Welcome to the forum.
Don't attempt to build your own NiMH or NiCD pack until you understand the thermal characteristics of the chemistry.
Otherwise you are bound for disaster.
I run NiMH all the time and love them.
I also use cheap Powerizer and Tenergy and they work fine.
I build my own packs and do break some rules yet they work.
1) Volts x Amps = Watts
I run 72V battery systems (60 cells in series) with a 72V controller.
At 72V I need half the amps than running a 36V system.
72V also means more speed (25-30 MPH).
NiMH (and NiCD) batteries will get hot as you draw more amps.
2) D-size takes heat better than C-size, C-size takes heat better than subC-size.
BUT ... You can NEVER EVER run cells in parallel within a single "pack" of cells.
A pack of cells must ALWAYS be in series.
(You can run packs in parallel but you can never charge cells in parallel).
3) Cell packs should generally be made in groups of 10.
Each 10-cell string is 12V nominal (and up to 15V while max charging).
So a 30-cell (series) pack is 36V nominal.
A 40 cell pack is 48V nominal.
A 60 cell pack is 72V nominal.
4) Charging ... (-) dV/dt AND (+) DT/dt
Ni cells need a minimum of 1.41 V/cell for charging.
As they reach full charge (over time t) their Voltage (V) climbs and the Temperature (T) increases.
But at FULL charge their V drops a little and the Temperature goes up quickly.
I ALWAYS install a max Temp sensor in my packs to terminate all charging current above 50 deg C (122 deg F).
BUT I am sneaky ...
I use a cheap 1.8A 36V SLA charger to charge my 30 cell packs with a separate charge lead connected to the 50C sensor.
The temp sensor is embedded in the "center of mass" of the pack,
My packs are nice and warm when fully charged (so I then let them cool down before use).
5) AHr Capacity is measured at the "C" rate / 20
A 10 AHr D cell is rated for 500ma over 20 hours.
But I usually pull 3-5 x the C rate.
I am lucky to get even 70% of the AHr C rate at 1 C. At 4C maybe 60% and they GET hot while being used.
But Hell ... I don't care ... the cells are cheap and packs are easy to build.
If you want to see some CHEAP NiMH builds I'll post the pics.
I also have an old thread on the subject ...
https://endless-sphere.com/forums/viewto ... 16&start=0