4S battery packs are a popular choice because they result in a raw battery voltage of 10.0-14.6V (for LiFePo4 cells) and 12.0V-16.8V (for LiIon/LiPo cells), depending on state of charge.
This makes them ideal for supplying power to 12V loads: by adding a buck-boost-converter, the battery pack can deliver stabilized 12V output at minimal losses.
A 4S BMS is required for basic battery pack protection. The BMS is neither a charger nor a discharger and simply cuts off the power in cases of short circuit (fire hazard) or over-discharge (battery cell damage).
Caveats
When building 4S battery packs, it is crucial to pick battery cells of same type and state-of-charge. Always make sure all four cells are fully charged.
If you have the opportunity, measure the battery internal resistance (requires a special meter and cannot be done with a normal multimeter), and make sure these resistances are roughly equal for all cells.
Using Adequate Battery Cells
If you use batteries of different type or age, or if one of the cells isn’t fully charged, yozr entire battery pack will perform as bad as the worst cell you used.
Let’s assume you forgot to charge one of the four cells. When you later charge the battery pack through the BMS, it will stop charging all cells once any cell reaches its top voltage. If cells aren’t equal, or if one cell initially wasn’t fully charged, it will no longer fully charge anymore because it is always lagging behind.
When you then later use your battery pack to power a load, the BMS cuts power once the first cell drops below the over-discharge voltage - which coincidentally is the worst cell or the one that initially wasn’t fully charged. So BMS cuts power even though the other three cells are still good to go.
With multi-string battery packs, you therefore easily cripple the overall capacity of your pack by using bad cells or by using cells that weren’t initially fully charged.
Balancing
The culprit with multi-string batteries is balancing: such packs work well only when all cells are balanced and charged to the same level. If only one cell is out-of-balance, this cell dictates the behavior and severely limits overall capacity.
A balancer can equal out cell imbalances:
- Simple BMS:
Simple and cost-effective BMS do not balance at all. That’s ok as long as you carefully pick battery calls that are absolutely identical in state, capacity, age, and make. - Passive Balancing:
Average BMS come with passive balancing, and you can also add dedicated passive balancer modules to simple BMS: if a cell reaches full-charge voltage before other cells, the excess power is wasted and converted to heat until all cells are fully charged. Passive balancing is not very efficient, and balancing currents are very small (in the mA range). While passive balancers can prevent imbalances from happening, they cannot typically heal gross imbalances that existed in the first place, i.e. due to differences in cell quality or their state-of-charge during assembly. - Active Balancing:
These typically are dedicated modules that can be added to a BMS. Active balancers actively feed extra charging current into cells that are behind. The balancing current is much higher than with passive balancers, often in the Ampere region.
Balancers May Not Always Be Needed
Even renown manufacturers with excellent quality record may not use balancers (i.e. Makita in their 5S tool battery packs). Instead, they solely trust in first grade battery cells of equal specs and quality.
This is much more important than balancing: balancing is merely straightening out problems that shouldn’t occur in the first place when using good battery cells.
So balancers maximize battery pack capacity when you must use lower grade battery cells (i.e. because you repurpose older batteries from different sources).
Example
I am frequently using 4S battery packs for projects involving 12V loads:
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(content created Feb 27, 2024 - last updated Oct 27, 2025)