The auxiliary battery is where a specialist conversion lives or dies. It powers the payload when the engine is off, and the wrong choice shows up as flat batteries, poor runtime or a system that costs far more than it needed to. The lithium-versus-lead-acid decision is not a straight upgrade in one direction. It depends on the duty cycle, the loads and the budget.
Usable capacity, not nominal capacity
The most important difference is how much of the rated capacity you can actually use.
- Lead-acid (including AGM) is typically limited to around 50% depth of discharge for reasonable cycle life. A 100Ah lead-acid battery gives you roughly 50Ah in practice.
- Lithium (LiFePO4) can be discharged to 80-90% routinely. A 100Ah lithium battery gives you 80Ah or more.
So a lithium bank often delivers well over twice the usable energy of a lead-acid bank of the same nominal rating. Sizing on nominal Ah alone is the classic mistake.
Weight and space
Lithium is dramatically lighter and denser for the same usable energy, often around a third of the weight of the lead-acid equivalent. On payload-limited conversions, welfare vehicles and electric platforms, that saved weight is directly useful. Lead-acid remains heavy and bulky, which is only a problem where weight and space are tight.
Charging behaviour
The two chemistries charge differently, and this drives the rest of the system design.
- Lithium accepts charge fast and holds voltage high through most of the discharge, so equipment sees a stable supply. It needs a charging source with a lithium-appropriate profile and a battery management system (BMS).
- Lead-acid charges more slowly, especially in the final stage, and its voltage sags under load, which can affect sensitive equipment late in a discharge.
A key constraint on lithium is temperature: most LiFePO4 cells must not be charged below freezing without damage, so cold-stored vehicles need charging that respects a low-temperature cut-off or a heated battery. Lead-acid is more tolerant of cold charging, though its capacity still falls in the cold.
Cycle life and total cost
Lithium costs more up front, often several times the price of lead-acid per battery. But it typically delivers many times the cycle life at usable depth of discharge, so on a vehicle that cycles daily the cost per usable kWh over the vehicle’s life is frequently lower. On a vehicle that cycles rarely, lead-acid’s lower purchase price may never be beaten.
Matching the battery to the system
Whichever chemistry you choose, the auxiliary battery has to be protected and charged correctly.
- Battery protection. A Battery Guard isolates non-essential loads before the battery is taken below a safe level, protecting both the auxiliary bank and, on split systems, the engine-start battery. On lithium, respecting the BMS limits matters as much as the low-voltage cut-off.
- Charging. Feed the auxiliary bank from a DC-DC converter with the correct charge profile for the chemistry, sized to the alternator and the duty cycle, rather than connecting it straight across the vehicle system.
- Monitoring. Track state of charge so the crew and any load-management logic can act before the bank is exhausted.
Choosing
Reach for lithium when the vehicle cycles hard and often, when weight or space is constrained, or when equipment needs a stable supply through a long discharge, provided the charging system respects its temperature limits. Stay with lead-acid when the duty cycle is light, up-front cost dominates, or the vehicle is stored cold without heated charging.
Size on usable capacity, match the charging profile to the chemistry, and protect the bank with a device sized to the load. Get those three right and either chemistry will do its job. Intellitec’s Battery Guards and DC-DC converters are UK manufactured.