Hi all. I am hoping to tap in to the collective experience on the list... I have a project ("advanced" shutter-release system for a camera - time-lapse, long-exposure, etc.) that is currently prototyped on a bread-board. The power will be derived from a single Li-Ion cell that will need to be recharged on occasion. I have the following components: The Li-Ion cell is 18650 sized 2000mAh. The charging controller is the LTC1733 chip from Linear Tech. I am using the LT1111-5 switching IC in DC-DC Boost-mode to get 5V for the "real" circuit. The "main" circuit contains an 18F4620 and an LCD 16x2 module as well as op-amps, switches, opto-couplers, and some other minor components. One of the critical components for this excercise is the REF194 chip from Analog Devices which is a 4.5V Precision voltage reference I use for all A/D conversions. Maximum draw of the whole "main" circuit is 40mA@5V. Typical draw is 25mA@5V. I am hoping to trim that down some more still. I have "everything working" in that when plugged in to the 5V wall-wart the charging chip activates and starts charging the cell. Simultaneously the power supply to the LT1111-5 chip gets swapped from the (now charging) cell to a 3.3V LDO (MCP1700) that draws it's power from the 5V wall supply. In other words, when plugged in, the circuit is powered from the wall. When unplugged it is powered from the cell. I know there is the inefficiency of the double-DC-DC conversion, but when using wall power that is OK, right? I have it set up that the main circuit is always turned on when plugged in at the wall. The 18F4620 always monitors the charging of the Li-Ion cell. Currently it measures the charging current (using output from the LTC1733), as well as the charging voltage. The PIC keeps control of the three status values the charging chip supplies (Charging status, Fault, and ACPower). It also keeps track of secondary values like the number of cell charges, the charge time, etc. The LTC1733 chip uses a Constant Power then Constant Current then Constant Voltage strategy for charging. The constant power part is because it is thermally limited internally to protect the IC, and this causes the current to drop when the IC operates at it's thermal cut-off temperature (which is at its worst case when the cell is at it's lowest charge - the power is the difference between the wall power-supply voltage (5V) and the cell voltage (2.5V) times the charging current (1.5A) = 3.75W). As the cell charges the cell voltage increases causing the power dissipation in the charging IC to decrease. Once the power requirement is such that it falls within the thermal capacity of the IC, it keeps a constant current mode... until the charging voltage hits 4.2V at which point it maintains the 4.2V charging voltage, and the charging current drops off steadily until the cell is charged. I have 3 questions... 1. Is there some way I can calculate a "degree of 'chargedness'". I want to get an idea of how close the cell is to being fully charged, and preferably to indicate something like "35minutes charging still required". 2. Is there a way to calculate the capacity of the cell during (or even at the end of) the charge process? It would be nice to say "The cell has been charged to 1800mAh capacity". This measure would be a good indicator of the aging of the cell. The unit does not have to be mAh, perhaps there is another more meaningful unit, something like mWh. There would also have to be a definition of "empty". Right now my circuit will automatically shut down if the cell voltage is less than 2.5V - unless plugged in). Cell capacity is normally measured by draining the fully charged cell at a constant current, then timing the discharge. Because the cell supplies a 5V DC-DC converter, even if my circuit had a constant 25mA drain, it would cause a variable drain on the cell as it's voltage dropped off with it's state of discharge.... 3. Should I even bother with the above challenges? It is secondary to getting this project going, but, since this is a hobby for me, the fun is in the learning, right? One thing I have learned is that the datasheets for Li-Ion cells all indicate a few curves for the capacity/discharge current of the cell, and the capacity decreases with an increase in the discharge current. Thus, a 2200mAh cell is only 2200mAh for a given discharge rate..... The rates given in the datasheets are normally much higher than the 30-50mA that I anticipate drawing (depending on cell voltage). Thanks in advance for any hints, pointers, references, or even "just forget about it" type comments ;-) Rolf P.S. Apologies for the long content -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist