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I want to hack my portable device, so that it does not use lipo battery anymore. I want to have it using just external power supply (don't ask for reasons).

Unfortunately after disassembling the device and removing lipo battery, the device does not work anymore. When connected to 5V DC the red led flashes, as if it was to inform me that lipo battery is dead.

No other functionality of the device is working.

That bring me to the question: What is the best way of fooling device that lipo battery is connected? The device obviously wants to charge the battery if it has low voltage.

My ideas so far are:

  1. connect 4.2V DC directly to the place where battery was connected (how to get 4.2V, having 5V (USB) as input voltage)

  2. be brave and connect 5V and hope that internal voltage regulator will not blow up and that none of the red-flashing-diode fail safe will not kick in.

  3. ???

Thanks, G

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  • \$\begingroup\$ Connecting 5v sounds like a bad idea - 4.2 would be OK (3v3 via a linear regular would be relatively easy, 3x1.5v batteries might also work in a pinch), but chances are it also has an extra pin or two for monitoring the battery status (i.e. temperature) which might also be required before it "powers up" - easiest way I think would be to leave the battery in there, drain it, and then use a 3.3v regulator to lower the 5v into the battery and the device, such that the battery is only there to fool the circuit into sending an "OK" status \$\endgroup\$
    – user2813274
    Jun 4, 2015 at 18:16
  • \$\begingroup\$ Agree with the above. Start with applying the same voltage the battery itself provides to the device. Don't apply more than 4.2V (or whatever the maximum voltage your battery ever applies to the device is). If its a LiPo cell, anywhere from 3.7V to 4.2V is likely to work. But, unclear if there are other terminals used by the device to detect if a 'valid' battery is inserted. \$\endgroup\$
    – jjmilburn
    Jun 4, 2015 at 18:22
  • \$\begingroup\$ Try to connect a large capacitor (1000 uF) instead of the battery and see if it wakes up \$\endgroup\$
    – PkP
    Jun 4, 2015 at 18:29
  • \$\begingroup\$ Ok, I get it. Thx :) I think I have some 5V power regs, they drop the voltage by 0.7V or so, so I would end up with 4.3V, should be fine for the circuit. \$\endgroup\$
    – pks
    Jun 4, 2015 at 18:44
  • \$\begingroup\$ Did the battery have an internal temperature sensor that you inadvertently disconnected? Many rechargeable packs have an internal thermistor which, if missing, would disable the device. \$\endgroup\$
    – Samuel
    Jun 4, 2015 at 21:29

1 Answer 1

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A Simple Success

I just cobbled together some parts on my bench and fooled a TP4056 LiPo charger into thinking there was a battery connected:

Fake LiPo

On a TP4056 LiPo charger IC (cheap in China), I connected two LEDs (D1, red, Vf=1.76V and D2, yellow, Vf=1.82V) in series with R2, a 22Ω current limiting resistor. R1 = 12kΩ programs the charge current to 100mA.

This made the charge IC happy; it is sitting on my bench and its output "charge" voltage is at 4.15V. It is working fine without (additional) load, as well as with ~100mA load (36Ω through 3.3V buck converter).

Caution

This will probably not work for all charge ICs. Many ICs will include protection logic which might get upset with your fake battery.

For example, some other hardware here uses a LTC4081 combined LiPo charger and buck converter IC. Even without a connected battery, all seems to be fine initially. After 4 hours the LTC4081 decides that no rising battery voltage can only mean defective battery, and disconnects power.

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