This is not complicated circuit. In the commerical product, voltage protection circuit and overfolow current protection circuit is needed.
Except that is probably a commercial circuit.
Most of these things (and I've taken apart 2 different ones) are as simple as is humanly possible (or simpler).
The sad fact about it is that these things are made by the lowest bidder and anything that can be removed will be.
Most importantly, there is no limit to the charge or discharge of the battery which does not lead to a long life -- especially if combined with extremes of weather. The only saving grace is that a normal cell and a battery holder are obviously cheaper than a battery with solder tags.