A truck parked for the night with the engine off is running a small hotel: a sleeper cooler drawing 600–900 W, cab lighting, a phone charger, sometimes a 12 V fridge. On a standard two-battery 24 V lead-acid set, that load comes out of batteries designed to deliver one short burst of cranking current and then be recharged immediately. They do it, for a while — and then the truck does not start in the morning.
This is the case for a dedicated parking battery: either a LiFePO4 pack added alongside the starter set, or a combined start-and-parking pack that does both jobs. What follows is what to settle before you buy either.
Two different batteries, two different jobs
A starter battery is specified for cold-cranking amps and for staying at high state of charge. A parking battery is specified for usable energy and for cycling: it will be taken to 80–90% depth most nights and recharged during the day. Lead-acid can be built for cycling, but the deep-cycle versions give up cranking current, and the cranking versions give up cycle life. That trade-off is the reason lithium packs have taken over this application.
Size it from the load, not from the label
The arithmetic is simple and worth doing before any conversation with a supplier:
- Usable energy needed = cooler power × hours of operation ÷ efficiency of the inverter and DC-DC path (typically 0.85–0.90).
- A 700 W cooler run for 9 hours needs about 6.3 kWh at the load, and roughly 7 kWh at the battery.
- At 24 V nominal, 7 kWh is about 290 Ah usable — around 340 Ah of nameplate capacity at 85% depth of discharge.
That is a larger pack than most buyers expect, and it explains the most common complaint about parking packs in hot markets: the cooler runs longer than planned, the pack is cycled deeper than planned, and the BMS cuts off at three in the morning.
The five numbers to put in writing
- Continuous and peak discharge current. Continuous has to cover the cooler plus everything else; peak has to cover the inverter start-up surge, which is several times the running current.
- Cranking current, if the pack also starts the engine. A combined pack must hold a cycling specification and a cold-cranking figure at the same time. Ask for both separately, and ask how each was measured.
- Charge temperature window. LiFePO4 must not be charged below 0 °C. In a cold climate the pack needs either a charge-disable input tied to cell temperature or an integrated heater. A datasheet that lists only a discharge range has not answered the question.
- Discharge derating above ambient. In a Gulf summer the battery box runs well above air temperature. Ask for the derating curve above 45 °C, not only a maximum figure.
- Cycle life at the depth you will actually use. A figure quoted at 80% depth of discharge is the relevant one for nightly cycling. A figure quoted at 100% depth with a low retention threshold is not comparable, however large the number looks.
Charging from the alternator is the part that fails quietly
Adding a lithium pack to a vehicle charging system is not a matter of running a cable. The alternator is regulated for lead-acid: it holds a voltage that will not fully charge a LiFePO4 pack and, in some vehicles, will stop charging altogether once the pack’s BMS disconnects. The usual answer is a DC-DC charger between the two systems, sized for the pack’s recommended charge current, fused at both ends, with a cable gauge chosen for the round-trip length rather than for the current alone.
Two failure modes are worth designing out at this stage. The first is a pack that is never fully charged because the charger was sized for the alternator rather than for the battery, so the cells sit at partial state of charge for months and the balancer never gets a chance to work. The second is a pack charged hard on a hot day with no temperature signal reaching the charger. Both are installation faults, and both look like battery faults to the driver.
Heat and cold are not symmetrical problems
Heat shortens life; cold stops charging. A pack installed in a chassis box in the Middle East or Sub-Saharan Africa will spend its service life at temperatures that accelerate calendar ageing, so the honest question is not “what is the maximum operating temperature” but “what capacity and life should I expect at 50 °C ambient, and what does the BMS do there”. In northern China and Central Asia the opposite applies: the pack will be asked to accept charge on a morning at −10 °C, and it must refuse — safely, and with a signal the driver can see — until the cells are warm enough.
Installation details that decide warranty claims
- Mount the pack where it is protected from road spray and direct sun, on a bracket that does not load the cell terminals.
- Fuse at the battery positive, as close to the terminal as practical, and again at the charger end of a long run.
- Keep the BMS alarm or communication line accessible, so a fault can be read without removing the pack.
- Record the installation: pack serial number, charger model, cable length and fuse rating. It is the first thing a supplier will ask for if a claim is made.
How Origin Drive Future supplies it
We build 24 V LiFePO4 packs for parking and for combined start-and-parking duty in the parking & start batteries range, and custom capacities and enclosures in the cells and packs range. Packs are built and tested against the target standards UN38.3 and IEC 62619; the document set for a specific model is confirmed in writing with the quotation, together with the charge and discharge limits at the temperatures your fleet actually sees. Send the vehicle type, the cooler load and the parking duration, and we will return a sizing proposal. Request a quote.