Every autumn the same question comes into the workshop: should I take the battery replacement as an excuse to go lithium? The subject deserves better than the usual marketing answer, because the two technologies do not fail in the same places.
Here is the comparison as it really stands, going into the cold season.
What exactly is being compared
Modern "lead" on a motorcycle means AGM: lead-acid with absorbed electrolyte, sealed, maintenance-free. Motorcycle "lithium" is almost always LiFePO4 (lithium iron phosphate), not the chemistries used in consumer electronics. That distinction matters: LiFePO4 is markedly more stable thermally.
| Criterion | AGM (lead) | LiFePO4 |
|---|---|---|
| Mass for equivalent capacity | Reference | –60 to –70 % |
| Purchase price | £50–110 | £100–300 |
| Typical service life | 3–5 years | 6–10 years |
| Monthly self-discharge | 3–5 % | Under 2 % |
| Tolerance to deep discharge | Poor, irreversible damage | Better, protected by the BMS |
| Starting at very low temperature | Consistent | Needs waking up |
| Dedicated charger | No | Yes, essential |
The genuine advantages of lithium
Weight. Two to four kilos saved depending on engine size, and carried high and towards the rear on most bikes. It is the most tangible gain on a light machine; it becomes marginal on a big loaded adventure bike.
Self-discharge. This is the winter argument. A LiFePO4 pack left three months without being ridden loses very little, where an AGM drifts down towards the sulphation zone. One caveat, though: the battery's own self-discharge is not the only drain. The bike's electronics (alarm, ECU on standby, clock) draw current continuously, and that is often what flattens the pack.
Voltage stability. Lithium holds a higher voltage for longer through the discharge, which gives a crisper starter spin at the tail end of the charge.
Service life. In normal use a LiFePO4 routinely more than doubles the life of an AGM. Per cycle it often works out cheaper, despite costing twice as much to buy.
The real drawbacks
Cold behaviour. This is the most misunderstood point. Below 0 °C, LiFePO4 chemistry accepts charge poorly and delivers less current until it has warmed up. In practice: in hard frost, switch on the ignition and leave the headlight on for thirty seconds before hitting the starter. That small current warms the pack and the bike then starts normally. Without that reflex, you will think the battery is dead.
By the same token, never charge a frozen lithium pack. A pack below 0 °C is brought back to room temperature first.
The charger. A lithium battery demands a charger with a LiFePO4 profile. A conventional lead charger applies an end-of-charge voltage and a desulphation mode that will destroy the pack, sometimes overnight. It is the number one cause of premature death.
The bike's regulator. On some older machines, the regulator-rectifier works by dissipation and regulates to a voltage designed for lead. Most modern packs cope thanks to their BMS, but on a 1980s or 1990s machine with loose regulation, checking the charging voltage with a multimeter is essential before leaving the pack permanently fitted.
Diagnosis. A lithium pack fails more abruptly: it holds its voltage and then the BMS cuts out. You get fewer warning signs than with an AGM that fades gradually.
