Hi again,Hi,
I would be very very careful when trying to use an output series diode to prevent battery back discharge into the charger. That is of course because then we have to rely on the diode voltage drop being constant in order to set the output voltage to the correct level. If the diode voltage drops, the output increases.
One idea that is sometimes used is to wire in the contacts of a relay in series with the battery. The relay coil is driven from the input voltage source, so if the source goes away the relay contacts open and the battery is disconnected completely. The relay coil is chosen based on the lowest level input that will properly run the charger so that it will open long before the charger starts to malfunction.
Another idea would be to wire the diode into the ground circuit and use an op amp voltage follower to track the B- battery voltage for the LM317. We would have to make sure though that the op amp input does not draw current when the LM317 no longer can supply power to the circuit. Of course this complicates the circuit though.
No, no and no.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?
A comparator is used to compare the voltage across a series resistor with a low voltage then Ohm's Law determines the amount of current.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 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?
I am sorry because in various ideas of your here I lost my sense and confused what you modified recently... My be recently you edited 'better charger than before'. But I think it has no auto-termination feature. It has no monitor indicator too.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.
I will add a charger monitor next but did not get to do that yet.
Hi,
Here is a drawing showing a simple charge monitor. It may not be as accurate as we would like however because it relies on the transistor base emitter voltage which varies with temperature. I used it on another charger though so here it is.
The monitor part is the R3, D1, Q1 circuit. Note that it is placed at the input to the LM317. Also note that the LED is either a single blue, single white, or two red's in series. The total voltage of the LED has to be around 3v or so and this makes a single red LED not possible because the voltage would be too low.
Also note that you should test it to make sure R3 is the right value for the current level you need to turn the LED off.
Also note that when the charge is started the LED will turn on fully, then when the current becomes low after the battery charges the LED starts to dim and then goes out completely.
Also note that the table shown shows resistor values for a low current charge which takes longer. You could lower the value of that first resistor to increase the max current but the other circuit is better for this.
No.can we say every general LEDs (which gives blue or white light) need 3V supply and red needs just 1.5 or less?
Hi,
- can we say every general LEDs (which gives blue or white light) need 3V supply and red needs just 1.5 or less?
- I feel previous circuit was little easy to handle. 30 ohms as 1st resistor gives just 0.3A. It's ok but for fine battery I need around 0.7A. Using 12 ohms there I will get 0.75A but dissipation on resistor will be 6.75watt.
How you feel, previous was little good for charging, right? Then cannot we combine the monitor there simply? (but just leave if you feel it's enough)thank you!
Oh yes, I understood. Simply I can use this 'monitor section' in the previous charger same as here. I attached a complete drawing, please check it.If you draw this monitor up with the previous circuit yourself i will be happy to check it for you.
May be there days are little difficult now a days. News said must colder days north America is facing. Takecare.Unfortunately i got a bad cold or flue so i am not doing too much for a few days or more.
I am little confused on R3, It is bypassed by D1 then....how....The monitor part is very simple. It measures the current using a transistor and turns OFF the LED when the current reaches a certain level set by R3.
Wow OK I want to see once. I have to charger various Li-ion cells including my good conditioned laptop Li-ion cells. So it wound be better if there is simple and accurate.It's not a super accurate monitor though so if you want a more accurate one i'll draw that up as soon as i can.
There was another post before 'my very last post' where I made this drawing, but I thought it is fool, so I made another. Ok Then my 1st drawing was good! OK I will adjust R1 for indicator, will increase R2 to 270 and use 9V instead of 7. I don't know how you didn't see it. I can see 20 posts at once here in a single page.Hi again,
I meant your very last drawing was drawn nice, and i only see one drawing. I took that drawing and made the corrections now.
hehehe don't worry for this time because I have two pieces of LM358, and I think one is enough for now. Think you!I almost forgot you have trouble getting some parts. We might need a special op amp for the more accurate monitor circuit, but i'll see what i can do.
What op amps can you get now?
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