Custom Chargers for backup battery

[dcbrown]

I'd like to take a stab at the theory of operations of the ifi supplied charger. Some things make sense... others leave me confused. Hopefully others can help correct and fill-in the pieces.

The LM317 regulator changes its output so as to maintain a 1.25v drop across R2 (220). Since the same current through R2 goes through R3,
Vout = 1.25 * (1+(R3/R2)) or about 8.3v. So far so good I think. R4 (22) limits the charge current that can be provided to the 7.2v battery. When the backup battery is near full, this would be ~45ma. This supply will be fairly constant as the regulator will hold Vout at 8.3v independent of input supply, as long as Vin is > ~9.5V

R1 (43) seems to be able to provide a constant charge current of ~110-130ma based upon the voltage differential of a 12v battery and the 7.2v battery independent of the regulated LM output. If or when the battery voltage drops to 10.8v - like under heavy discharge of a running robot - current available via R3 will drop down to about 80ma.

This is one of the things that is confusing... why go through the trouble of a regulated supply if the simple resistor is capable of supplying upwards of 2x or more than the regulated circuit is capable of?

So there is a total current available of ~150-170ma draw from the 7.2v circuit as shown?

Why D1? I'd be tempted to replace this with a LED until competition to show when the 7.2V is in discharge. That is, if you disconnect the 12v battery the regulator circuit input supply voltage will be back fed through R1. In addition, D1 will bypass R4 and 7.2V will be applied to (R2+R3). This equates to an approximately 5ma current constant discharge. So, if you are like us who routinely "safe" the robot by unplugging the main battery but don't disconnect the backup, then we'll end up draining/discharging the 7.2v battery through this charging circuit overnight. A small red led where D1 is would provide a visual reminder that this is occuring.

Many regulator circuits worry about sudden current discharges and transient spikes delivered back into the output pin of the regulator and put a protection diode across the input/output pins. D1 in essense is across the pins but in the reverse direction of what is needed so the regulator isn't protected. I guess if you burn out the chip you might not notice because you still have R1 delivering 100ma+ charging current?

Ok, pull the backup battery and what happens. In an open circuit with a load <~80ma through R1, the 'batt' point floats above 8.7V and the zener diode draws current to clamp it at 8.7V. The zener could see current spikes up to ~180ma as it does this.

I'm sure I have some of this incorrect and would like feedback.

thanks,
bud

http://www.ifirobotics.com/docs/firs...up-charger.pdf