The car is supposed to work with 6 AA-cells in series.  Only rechargeable cells are an option here.

NiMH

• Putting six of them in series, also adds all their source resistances in series.  When high current is drawn, the output voltage will drop a lot, leading to power loss.
• The Goldpeak cells I use suffer from high self-discharge.  Panasonic Eneloop should be better in this respect.

Li-ION

Setup 1

1. Using a LiitoKala NCR18650B
2. Charger : TP4056
3. Protector : the single 8205A doesn't allow enough current.  So the over-current protection had to be bypassed.  This also disables under-voltage protection.  When someone forgets to turn off the car, the battery will be drained to death (as has happened already)
4. Boost circuit to boot the Li-ION cell voltage to 9V using XT1208 (advertised differently).  This seems to work well.

Setup 2

• Using 1 or 2 18650 cells.
• Charger functionality removed
• Li-Ion protection : see Power Management.

The charger has an XT60 connector, so charging is easy.  There's no longer a risk of incorrectly plugging in the battery after charging.