Hello,
Is it safe to connect single Li-ion cells in parallel, if each cell is protected with external circuit? By protection I mean over-discharge and over-current protection.
I'm not intended to charge them in this configuration, only discharging is necessary.
Thanks in advance.
The problem is that individual protection circuits aren't designed to work in parallel. If the cells have even a small voltage difference, a large equalization current can flow and the protection boards won’t coordinate with each other. So it’s not considered safe unless the cells are voltage-matched first or you use a single shared BMS for the whole pack
If the cells are identic and you charge them to same voltage, yes. Connect them initially through 100ohm resistor to balance any residual voltage mismatch.
No never!
If you use individually protected cells, the protection, for whatever reason, may trigger at different points. Then if you later reconnect power, or clear whatever triggered the protection, the two cells may well by then be at very different voltage levels, and the current flow could be large. So I think parallel cells need to be unprotected cells, but then protected as a batch. Once you connect them together, you don't want to separate them, or have their voltages differ ever again for any reason. That means you can't replace individual cells, or charge them individually.
I don't know if it's ever safe to connect lithium cells in parallel, but I've seen examples of it being done in commercial products. But you have to know what you're doing, because the consequences of doing it wrong could ruin your day.
I had 6 100AH 12V LiFePO4 connected in parallel in my RV, as do most RVs these days. My best guess is there are 10's of thousands of these rigs in the USA alone. A significant majority are under the bed or in what we call the basement, roughly under the bedroom. One extremely popular brand is the Battle Born by Firefly. The car-sized 100AH battery is made up of the standard 18650 type cells welded into groups of 4, yielding a nominal 12.8V. Assuming each 4 pack is 2000mAh they then weld 50 of the 4 packs to deliver 10,000mAh or 100AH.
NOTE NOTE NOTE There is a very high quality and sophisticated electronics board(s) inside the battery to provide the high temp, low temp, over discharge protection. Also the batteries are externally monitored sometimes in 2 places and everything is connected to a master computer so to speak(Inverter/Charger).
Summary:
The batteries have parallel cells inside, and they themselves are in parallel BUT with a full BMS and balancing circuitry.
Bottom Line I agree with @jim-p , NEVER as a hobbyist should you connect Lithium cells in parallel. The only exception is if you have commercial BMS and balancing boards of high enough quality that you trust your life with. They will cost more than the batteries.
Solution Connect a few in series and use a voltage regulator.
A good resource is Battery University
True but in practice equal cells behave just well. Let's say they have 10A discharge limit on protection circuit. In parallel connection the load current splits between cells unevenly. You apply 18A load and the one with lower Rint trips at 10A. Other one trips immediately for 18A.
That's the ideal way to go.
I don't see reason to demonize parallel connection (except with non rechargeable cells).
DIY lithium battery circuits always need lot of care.
I would not recommend to build parallel battery for higher discharge current approach, but for longer battery life purpose I don't see fundamental problem.
Of course it is, battery manufacturers do it all the time but I'm 100% sure that @amazed is not a battery manufacturer, that's why I said no
Get a grip. Let’s stick to informed, evidence-based practice instead of blanket ‘never’ rules.
And it is nonsense to say no individual should never do the same.
I won't argue that you don't have to know what you are doing. As it is in many cases of things we do all the time in every area of life.
You might a well tell ppl to never fry an egg on a gas range.
TBH I've seen more spectacular and frighteningly dangerous activity from commercial products with these amazing cells and batteries than from informed hobbyists using them properly.
a7
Show this evidenced based practice you talk about.
FWIW
Here are the results of my google-AI "deep dive" --
active circuit for parallel 18650 load sharing
In a parallel 18650 configuration, an "active circuit for load sharing" is generally not required because the batteries naturally share the load as they are connected to the same voltage potential (bus). The key safety and performance consideration is proper cell management, primarily during initial assembly and charging/discharging cycles.
Here are the main components and considerations for a safe and effective parallel 18650 system:
Inherent Load Sharing)
Automatic Balancing: When healthy, matched 18650 cells are wired in parallel, they naturally equalize their voltage, and current is drawn proportionally to each cell's capacity and internal resistance. No complex "active load sharing" circuit is necessary for the parallel bank itself.
Required Active Circuits for Safety and Function)
While the load sharing itself is passive, specific active circuits are essential for overall safety and management of lithium-ion batteries:
Battery Protection Circuit Module (PCM or BMS): This is the most crucial circuit for a lithium-ion battery pack. It protects the cells from:
Over-charging: Prevents individual cells from exceeding their maximum voltage (typically 4.2V).
Over-discharging: Cuts off the load before cells drop below their minimum safe voltage (typically 2.8V-3.0V).
Over-current/Short-circuit: Protects against excessive current draw.
For a parallel configuration, you use a BMS designed for a single cell group (known as a 1S configuration), but rated for the total current of all the parallel cells.
Load Sharing (Charge/Discharge Management): If you intend to draw power from the batteries while they are simultaneously being charged by an external source (like a solar panel or USB power), a specific "load sharing" or "UPS" circuit is beneficial. This type of circuit (often found in modules like those based on the TP4056 or dedicated BQ-series ICs) ensures that:
The external power source primarily powers the load.
The charging circuit is not confused by the load current, allowing for proper charge termination and preventing the battery from being overcharged or constantly topped up unnecessarily.
Active Cell Balancers (Optional): In a purely parallel configuration, cell balancing is less critical than in a series setup, as the cells self-balance to the same voltage. However, if you are building a complex battery pack with both series and parallel components (e.g., 2S2P, 3S3P, etc.), active or passive cell balancing circuits become vital for maintaining long-term pack health.
Best Practices for Assembly and Operation
Use Matched Cells: Start with cells of the same capacity, age, and internal resistance.
Pre-charge to Equal Voltage: Before connecting cells in parallel, ensure they are at roughly the same voltage to prevent high initial inrush currents that can cause damage or fire.
Integrate a Protection Board: Always use a suitable BMS/PCM board with over-voltage, under-voltage, and over-current protection.
Consider Fusing: For larger parallel banks (after about 25 cells), individual cell fusing is recommended for safety in case a cell fails.
When you say "load sharing" is generally not required because the batteries naturally share the load as they are connected to the same voltage potential (bus)" - aren't you assuming that they will discharge identically?
When batteries are connected in parallel,
they will not discharge identically in terms of the amount of current flowing from each individual cell at any given microsecond. However, they naturally share the load because the connection forces them to maintain the same voltage potential across their terminals.
The variations in current draw between cells are a function of their internal resistance and current state of charge (voltage):
Internal Resistance Determines Current: The cell with the lowest internal resistance will momentarily supply slightly more current than cells with higher resistance. This is governed by Ohm's law (I = V/R), where all cells share the same voltage (V).
Voltage Potential as the "Bus": The parallel bus acts like a shared reservoir. Any voltage drop caused by the load is instantaneously distributed across all parallel cells.
Natural Self-Balancing: If one cell's voltage drops slightly faster due to supplying more current, the other higher-voltage cells will naturally "push" current into the slightly lower-voltage cell until equilibrium is restored across the common bus. The current flow constantly shifts dynamically to maintain this voltage balance.
The "load sharing" provided by a dedicated active circuit in non-battery applications (like power supplies) is usually for ensuring even wear and preventing one power source from failing due to overload while others sit idle.
For parallel lithium-ion cells, this active circuitry is redundant because the physical properties of parallel connection achieve the same goal: they inherently act as a single, larger, aggregated cell. The system is self-regulating, provided the initial conditions (matched cells) are met [1]. The primary focus remains on the necessary safety circuits (BMS) for protection, not for balancing the current flow itself.
What precision is required for "matched cells"? What if the cells aren't 'matched' closely or closely enough?
For optimal safety, performance, and longevity of a parallel 18650 pack, "matched cells" means they should be as close to identical as possible in three key aspects:
capacity, internal resistance, and initial state of of charge (voltage).
Required Precision for Matched Cells
Capacity (mAh): Ideally, cells should come from the same manufacturer, model, age, and production batch. When building a DIY pack from salvaged cells, they should be tested and grouped so that the total capacity of each parallel group (P-group) is within a very small margin of difference (e.g., within 1-2% of the total group capacity).
Internal Resistance (IR): This is critical for load sharing. Cells with a very high difference in IR will not share the current load evenly. The cell with lower IR will work harder, heat up more, and degrade faster. Legitimate battery pack manufacturers often reject 20-30% of cells to ensure close matching, which highlights the importance of this metric.
Initial Voltage (SoC): This is a critical safety requirement before connecting cells in parallel. Cells must be charged to a nearly identical voltage (e.g., within 0.1V, preferably within a few millivolts [1-5mV]). A large voltage difference can cause a dangerous inrush of current between cells, leading to excessive heat, potential venting, or fire.
Consequences of Mismatched Cells
If cells in a parallel bank are mismatched in capacity or internal resistance, the overall performance and safety of the pack are compromised:
Reduced Overall Capacity: The entire battery pack will perform only as well as its weakest cell. The stronger cells will be limited by the weaker ones, so you cannot access the full theoretical capacity of the pack.
Uneven Wear and Faster Degradation: Cells with higher internal resistance or lower capacity will be stressed more during charge and discharge cycles, which accelerates their aging and wear.
Internal Current Circulation: There will be constant, dynamic current flowing between cells as they try to balance their voltage, even when no external load is applied. This "parasitic" current generates heat and wastes energy.
Over-discharge/Over-charge Risks: In a series-parallel (e.g., 2S2P) configuration, a weak cell can be driven into an over-discharged state or overcharged condition without the Battery Management System (BMS) detecting the individual cell's dangerous state, potentially leading to permanent damage or a safety hazard.
Potential for Thermal Events: In the worst-case scenario, a failing or severely mismatched cell can short internally, causing other cells in the parallel bank to dump their energy into it, leading to significant heat generation and a fire risk.
Conclusion: It is highly recommended to use new, identical cells from a reputable manufacturer for safety and performance. If using salvaged or non-identical cells, rigorous testing for capacity and internal resistance, and meticulous matching of parallel groups, are mandatory safety procedures.
"Meticulous matching" is definitely not trivial. Which would be more difficult - a painstaking selection process or implementing an active, current-sharing circuit?
Thinking
Searching
Implementing an
active, current-sharing circuit specifically for parallel lithium-ion cells would be far more difficult and complex than a painstaking selection process [1]. The selection process, while manual, relies on existing, standard battery analysis tools. An active circuit for dynamic current balancing in a DC battery bank, however, requires significant engineering expertise.
Here is a breakdown of why:
Difficulty of Implementing an Active Circuit
An active current-sharing circuit for parallel cells is a sophisticated engineering challenge that generally involves:
Complex Power Electronics Design: You would need specialized power electronics that can monitor the micro-current and voltage of each individual cell in real-time and actively shift current using components like inductors, capacitors, and FETs.
High-Speed Control Logic: The circuit must react instantaneously to changes in load and internal resistance to balance the current flow, requiring fast microcontrollers or application-specific integrated circuits (ASICs) and intricate firmware.
Thermal Management: The act of transferring current generates heat through circuit inefficiencies, which must be managed through appropriate design and cooling, adding complexity and bulk to the system [1].
Cost and Efficiency Trade-offs: Such a circuit would introduce additional cost, complexity, and energy losses (reduced efficiency) into the system.
Difficulty of the Selection Process
The "painstaking selection process" is primarily a manual and time-consuming task, but it is achievable with common tools:
Tools: The process uses readily available and relatively inexpensive professional battery analyzers/chargers (e.g., medical or industrial-grade chargers with analyzing functions) [1].
Process: It involves testing cells over several charge and discharge cycles, recording data (mAh capacity, IR), and physically grouping similar cells together. This is a labor-intensive sorting process, not a design challenge.
Conclusion
The selection process is a one-time setup cost in terms of time and labor. The active circuit design is a significant engineering project with ongoing costs and potential points of failure, which is why the industry standard relies on matched cells and passive safety mechanisms for parallel banks.
I love it! 
A nice load of gobbledegook
I use this for using series connection of 3 l8650’s. 2Pcs 3S 11.1V 12.6V 25A W/Balance 18650 Li ion Lithium Battery PCB Protection Board. Notice it has balance in the description. I used four cells in parallel with this board.
I would remove "current state of charge" and replace it with "charge capacity." These are the two parameters that must be measured and must be exact for the battery to be good and safe.
If they are different, the differences will make one cell "work" (energy over time) harder, have a shorter life, and cause heat. Heat degrades insulation. Lack of insulation causes polarity shorts.
The rest (of the nay-saying) is boogyman hearsay.
Help me out here.
These are the two parameters that must be measured and must be exact for the battery to be good and safe.
I was trying to get 'LLM' to see the light. "Matched closely", "standard battery analysis tools",(available to the 'layman'?), reputable manufacturers - well, rather.
But even "measuring exactly" - how to do that, to what what precision?
Certainly an active current-sharing circuit will have a range, too, tolerances and so on.
I don't manufacture batteries.
You need help ?
You are right... "matched closely" is correct. I do not know the standard precision, but not "units" and maybe not "tenths" but "hundredths" seems a good start.
I used "exact" because "closely" can be taken and run-with like "an inch is as good as a mile."
A battery manufacturer says about certifications; "cell level is easier to pass but does not guaranty a safe pack" and "pack level is harder to pass but is safer as a pack"... and "neither guarantee total safety."
So, I would go with "precision at hundredths" and I will be as safe as a full-scale manufacturer.