Hi again,
- If I used two cells in series to charge through your circuit then I should have to use double voltage (8.4V) and double current (0.7A per cell X 2= 1.4A). And also, need double current (0.7X2= 1.4A) but same voltage (4.2V) to charge two cells in parallel, am I right?
- Are there any circuit in your hand which indicates when charging current level is lower than 5mA OR, when charging voltage tends to reach 4.2V? It would be a nice 'battery fully charged' monitor, isn't it?
Hi,
As audioguru mentioned, there are problems that come up when charging in series or parallel. The series problem is that of voltage sharing, and the parallel problem is that of current sharing.
Commercial chargers will do this by monitoring each cell individually and halting the charge on any cell that gets fully charged. For a hobby charger we'd have to build that functionality into it, but it's not as simple as just hooking two in series or two in parallel.
In theory if the two cells were exactly the same at all times then we could do that, but we dont really know if they are the same at all times because their characteristics change over time and charge state. This is why i never recommend charging in series or parallel. There are people that do it, but it's kind of dangerous because if something does wrong it's fire and smoke on the way.
So the best bet is to simply build two chargers if you need to charge two cells. These chargers are so cheap to build it cant possibly hurt to build two. That wey each cell is assured of being charged safely.
Yes we can add a charge termination monitor that lights an LED. It should monitor charge current really because for a proper charger the voltage is already monitored by the charge circuit itself. Once the charge current gets down to a small percentage of the normal current level, we can light an LED.
This would not be hard to add and if you like i can add one or you could come up with your own idea. My first idea is to use a simple comparator and sense resistor to monitor the charge current, or if you dont mind a little less accurate we could use a simple transistor and sense resistor where when the current got low the LED would turn off (a little more circuitry and it would turn on if that's what you really want).
I am going to redraw my original circuit anyway so i'll add a charge monitor to it. I have another circuit too that has a charge monitor. There's a slight change to the original circuit that can make it work better so i'll show that too.
LATER:
I have redrawn the schematic to show the more correct connection for the adjustment resistors. Note that this one has the resistor R3 connected to the sense resistor R4 instead of directly to ground. That's the correct way to do it. It will work the other way too, but will take longer to charge to the full level.
I'll see about adding the charger termination sense next.
One quick note...
When measuring the cell voltage it must be measured right across the cell because with this charger the negative terminal of the cell does not connect to ground. Also, the input voltage must be slightly higher because the sense resistor takes about 0.6v, and that means the output of the LM317 (shown here as another similar IC) will be higher than 4.2v when the cell is fully charged although the cell voltage is really 4.2v.
Of course the better max cell voltage setting is 4.15v though.
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