I have low power system which requires dual batteries to overcome the loop holes in the current system. Currently I have two batteries: Primary Battery Li-Ion 3.7V 1800mAh and a secondary battery Li-Ion 3.7V 560mAh. Both these batteries are connected to a load sharing circuit wich uses LTC4413 IC to control the switching between batteries. Basically what LTC4413 does is it constantly compares the voltage of the two batteries and keeps switching them ensuring both batteries discharge at the same rate.

But my application need is different. I am trying to design a circuit that only switches to back up battery under two conditions: 1)If primary battery is disconnected OR 2)If the voltage of Primary Battery is lower than 3.3V. Ideally I would like to have a STATUS pin which can transmit information (Low/High) to the uC regarding the which battery (Primary or Secondary) it is connected too.

During my research on internet, I have come across several circuits that can switch from battery 1 to battery 2 in absence of battery 1 but the 3.3V condition is vital and challenging. I am also trying to implement without a uC to control the switching. However if a uC is present with very low power characteristics in order of uA, I might use it as a last resort.

I would be obliged if somebody can suggest an IC that can do the job or circuit with low power loss.


  • \$\begingroup\$ What chip did you find that did the switching from bat1 to bat2? \$\endgroup\$
    – Andy aka
    Jan 15, 2015 at 19:40
  • \$\begingroup\$ Maybe two diodes (diode OR) can do the job? \$\endgroup\$ Jan 15, 2015 at 21:14
  • \$\begingroup\$ Perhaps a power supervisory IC? It holds an enable signal low depending on the voltage of one cell. Use this enable to control your switching. Normally you'd use these ones to keep an IC in reset while power rails deal with inrush currents. But you can also use them to initiate a safe shut down or sleep mode when it senses voltage is dropping. \$\endgroup\$
    – lm317
    Jan 15, 2015 at 23:10
  • \$\begingroup\$ If your primary battery disconnects the resulting voltage from it is 0v, so you only have a single constraint: "2)" lower than 3.3v. \$\endgroup\$
    – Nedd
    Jan 16, 2015 at 7:40
  • \$\begingroup\$ All that you need is to use a relay and a hysteresis circuit NE555, when the voltage in the primary battery is lower than 3.3V the relay switches to the secondary battery. \$\endgroup\$
    – R Djorane
    Jan 16, 2015 at 8:45

2 Answers 2


LTC4415 is the IC to be used for such application. An enable pin can be used to set the threshold at which you desire the switch over. Here is the Datasheet. http://cds.linear.com/docs/en/datasheet/4415fa.pdf


For the 3.3v level detection create a simple voltage comparator using a 3v single supply op-amp and a 2.5v reference chip:

One op-amp input comes from a high value potentiometer center pin. The top of the potentiometer is powered from the 2.5v reference chip. The other op-amp input connects to the center of a 2-resistor voltage divider that is powered by the primary battery. Power for the op-amp and the reference chip is derived from the OR'ing of both batteries using two low Vf diodes (this assures no lose of power). Tune the potentiometer to make the op-amp switch at the 3.3v level of the primary battery. By swapping the op-amp inputs you can achieve either a high or low switched signal. Connect that switching signal to the battery switching chip you seem to already know of.

Here is an example of an ultra low power single supply op-amp with a voltage reference built in: http://datasheets.maximintegrated.com/en/ds/MAX951-MAX954.pdf


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