battery-charging
I am not that familiar with electronics, it's just a hobby.
I am reviewing the TP4056 and DW01A ICs that are used for battery charging.
Actually there is nothing special about these ICs but I was wondering how the red marked area works.
I know the two DW01A pins prevent overcharging and discharging, but how does it work? I searched for an explanation but nothing found.
The use of 4 x AA Alkaline would usually be safe BUT does exceed the USB spec and damage may occur in some cases. I have seen IC's in this role with max operating voltages of 5.5V (which is ludicrous) - you'd hope designers had more sense, but it can't be guaranteed.
While some devices may use converters between charge input and battery, many don't (probably most). A LiIon battery has max charging voltage of 4.2V so a 5V nominal USB input will usually meet this need with enough headroom for a linear regulator.
An Alkaline cell can be nearly 1.6V when fully charged - about 1.55V is common or 6.2V for 4, and up to 6.4V may be seen. There is not much energy in this initial high voltage "tail" and voltage falls to 1.5V or below very quickly.
So, you should be safe, but YMMV, alas.
A solution would be to use an LDO (low dropout voltage) regulator OR a clamp regulator which takes the peak energy out of the battery or a series diode to drop 0.4 to 0.8V (Schotky / Silicon).
LDO is best solution but you want as little drop as possible.
Clamp to drain peak battery voltage is unusual but viable. A zener could be used but is too inexact. An eg TL431 clamp regulator in a TO92 or other largish package (to get OK dissipation capability) ould do. A TL431 plus a transistor would be safer.
Series diode is cheap and easy but prevents full battery use. Say minimum usable battery voltage is 4.6V (may be higher). At 1.15V/cell there is still some battery capacity left. Adding a Schottky diode increases minimum battery voltage to 4.6 + 0.4 = 5V or 1.25 V/cell. Some capacity wasted. At the top end a 0.4V drop diode results in Vbattmax of say (1.55V x 4 - 0.4) = 5.8V or 1.45V/cell."Almost certainly safe".
Using NimH works but is more marginal at bottom end and safer at top end. At 4.6V, V per cell is 1.15V where NimH still has modest energy left. At top end Vmax = say 1.35V, maybe 1.4V for short periods at start. 4 x 1.4V = 5.6V. Very probably safe.
I don't know whether this is a 100% perfect solution as per my requirement or not but seems to be working good and very much similar to the solution I was looking for.
Solution is Load sharing: charging the battery and have the main circuit run normally
We can use any charge controller ic I am considering here is MCP73831 because of low cost and good features and 5-Pin SOT-23 package, just simple to use.
Will have to test this for charging cycles per day because my requirement is to have low number of charging cycles(as charging/discharging continuously is not preferred and my circuit will be at remote position and will be powered ON 24x7).
For time being this solution looks good to me.
References: Thanks Microchip for good explanation on Load sharing concept ww1.microchip.com/downloads/en/AppNotes/01149c.pdf
and a good blog from
blog.zakkemble.co.uk/a-lithium-battery-charger-with-load-sharing/
as I have lower reputation so cannot share more than two links hence links posted as a string(sorry...)
Please correct me if I am wrong.
Best Answer
Monitoring the battery voltage
The DW01A monitors the battery voltage using its power supply pins (pin 5 and 6 of DW01A ) as shown above. The comparators shown on the left side trigger in case of overcharge (battery voltage too high) or overdischarge (battery voltage too low).
The battery voltage is not measured directly. There is an RC filter (R5 & C2) that filters out spikes in the battery voltage due to e.g. inrush currents of the load.
Monitoring the battery current
When the battery is being discharged, the battery's current will flow B+ to OUT+ through the load, entering OUT-, flowing through the dual n-channel mosfet FS8205A back to B- as shown with the red arrowed line.
When the battery is being charged, the charge current flows as shown with the blue arrowed line. (I omitted the load current, note the load current is not measured while charging.)
The FS8205A has a drain-source on-resistance \$ R_{DS(ON)} \$. The current through this resistance will cause a voltage drop with respect to GND (pin 6 of DW01A), which is applied via R6 to pin 2 of the DW01A(1). The comparators on the right side of the DW01A all have a small voltage source on the + input, so they can measure positive and negative currents through the FS8205A.
The charger detector, short circuit detector and overcurrent detector will trigger based on the voltage drop across the FS8205A. So, the \$ R_{DS(ON)} \$ of the FS8205A determines at which current this will happen.
(1) Theoretically, there will be no current flowing through R6, and therefore no voltage drop across R6, because the inputs of the internal comparators is theoretically infinite. In practise, there will be a (negligible) leakage current. The purpose of R6 is to protect pin 2 of DW01A against ESD.