Battery replacement

Need operating voltage range and max current. 2 serial (too low) or 3 serial (too high), N parallel.

Recall it. Cancel vacation.

And do you need fast charging? I am current working on a 4.2V fast charger. The DC supply can pump out 2A, but the charge controller can only handle 300mA. So, i am thinking about by-passing it with a mosfet below 3.7V and pump max. current into it. That's for each cell by itself. I have one charger per cell.

Look into NiMH, nickel metal hydride, a direct replacement for NiCad. You should be able to find the exact same form factor and capacity. May need a new charger though. Stay clear of lithium, no justification for it.

Why? Lithium batteries are cheaper and higher capacity.

Voltage is not compatible with his system, unlikely to find compatible form factor, (this thing moves so not a lot of leeway there), they're pricey, requires more expensive charger, and they're hazmat to the max. NiMH should be a drop -in. They used to have excessive self-discharge but that problem has been fixed.

It might not be optimal, but you can limit lithium batteries to upper or lower usage range and it can still be higher cap than NiCad or NiMH.

DC module is around $3 and the 3582 charger chip is around 25 cents. But the charge current is low with unmodified circuit. With zener and by-pass mosfet, i can build one for around $5.

True for Tesla, but the OP is not building millions.

As ironically stated by the John Doe snipped-for-privacy@message.header troll in message-id <sdhn7c$pkp$ snipped-for-privacy@dont-email.me who has posted yet another incorectly formatted USENET posting on Fri, 13 Aug 2021 19:12:52 -0000 (UTC) in message-id <sf6g7k$7b1$ snipped-for-privacy@dont-email.me.

On a sunny day (Fri, 13 Aug 2021 11:47:50 -0700 (PDT)) it happened Fred Bloggs snipped-for-privacy@gmail.com wrote in snipped-for-privacy@googlegroups.com:

8.4 / 1.2 = 7 eneloop AA No self discharge, safe, does not catch fire, charging is simple at 1.6V constant current.

Else I would go lipo, there are chargers for that, lipo performed better in my drone than the li-ion stuff. You only see lipos in drones. Some lipo batteries have build in protection.

4.2V top, about 3.8 nominal, with 3 just more speed for the car..? Very high current capacity, If the car does not want more voltage you may need some switcher / regulator, that reduces efficiency. Need more data.

Consider LiPo in stead. They are less expensive, higher specific energy, and can output tremendous current. They are used in pretty much all hobby remote control applications these days. You can have them in a huge variety of sizes and shapes, so you should be able to find one that will fit.

The problem in your case, is that the voltage you need cannot be achieved with any Li based battery. A two-cell battery would have a nominal voltage of 7.4 V, while a three-cell would have 11.1 V. You would have to choose between poor performance or spectacular performance with the risk of cooking the motor.

Personally, I'd get the three-cell, and when something burns out, replace it with something better. But for a Christmas toy, that may not be worth the money.

NiMH has a higher internal resistance than NiCad and may not work as well in a high current drain application like a car. That's why they still use NiCad in many tools rather than NiMH. Of course, many are now lithium ion. So clearly not a bad choice.

Rick C snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

NiCad sucks and has been phased out for half a decade.

Nearly every small application these days is Lithium ion.

Switched out my 6Ah SLA motorcycle battery for an 8Ah LiFePo battery that was the same form factor all but just 1/4 inch taller. It operates better than the other. It should... it was two to three times the price.