I currently have a project requiring the use of a battery. My project has to be able to run off the battery power, but when an outside source is connected, the battery should be able to charge off this source, but the source also has to power my project.

I am looking into using an SLA battery such as this one, because in my lack of experience, it seems to be easier to deal with charging than a battery such as a LiPo battery. Since this is the point in which a screw up could result in a destruction of my project, and I am inexperienced in the area of batteries and charging beyond the basics of electronics, I would like an outside opinion from someone knowledgeable in this area, to let me know if it would work or not.

I attached an image of what I think the layout of my charging system would be. The battery would have the ground connection for the project, and the 12V connection. The relay would go on the ground side of the battery, after the connection to the project. This way, the battery can charge/discharge according to the relay. The voltage monitor would determine when the relay needs to be on or off. The relay and voltage monitor would have connections to the Arduino Nano, acting as the controller for the charger. The Nano would supply 5 volts to the voltage monitor. Also, the Arduino Nano is powered from the same charger, via an inline voltage regulator.

Essentially my question is: Will this work before I go and do it and mess something up? Or is there possibly a better way of doing this?

Planned Schematic

Voltage Monitor


Regulator to Power Nano

  • \$\begingroup\$ Welcome to EE.SE. Use 2 x <Enter> for paragraph breaks. \$\endgroup\$
    – Transistor
    Aug 15, 2018 at 22:09

1 Answer 1


It is extremely common to connect a battery charger, a SLA battery, and a load in parallel - millions of cars are wired that way.

If the charging source can supply more current than the load needs, the excess goes to charge the battery.

If the charging source can't supply enough current to supply the load, the battery will discharge to make up the difference.

The battery will switch between charging and discharging automagically as the load or charging source change - no need for any external control.

  • \$\begingroup\$ Would a charger like this work then? As in I wire it so the charger charges the SLA battery but powers my arduino too? amazon.com/JOYLIT-100-240V-Switching-Adapter-Flexible/dp/… \$\endgroup\$
    – Matt
    Aug 15, 2018 at 23:16
  • \$\begingroup\$ I also want to be able to have the charger connected without worrying about damaging the battery. What I have found on this is saying that leaving it continuously plugged in will damage the battery, whcih is something I really cannot have in this situation \$\endgroup\$
    – Matt
    Aug 15, 2018 at 23:22
  • \$\begingroup\$ You need a proper "smart" charger that will reduce the charging current as the battery approaches full charge, and that will not over-charge the battery if left on indefinitely. A simple constant-voltage power supply is not suitable for this purpose. \$\endgroup\$ Aug 15, 2018 at 23:32
  • \$\begingroup\$ That is why I want to shut it off at a certain voltage, ten turn on at a certain voltage. I can't use a smart charger because I have to power an Arduino board , and a 12V solenoid, whether the battery is charging or not. \$\endgroup\$
    – Matt
    Aug 15, 2018 at 23:37
  • 1
    \$\begingroup\$ The smart charger on my boat works just fine, powering a fridge, electronics, lights, etc. while keeping the batteries topped up. Do a web search for "float charge SLA" for more information. \$\endgroup\$ Aug 15, 2018 at 23:45

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