How to charge a 18650 battery pack with BMS

Question

I am planning to built a batterypack using unprotected 18650 cells (2600mA) to power a DIY boombox with the TA2024 AMP. The pack will initially be 3s2p (expandable to 3s4p using battery holders that can hold 4 18650 cells). I want to use the following protection module/BMS

http://www.ebay.com/itm/271808179403?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Now my question is if I can just use a 12 volt 2A switching power supply and connect this to the BMS to charge the pack or do I need a more inteligent Li-Po/Li-ion charger.

The battery is: https://www.fasttech.com/products/1420/10002357/1170800-authentic-samsung-icr18650-26f-18650-3-7v-2600mah

Thanks for your reply.

Answer 1

I know this is an old question, but I've been doing some similar things lately and so I'd like to give you a different perspective. First of all, I fully understand why you're selecting these boards and cells on e-bay: to save money. I'm with you there 100%, and I think you obviously know the risks. There are good buys and bad ones as well as mis-representations, and as long as you understand this, the low prices do justify the experimentation. That said, let me answer directly...

I have some experience with battery management chips and circuits, and what you're suggesting "should" be fine, though you would need a supply that at least can be adjusted to a little higher, like 12.6V. Understand, however, that some inexpensive switchers (like wall adapters) can generate a lot of ripple, which I have found can confuse BMS circuits. If the BMS can't do exactly what it was designed to do, all bets are off, and you may never know. So if you use a switcher, consider adding some extra filtering. Alternately consider a higher voltage with an analog regulator, as it will provide current limiting. Its hard to know what inward current limiting exists on some of those e-bay BMS boards, and Lord knows you won't easily carry on a technical conversation with most of the sellers. But current limiting will give your batteries a bit more longevity, at the cost of a longer charge time, but in any case the current must be known by actual measurement. And be aware that picking a 12V 2A switching supply does not guarantee the battery and BMS board combination won't attempt to draw more, which could damage the supply too.

To remedy that, you can often get away with a higher voltage, even without a regulator, using a simple resistor ballast, and solve a multitude of problems. As long as there is something to limit the current. The reason is that these BMS boards effectively go to open-circuit, thus isolating the battery, once full charge is reached. I've verified this many times with a bench supply set to 16V with a 10 ohm power resistor in series, with using the exact same BMS board you linked! The voltage at the input/output point clamps down to under 12V until the battery reaches full charge, and then springs back to 16V, while the cell combo sits at 12.6. Granted, this is not the fastest charge, so eventually you can go to a lower resistance. Sometimes an auto turn signal lamp works well in this kind of circuit, because the bulb can offer an indication of full charge when it completely goes out. I know that sounds like a kludge, but what I'm saying is the BMS board will give you a lot of latitude for making a charging supply, as long as you're careful and measure whats going on while experimenting.

Finally, while we want to avoid repeating other people's answers here, I will second that you be doubly mindful of safety. When experimenting with inexpensive "e-bay" or "ali-express" Li-ION cells, BMS boards, and home brewed charging systems, always be aware that things can go horribly wrong, causing burns, fires, and more serious injury. Do all initial charging in safe, fireproof environments, with adequate protection to yourself and surroundings.

Answer 2

Be careful here. That BMS board appears at first, to be for lithium batteries. "variety of capacities in various shapes 3.7V lithium batteries." Charging voltage is stated as 4.25-4.35v per cell.

The word "18650" is ambiguous - these can be several very different types of cell, most of which are incompatible with each other. Lithium, Lithium-Ion, Lithium-Polymer, Lithium-Iron-Phosphate, etc. Further in the specifications, this item says "using Japan's Ricoh rechargeable lithium iron phosphate battery protection IC"... so is this for lithium cells, or lithium iron phosphate cells? If a BMS is connected to the wrong type, the best-case scenario is reduced battery capacity/life. The worst-case scenario is a fire and explosion!

I'd strongly recommend avoiding ambiguous products from eBay. The specifications and text given do not correlate to any one composition of 18650 cell. In other words, it is a total guess to whether or not it will work, let alone be safe and reliable. If you don't already have the datasheet for your selected batteries, get it, and make sure none of the charging parameters will be violated (such as trying to force 4.35v into a 3.2v lithium-iron-phosphate cell.)

As for 18650's in series/parallel, using a "BMS" board to charge them is a good idea. The more cells that are added, the more "finicky" the total design becomes. Charging each cell individually is the best way to ensure nothing dangerous happens. Two or four BMS boards could be powered from one supply, however note that in the case for Lithium batteries, 4.35v * 3 = 13.05v. Depending on the BMS, it may require more voltage than that also. The linked BMS provided no input specifications.

There are many options of battery management IC's available instead, if you wanted to know definitively that your charging solution will work reliably. Look at the various manufacturer's product offerings for ideas, such as Microchip, Linear Tech, Texas Instruments, etc.

And most battery management solutions may be capable of 4-5A "fast" charge; so four of those simultaneously would be 20A. So theoretically you could either "slow-charge" them all with 2A, or "fast-charge" them all with up to 20A (assuming the batteries are rated for this.) I'd personally recommend not fast-charging them, as it would require a massive power supply and will generate much more heat.

One last note about lithium batteries. Some can have self-discharge rates as high as 30% or more per month. So if your device isn't used in 3-4 months, it may be completely dead.

Answer 3

Short answer is no. That BMS board only provides limited protection and doesn't handle charging.

You'll need a balance charger that can handle three cells (3S). Since this is a huge selling point, and that Ebay listing doesn't mention charging at all, you can be assured that that product doesn't provide it, otherwise they would have mentioned it.

I'd also second rdtsc's answer and warn that 99% of the cheap Chinese electronics sold on Ebay are, at best, mis-advertised, and at worst, complete garbage that the manufacturer couldn't sell due to defects. Some young entrepreneur probably had the bright idea to dig them out of a dumpster and sell on the Internet. I say this as someone who's learned this the hard way. I've come to regret almost all of these types of products I've bought off Ebay.