My understanding (correct me if I am wrong) is that those three boards differed as follows:
- The VBAT "label" (on the schematic) existed for RTC backup trickle powering only.
- Physically VBAT was brought out of the board on a pin one row inward of X22.
- No label or pin was called VBACK, VBACKUP.
- PYBLITE1.0 and PYBV1.1:
- The VBAT "label" (on the schematic) was re-assigned to serve as the label for the rail on which full-board power input from a LiPo would come. FETs were added to protect LiPo from over-charging by VBUS
- Physically VBAT was brought out on a new pin two rows inward of X23.
- A new schematic label "VBACKUP" was created (sometimes also called VBACK on labelled images of the board) to cover the RTC backup trickle powering role previously done by VBAT.
- Physicsally VBACKUP was brought out on a pin one row inward of X22 -- i.e. in the same location as VBAT had previously emerged. This means that though the *name* of this pin had changed from VBAT to VBACKUP, its physical role had remained the same: RTC powering.
What is very much less clear to me, however, is whether the newer PYBLITE1.0 and PYBV1.1 design intends also to be able to CHARGE a LiPo that has been connected if USB power has also been provided.
There are few clear statements about this on the forum. Some of those I have found appear to be contradictory, while others don't make clear enough which board versions they are talking about or are vague. My PYBLITE1.0 project normally is normally powered by a big LiPo connected to VBAT. Thus far, whenever I've wanted to charge the battery, I disconnect it, charge it externally, and while it is being charged I can power (or program) my PYBLITE1.0 via USB. Occasionally I have both connected at once, and indeed, the LiPo doesn't explode or overcharge. So far so good.
However, when I remove the LiPo and connect USB power, and then measure the voltage on VBAT I see only 0.72 volts, while V+ measures about 4.38V (which I imagine means VBUS is a diode-drop higher, or so).
I don't really understand why VBAT is so low in such a scenario.
PYBLITEv1.0b's schematic makes the statement statement that "Q1 prevents VBUS (5V) from overcharging Lion / Lipo batteries". I therefore imagined that the circuit (which I admit I don't understand) involving Q1, VBAT, V+ and VBUS intends to leave VBAT as high or as close to VBUS as it can, while ensuring that it nonetheless does not exceed some safe limit (4? I can't see a value written down anywhere) -- so that charging of the battery could take place via USB. However my LiPo is certainly not going to charge if all it sees is 0.72 volts. So either (a) LiPo charging via USB is not something that that circuit is intended to alllow, or (b) the circuit behaves differently if it senses a LiPo battery present.
I would be grateful if anyone could help explain:
- whether VBAT on the PYBLITEv1.0 or PYBv1.1 is intended to be able to allow USB->LiPo charging (or not)
- If it is not, then what extra circuits or solutions do people tend to add on to their projects so that LiPos connected via VBAT do get charged when the user plugs in a microusb cable ....