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12V DC UPS

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PlainName:
Hello :)

Just to shortcut even more angriness, perhaps I can butt in and point out:


--- Quote ---A BMS is a safety cutout no more and no less is is not now and never will be an appropriate method to terminate a multicell Lithium Charge process.
--- End quote ---

My understanding (of the suggestion, not batteries) is that the BMS is intended to stop the cell being depleted below 2.8V (or whatever the safe minimum is). It's not intended to be part of the charging system, which is what the constant voltage and resistor is. The CV/R is thus 'safe' because the battery will be disconnected by the BMS on over-discharge, so the <2.8V and 0V situations won't arise. Maybe a small fuse just in case would round it off.

You're welcome  ^-^

tom66:
The point is many BMS devices only provide balancing and overvoltage/overcurrent protection.  They are not smart enough to provide full protection for the cell, like adjusting the current limit at low state of charge.  That is the responsibility of the charging IC / controller.

PlainName:
Maybe those would be the wrong BMS to use, then. I mean, if your design is based around a 5V zener it's a bit pointless to note that all those snazzy diodes and diacs and what have you would really suck at the job.

But I reckon I am best out of it now...

beanflying:
Even the basic BMS boards typically provide over and undervolt battery protection including evilbay/AliX rubbish.

Because of the low impedance of most Lithium cells you can't charge as Constant Voltage from near flat (circa 3V) toward a 4V or better a 4.2V charged state. You need to use a controlled current source for the initial bulk charge a resistor and diode is not such a source. Toward the top for the last 10% of capacity (generally over 4V) this is where a Constant Voltage type of charge is done.

And before anyone bleats again about my use of the generic 'Lithium' rather than a specific type be that ion, polymer, Fe ..... it is for a reason not all Lithium Rechargeable Cells are the same but most share similar charge and use conditions.

NiHaoMike:

--- Quote from: beanflying on July 27, 2022, 02:03:16 am ---Because of the low impedance of most Lithium cells you can't charge as Constant Voltage from near flat (circa 3V) toward a 4V or better a 4.2V charged state. You need to use a controlled current source for the initial bulk charge a resistor and diode is not such a source. Toward the top for the last 10% of capacity (generally over 4V) this is where a Constant Voltage type of charge is done.

--- End quote ---
The resistor value is chosen to keep the current below the max charge rating of the battery. The current will go down as the battery charges, that's perfectly fine if fast charging is not the goal.

The 4.2V figure is to get the full rated capacity which is not desirable in this case. A lower voltage reduces the usable capacity but also reduces the degradation over time so in the long run it ends up cheaper. It's a completely different use case than for a drone where weight reduction is everything.

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