Electronics > Projects, Designs, and Technical Stuff
UPS - Charging a Li-ion battery whilst also powering a load?
magic:
I wonder how laptops maintain their batteries?
I have an over 10 y.o. laptop which must have accumulated a few years of total uptime by now being almost constantly plugged in. It still manages more than half of its original battery life on the odd occasion that power goes out.
edit
Partly answering my own question: current battery voltage reported by ACPI is 15.415V. I suppose that means four cells in series and 3.85V per cell.
Buriedcode:
--- Quote from: magic on March 22, 2020, 03:49:38 pm ---I wonder how laptops maintain their batteries?
I have an over 10 y.o. laptop which must have accumulated a few years of total uptime by now being almost constantly plugged in. It still manages more than half of its original battery life on the odd occasion that power goes out.
edit
Partly answering my own question: current battery voltage reported by ACPI is 15.415V. I suppose that means four cells in series and 3.85V per cell.
--- End quote ---
Because the battery isn't always charging. And when plugged in the laptop is powered by the DC input, so the battery is effectively only powering its support circuitry. Laptop batteries' life is based on charge/discharge cycles, as well as whatever limit the BMS has set. Often the limits are set so that the battery pack stops allowing charge, so the user replaces the pack before performance degrades badly, or just so the user replaces the pack every few years for safety purposes. If your laptop spends the majority of its time plugged in, then that isn't very many charge/discharge cycles used.
Buriedcode:
To the OP, I may be missing something here but aren't you just talking about load sharing?
https://ww1.microchip.com/downloads/en/appnotes/01149c.pdf
Most, if not all lithium chemistry chargers use current draw at constant voltage for charge termination. SO it charges at constant current, then when maximum cell voltage is reached, it holds that voltage, allowing the cell(s) to determine the current. As the cell charges, this current decreases, and "end of charge" is usually when this current is C/10 or C/20 where C is the charging current.
The upshot of that is, any load on the system also draws current, and if this is larger than C/10 - say you charge at 500mA. C/10 = 50mA, so any load greater than 50mA will mean the charger never stops, and could mean, that you're constantly putting energy into the battery even when it is at max capacity. Whilst this current may be small, and would probably just cause slight heating, it is still generally a bad idea.
So it is best to keep the charger circuit directly connected to the battery, but isolate the load with a MOSFET, so when charging, the only thing the charging circuit "sees" is the battery.
SiliconWizard:
--- Quote from: Buriedcode on March 22, 2020, 04:25:27 pm ---
--- Quote from: magic on March 22, 2020, 03:49:38 pm ---I wonder how laptops maintain their batteries?
I have an over 10 y.o. laptop which must have accumulated a few years of total uptime by now being almost constantly plugged in. It still manages more than half of its original battery life on the odd occasion that power goes out.
edit
Partly answering my own question: current battery voltage reported by ACPI is 15.415V. I suppose that means four cells in series and 3.85V per cell.
--- End quote ---
Because the battery isn't always charging. And when plugged in the laptop is powered by the DC input, so the battery is effectively only powering its support circuitry. Laptop batteries' life is based on charge/discharge cycles, as well as whatever limit the BMS has set. Often the limits are set so that the battery pack stops allowing charge, so the user replaces the pack before performance degrades badly, or just so the user replaces the pack every few years for safety purposes. If your laptop spends the majority of its time plugged in, then that isn't very many charge/discharge cycles used.
--- End quote ---
Yup. Laptops just charge the batteries when they are under a certain level too. For instance, if you're on DC supply, it will provide power to the laptop AND will charge the battery ONLY when it is under a certain level (depends on the laptop and configuration - can be anything from 80% to 95% or even 100%) There's always some kind of "hysteresis" implemented with usually two thresholds (the battery's state under which the charging kicks in, and the state over which it stops charging.) If you constantly leave your laptop on DC adapter, the battery will recharge only when it has self-depleted to a given state as I just said, and since a Li-ion battery in good shape has a relatively low self-discharge, it doesn't happen very often.
So it requires some kind of "intelligence" - granted it's pretty simple overall, but it will still require some logic to do this. If you're trying to build a PSU, you should definitely control the charging in a similar way probably with some MCU controlling the charging circuit.
magic:
--- Quote from: SiliconWizard on March 22, 2020, 04:51:55 pm ---Yup. Laptops just charge the batteries when they are under a certain level too. For instance, if you're on DC supply, it will provide power to the laptop AND will charge the battery ONLY when it is under a certain level (depends on the laptop and configuration - can be anything from 80% to 95% or even 100%) There's always some kind of "hysteresis" implemented with usually two thresholds (the battery's state under which the charging kicks in, and the state over which it stops charging.)
--- End quote ---
I have noticed it too. Normally the "charged" LED is lit, but sometimes the "charging" one lights up for a short time. I just set up a script which logs battery voltage on that laptop so I will see what the thresholds are.
At any rate, laptops are proof that with proper techinque a perfectly viable UPS can be built around Li-ion.
update
Charging kicked it an 3.81V per cell and got it up to 4.17V per cell, although I don't know the exact algorithm and termination condition used.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version