Battery charger schematics

Some comments on this schematic:

• Q1 and Q4 are small signal transistors like the BC547.
• The transistor pair Q2-Q3 I used was BD140 - 2N3055, just because I had some. A power PNP transistor that is able to handle the charge current, and has
  an amplification (Hfe) > Icharge / (Uadapter/R4) (eg 1A charge, Uadapter =12V, R4 = 1k5 yields Hfe of 125) will do the job instead of the Q2 Q3 pair.
• D1 is a small signal diode, like 1N4148; it prevents overvoltage at the input of the comparator.