Safe to hook LiON battery to Vbat?

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froeber

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Hi, I think this is a question for Paul. I have a Teensy 3.0 and I want to protect the RTC from power loss by connecting our LiON battery to the Vbat pin. And that is what I have done on my first prototype. But before I build up more boards I'm wondering if what I did is "safe" and couldn't harm the Teensy processor.

My thinking is that section 4.4 of the Freescale MCU data sheet says that the max operating value allowed for Vbat is 3.8V. Based on how they define limits in the data sheet it implies that if you go over 3.8V you could break the chip. And my (fairly standard) LiON battery can put of 4.2V max or so. I see that there is a dual diode between the Vbat external pin and the MCU Vbat pin. But I saw a post where Paul said that diode was a PMEG4010CPA Schottky diode. And checking the data sheet for it, the forward voltage drop across it is below .4V and, for low current, much below .4V. So I would think I could be driving the MCU Vbat pin with too high a voltage. But my board isn't dead yet and I've been doing this for months.

I'm wondering if anyone has any thoughts on whether this is safe or if I should add some external part between the battery and the Teensy VBat pin?
 
If you are having to use extra parts to reduce the voltage of the li-on battery, I would think it might be as simple to use a coin cell battery that it was designed for. That way it keeps the time, even if the main battery is discharged or you are changing batteries.
 
Thanks for the suggestion Michael but we have the LiON battery to power the whole board too. I like having the VBat connection from the battery in addition to having it hooked to Vin so that I can shut down the processor when I detect battery getting low but still have the RTC keeping time until someone plugs in USB which recharges battery.
 
VBAT is only for the RTC -- it doesn't power the whole system.

Connecting a Li-Ion to VUSB (or VIN if you break the link between them) is safe -- this drives VREGIN which can go to 5.1 V (and down to 2.7 V) -- see p51, '6.8.3 USB VREG electrical specifications' of https://www.pjrc.com/teensy/K20P64M72SF1.pdf.

Just be careful not to over discharge the Li-Ion
 
@Jp3141, I'm using the Teensy 3.0 so the older data sheet. But I think the section references I give are the same as for the Teensy 3.1 data sheet you quote. Section 4.4 of data sheet has the full operating range of Vregin input to processor. It can go from -.3V to 6V. Of course, I realize processor won't run across that whole range. But is will run well below 2.7V. Table 1 in section 5.2.1 shows that the Vbat and Vdd can range from 1.71 to 3.6V and things will still work. By hooking battery to both VRegin and Vbat, as I tried to indicate above, I can put the processor into VLLS0 deep sleep shutdown mode when battery voltage drops to something like 2.2V to reduce current draw on battery by turning off regulator and therefor removing power from processor (since regulator output is what drives Vdd).

The battery will still be powering the RTC through the Vbat connection. And since the RTC pulls very little current it should take quite a while to drop the voltage from 2.2V down to the 1.71V lower limit of RTC operation. I think once the battery voltage drops below the 1.71 V the RTC should stop running and I'm hoping stop drawing current, thereby avoiding over discharging the LiON battery.

But to get back to my original question, why can I hook the 4.2V LiON battery to the Vbat pin as I have been doing and not cause processor problems due to exceeding the 3.8V upper limit on Vbat?
 
froeber said:
But to get back to my original question, why can I hook the 4.2V LiON battery to the Vbat pin as I have been doing and not cause processor problems due to exceeding the 3.8V upper limit on Vbat?
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No, this is not a good idea.

The datasheet says the max for Vbat is 3.6V. The current is so very low that the series diode will drop almost none of the 4.2 volts. It might work anyway, but applying more then 3.6V to Vbat is exceeding the maximum spec.

Vbat is really intended for a 3.0V lithium coin cell.
 
On the Teensy, VBAT is already supplied (via a diode) from VOUT33 -- this is the 3.3 V regulator that is powered from VREGIN -- your Li-Ion cell. See the schematic here: http://pjrc.com/teensy/schematic.html. So to run, you just need to connect your Li cell to VIN (or VUSB).

Undervoltage on VBAT is quite low (from the datasheet -- VPOR_VBAT is between 0.8 and 1.5 V), so it will likely work until your Li-Ion is excessively low. The best way to maintain the performance of your Li-Ion (by not overdischarging it) is to shutoff the system at a lower votlage (~ 2.8 may be reasonable). This requires an additional, external circuit.
 
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