I'm designing a charger system for my lawn mower robot.

The idea that I had is based on a relay. As soon as the robot reaches the charging station, with "Volt Sense" pin it detects a charging voltage, so enables the "enable charge" output. In this way the the relay coil is turned on and so the battery switches from the normal circuitery to the charging one. I didn't put the charging part of the system, but that one is quite easy, since I have a lead acid battery.

The enable charge pin is there also to control the charging of the battery. When it's fully charged, the pin is switched off, so the battery can go back to normal functioning mode.

I will add a current measurement IC, so when the current drop to very low values, the battery is considered charged.

I'm not sure if the following schematic is ok, could you please tell me your opinion? Thanks

EDIT: I added the charge measurement pin.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Why use a relay at all? D2 alone should do everything you need. \$\endgroup\$ Oct 28, 2015 at 14:12
  • \$\begingroup\$ how it can switch from normal usage and recharging mode by only using D2? \$\endgroup\$
    – Val
    Oct 28, 2015 at 15:17
  • 2
    \$\begingroup\$ You don't need to switch - when power is present on the charge pin, it will charge the battery and supply power to the rest of the circuit. \$\endgroup\$ Oct 28, 2015 at 15:21
  • \$\begingroup\$ see my comment below, please \$\endgroup\$
    – Val
    Oct 30, 2015 at 7:35

1 Answer 1


Charging a lead-acid battery is a bit more complicated than your on-off switching logic, if you want to do it right. You'll have to provide constant current first, then constant voltage, and finally a pulsed charge in the end.

The main issue you may face with your schematic is when you start charging the battery after a deep discharge. Since you charge with constant voltage, that may result in high charge current, which may damage the battery itself, the relay and D2.

  • \$\begingroup\$ In the schematic, beyond "recharge pin+" there is a circuit for recharging, not a direct connection to a power supply. However, the robot will never reach a deep discharge, since it will come back home at about 70% of the battery level. \$\endgroup\$
    – Val
    Oct 28, 2015 at 15:19
  • \$\begingroup\$ If you have a dedicated charging circuit, why to you need the circuit figuring in the question? \$\endgroup\$ Oct 28, 2015 at 15:29
  • \$\begingroup\$ because I thought that it needed something to switch from normal usage to charge mode, in order to disconnect the normal circuitery.. \$\endgroup\$
    – Val
    Oct 29, 2015 at 8:08
  • \$\begingroup\$ Why would you need to disconnect the normal circuitry while charging? It cannot tolerate 14.5 Volts? Anyway... yes, your schematic will do what you want, provided the arduino is correctly programmed. \$\endgroup\$ Oct 29, 2015 at 9:03
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    \$\begingroup\$ @user3318528 You can measure the voltage on the charge pin with your resistor divider before the diode. You can measure the battery voltage with another resistor divider after the diode. Besides, if your Arduino powers off the rest of the circuit using the relay, won't that power off the Arduino too? \$\endgroup\$ Oct 30, 2015 at 9:51

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