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Understanding Batteries in Series and Parallel: Choosing the Right Configuration
Quick answer: Series adds voltage; parallel adds capacity. Two 12.8V 100Ah batteries in series give 25.6V 100Ah; in parallel they give 12.8V 200Ah. Both store the same 2,560Wh. The rule that follows is the one that actually matters: batteries in series carry identical current and must be closely matched, while batteries in parallel share current and divide it according to their internal resistance.
Series: voltage adds, capacity does not
Batteries in series are connected end to end — positive of one to negative of the next — so the same current passes through all of them. The voltages add and the amp-hour capacity is set by the weakest battery in the string, since every one of them has to pass the same charge.
| Connection | Voltage | Capacity | Energy |
|---|---|---|---|
| 1 x 12.8V 100Ah | 12.8V | 100Ah | 1,280Wh |
| 2 in series | 25.6V | 100Ah | 2,560Wh |
| 4 in series | 51.2V | 100Ah | 5,120Wh |
| 16 cells in series (3.2V 100Ah) | 51.2V | 100Ah | 5,120Wh |
Parallel: capacity adds, voltage does not
Batteries in parallel are connected side by side — positive to positive, negative to negative — so the voltage is unchanged and each battery supplies a share of the total current. The amp-hour capacities add, and so does the available discharge current, which is why parallel connection raises both runtime and the peak the system can deliver.
| Connection | Voltage | Capacity | Energy |
|---|---|---|---|
| 1 x 12.8V 100Ah | 12.8V | 100Ah | 1,280Wh |
| 2 in parallel | 12.8V | 200Ah | 2,560Wh |
| 4 in parallel | 12.8V | 400Ah | 5,120Wh |
| 2 x 3.2V 15Ah cells in parallel | 3.2V | 30Ah | 96Wh |
Note that series and parallel can reach the same stored energy from opposite directions: 25.6V 100Ah and 12.8V 200Ah are both 2,560Wh. The choice between them is therefore not about energy at all — it is about what system voltage the equipment needs and how much current the wiring can carry.
Choosing between them
| Requirement | Configuration | Reason |
|---|---|---|
| Higher system voltage | Series | Reduces current for a given power; thinner cable |
| Longer runtime | Parallel | Adds amp-hours at the same voltage |
| Higher continuous current | Parallel | Current divides across the batteries |
| Standard 48V storage | Series-parallel | 16S for 51.2V, parallel groups for capacity |
| Small 12V pack | 4S (series) | Four 3.2V LiFePO4 cells give 12.8V |
The two are combined in almost every practical pack. A 48V 200Ah LiFePO4 battery is sixteen cells in series (51.2V) with several cells in parallel at each series position to reach 200Ah. Series sets the voltage; the parallel groups set the capacity and the current.
Balanced charging and the cell-matching rule
In a series string the current is identical in every cell, so a cell with lower capacity or higher internal resistance is driven to the top of its voltage range first on charge and the bottom first on discharge. The BMS can compensate for small differences by balancing, but it cannot compensate for a mismatched cell — it can only slow the drift.
- Match by capacity, resistance and voltage. Cells in one string should be graded, not selected at random.
- Do not mix old and new. An aged cell in a new string will be driven to the limits every cycle and fail first.
- Do not mix chemistries or capacities in one string, even if the nominal voltage matches.
- Keep the strings equal in parallel banks. Equal cable lengths keep the current division between parallel batteries balanced.
Failure modes and safety
The failure behaviour differs between the two configurations, and it is worth knowing which one you are exposed to. A failure in a series string stops the whole string, because the current path is broken — a clean, obvious failure. A failure in a parallel bank is more subtle: the remaining batteries continue to supply current into the faulty one, which can drive a short-circuited cell into a thermal event. Parallel banks therefore rely on per-battery protection rather than on the circuit simply opening.
Assembly practice follows from that: follow the manufacturer’s connection sequence, keep cable lengths matched in parallel strings, use correctly rated fuses on each parallel branch, and never make or break a series connection with the pack under load.
Practical wiring and cable sizing
Series and parallel differ in what the cable has to carry, and that difference is often the reason to choose the higher voltage in the first place. In a series string the whole pack current flows through every connection, so the cable gauge is set by the system current; raising the voltage lowers that current for the same power and allows thinner, cheaper cable with less resistive loss. In a parallel bank the total current divides between the batteries, so each branch carries a share — but the branch fuses and cable must still be rated for the fault current the other batteries can deliver into a shorted branch.
Series and parallel FAQ
What does connecting batteries in series do?
It adds the voltages and leaves the capacity unchanged. Two 12.8V 100Ah LiFePO4 batteries in series give 25.6V 100Ah. The current through every cell in the string is identical, which is why the cells must be matched.
What does connecting batteries in parallel do?
It adds the capacities and leaves the voltage unchanged. Two 12.8V 100Ah batteries in parallel give 12.8V 200Ah. The available current also doubles, because each battery supplies part of the total.
How do I work out the energy of a pack?
Voltage multiplied by amp-hours. A 25.6V 100Ah pack is 2,560Wh; a 12.8V 200Ah pack is the same 2,560Wh. Series and parallel reach the same stored energy from opposite directions, which is why choosing between them is about the system voltage and the current, not the energy.
Why must series batteries be matched?
Because they carry the same current. A lower-capacity cell reaches the top of its voltage range first on charge and the bottom first on discharge, so it is repeatedly driven to the extremes. It degrades faster, the imbalance widens, and the whole string’s usable capacity is limited by that cell.
Can I mix old and new batteries in a string?
No. A new cell in series with an aged one is a guaranteed imbalance: the aged cell has lower capacity and higher resistance, so it hits the limits first every cycle. The same applies to mixing chemistries or capacities in one string.
How do I choose between series and parallel?
Choose series to reach the system voltage the inverter or motor requires, and parallel to reach the capacity and current. In practice you do both: a 48V 200Ah pack is sixteen LiFePO4 cells in series and several parallel groups per series position.
What happens if one battery fails in a series string?
The whole string stops working, because the current path is broken or the faulty cell is driven outside its limits by the others. This is why cell-level monitoring and a BMS that can isolate and report the failure matter, rather than relying on a pack-level voltage reading.
Does the order of connection matter?
For the electrical result, no. For safety during assembly, yes — connect in the order the manufacturer specifies, use matched cable lengths in parallel strings so the current divides evenly, and never make or break a series connection while the pack is under load.
Explore related pages
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- 12V LiFePO4 batteries
- 18650 pack calculator
- Custom battery pack design
Ask about a pack configuration
Tell us the system voltage, the capacity and runtime you need, and the continuous current — we will recommend the series-parallel arrangement and supply a specification.
