Folks, I build small alarm systems with are powered by 1.2AH 12v SLA batteries. They are constantly charged from a small charger using an LM317 which which charges at a constant voltage and can switch to constant current should the current exceed a setpont. The circuit is very basic , works well, available all over the internet.

I want to start using 3x Li-Ion cells in series and I’m a little confused about charging them with a constant trickle. I’ve bought 3x 18650 batteries plus a 3S BMS board, the data on the BMS board says it provides protection from over charging, over discharging, and over current. Is it safe to think all I need is a DC supply of around 13v with a current limit within the cells spec and they’ll happily trickle charge indefinably whilst the BMS board looks after the individual cells health ?

Also , I spotted some charger boards on Aliexpress which operate from USB and are able to charge 3x cells. Has anyone used these, and have an opinion ?

Regards
Malcolm
South Africa

 

Lithium-Cells really don't like sitting at full-Charge for long periods. ( ~4.2-Volts ).
You will need a special Charger that only takes them up to around 3.8-Volts.
This will reduce the maximum amount of Power they will be able to deliver,
but at least they won't swell-up and catch on FIRE.

I would recommend that You stick with the Tried & True SLA Batteries.
Lithium-Cells are just not well suited to standby duty,
they need to be "Pumped" continuously to get all of their advantages, and keep them safe.
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I also see no compelling reason to change battery types.
Why do you want to?
SLA's are well suited to that standby task and Li batteries are not.
The main advantage of LI's are that they are smaller and lighter for a given amount of energy capacity as compared to SLA's.
That wouldn't seem to be needed for this application.

But if you really want to use them, then your charger voltage should be limited to to maintain their charge some below their full level, as LQC noted.
The Battery University website states here "In terms of longevity, the optimal charge voltage is 3.92V/cell", so the maximum charge voltage for the three batteries should be 3.92V * 3 = 11.7V.
Your 317 charger adjusted for that voltage and the desired current limit may work for that.

 

Gents,

Thanks for the advice. I'm guilty of not fully describing why I'd like to switch to Li-Ion.
South Africa has major problems with the AC supply. Today is a good day, we'll have power for 19hrs of the day, the rolling blackouts have been with us for a year and everything points to them getting worse.

So the reasons to switch to Li-Ion is packing more capacity into the same space and ride through longer outages.
And hopefully have batteries lasting longer with LI-Ion's better cycling specs.
Talking about trickle charging LI-Ion batteries wasn't the best description.. in the "civilized world" , they may be called on a couple of times a year, but here is a daily event.

I'm going to setup some tests to monitor the batteries voltage and temperature, and take precautions should anything fail spectacularly.

 

And hopefully have batteries lasting longer with LI-Ion's better cycling specs.

For that you still want to charge it below the maximum for maximum cycle life.
Look at the Battery University website I referenced for rated charge cycles versus maximum charge voltage.
It's a tradeoff between the number of cycles and the maximum charge capacity you need to use.