I want to power my project with a regular 3.7v lipo battery, but I would like to charge it about 3.9v to maximize the life of the battery, I would also like to be extremely conservative at the lower limit and cut off at 3.1v at the most. I have plenty of power as I am not worried about capacity loss, however I may not have to replace the battery for ages.
I am not able to find a charger where I can set these values as All I can see has a fixed voltage end of 4.20v with no possibility of change.. All I can find are exceptional, complex and expensive ICs with thousands of other features and eventually also with with the ability to change the charge / discharge voltages.
Can you recommend a simple IC or preferably, a ready-made pcb / board on which I can set these values? I am not looking for anything extraordinary, at least 0.3A charging rate and 5V input.
Edit: looking for a IC/board to embed in my circuit rather than an external charger.
Why do you think this will make a significant difference, given that battery life is determined almost entirely by the rate of power draw and number of charge/discharge cycles?
It explains that charging a lithium-ion battery to 3.9 V instead of 4.2 V can increase cycle life dramatically, from around 500 to as much as 2400–4000 cycles.
Although each cycle delivers less energy (only ~70%), the much longer lifespan often results in greater total energy over time. This approach also keeps the battery healthier and more stable, making it a good choice when capacity isn’t the main concern.
Charging the lithium battery to only 3.9v was the strategy I adopted for the $50SAT project.
An LTC3105 solar charger was used, set for a 3.9v limit.
The $50SAT satellite went through about 13 charge cycles a day, dark and light parts of the orbit etc. It stopped transmitting after 20 months when the solar panels degraded, at which time the battery had gone through approx 8,000 charge cycles.
I doubt you went through 13 charge cycles. A charge cycle is defined as starting at 3. V going up to 4.2V and back down to 3.0V. If you forced a limit of less, the battery would have to charge and discharge many more times to be algebraically equivalent to a 'normal' cycle. This means that you may have had 8,000 round trips from 3.0 to 3.9, but that is the equivalent of the normal 2,000 to 5,000 full cycles. It's an area under the curve situation, and I actually am suspicious (without doing the math) that you got on the low side. What you may not know is that in this UPS or charge while discharging circuit, there are many published versions that are incorrect. A correct circuit is published here on the forum. Sorry, I don't have a link, but you will know you found it when the poster's name is Sherman
I remember from a Youtube series on building a CubeSat for under $1000 that the battery situation was a bit unusual. The battery had to be sized so it could be recharged within the 45 minutes of "daylight" in a low-Earth orbit. And that meant that it could only discharge about 20% at "night" so recharging current wouldn't be excessive relative to its size. Or something like that.
That series also went through the schematics of each section, including the power supply. I was impressed by how careful and thorough the design process was.
@srnet, I was unable to bring up the $50SAT website, but would be interested in any schematics that might be available, if you have a link for that.
As for the original question, they do make 4.1V chargers for the older lipos, but I don't know of a chip that lets you adjust the termination voltage. But I wonder what would happen if you powered a standard linear charger like the TP4056 with, say, 4V. It couldn't charge to 4.2V because current can't flow uphill, but would it charge at all? Would it terminate properly? Or would it even need to terminate (can you continue to apply 3.9V to a lipo indefintely)?
Following up on this, I set up a TP4056 charger powered by an LM317 power supply adjusted to 4.02V output, and started charging an 18650 that was about 3.78V. The current setting resistor in the TP4056 circuit was 1.2k, which provides about 1A constant current.
The cell has now charged up to 3.93V, and appears to be stuck there. The red charging LED is still lit, so it hasn't terminated, but the charger is only drawing 7mA (the termination current should be 100mA).
So if the charger is linear, you might be able to use standard 4.2V parts, but just power the whole thing at 4V. I just don't know if continuing to send 3.93V into the cell is a problem.
I let the charger continue to run for several more hours, and with the input voltage at 4.00V, the 18650 settled in at 3.96V, and never went higher. So I think that makes sense. The TP4056 apparently stays in constant current mode, and never switches into constant voltage mode, presumably because the switchover is normally triggered by reaching 4.2V, which in this case never happens. And of course it never terminates.
If I remove the input power, battery voltage drops to about 3.94V. That means some very small current continues to flow into the battery as long as charging power is connected, even though its voltage is not increasing. But does that matter?
Big Clive has a video claiming you can continue to apply 4.2V to a fully charged cell, and it won't hurt anything. This goes against everything I've ever seen related to lithium charging. I've never seen a charger IC that does not terminate charging. None of them "trickle" charge at 4.2V. Clive says the cell won't be charged above 4.2V, but I think the real question is whether current continues to flow into the battery. He doesn't address that.
But does it matter if you trickle charge at a lower voltage - at 4V, or even at 3.7V? I don't know.
If you connect 4.2volt to a battery that is charged to 4.2volt, then no current will flow.
"Trickle charge" is when you apply a constant current to a battery, from a higher voltage source. This was done with NiCad batteries.
A TP4056 does not trickle charge. Current tapers off between 4.0 and 4.2volt. Then no current flows when the battry equalises.
Leo..
Well, my experiment suggests that when charging at 4V, the battery never quite gets to 4V, and current continues to flow into the battery - indefinitely. So in effect it is a trickle charge. I doubt it would be any different at 4.2V.
My experience is that there is very little self-discharge with a good LiPo battery, even above 4volt, so there is almost no trickle- charge. Maybe you have a poor quality battery.
Leo..
The Tp4056 doesn't terminate charging. It keeps on trying to reach that 4.2volt.
At a linear reduced current between 4 and 4.2 volt.
It's not clear why OP wants a longer battery life, and how long, and under what conditions.
Cellphones with LIPo last >4 years under 4.2volt abuse.
As said, LIFePo4 could be a better choice.
Leo..
"The charge voltage is fixed at 4.2V, and the charge current can be programmed externally with a single resistor. The TP4056 automatically terminates the charge cycle when the charge current drops to 1/10th the programmed value after the final float voltage is reached."
. Can you confirm the IC you used? Searching for LTC3105 I can only find a Step Up