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Hot-swap dual battery setup.

Discussion in 'Power Electronics' started by peteb2, Nov 25, 2015.

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  1. peteb2


    Nov 9, 2015
    Hi all. I need to design something that will allow a piece of equipment to run on one battery then once that batterie's terminal voltage has dropped to a specific point (indicating eminent discharge), auto switch to another (fully charged battery) meaning the discharged one can now be swapped out for a replacement charged unit. There would need to be a visible indicator showing which battery has to be swapped out.

    The device is 12V but can draw maximum high as 5.8A say 6A around 70W. I don't really want to use mechanical relays because the environment can be quite rough. The batteries are Li-ion 14.V 6.0Ah, 87Wh units, the equipment well able to tolerate the few volts over 12V.

    My research so far I assume some kind of MOSFET will let me switch such currents such as the 'Power Trench" devices from Fairchild (FDS6612A)...

    Obviously the circuit needs to be fairly efficient (pointless if it was to use more energy than the device being powered by the battery).

    I could simply go for a 'Smart' power source selector Dual-Battery i.c. as in the MAX1538 (as i have found doing a Goggle) but my concern is that the circuit could never be 'patched up' in the field if it was ever to become damaged...

    Of course if i use a bunch of op-amps to sense Vterminal and others to act as comparators , while effectively having the batteries in parallel for their combined and a visible alarm to show the one that needs to be replaced, switching it out of circuit etc then its hardly going to be all that efficient... Maybe an Arduino, maybe a Raspberry Pi?

    Interesting quandary.... What do others think...
  2. dorke


    Jun 20, 2015
    I think you can use the LTC4417 for this job ,
  3. pgib8


    Jul 26, 2015
    Off each battery, use a zener diode with a pull-down resistor. once the voltage of the battery falls below the threshold, it turns off the mosfet and stops using that battery and at the same time switch on another mosfet to use the other battery. but if both batteries are about dead, the thing might just start to oscillate :)
  4. AnalogKid


    Jun 10, 2015
    Actually, the efficiency of the circuit in terms of circuit power consumption or loss relative to the energy delivered to the load is almost completely dependent on the switching devices, not the amount of control goop behind them. With one power MOSFET for each battery, the power system efficiency will be over 99%.

    The method in post #3 will start oscillating as soon as the first battery goes low, and lead to having two dead batteries and no time to swap anything. The way around that is to have a latch. Once one battery's comparator says it's time to switch over, it clocks a latch so the second battery stays online no matter what happens to bat1. This is important because once bat2 takes the load, the output voltage of bat #1 will rise up to an operational voltage level again - but only at no load.

    There are a ton of ways to do this, but I think a microcontroller is massive overkill unless you also want an LCD battery voltage display and have it play happy birthday once a year. The KISS method has one comparator, one flipflop, one reset switch, two power MOSFETs, and some passives. The comparator and ff can be two halves of an LM358, or go all discrete, or fold in CMOS logic, or Linear Tech has some battery management and ideal diode controller chips, or or or...

    Reset switch puts bat1 online.
    Low bat1 latches bat2 online and signals a recharge.
    When bat1 is recharged and installed, push RESET to bring it online.
    Pull, recharge, and install bat2 for the next cycle.

    Or you can replace the reset switch with a second comparator and have the system toggle between the two batteries automatically.

    Last edited: Dec 3, 2015
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