How to Write Electrical Requirements a Pack Builder Can Quote
An electrical requirement sheet a pack builder can quote is seven linked decisions, not a spec table: the voltage window, capacity, continuous current, peak current, cycle life, DCIR, and temperature derating. Write each one as a number with its condition attached, meaning the discharge rate, the temperature, the voltage window, and the duration it applies over. Any condition left out gets filled in by the builder with the version that is cheapest to build, and the quote that comes back is a price for that assumption rather than for the pack you had in mind.
I keep one quote in my desk drawer for exactly this reason. Single page, price near the top, a short block underneath headed Assumptions. Every line in that block was a question I had failed to write down: a capacity figure resting on one discharge rate and one temperature, a continuous current number that matched the BMS board instead of the pack, a cycle count with no end-of-life threshold under it. Nothing on that page was false. It answered the sheet I actually sent.
Now the price is the last thing I look at. Assumptions first.
Four numbers, not one voltage
Most requirement sheets I get handed carry a single voltage. I write four.
Maximum charge voltage. Two different things hide under that heading: the constant-voltage setpoint your charger terminates at, and the point where the BMS opens the charge path. They are not the same number, and if you write only one, a builder can quote to the protection trip point and be technically correct. State both, and state the tolerance you can live with.
Nominal voltage. Partly a reporting convention, but your runtime estimate is built on it, and if the builder reports nominal at a different point on the discharge curve than the one you assumed, two quotes carrying the same capacity will show different runtimes on paper. Naming it takes one line and removes a degree of freedom.
Discharge cut-off. This is where the quiet disagreements live. Specify it at the pack terminals, under the stated load, at the stated temperature, not at rest. Under load the terminal voltage sags, so an assembled pack reaches the cut-off number earlier than the same pack measured on a bench. A cut-off specified at rest gives you less runtime than you think, because the device shuts down while there is still charge in the cells.
Recovery voltage. The point at which the load is allowed to restart, or at which charging resumes. Left unwritten, that hysteresis belongs to the builder, and you meet it at the end of a discharge when the load drops out, recovers, restarts, and drops out again on a short cycle.
Then say where the measurement happens. Pack terminals, or the far end of the output cable? Harness and connector inside the spec or outside it? On a pack with a long cable, that one line moves the number more than a cell change does.
Capacity and continuous current both need a condition
Capacity goes in as a minimum, at a named discharge rate, at a named temperature, across the voltage window above, at beginning of life. Minimum is the word that does the work. Rated capacity describes a cell at reference conditions on a datasheet; delivered pack capacity is a different quantity, because it also contains the interconnects, the BMS path, the pack's own heat, and the plain fact that in a series string the weakest cell ends the discharge for everyone. If you want watt-hours, ask for watt-hours, and say whether the BMS's own draw sits inside or outside that number. At low discharge rates it is not a rounding error.
Continuous discharge current is the one buyers get wrong most often, because there is a number printed on the BMS board and it looks like an answer. It is a component rating. What the assembled pack can pass continuously is set by the whole thermal path: busbar and tab joints, cell spacing, how heat reaches the enclosure, the connector, the harness, the fuse. So write the requirement with a duration and an ambient. Twenty minutes in moving air is not the same ask as two hours inside a sealed enclosure. Give the builder the run time, the duty cycle and the ambient, and ask what the pack's thermal steady state is at that point, not just whether the protection FETs will stay closed.
Charge current deserves its own line at its own temperature. Plenty of sheets describe discharge down to the decimal and then leave charging as a single number that turns out to be unachievable when the pack is cold.
Peak current needs a duration and a count
A ten second pulse and a two second pulse are different requirements. They can pull different cells and occasionally different interconnect hardware. Write the amplitude, the duration, how many pulses inside a burst, how often the burst repeats, and whether the pack is warm or cold when the pulse arrives. Resistance is higher cold, so a pulse that is comfortable on a bench can sag into cut-off on the first start of a winter morning.
State the voltage floor during the pulse, too. It is not enough for the pack to survive the current; the terminal voltage has to stay above your load's minimum input for the entire duration. That floor is what ties peak current back to the voltage window and to DCIR.
And name the load. Motor inrush, contactor pull-in, a radio transmit burst, a heater bank switching on. Builders read the shape of the load off that description, and the ones who are actually engineering will tell you whether the number is realistic before you have paid for a prototype.
Cycle life and DCIR: the two numbers that age
Cycle life is a sentence, not a number. A thousand cycles means nothing until it says: to what end-of-life threshold, at what depth of discharge, at what charge and discharge rates, at what ambient, and whether the cycle includes a rest. Eighty percent of the minimum rated capacity is the usual convention, but write it, because some quotes read eighty percent of typical instead. Charge termination belongs there too, constant current then constant voltage down to a stated cut-off current. Two quotes can both say a thousand cycles while describing two different tests, and nobody finds out until the third year in the field.
Whether the pack parks at full charge is worth stating as well. A duty cycle that sits at high state of charge for weeks is a harder requirement than the same cycling in the middle of the window, and it is better to say so up front than to discover it.
DCIR is the number that decides how much of your capacity you actually get to use. It sets how far the terminal voltage sags under load, and therefore how early the pack reaches cut-off. A pack can hold plenty of energy and still strand most of it if the sag pushes it under the cut-off voltage while the chemistry still has charge left. Specify DCIR with the state of charge it is measured at, the pulse length, the temperature, and whether it is beginning of life or end of life, because resistance climbs with age and falls as the pack warms. A DCIR quoted at mid state of charge and room temperature flatters a pack that has to work at low state of charge on a cold morning. Ask for the definition and the measurement points, since some builders report it across the whole pack and some across the cells with the interconnect and protection path excluded.
Temperature derating: name the edges
Name the coldest and hottest conditions at which the specified current and capacity still have to hold. An operating range on a datasheet usually means the pack will not be damaged across it, which is a different claim from the pack will deliver the spec across it. Put the performance edges and the survival range in separate lines.
Then say whether charging happens at those temperatures. Low temperature discharge is a sag problem. Low temperature charge is a different and more serious problem, because the cells will accept current they should not. If the device sits on a dock or a roof overnight, that one sentence narrows the cell choice more than any other line on the sheet.
Say whether the pack is heated, insulated, or expected to warm itself under load, and give the storage temperatures. If you specify only the two ends, a builder can quote a pack that hits both and is marginal in the middle, which is where most products actually spend their time.
How the seven lock together
These are seven decisions and not seven lines because they move each other. Push the peak current requirement up and every cell on the table sags further under that pulse, which eats into your voltage floor and can force a larger, heavier pack for the same delivered energy. Drop the cut-off voltage to capture more usable capacity and every volt of sag becomes more expensive, so the DCIR line tightens. Ask for a longer cycle life and the builder will reach for a cell built for longevity, which may not be the one with the pulse capability you wrote two sections earlier. Ask for the full rating at a temperature your product sees twice a year and you buy margin you will never use.
Which is why a sheet of maximums gets one of two responses. The builder prices all of it, and the number comes back high, or the builder quietly picks which lines it will meet and lists the rest in the assumptions block. Most pick the second.
So read the assumptions before the price. If a builder never asks about temperature, pulse duration or cycle life conditions, they are not engineering your pack, they are selling you cells in a box. Write the negative requirements too. If peak current is trivial for your load, say so. If charging never happens below a certain temperature, say that. Requirements you do not have cost money the moment somebody assumes you do.
Keep it to one page, one condition per line, version and date at the top. Then ask the builder to restate all seven conditions in the quote next to their numbers. If a condition cannot be repeated back, it is not in the quote.
