How to use this battery charging calculator
- Chemistry, this sets every per-cell voltage limit and the maximum safe charge rate.
- Cells in series (S), how many cells are wired end to end. This alone decides all the pack voltages.
- Cells in parallel (P), how many series strings sit side by side. This decides capacity, never voltage.
- Capacity per cell, the mAh printed on a single cell, not the pack total.
- Charge rate, 0.5C is a sensible default. The tool warns you if you exceed what the chemistry tolerates.
- Depth of discharge, how flat you run the pack before recharging. 80% for lithium, 50% for lead-acid.
If you want the pack figures themselves, voltage, mAh, Wh, weight and runtime, theBattery Pack Calculator covers that side. This page picks up where it leaves off and deals only with charging.
Series sets the voltage, parallel sets the capacity
Every number on this page comes from that one sentence. Cells in series add their voltages together, so the charge voltage, the nominal voltage and the empty cutoff are all just the per-cell figure multiplied by S. Cells inparallel add capacity, so they change how long a charge takes and how much energy you store, but they never move a single voltage.
That is why a 3S2P pack and a 3S1P pack charge to exactly the same 12.6 V. The 3S2P simply holds twice as much, so at the same current it takes twice as long, and at the same C-rate it draws twice the current and finishes in the same time.
Per-cell charge and cutoff voltages
| Chemistry | Nominal | Charge to | Stop discharge at | Max charge rate |
|---|---|---|---|---|
| Li-ion (NMC / NCA) | 3.6 V | 4.2 V | 3.0 V | 1C |
| LiPo | 3.7 V | 4.2 V | 3.0 V | 1C |
| LiFePO4 | 3.2 V | 3.65 V | 2.5 V | 1C |
| NiMH | 1.2 V | 1.45 V | 1.0 V | 0.5C |
| Lead-acid (AGM) | 2.0 V | 2.4 V | 1.75 V | 0.3C |
These are the mainstream values chargers and BMS modules are built around, kept deliberately below absolute-maximum ratings. Charging a lithium cell past its per-cell ceiling is the one mistake that turns a battery project into a fire, so treat the charge voltage as a hard limit rather than a target to round up.
What a C-rate actually means
A C-rate is charge current expressed relative to capacity, which makes it portable across pack sizes. 1C is a current numerically equal to the capacity: 6 A into a 6 Ah pack. 0.5C is half that, 3 A, and takes about twice as long. Charging slower is almost always the right trade for a pack you want to keep, because heat is what ages cells and current is what makes heat.
Why charging takes longer than the simple division
Divide capacity by current and you get the constant-current time, but that is only the first phase. A charger runsconstant current until the pack hits its charge voltage, which gets you to roughly 80%, then switches toconstant voltage, holding that level while the current tapers off toward zero. That tail is slow on purpose, and it is what fills the last stretch safely.
This calculator adds a per-chemistry taper factor for that phase, about 20% extra for lithium and considerably more for lead-acid, whose absorption stage is notoriously long. It is why a pack that looks like a two hour charge on paper is closer to two and a half in practice.
Depth of discharge and how long the pack lasts
Depth of discharge is how much of the pack you use before putting it back. It is the single biggest lever you have over lifespan, and it works in the opposite direction to intuition: using less of the pack each time gets youmore total energy over its life. A lithium pack cycled to 80% typically lasts several times longer than the same pack run flat every cycle, and lead-acid is conventionally held to 50% for exactly this reason.
The usable capacity figure in the calculator reflects your chosen depth, so if you need a genuine 100 Wh of daily use at 80% depth of discharge, you need a pack of about 125 Wh, not 100 Wh.
Safety note: Lithium cells can catch fire if over-charged, over-discharged, shorted or damaged. Any multi-cell lithium pack needs a proper BMS with cell balancing, cells matched from the same batch, and a charger set to the correct chemistry and voltage. This calculator does the electrical arithmetic only. Charge on a non-flammable surface, never unattended, and if you are not confident in safe assembly, buy a finished pack with built-in protection.
Frequently asked questions
What voltage should I charge my lithium pack to?
Multiply cells in series by the per-cell charge voltage. Li-ion and LiPo charge to 4.2 V per cell, so 3S is 12.6 V. LiFePO4 charges to 3.65 V per cell, so 4S is 14.6 V. The parallel count never changes voltage, only capacity.
What is a C-rate and what charge rate is safe?
A C-rate is charge current relative to capacity, so 0.5C into a 6 Ah pack is 3 A. Most lithium cells accept up to 1C, NiMH prefers 0.5C or less, and lead-acid is usually limited to about 0.3C. 0.5C is a good default that trades a little speed for longer life.
Why does charging take longer than capacity divided by current?
Charging has two phases. Constant current gets you to about 80%, then constant voltage holds the charge voltage while current tapers away. That taper adds roughly 20% for lithium and much more for lead-acid.
What depth of discharge should I use?
80% for lithium and 50% for lead-acid are the usual figures. Shallower cycles dramatically increase how many cycles you get, so using less of the pack each time yields more total energy over its life.
Does the parallel count change the charge voltage?
No. Series adds voltage, parallel adds capacity. A 3S2P pack charges to the same voltage as 3S1P, it just holds twice as much and takes twice as long at the same current.
Do I need a BMS to charge a lithium pack?
Yes, for any multi-cell lithium pack. Cells drift apart over time and without balancing one can be overcharged while the pack total still looks right. A BMS enforces the per-cell ceiling, the low-voltage cutoff and the current limit.