Battery level indicators generally detect the voltage levels of the batteries to give a result. So, tester circuits must not be a heavy load during the measurement process.
http://www.circuit-projects.com/battery-circuits/economic-battery-sufficiency-tester-by-bc547.html
This very simple circuit uses a transformer ,two diodes , a capacitor and an ammeter.To charge a battery just connect the plus and minus terminals of the circuit to the corresponding terminals of the battery.
http://www.electronic-circuits-diagrams.com/psimages/powersuppliesckt7.shtml
the charging current is controlled by the current control circuit.
http://www.hobby-elec.org/e_car2.htm
for Battery Operated Hi-Fi Preamps
http://sound.westhost.com/project98.htm
This automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor. It starts charging when the battery voltage drops beyond a preset value, and it stops when an upper
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=1823
http://www.aaroncake.net/circuits/charger1.htm
http://ourworld.compuserve.com/homepages/Bill_Bowden/page7.htm#charge.gif
The schematic for this charger is pretty simple. You can charge from 1 to 20 +/- nicads at a constant current of from 20 to 200 ma +/-.
http://www.uoguelph.ca/~antoon/gadgets/ccharger.htm
lead-acid battery charger that works with either gelor wet-cell, lead-acid, 12V batteries
http://www.edn.com/archives/1996/091296/19di3.htm
This circuit is designed to peak charge Rx and Tx batteries. It's programmed for a C/2 charge rate for 250mAh and 500mAh batteries (charge currents of 125mA or 250mA). It'll charge Rx from 12V at the field, Tx from a 15V supply (like a car w/engine running). Use a 15-18V supply to charge at home (you should be able to find wall cubes with this rating). It is based on a Maxim IC, the MAX713.
http://www.uoguelph.ca/%7Eantoon/gadgets/peakchrg.htm