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I have a requirement to provide a DC voltage to a PWM motor control board. I wish to use 2 x 18650 Li-ion cells in series OR a 12 DC regulated bench power supply. I require the battery pack to be charged if the 12 V DC PSU incoming level is present and also to feed this voltage to the MCB. If the 12V DC level is not present then the battery voltage should feed the MCB. These 2 Li-ion cells are connected to a balanced charging circuit which provides protection from over charging or over discharging. When the MCB is not in use i.e. switched off, I do not want any parasitic drain on the battery pack. I have designed a circuit which uses 2 x 12 V 6 pin relays to disconnect the charging circuit when the PSU is not providing power.
My problem is that the relays fail to unlatch when the PSU is switched off because they experience the battery pack voltage and as a consequence the small parasitic current flowing through the charging circuit flattens the batteries over a few days. If I could force the relays to unlatch by temporarily zeroing the relay supply voltage then the circuit would work. I have proved this by manually disconnecting the battery pack and observing the result i.e. the MCB sees the battery voltage and the charging circuit is disconnected.

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What suggestions can you make as to how I might make this circuit work or achieve the desired result in a different way?

I have redesigned the circuit so as to use the 2S charging circuit correctly i.e. input power and output power uses the same P+ and P- terminals. I have disassociated the relay coil from the PSU supply by using a MOSFET to energize the relay coil. I have also simplified the design by only switching the positive rail consequently I only need one relay. I have inserted a diode between the 2S circuit and the LM2596 to prevent any parasitic drain when the PSU power is absent. When the PSU supply is present the relay is called which disconnects the batteries +ve line from the motor control bus bar and connects the +ve PSU supply instead.
This should not have taken me so much time to resolve but I have learnt a good lesson about relays and how to design a better circuit.

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2 Answers 2

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You could connect the relay coil positives before the Schottky diode. This would switch the relays when +Ve is applied, and when it’s not the diode would block the battery voltage from energizing them.

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  • \$\begingroup\$ I've checked up on the circuit I am using Battery Management and I have not wired it up as required. The supply and load should be connected to P+ and P- so I will have to redraw the circuit AGAIN! Thanks for your suggestion. \$\endgroup\$ Commented Nov 4, 2023 at 0:09
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If the battery protection circuit truly protects against over-discharge then I would expect that the battery should connect to B+ and B- and the load (motor) and the charger both should connect to P+ and P-.

With this design the load is automatically powered by the higher of the charger voltage and the battery voltage. I would place a diode between the output of the charger and the load to ensure that the output of the charger does not draw current from the battery as I have seen all sorts of bad layouts on LM2596 dev. boards.

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  • \$\begingroup\$ Yes you are correct - I need to reconsider/draw this circuit with that in mind and see if the parasitic drain still exists. Thanks for your comment. \$\endgroup\$ Commented Nov 4, 2023 at 0:10

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