Charging Li-Ion while providing load.

Hello. I'm building a 12v bluetooth speaker and am trying to design the battery and charging system. I'm thinking about using 3 of the Panosonic NCR18650B in series to start, and then perhaps upgrade to another 3 pack in parallel later if the battery life isn't good enough. This is my first experience building something with Lithium batteries and it's a bit intimidating. I've done quite a bit of reading, but i wanted to come here and ask a few questions in hopes of getting this right the 1st try.

The BMS i've chosen (linked here) shows "charge/discharge" on one of the images for the P+ and P- terminals. I'm hoping this means that I can connect my incoming power AND my load (12v amp & bluetooth module) to those terminals and it will allow me to use the amp while charging. In the description it says "Charge with 12.6-13.0V Power supply (not included)." With that in mind, here are my questions:

1) I was working under the assumption that i would need a battery charger or charging circuit before the BMS, but the description makes it sound like i can just put a power supply directly to the BMS. Would one of the 12.6v li-ion plug-in style chargers (like this one) be ok to use with this BMS? Would a charger like that cause interference noise in the speaker?

2) When connecting the power and load to the BMS, do i need any kind of diode in line with the load?

3) I was planning to get the protected versions of these batteries, but is that necessary if i use the BMS and charger that also offers protection? If I was to use the protected batteries, does that eliminate the need for the BMS?

I have built NiMH chargers using the Maxim ICs in the past that allow for charging and load as the same time without any interference, and I know that option is available for Li-Ion as well. However, i would REALLY like to avoid making my own charger if possible.

Thanks for any advice you can offer!

I was working under the assumption that i would need a battery charger or charging circuit before the BMS, but the description makes it sound like i can just put a power supply directly to the BMS.

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The board does the job of both charger and protection; you will just need a suitable regulated power supply with reasonable current capacity.

I was planning to get the protected versions of these batteries, but is that necessary if i use the BMS and charger that also offers protection

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They are expected to work with cells that are unprotected. If your cells are protected, you get no additional benefits at all.

They are being used exactly the same way you will use a supercap as a backup power.

c_mitra said:

The board does the job of both charger and protection; you will just need a suitable regulated power supply with reasonable current capacity.

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That's great! The downside is that I'll have no external way of knowing when the batteries have fully charged. There are S1 & S2 solder places on the image. They have a line drawn to where text is supposed to tell you what they do, but it is missing. Could this be for indicator LEDs?

c_mitra said:

They are expected to work with cells that are unprotected. If your cells are protected, you get no additional benefits at all.

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That's also great.. i believe the unprotected cells are a bit cheaper.

I'm confused by the supercap comment. What does that mean?

Thanks for the response!

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If i decide i want to double the capacity, would be it acceptable to use this setup twice and connect the charge/discharge of both BMS boards together? That is, if i already have a 3s battery with the BMS built... could i just build another identical setup and combine them? Or will i need to rebuild the pack to a 3S2P configuration and buy a different BMS?

It is not clear because I do not have the specs. But in the photo I see they are connected to a 15 cell pack - 5x3 configuration. A 3seriesx5parallel pack is seen connected to this board and you need to consult the seller for the specs.

So i posed my plan for this project (image below) on the diyaudio forums. Someone raised the concern that my BMS is overrated for my project, at 25A. I wouldn't expect to use more than a few amps at maximum, but it never occurred to me that the BMS being rated that much higher than my project could be a problem. He said "Using overrated BMS module may cause over current shutdown protection not able to work." I tried to a quick search on this and found no information warning against it. Has anyone heard of this being a problem?