Amprius SA504 Drone Battery Pack Design
1 Amprius SA504 drone battery pack design
1.1 Amprius SA504 cell positioning
The Amprius SA504 (also known as the Amprius SiCore SA504) is a 10.8 Ah pouch lithium-ion cell from Amprius's silicon-anode range, rated for 66.3 A (6C) continuous discharge and 88.4 A (8C) for 30 s, with 386 Wh/kg energy density. A multirotor draws most of its energy in hover and forward flight at moderate power, with short peaks at take-off, in manoeuvres and on landing, so energy density sets the flight time and current capability sets the peaks. This paper uses the datasheet's 60 °C temperature cut-off and 2.5 V lower voltage limit throughout.
Our initial capacity check on the Amprius SA504, a C/30 discharge run before the 8-week test programme that builds the Voltt data and models, measured 11.31 Ah. This is 4.7% above the 10.8 Ah datasheet minimum, which covers cell-to-cell variation, so size the pack against the minimum.
Figure 1.1 places the Amprius SA504 against the 532 cells in the Voltt library. At 386 Wh/kg it sits toward the high energy end of the field, the region that sets drone flight time, and its power density places it between the 5C and 10C lines, consistent with its 6C continuous rating. Cells above it offer more power density at lower energy density, which a drone pays for in flight time; cells below it at similar energy density deliver less power for take-off and manoeuvre.
Figure 1.2 shows how the Amprius SA504 power density and energy density change as the discharge rate increases. The two trade against each other along a single curve, and the marker style shows which limit ended each discharge. At lower rates the discharge ends at the 2.5 V lower voltage limit. At higher rates the cell reaches 60 °C first, and the discharge ends with usable capacity remaining. The discharge rate at which the cell becomes thermally limited rather than voltage limited depends on the cooling as much as on the cell, and sections 2.2 and 2.3 quantify it.
1.2 Amprius SA504 pack design questions
Cell selection sets the ceiling on pack performance and the floor on pack cost, which makes it a strategic decision for any drone battery pack programme. Energy density sets pack mass, and on a drone every gram of battery is a gram of payload or flight time; the peak current demand sets the number of cells in parallel. The thermal architecture follows from the heat the cell generates at the mission's discharge rates and from the cell surface area the mechanical design can expose to the cooling. On a drone, where the pack often sits in an enclosed battery bay with little airflow, a cell chosen on datasheet headline figures cannot be changed late in the programme without redesigning the pack and the airframe around it.
A datasheet reports capacity at a few discharge rates, under cooling conditions that are rarely stated and rarely representative of a packed module. It does not report dynamic behaviour: peak temperature under sustained load, voltage sag across the discharge, or pulse power at low state of charge. Those gaps decide whether a pack meets its requirements. Closing them by test means cyclers, thermal chambers, instrumentation and weeks of qualification for every candidate cell.
Voltt is About:Energy's battery cell selection platform. It provides validated models for more than 50 high-performance cells, each parameterised against measurements from About:Energy's laboratory so that it responds as the measured cell does rather than as the datasheet implies, and free data on more than 500. In this paper we use Voltt to answer five drone pack design questions for the Amprius SA504 before any hardware is built: the current the pack hardware must be rated for (section 2.1), the usable capacity at each cooling level (section 2.2), the cooling each discharge rate needs and on which face of the pouch (section 2.3), the BMS pulse limits and minimum state of charge (section 2.4), and whether a 12s2p pack completes a full drone flight (section 3).
2 Amprius SA504 cell performance
2.1 Amprius SA504 pack current sizing
The Amprius SA504 is characterised under constant current and constant power discharge, the two control modes a pack sees. Datasheets report, and cells are compared, at constant current. A multirotor loads the pack at constant power, since the electronic speed controllers (ESCs) draw the current needed to sustain thrust. At constant current, power falls as terminal voltage falls; at constant power, current rises to sustain the power.
The discharge curves overlap at the start of discharge and separate as the discharge rate increases. The separation is polarisation: under load, terminal voltage sits below open-circuit voltage by approximately the product of current and internal resistance, and the offset grows with current. At the highest rates the discharge does not reach 2.5 V. Resistive heat generation scales with the square of current, and a 60 °C cut-off leaves only 35 °C of headroom above a 25 °C start, so the Amprius SA504 reaches 60 °C with usable capacity remaining, and the discharge ends on the temperature limit. The cell is thermally limited within its own continuous rating: the energy remains in the cell, but the pack cannot reject heat fast enough to extract it. Adding cells in parallel lowers the current per cell but does not change the heat each cell must reject at a given C-rate.
The control mode also sets the pack hardware current rating. Under constant power, current rises as terminal voltage falls, so the peak current occurs at the end of discharge, where the cell is least able to supply it. Wiring, connectors, ESCs and the BMS current sense must be rated for this end-of-discharge current, not for the nominal value. On a cell with a lower current rating the same profile would exceed it. The options are then a minimum state of charge limit that ends the discharge before the current rises, or more cells in parallel to reduce per-cell current.
2.2 Amprius SA504 usable capacity and cooling
Recording which limit ended each run shows whether a shortfall in usable pack energy comes from the cell or from the thermal design.
Figure 2.2 shows Amprius SA504 retention against discharge rate for three cooling levels in both control modes. Filled markers ended at the 2.5 V lower voltage limit, hollow markers at the 60 °C limit. Retention slightly above 100% at low rates is expected, because the reference is the 10.8 Ah datasheet minimum beginning-of-life capacity rather than a typical value. It is not margin to design against.
Two effects act across the range. At low rates a better-cooled cell runs cooler and has higher internal resistance, so it polarises more and delivers marginally less capacity. This is why the curves cross at the low end. As the rate increases, heat generation exceeds heat rejection, the cell reaches 60 °C before 2.5 V, and retention drops sharply. The rate at which this happens, the thermal limit onset, moves from 1.45C at 10 W/m²K to 4.21C at 50 W/m²K and 6.80C at 100 W/m²K: a factor of 4.7 across the cooling levels simulated.
The thermal limit onset is therefore set by the cell and the thermal design together. Internal resistance sets the heat generated at a given current; the cooling configuration sets the rate at which that heat is rejected. On the Amprius SA504 the cooling decides whether the continuous rating is usable at all: at 50 W/m²K the cell sustains 4.21C, below its 6C rating, and only the 100 W/m²K level holds it above 6C.
2.3 Amprius SA504 thermal management
The Amprius SA504 datasheet states a temperature cut-off but not the cooling needed to stay below it. That cooling requirement is an input to the battery thermal management design, and simulation is the fastest way to obtain it.
2.3.1 Amprius SA504 continuous C-rate limit
Figure 2.3.1 plots Amprius SA504 peak cell temperature against discharge rate for the three cooling levels, with the 60 °C limit marked. Peak temperature rises with rate, and better cooling lowers it at any given rate. The rate at which each curve meets the limit is the highest that cooling level sustains continuously: 1.45C at 10 W/m²K, 4.21C at 50 W/m²K and 6.80C at 100 W/m²K under constant current.
The constant current panel is the basis for comparing cells against datasheets; the constant power panel is the basis for sizing a drone's propulsion pack. Under constant power the limit is reached at a lower nominal rate, because current rises through the discharge (section 2.1), so the end of the run generates more heat than the nominal rate implies. At 50 W/m²K the cell sustains 4.21C under constant current but 126 W under constant power, which is about 37 A, or 3.4C, at nominal voltage. Where the load is specified in power, size the cooling from the constant power results.
2.3.2 Amprius SA504 cooling by contact geometry
Figure 2.3.2 Amprius SA504 cooling requirement: the heat transfer coefficient required on the contact area across the discharge rate range for four pouch contact geometries, against natural convection, forced air and forced liquid reference levels.
Figure 2.3.2 inverts the previous result: it gives the heat transfer coefficient required at each discharge rate to hold the Amprius SA504 below 60 °C.
Every discharge rate needs some cooling. With no heat removal the cell reaches 60 °C at every rate simulated, because a 25 °C start leaves 35 °C of headroom, so the Amprius SA504 has no region where it tolerates an adiabatic installation. On the whole surface the requirement rises steadily with rate and leaves the forced air range at 6.6C, just above the 6C continuous rating. The design question is the rate at which the requirement exceeds the capability of the chosen cooling technology, which is why the natural convection, forced air and forced liquid levels are marked.
Four contact geometries were simulated, because a real pack does not cool the whole cell surface: the whole surface (100% of the pouch body area), both large faces (79%), one large face (40%), and the edges only (21%). Voltt resolves the requirement onto the cooled surface. Cooling both large faces needs 1.26 times the whole-surface coefficient on the contact area, one large face 2.53 times, and the edges only 4.78 times. Voltt models the cell as a single lumped thermal mass, so these multipliers scale with contact area alone.
Contact geometry therefore has an influence on the cooling requirement, alongside the choice of cooling technology, and on a thin pouch that influence is moderate: the large faces are most of the cell's area, so cooling both of them costs 26% over the whole surface. Neither arrangement is better in general, since each has system-level trade-offs in packaging, mass, manifolding and cell spacing. The cooling technology and contact geometry are chosen together and early. At a given discharge rate an edge-cooled design needs 3.8 times the coefficient of a design cooled on both faces, so it leaves the forced air range at a much lower rate. Liquid cooling raises the achievable coefficient and extends the range over which edge cooling is viable, but it does not remove the difference between the two geometries. Where packaging fixes the contact area first, the cooling technology is sized to that multiplier.
2.4 Amprius SA504 pulse power and BMS limits
Continuous ratings do not cover the peak demands of a flight. A rapid climb, holding position in a gust, an aggressive avoidance manoeuvre, lifting a payload or landing in wind each demands a short pulse well above cruise power, at whatever state of charge and cell temperature the pack is at.
State of charge has the strongest effect on Amprius SA504 pulse power. At high state of charge pulse power is limited by the 88.4 A datasheet pulse current, which the datasheet rates for 30 s and which is applied to the 10 s and 60 s pulses by assumption. Above 35% state of charge the envelope stays above three quarters of its peak. Below roughly 25% state of charge, pulse power drops sharply: open-circuit voltage falls, internal resistance rises, and the cell can no longer hold terminal voltage above the lower limit at the pulse current. This paper calls that transition the knee, and places it at the state of charge where pulse power has halved. At 35 °C that is 13.5% for a 10 s pulse, 15.6% for 30 s and 18.3% for 60 s.
Temperature affects both the pulse power and the position of the knee. Above the knee the isotherms converge, since the cell is already at its current limit. Below it the limit is terminal voltage under load, and internal resistance falls as the cell warms, so between 35 °C and 50 °C a warmer cell delivers more pulse power at low state of charge and holds its knee to a lower state of charge. The exception is the longest pulse: from 40 °C a 60 s pulse self-heats into the 60 °C cut-off and becomes temperature limited, so warming the cell further lowers it rather than raising it. A pack that enters the landing phase colder than it should, for example after a winter field launch or a long flight in an open bay in cold air, has reduced pulse capability where a pulse is most likely to be needed, which is the case for thermal conditioning: pre-warming packs before launch, and insulating the bay.
Pulse duration has a smaller effect than state of charge. Above the knee the 10, 30 and 60 s envelopes sit close together, since all three are limited by the same 88.4 A current. Below it a shorter pulse delivers more power than a longer one, because the cell has less time to polarise and self-heat: at 35 °C the knee moves from 13.5% state of charge for a 10 s pulse to 18.3% for a 60 s pulse. For the BMS this gives a simple rule: above the knee a single 88.4 A current limit applies at any duration, and below it the limit must be a function of temperature and duration.
Voltt provides the full Amprius SA504 pulse map as a lookup table indexed by state of charge, temperature and duration, for use in BMS limit tables.
3 Amprius SA504 drone pack mission
Sections 2.1 to 2.4 characterise the Amprius SA504 under constant loads, which define its operating limits. A flight holds no load constant for long. Power varies across take-off, climb, cruise, approach and landing, each transition produces a voltage transient, and heat accumulates across the profile. Verifying that a pack meets its requirements therefore requires simulating the full duty cycle.
The duty cycle is the Voltt library quadcopter flight on a 12S pack, the common bus for heavy-lift and industrial multirotors. The cruise was solved so that the flight lands at a 25% state of charge target, extending the recorded hover to fill it, so the mission length reflects the energy this pack holds, while take-off, climb and landing power come from the profile. 12 cells in series give 40.8 V nominal and 50.4 V at the 4.2 V charge limit. At the 1.94 kW peak each of the 24 cells supplies 81.0 W, which is 24.6 A, or 2.3C, within the 66.3 A continuous rating.
Amprius SA504 pack configuration
12s2p, 24 cells
40.8 V nominal, 21.6 Ah
0.90 kWh total energy
2.34 kg of cells, before structure, busbars, cooling and enclosure
Simulation conditions
100% initial state of charge
25 °C initial cell temperature, 20 °C ambient
Natural convection in an enclosed compartment, 15 W/m²K applied to the whole cell surface
2.5 V per cell voltage cut-off, 30 V at pack level
60 °C thermal cut-off
Alt text:
Pack voltage starts at 49.0 V, drops at take-off as polarisation builds, then falls steadily through cruise as state of charge decreases. The minimum of 35.8 V occurs during landing, where the highest power since take-off coincides with the lowest state of charge of the flight. This is 2.982 V per cell, 0.48 V above the 2.5 V limit, so the pack is not voltage limited at any point. The flight draws 706 Wh of the 0.90 kWh installed, with a peak demand of 1.94 kW at take-off.
Amprius SA504 cell temperature rises to 25.9 °C through take-off and climb, then climbs steadily through cruise to 36.2 °C, because at 15 W/m²K the heat generated at cruise power exceeds the heat rejected and the cell never reaches a steady temperature within the flight. It enters the landing heat soaked and peaks at 40.6 °C, a rise of under 16 °C that leaves 19.4 °C of margin to the 60 °C limit. A longer cruise would start the landing hotter, with less margin, and reach a higher peak.
Two results follow from the duty cycle, one thermal and one electrical. Under natural convection the heat accumulated through the cruise sets the temperature at which the landing begins, so the thermal system is sized by the length of the flight as well as by the take-off and landing peaks: a longer flight needs more cooling to land with the same margin. The landing is also where pulse capability is lowest, because its peak demand coincides with the lowest state of charge of the flight, so pulse capability at landing informs the state of charge reserve. The cell enters the landing at about 36 °C, and at 35 °C section 2.4 places the knee at 18.3% state of charge for a 60 s pulse. This mission lands at 24.6%, above the knee but on the declining part of the envelope: a 60 s pulse there delivers 69% of its peak value, and is limited by terminal voltage rather than by the current rating. A hold in hover, a headwind on the return leg or a missed landing each draws more energy and moves the landing closer to the knee.
The mission completes within both limits. The flight lasts 40.6 minutes and lands at 24.6% state of charge, above a typical 15% low-battery threshold at which a drone's autopilot forces a return or a landing.
4 Amprius SA504 conclusions
The Amprius SA504 combines 386 Wh/kg energy density with a 6C continuous and 8C pulse current rating. Across the lower and middle part of its discharge rate range it delivers close to full capacity, with each discharge ending at the 2.5 V lower voltage limit.
Thermal management is significant to how much of the current rating is usable. The 60 °C cut-off leaves 35 °C of headroom, so every discharge rate needs some cooling, and above the thermal limit onset the discharge ends at 60 °C with delivered capacity falling. The highest sustainable discharge rate varies from 1.45C to 6.80C across the cooling levels simulated, and running the full 6C rating continuously needs the upper end of the forced air range on the whole surface. The pouch contact geometry changes the required heat transfer coefficient by up to a factor of 4.8.
At pack level the 12s2p configuration completes the 40.6 minute mission and lands at 24.6% state of charge, drawing 706 Wh of the 0.90 kWh installed. Minimum cell voltage is 2.982 V and peak cell temperature is 40.6 °C, both inside their limits.
Three limits bound the operating envelope, and none appears on a datasheet. First, the 6C continuous rating needs forced air at the upper end of its range on the whole surface, and liquid cooling on a smaller contact area. The cooling technology and the contact geometry are therefore one decision that's made early: the technology sets which contact geometries are viable at the pack's continuous rate. Second, under natural convection the cell heats through the whole cruise, so the flight length sets the landing temperature and with it the thermal margin. Third, below roughly 25% state of charge pulse power falls faster than an energy-based reserve calculation assumes, so the low-battery threshold that triggers a return or landing must be set on pulse power as well as on remaining energy.
All three are pack design inputs, and none is on a datasheet. The workflow in this paper produced them from a validated cell model before any cell was bought or any test rig built: cell positioning, the current rating under constant power, usable capacity at each cooling level, the cooling requirement on each face of the pouch, the pulse map for BMS limits, and a solved mission on a sized pack. Run in Voltt, the same workflow screens each candidate cell in the concept phase, fixes the contact geometry and cooling technology before the mechanical design, and sets BMS limits before the first pack is built. That greatly accelerates pack development against sourcing and testing each candidate cell, which takes weeks per cell.
To evaluate the Amprius SA504 against your own mission profile, open the Amprius SA504 in the Voltt battery library or explore Voltt at About:Energy.
5 Amprius SA504 FAQs
What is the Amprius SA504?
The Amprius SA504, also sold as the Amprius SiCore SA504, is a 10.8 Ah pouch lithium-ion cell released in 2025. It is rated for 66.3 A (6C) continuous and 88.4 A (8C) for 30 s, at 386 Wh/kg, which places it toward the high energy end of the Voltt library for drone endurance.
How many Amprius SA504 cells does a drone battery pack need?
The pack simulated here is 12s2p: 24 Amprius SA504 cells, 40.8 V nominal and 0.90 kWh, with 2.34 kg of cells: a 12S pack for a heavy-lift or industrial multirotor. At the 1.94 kW peak each cell carries 24.6 A, within its 66.3 A continuous rating, so the current rating did not set the parallel count.
What is the Amprius SA504 maximum discharge current?
The Amprius SA504 maximum continuous discharge current is 66.3 A, which is 6C, and its pulse rating is 88.4 A for 30 s, with a temperature cut-off at 60 °C. How much of the continuous rating a pack can use depends on its cooling: between 1.45C and 6.80C across the cooling levels simulated.
Is the Amprius SA504 suitable for drone battery packs?
In simulation, the 12s2p Amprius SA504 pack completes a 40.6 minute drone flight with a 1.94 kW peak demand. It lands at 24.6% state of charge, with a minimum cell voltage of 2.982 V and a peak cell temperature of 40.6 °C, both inside the cell limits.
How much cooling does an Amprius SA504 battery pack need?
Every discharge rate needs some cooling, because with no heat removal the Amprius SA504 reaches its 60 °C cut-off even at 0.1C. The highest continuous rate is 1.45C at 10 W/m²K, 4.21C at 50 W/m²K and 6.80C at 100 W/m²K. The simulated drone mission runs at 15 W/m²K, natural convection in an enclosed compartment.
Should an Amprius SA504 pack be face cooled or edge cooled?
Both large faces cover 79% of the Amprius SA504 pouch body and need 1.26 times the whole-surface heat transfer coefficient. One face needs 2.53 times it and the edges only 4.78 times, so an edge-cooled pack leaves the forced air range at a much lower rate. Choose the contact geometry and cooling technology together.
How does state of charge affect Amprius SA504 pulse power?
Amprius SA504 pulse power stays above three quarters of its peak above 35% state of charge, then falls sharply. At 35 °C it halves at 13.5% state of charge for a 10 s pulse, 15.6% for 30 s and 18.3% for 60 s. A warmer cell holds more pulse power at low state of charge, up to the point where a long pulse self-heats into the 60 °C cut-off.
What state of charge reserve should an Amprius SA504 drone pack keep?
The simulated mission lands at 24.6% state of charge, above the 60 s pulse power knee at 18.3% but on the declining part of the envelope, where a 60 s pulse delivers 69% of its peak at 35 °C, close to the cell temperature at landing. Set the low-battery threshold on pulse power as well as on remaining energy, and allow for a hover hold or a headwind on the return leg moving the landing closer to the knee.
