EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ct1305 on October 02, 2022, 09:24:42 pm
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Hey guys,
Have posted previously about trying to use an off the shelf 18650 charger/5v boost board however I would like the challenge of creating my own that’s more suited to the low current draw application.
Specs I would like are:
2 x 18650 cells in parallel for higher capacity
5v 2a input for charging
5v 500mA boosted output
4 x leds showing battery percentage/flashing when charging
Push button on/off to turn the boost/charger on and off
The required output current is only in the 50-100mA range
I am slightly lost on selecting the correct IC/IC’s for this as these seems to be a lot of options. The goal would be to add this to a section of the breakout PCB. Would anyone be able to point me in the right direction with the selection process and schematic?
Thanks in advance
Chris
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I think I linked to my video in your other thread, which includes a circuit using the TP4056 charger and the MT3608 boost converter. But it just has a plain on/off switch ahead of the boost converter, and no blinking lights showing state of charge. (However, the TP4056 has outputs for "charging" and "charged" LEDs.)
One potential problem with designing your own boost converter is that board layout seems to matter a good bit, presumably because of the inductor. I avoid that problem by using modules which someone who knows more than me has designed. There are modules for both the TP4056 and the MT3608, and there's the J5019 module which combines both (but has no protection, load sharing, or on/off switch).
It's not clear to me how the charging percentage is determined while charging. You can't just measure voltage because the final charging stage has voltage fixed for quite a while.
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Hi,
You dont have to measure charging percentage you just have to measure voltage, and keep the current below the manufacturers specification.
When charging these kinds of cells the voltage has to be limited to 4.20 volts (or slightly higher if it is a special type that goes up to 4.35 volts but that's more rare).
The current stays limited at some value like 700ma or lower (can be much higher depending on the cell).
The voltage will be low, typically 3.5v or so when the charging starts, and the current will be max (let's say 700ma for now). As the cell charges, the voltage rises slowly with a good cell. As the voltage gets near to 4.20 volts (or 4.15v to extend cell life) the current starts to drop due to the control circuit sensing the voltage. The closer the voltage gets to 4.20v the lower the current, until very very close to or at 4.20v the current drops to some low value like 50ma or 20ma (depending on cell). That's when the charging process has to be terminated to keep the cell from being damaged.
So there is a bit more to this then charging NiCd cells where you can over charge them somewhat and not do too much damage. The control circuits are not that difficult to understand nor design though so it's something i think anyone can do with a some decent amount of reading on the subject.
The most important is not to go over the 4.20v spec i think the max is 4.25v but i never go that high.
Not sure why anyone would need a boost circuit for this though i would think a higher voltage would always be available, like 5v for example from a cheap wall wart. One exception is if you wanted to charge from a single or two AA cells where the voltage is only 1.5 to 3v or something, but i think that is a waste of batteries unless it's something you really can not do without.
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Check out Ali Express
18650 boost PCBs ready made.. and cheap like borscht
No point in re-inventing the wheel