Where the heat comes from
Nearly all of the round-trip loss leaves as heat inside the enclosure. Cell internal resistance dominates and scales with the square of the current, so doubling the power roughly quadruples the cell contribution -- which is why a peak-shaving duty and an energy-shifting duty with the same daily throughput are not the same thermal problem.
Add the busbars, contactors and fuses in the current path, the auxiliary loads that run continuously, and, on an outdoor cabinet, solar gain on the surfaces facing the sun. Sum those at the worst duty rather than at the average, because that is the condition the cooling has to hold.
The constraint is spread, not average
A cabinet can sit comfortably inside its average temperature limit and still be badly cooled. What matters is the difference between the hottest and coolest cells, because that difference is what drives divergent aging, misleading state-of-charge estimation and balancing that never finishes.
Cells at the end of an airflow path see air that has already collected heat from everything upstream. That gradient, not the fan capacity, is usually what sets the practical density limit of an air-cooled design.
What an air-cooled cabinet costs to keep working
Air cooling has fewer failure modes in the electrical sense and no fluid to contain, which makes it a reasonable default for moderate density and a service team without thermal specialists. Its running costs are maintenance costs.
- Filters that block gradually and degrade cooling before any alarm triggers
- Dust, salt mist and vegetation at the intake, which are site-specific and often underestimated
- Fan redundancy, since a single failed fan can strand one part of the pack
- Recirculation between cabinets placed too close together, which quietly raises every intake temperature
- Acoustic limits, which can decide fan selection outright near occupied buildings
What a liquid-cooled cabinet costs to keep working
Liquid cooling moves heat from the cell surface through cold plates, so it holds a much tighter spread and supports densities air cannot reach. That capability is real and it is the correct answer for high-rate duty in hot ambients.
It also adds pumps, manifolds, hoses, coolant chemistry and its service interval, leak detection and the response the system takes when leak detection trips, pressure management, freeze protection where winters demand it, and a service procedure that requires the circuit to be broken and refilled correctly. Liquid cooling is better when those obligations are engineered and staffed, and worse when they are not.
What decides it in practice
Rate and ambient decide more than energy. A cabinet cycled once a day in a temperate climate is usually an air-cooling problem regardless of its kWh. A cabinet expected to deliver high power repeatedly, or to work through hot summers with limited derating allowance, moves toward liquid.
Derating is the honest tiebreaker: if the air-cooled design meets the requirement only by derating on the days the site most needs power, it does not meet the requirement.
- Worst-case charge and discharge power, and how long each event lasts
- Events per day, and the recovery time between them
- Ambient range, altitude, solar exposure and whether the location is indoors
- Cabinet spacing and the airflow path actually available on site
- Allowed cell temperature range and maximum permitted spread
- Who maintains it, how often they attend, and what spares are held
Cooling is a controls interface, not just hardware
The BMS, the cooling controller, the EMS and the fire-protection system have to agree on which temperature signals are authoritative, at what thresholds derating begins, who is allowed to command a shutdown, and what each does when the link between them is lost. A cooling system specified without those definitions is a mechanical drawing rather than a design.
Final equipment and services are set by the written project quotation and the local engineering requirements that apply to the site.
Questions this raises most often
Close to the whole round-trip loss. Cell resistance dominates and scales with the square of the current, so estimate at the worst duty rather than the daily average, then add busbars, contactors, auxiliaries and solar gain.
It holds a tighter temperature spread and supports higher density, and it adds pumps, coolant maintenance, leak detection, freeze protection and a stricter service procedure. It is better where that duty exists and those obligations are staffed.
Spread drives divergent aging, unreliable state-of-charge estimation and balancing that never catches up. A cabinet can meet its average limit and still be poorly cooled.
Power rate, event frequency and ambient conditions -- not stored energy. If the air-cooled design only meets the requirement by derating on the hottest days, it does not meet the requirement.
Take it to a part number
Ratings vary between codes that read almost the same, so settle the decision here and then confirm the figures on the product page for the exact code.