteries?

Don't mix old and new batteries. If one battery exhausts its charge earlier than the others, it will be destroyed by polarity reversal.

I would go with the AA NiMH option. You can buy battery holders in many different sizes, 2,3, 4, 6, 8, 12 etc. NiMH batteries can be trickle charged continuously or fast charged in a charger designed for NiMH cells.

Everything you need to know about rechargeable batteries can be found at

Thanks, I guess I'll grab a couple of those 6xAA boxes then.

Still It would be nice to reuse that old NiCd 5xAA pack but I'm not sure how to charge it.

Without any connector at all, just wires.

I have only that smart NiMH charger (smart - it knows when to stop charging NiMH batteries) and also an old simple dummy 2xAA charger (dummy - I have to stop charging when I think it's enough) but, of course, I'll have to hack something together to attach that NiCd pack to the charger.

Will a single AA battery slot of a charger (and which one - dummy NiCd or smart NiMH?) be able to charge entire series of 5xAA NiCD batteries? Or it won't work at all or even be dangerous?

If it takes entire night, it's ok, I just don't want to buy a new charger for just that old NiCd pack but I want to make use of it.

That 700 mAh pack doesn't have much capacity (NiMH AA batteries store 2500 mAh), but it is safe to charge it at "0.1C" (70 mA) for 12 hours or so.

To charge the yellow pack, you will need a series resistor and a power source, say a 9V or 12V wall wart. Assuming that the pack voltage is about 6V, choose the resistor to provide 70 mA based on your power pack using Ohm's law (R = V/I). Connect the wall wart plus to the resistor, and the resistor to the red battery lead. Black is minus.

Example (9V-6V)/0.070A = 43 Ohms. A 33 Ohm 1/2 Watt resistor would be fine, and charge a bit faster, but it will get hot as it will dissipate 0.27 Watts of heat. If the resistor starts to smoke, one or more cells in the pack is internally shorted, making the pack useless.

If you don't have a multimeter, now is the time to buy one.

Awesome, thank you, now I will know the missing piece of information of battery charging - the charge rate C and how to apply it in practice.