I want to power a small project with 4 little and cheap dc-motors (tt Gearmotor), a Microcontroller and some sensors. Maximal current draw is 2 A, most of the time 1A. For that I want to use two 18650 Li-Ion Batteries (I still have a bunch left over from a previous project) in 2s1p configuration and now I am looking for a simple (all-in-one) solution that can provide
undervoltage-protection during discharge
1-2A continuous discharge current, but more is better
charger with current limit, individual overvoltage-protection and prevention of trickle-charging
balancing (active if possible, at least passive)
charging with standard 5V-USB-Charger (necessary to step up 5V for charging)
Is it really possible that there is no single board that checks all my boxes? For a single Li-Ion-Battery there is an all-in-one-solution with the TP4056 as long as your current draw isn't too high. Is there nothing comparable for a 2s configuration on the market out there?
What I've found during my research so far is that there are several single components that only meet some of my requirements and have to be combined to achieve what I'm looking for:
undervoltage protection, but charging without current limit (5A in datasheet is too high): Battery Charge Discharge Protection Board, don't know if triggle charging is prevented...
passive balancing, but without undervoltage protection (2,5V measured on single cell and still running): BMS Board [Balanced Version]
USB-C charging-Board with step-up and current limit: USB-C 2S Charger
I would appreciate any advice in this regard. If there's really no one-board-solution out there what might be the best combination of boards to meet all my requirements? As I need it in an educational context, I would also prefer a cheap and simple solution, as it should not be too expensive for the students and the fewer parts we need, the easier it will be to realize. This is also the reason why I want to avoid the need for an external battery charger.
The 2S configuration really complicates things. You need to boost voltage for charging, and you need balancing. Another approach would be to use a 2P configuration, charge directly from a 5V supply, then boost the output as needed. You would have one switching device, as in the original case, but no balancing required. I don't know of an all-in-one solution for 2S, but they are available for 2P, and they even include load sharing.
Edit: This 2P version might work, but it's pretty expensive, and it has no way to turn it off except remove the batteries.
Thank you very much for the quick reply. I have already experimented with a 2p configuration in an old project. At that time, I was unable to find a suitable step-up converter that could deliver a stable 2A. Does anyone here have any experience with a step-up converter that reliably achieves 6-8V at 2A output and can recommend something?
It would be best if it has been tested, as unfortunately I no longer trust the data sheets here regarding the output current. When I tested various step-up-modules back then, their voltage always collapsed very quickly at 1A output current at the latest, even though the data sheet promised more. Since then I have never considered this approach again if higher output voltages than 3,3 V are required. But according to your link, there already seem to be modules in this direction. Do you know whether the 3A really refers to the output current and not the input current?
Unfortunately, there is only a 5V and 9V solution. Something in between would be nice (the motors are actually only intended for 6V). Unfortunately, the solution isn't cheap either, but if I can save a DIY solution with lots of different components, it might be worth considering again...
I would be grateful for any further thoughts on this subject...
The output voltage of the converter would be determined by a resistor divider coming off the output. So to get 6V, you could get the 5V or the 9V version, and change out one resistor (or possibly just add one in parallel with one of the existing resistors).
I don't know about the accuracy of the current spec. But I'm sure the 3A is intended to be the output current spec. One of the listings shows this:
"2、Boost circuit: peak power up to 20W, rated power up to 15W, BOST boost circuit, built-in 12A high current MOS, efficiency up to 96%, module static power consumption 25mA, optional 5V/9V/12V output. (Load normals are recommended <15W. Output 5V: current 3A Output 9V: current 1.6A Output 12V: current 1.2A )"
But you would have to get one in hand and test it to have any confidence in what it will do.
Well I don't know if this particular module would be good for what you want. I just never had any luck finding modules that deal with with 2S and provide balancing.
Perhaps there are actually no good 2s-options because there is little demand or similar.
That's a good idea to simply adjust the resistors. Sounds promising. Thank you.
It's a bit annoying that your 2p all-in-one suggestion is primarily intended as a UPS and therefore can't really be switched off without removing the batteries, but maybe there's a solution for that, that isn't too complicated. I'll probably have to order one or two and see if it works for my application if no other good suggestions come up. In any case, thank you very much for your help!
In case it might be enough, the single-cell version of this module does have a provision for installing a switch. In fact, you have to install something for the module to work at all. But if you need 2A at 6V, then the boost converter may be sucking in something approaching 4A from the battery, so I suspect the two-cell version is more likely to work for a reasonable time. But looking at the pictures of the two-cell, I don't see any reasonable way to modify it so it can be turned off.