Is there way to construct an 18650 battery pack with built-in balance charging? Or is balancing not very necessary?

Question

I am a DIY enthusiast, especially in electronics. I'm currently building a portable Playstation 2 Slim with an IPS display, so that it would look like a Wii U controller. It's my first large electronic endeavor.

However, being portable, I'd need it to be powered by rechargeable batteries. I'm very confused on how to approach the power part of this project. I'll try my best to be detailed as possible! I look forward towards your answers!

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The Little Details

Looking around, I concluded that a battery pack of 18650s (3S) would do a good job, it seemed straightforward too. Connecting three of them in series will produce a maximum of 12.6 V. It's also a bit common, therefore a lot of information about the batteries can be found, and even be salvaged from laptop battery packs.

I have some details on the electronics included in the project so far:

Sony Playstation 2 Slim (Model 75003)

• Operational Voltage: 8.5 V
• Power Consumption: 6 A Maximum

Innolux N070IDG (Yeh, I love nice screens :D)

• Type: IPS LCD
• Resolution: 1280x800
• Size: 7 inch Diagonal
• Operating Voltage: 9-12 V (best at 12 V)
• Consumption: 190-210 mA (full brightness) (Indicated from bench power supply)
• Display Interface: Includes Interface board of HDMI, VGA, 2 x AV.

PAM8403 Audio Amplifier

• 2 Channel
• Output: 3 W output per channel at 4 Ω.
• Voltage: 5 V

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The Batteries

I managed to obtain 6 x 18650 batteries from an old laptop. After some searching, they seem to be Sony SF US18650GR 2400 mAh Li-ion batteries. So I concluded that this seems good enough as a start, three of them.

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The Problem

I wanted to use this 3S battery pack with a BMS. After I got the BMS, just as I was about to assemble the pack, I researched some more.

It seems that BMS's do NOT balance cells. I thought, since it has an under- and overcharge protection, it will all charge the cells at 4.2 V each, and when a cell is full but the others are not, it will stop charging for that particular cell and continue on the cells that aren't full. But I seem to be wrong, and it can still be out of balance.

I was wondering: most consumer devices we use simply use a DC power charger/supply to re-charge devices, such as laptops or portable speakers, etc. For sure, they must've designed a balancing circuit inside the battery pack or in the device - or aren't they balance-charging either?

Most tutorials mention that the use of a balance charger with a balance connector is the only way to maintain performance. I find it rather inconvenient to carry around a balance charger and remove the battery pack from the device to re-charge it.

My question is

Is it possible to design a battery pack that has necessary protection features such as under-/overvoltage and over-current protection, and design it in a way that it charges through a simple DC barrel charger?

Or is balance charging something not completely necessary?

I'm just really scared about using lithium batteries. I don't want to put myself or anyone in harm.

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My Possible Solutions

Since I'm not very experienced with lithium batteries, and to me, it seems balancing is so critical, I thought of a few solutions that I hope will be OK to do, I welcome your feedback on them.

Solution A
Use only a 1S3P (or more in parallel) pack instead and use a TP4056-based USB 5 V charger. Pairing with 3 BOOST converters to power the LCD, PS2 and other electronics at their own voltages, WITH a 1S BMS. (My worry is that my battery may not be able to handle the current draw.)

I'm aware that I'll have to do calculations based on the efficiency of the boost converters as well, to get an accurate current draw from the batteries.

Solution B
My initially decided method, I think the diagram is self explanatory. But I'm hesitant to use this method as I discovered it does not balance the cells (and ruins their life) and can be dangerous.

Solution C
Individually protect each cell with a 1S BMS, AND use a 3S BMS together. It sounds ridiculous, I guess. But somehow I think it will work, but not as great or wouldn't be recommended.

Solution D
The proper balanced method, which would need usinh a bulky balance charger and the inability to use the device while charging (removal of the pack needed to charge). It's really inconvenient, in my opinion.

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Well, thanks for reading, I hope that wasn't too lengthy. I really hope I get an answer out of this once and for all. Because I don't usually ask, I just research. Now I really need help as it can be dangerous if this goes wrong.

Please let me know what you think and what solution is best. I'll try to answer back to the best of my ability.

I'm also eager to know, about what mistakes there could be in my 'possible solutions', so that I could avoid or correct them in the future.

Answer 1

I managed to obtain 6 x 18650 Batteries from an old laptop.

This is your first problem. Those old batteries are probably tired and will struggle to supply the required current. Individual cells may have different internal resistances and capacities, so balancing is advised.

Solution A - Use only a 1S3P (or more in parallel) Pack instead and use a TP4056-based USB 5V Charger.

Bad idea. The battery will charge very slowly, and the booster will waste power. The pack and wiring will have to handle 14A+ discharge current.

Solution B (BMS and '12.6V' charger)

If the BMS includes balancing then it should work, provided the '12.6V' charger is designed for 3.7V Lithium cells. Without balancing, some cells could reach peak voltage before others and then the BMS would terminate the charge early, resulting in a partially charged, out of balance battery.

The BMS won't cut on discharge until at least one cell has dropped to a dangerously low voltage. After a few cycles the cells will start dying. To protect the battery you should install an alarm or cutoff that doesn't let any cell go below 3.2V.

Solution C - Individually Protect Each CELL with a 1S BMS, AND use a 3S BMS

Overkill, but perhaps (depending on the balancers) not enough! Many balancers work on the principle of bypassing charging current when the cell reaches peak voltage (4.2V). The problem with this method is that if the balancer can't bypass all the current then the cell will continue to be overcharged (until the protection circuit kicks in).

Solution D - The Proper Balanced Method , which would need a use a of bulky balance charger

Again, how well this will work depends on the particular charger. Some contain 3 isolated circuits that charge each cell individually. This is the most reliable method of balance charging, but the control panel has to communicate with all 3 chargers while maintaining isolation, so it is mostly used in simple low-end chargers that may be unreliable.

More sophisticated balancing chargers have an LCD screen and are fully programmable. Their balancers usually work throughout the charge cycle so the cells start to become balanced before reaching peak voltage, but most of them have relatively weak balancers. The main advantage is that the LCD screen shows you the cell voltages, so you can cut the charge rate down to help balance the pack if necessary. The display also shows how much charge is put in, so you can gauge the health of the pack.

A good balance charger may be bulkier, but will be more powerful and gives you much more control and flexibility. Many can also do Nicad/NiMH, LiFPO4 and Lead acid batteries. One charger may be all you need to charge many different devices.

Answer 2

I think you mis-interpret how your BMS balances cells during charging.

For a 3 cell BMS, there are typically FETs across each cell. When a cell is getting close to fully charged, the FET is used to bypass some of the charge current (it does not turn off charging to the individual cell usually). Balance current bypass are typically a very small fraction of the charge current...perhaps as low as 1/10 the pack charge current, but this is enough to balance out relatively small differences in cells. In addition to being able to bypass some current around a given cell the BMS can turn the charge current for the whole pack off.

For BMS implementations where the charge current gets high (many Amps) they use a charge pump technology to divert power from an overcharging cell to an undercharged cell or back to the supply capacitor. Like this from Linear. This improves power efficiency, but this is not the typical BMS you buy from Ebay with simple charge diversion.

Read this for an introduction to BMS methods.

The BMS you show is a simple voltage threshold unit. There are others (just as simple) that balance 2S, 3S, 4s and 5S packs. Here's an example for 3S:

This guy (on Ebay) has a large number of boards (quality unknown of course), but worth looking at the board details to see what variations of the boards implement both overcharge (balance overvoltage) and undervoltage methods or short circuit protection for multi cell packs.

Assuming your 3 Cell BMS is able to control overcharging, your method B) looks quite appropriate for your battery pack.

Answer 3

If you're going to charge Lithium batteries in series, then yes, you do need to balance them.

You can easily find ready-made balance charging circuits for sale on the Internet. It is also possible to salvage one, for example from a laptop battery pack. Neither of these solutions have to be particularly bulky.

Designing your own is certainly possible, but that is a project in its own right. So to your solutions:

• Solution A is easy, safe (provided that you have enough batteries in parallel) and workable, but you need a beefy boost converter, and it will not be the most power efficient.

• If solution B doesn't balance the cells, it doesn't seem good to me.

• Solution C seems clunky, but whatever works, works. One question that arises, is when one of the 1S BMS detects overvoltage, what does it do? How will it behave in the overall circuit? If it goes open circuit, that means that the two other batteries will stop charging as well.

• Solution D is the proper way to do it IMO. A dedicated balance charger on a circuit board isn't necessarily clunky at all, it can easily be smaller than Solution C.

Answer 4

Perhaps the best solution could be combining the use of the protection board and incorporating a set of balance charge wires in the battery pack itself.

This approach would protect the cells on discharge via the protection circuit (cheap 3s "BMS") while also relying on the external balance charger (e.g. IMAX B6 or B3) to properly balance charge the cells. Overdischarge protection offered via the protection board would never be used if going this route, since you would only be charging via the balance charge wires and external balance charger.

If you were really in a pinch and were without your balance charger, I suppose you could plug in a 12.6 V charger to the DC-in, but 99.99-100% of the time, it would think it would be better to charge via the balance charge wires.

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As a follow-up to my previous response diagramming the wiring of the balance leads in addition to the protection circuit, I found a DIY video on YouTube showing the exact configuration be put into action for a 3s li-ion pack. See approximately 4:30 into the following video for the build instructions: https://www.youtube.com/watch?v=hwhqn4BmC2I