Electronics > Power/Renewable Energy/EV's

Trickle charge lithium-ion battery after it is fully charged

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I am working on a multichemistry battery charger as a feature on a product. The charger is slow, 0.1C or less.

One of the issues I face is that I have to accurately and quickly determine if the battery is present. This would be fine with a normal Li-ion battery, as I could just turn off the regulator and measure the voltage once it's done charging, or measure the current while it is still charging. However, the customer has several types of battery packs, some of which have built-in fast charge regulators and series diodes to act as protection. So, if I turn off the regulator once charge is terminated, I am left with an indeterminate (could be zero!) voltage reading, as I am reading the input of a circuit and not reading the cell directly.

A solution I am working on that works quite well is to give the battery a few mA of charge current through a series diode. Then, I get a nice 0.2V if the diode delivers current to a load ("battery present") or if there is no load, a pull-down resistor ensures the reading is zero.

Is there any issue of pumping a few mA into a 2500mAh Li-ion battery if it has already charge terminated? I read online that the self-discharge is 5% in the first 24 hours, so presumably if I kept my current under 5%/24 hours (5.2mA), I shouldn't overcharge the battery?

The image below is a simplified schematic of the battery charger showing the series diode I use to sense battery presence. (The problematic diode is the one in the battery pack, not the one shown here)

I asked the same here a while back and the answer I got was, afaik - a couple of mA cannot hurt provided the voltage at the cells will not exceed their max voltage (4.20V in your case)..


You could apply small current once in a while, say for a few ms every few seconds. Trickle charging lithium batteries is not safe. There is barely any self discharge, so if battery is left like this for a prolonged time, it's really a safety hazard.

Thanks, and reading that thread I realized I should not use the term "trickle charge" because the point of this charge current is distinct from a typical NiCad or NiMH trickle charge.  I figure if I terminate at 4.1V, or 4.0 maybe, then set the regulator to maybe 5mA current limit or something like that, I can keep within a pretty safe margin. But to ensure the current is always available, I'd have to set the voltage higher than I would expect the battery voltage to go, maybe 4.5V?

I was also thinking about the pulse current thing, since I have access to a microcontroller. This would lower my average current significantly, if I did a short duty cycle. Say I pulsed 10mA every half second for 50ms, I'd get an average current of 1mA, which I would think is well below the self-discharge rate of the battery, not?


--- Quote from: drkntz on September 08, 2023, 02:58:15 pm ---average current of 1mA, which I would think is well below the self-discharge rate of the battery, not?

--- End quote ---
depends what battery. If it's say 2000mAh lithium cell, its self discharge will be way lower and 1mA is more than enough to overcharge if enough time is given. I personally would not make an average current more than 10uA. There is no reason to apply current for whopping 50ms unless initial detection already happened and you want to verify more reliably.


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