# How should i charge 3 lithium ion batteries?

I'm new to battery charging and I was wondering. What is the best way to charge and manage 3 18650 cell batteries? The load is about an amp and I have a 3.7 amp power supply to charge them. I'd like to be able to charge the batteries and also power my device.

I plan on using the batteries in series, i'm aware that i need to use a 3S balance board, and a constant current converter to charge them, but is it safe to draw power from the batteries while charging? and where do i connect the load to?

• I have a 3.7 amp power supply to charge them You don't mention the output voltage of that power supply which is a quite important parameter. I really suggest that you browse the Internet and see how others do this. There is plenty of information to be found usually related to RC cars etc. It is safer to understand what you're doing rather than rely on "connect A to B" instructions as there you can easily make a mistake and ruin your day. Aug 26, 2019 at 6:57
• How should i charge 3 lithium ion batteries? The answer is : carefully! Mistakes may be highly explosive.
– J...
Aug 26, 2019 at 16:31

i'm aware that i need to use a 3S balance board, and a constant current converter to charge them

You need more than just a current source. The charger must limit the voltage to 4.20V per cell (12.6V total), and (if you want to get more than 80% charge into the battery) then continuously reduce the current at that voltage until it reaches ~1/10 the set charge rate, then shut off. If you are using a basic CC/CV (Constant Current / Constant Voltage) converter then it probably won't shut off, so set the 'float' voltage to 4.15V per cell.

To prevent over-discharge you should have a cutoff circuit that disconnects the load when the battery reaches 3.0V per cell (9V total). If the battery gets very low (<3V/cell) it should be charged at a lower rate until the voltage reaches ~3.7V per cell (11.1V total).

is it safe to draw power from the batteries while charging?

Yes, but the battery won't charge if the load current is higher than the charging current. Assuming you have 3.7A at >12.6V available and the load only takes 1A, there is 2.7A free for charging. However the charge current must not exceed the battery's rating, which might be eg. 1.5A. So you could set the charge current to 2.5A, but if the load is turned off then the battery would get 2.5A which is too much. Therefore the charger has to monitor battery current separately from the load, or charge at 1.5A which will only get 0.5A into the battery while the load is on.

and where do i connect the load to?

To the battery through the 'balance' board.

Alternatively you could power the load from the power supply when it is on, and charge the battery at the same time with its output isolated from the load. This has the advantage that the battery can be charged at full rate even when the load is on, but the circuit is more complex.

If the load can work at slightly higher than 12.6V then a simple diode switch-over circuit might suffice. Details are important though. With the charger and load sharing a common ground the charger must monitor current in the battery positive lead, not negative, otherwise load current would upset the charger circuit.

Li-ion batteries can explode if they are overcharged, so don't charge unattended until you are sure that the circuit is working properly.

• Hello, each of the batteries have their own protection circuits, so I think the overcharge and overdischarge problems are solved. Thanks for your helpful response! Aug 26, 2019 at 10:30
• @Ramsom careful with relying on those in series, because say an overcharge circuit trips and the battery ends up disconnected on one end - the protection circuit might expect say up to a 5v charge voltage, but if it remains in series with other cells that aren't fully charged, it could end up recieving much closer to your full charger voltage (i.e. say you charging the entire pack with a 15v charger, relying on the cells to turn off individually once charged - if the first cell charges to 4.2v and shuts off, while the others are at 3.5v you will have (15-3.5*2)= 8v on the first cell) Aug 26, 2019 at 14:22
• @Ramsom For something that can catch fire, you might consider having the battery's protection circuits be the backups to your own. Two things that have to fail before your house/garage/whatever burns down. Aug 26, 2019 at 15:13
• Protection circuits trip at voltages above and below the recommended operating voltages of the cell. They should be used only for 'last-ditch' protection against failure of the charging and cutoff circuits, not to determine end of charge and discharge voltages. Aug 26, 2019 at 20:42