Questions
The things worth understanding before building a pack from a number a web page gave you.
Why some questions get a refusal instead of an answer
Several genuinely popular searches ask how to build packs that are unsafe — lithium without a BMS, series lithium without balancing. This site will not produce those designs, and it does not try to rank for them either. Ranking for a build we refuse to design would either waste the searcher’s time or, worse, be exactly what they wanted.
Questions
Do I need a BMS?
For any lithium chemistry, yes, and this site will not design a pack without one. A BMS is the only thing that stops one cell being driven under-voltage on discharge or over-voltage on charge, and the divergence between cells is invisible from outside the pack. Lead-acid and NiMH can be built without one; lithium cannot.
What does 3s2p mean?
Three cells in series, then two of those strings in parallel — six cells in total, at three cells’ voltage and two cells’ capacity. Series multiplies voltage and keeps capacity; parallel multiplies capacity and current capability and keeps voltage. The series number always comes first.
Why is the usable capacity lower than the cells add up to?
Because running a lithium cell flat every cycle destroys it quickly, and because cells lose capacity as they age. The default here is 80% depth of discharge with a 20% ageing margin, so a 12 Ah pack delivers about 7.7 Ah of real work. Both figures are inputs you can change, and the result always says which were used.
What is C-rate?
Current expressed as a multiple of capacity. A 3 Ah cell at 3 A is 1C; the same cell at 15 A is 5C. It is the number that decides whether a cell is being used gently or punished, and the rating that matters is the one on that cell’s own datasheet rather than a rule of thumb.
Why does the pack voltage drop when I put a load on it?
Every cell has internal resistance, and current through resistance drops voltage. Series multiplies that resistance and parallel divides it, so sag is a property of the arrangement as much as the cells. Resistance also roughly doubles near freezing, which is why a cold pack sags far more than the same pack indoors.
Can I charge lithium in the cold?
No. Charging a lithium cell below 0 °C plates metallic lithium onto the anode. It is cumulative, invisible from outside, and eventually shorts the cell internally. Discharging cold is fine — it is only charging that does the damage, and a BMS worth having refuses to.
Are these designs safe to build?
They are a design aid, not an engineering sign-off. Every result states the assumptions it used so you can check them, marks which figures rest on a typical value rather than a measurement, and always includes the protection the pack needs. Anything that will be charged unattended should be reviewed by somebody qualified.
Do you recommend where to buy cells?
Not currently. Every cell page links to the manufacturer’s datasheet and nothing else. If supplier links are ever added they will be disclosed on the page, and the ranking will still be computed from electrical fit alone — a designer that recommends the cell paying the most commission is worthless, and it would look exactly like one that does not.