Nobody calls their battery supplier because the cells failed.
They call because the truck now shuts down at what the display insists is 40 percent. Or because the charger in the corner of the warehouse faults every third cycle and someone has started resetting it by hand. Or because a machine that passed its stability check carrying a 900 kg battery now carries one weighing 380 kg, and the safety manager wants to know who signed off on that.
Swapping lead-acid for LiFePO4 is one of the easiest upgrades to justify on paper and one of the easiest to get wrong in the field. The chemistry is the settled part. What trips up projects is everything around the cells: the charger, the machine controller, the mass distribution, the temperature at 6 a.m. in an unheated yard, and the paperwork that travels with the pack. Here is what I would want an engineer or a buyer to understand before signing the order.
Rated Amp-Hours Lie: Usable Capacity in Lead-Acid vs LiFePO4
A 100 Ah lead-acid battery and a 100 Ah LiFePO4 battery are not the same battery with different labels. Three effects stack up.
Depth of discharge is the obvious one. Lead-acid is conventionally held to around 50 percent depth of discharge for reasonable service life, while LiFePO4 is routinely specified at 80 to 90 percent. Treat those as planning figures, not guarantees. What matters is the cell datasheet and the cycle-life curve behind it.
Peukert's law is the reason so many lead-acid runtime estimates are optimistic. A battery rated 100 Ah over a 20 hour discharge may deliver considerably less if you pull it down in an hour, with published examples landing in the 60 to 80 Ah range depending on the battery and the test method. LiFePO4 has a Peukert exponent close to 1, so delivered capacity stays roughly flat across normal rates.
Round-trip efficiency finishes the picture: lead-acid commonly lands around 80 to 85 percent, LiFePO4 above 95 percent. In solar or generator-charged installations that gap is real money and real charging hours.
A working rule of thumb: a LiFePO4 pack rated at roughly 50 to 60 percent of the lead-acid nameplate amp-hours often delivers equivalent real work in a high-rate application. That is not a specification. It is the reason your sizing conversation should start with the duty cycle rather than the old label. Anyone who quotes a replacement from nameplate Ah alone has skipped the engineering.
Cycle life works the same way. Lead-acid in motive service is commonly quoted around 300 to 500 cycles at 50 percent depth of discharge, while quality LiFePO4 packs are specified in the thousands. Those lithium numbers are test-condition numbers. Ask what temperature, what rate and what end-of-life threshold they were measured against, because 3,000 cycles to 80 percent capacity at 25 °C is a very different promise from the same figure at 40 °C.
Can You Charge a LiFePO4 Pack With Your Existing Lead-Acid Charger?
Usually not, and this is the most common cause of a battery returned as defective when it is nothing of the kind.
Lead-acid chargers do things that make no sense to lithium. Equalization and desulfation modes deliberately push elevated voltage, often above 15 V on a 12 V system, to stir electrolyte and break down sulfate on the plates. LiFePO4 does not sulfate. What it does instead is trip the BMS overvoltage protection, which opens the charge path. If your operator sees the charger cycling and faulting, that is usually what is happening.
The opposite failure is quieter and worse. Plenty of lead-acid chargers terminate around 13.6 to 13.8 V on a 12 V system, a sensible float voltage for lead-acid that leaves a LiFePO4 pack well short of full. Nobody sees a fault. They just report that the new battery does not last as long as promised, and they are right, because it never fully charged. Chronic undercharging also starves the BMS of the balancing window it needs at the top of charge, so imbalance quietly accumulates.
There is a third trap. Return-to-bulk thresholds in lead-acid chargers are set against lead-acid resting voltages. LiFePO4 rests higher, near 13.3 to 13.6 V on a 12 V pack, so a charger may decide the battery is already full and never restart a proper cycle.
My position is blunt: budget for the charger from day one. It is a small fraction of project cost and a disproportionate share of the failures. Either specify a charger with a genuine LiFePO4 profile, or confirm in writing that the existing unit can be reprogrammed to a CC/CV profile matched to your pack with equalization and desulfation disabled. Verify the actual termination voltage rather than trusting the mode label on the front panel.
The Flat Discharge Curve Will Break Your Fuel Gauge
Lead-acid voltage falls steadily as the battery empties, and a generation of machine controllers was built on that assumption. In a 48 V system, pack voltage starts near 50 V when full and slides into the low 40s as it depletes. Controllers read that slope to estimate remaining charge and derate power near the bottom. The assumption was so fundamental it often went undocumented.
LiFePO4 destroys it. The pack enters the cycle at a higher voltage, holds close to nominal across most of the usable range, then falls off sharply near the end. Roughly 80 percent of the stored energy sits inside that flat plateau, which means voltage tells you almost nothing about state of charge in the middle of the cycle. What the operator sees is a gauge reading comfortable, and then a machine that stops.
There are three honest responses, and you should decide which one you are buying.
Accept it and retrain operators. Fine for a walkie pallet jack, not fine for an aerial platform where an unexpected shutdown at height is a safety event.
Put a coulomb-counting monitor or a smart BMS with CAN or RS485 output in the pack and feed real state of charge to the display. Right answer for most industrial equipment, and worth the cost, because it also gives you fault logging.
Engineer the pack to sit inside the legacy platform's electrical envelope so the machine never sees anything it cannot interpret. This is where the better drop-in suppliers have landed: constrain the working voltage range and the continuous and pulse current limits to what the old controller tolerates, and let the BMS handle the rest internally.
One detail gets missed constantly. Check the machine's low-voltage cutoff against the pack's BMS cutoff. Set for a lead-acid curve, it may cut power while the pack still holds real charge. Set below the BMS threshold, the BMS opens the contactor first, dropping the load abruptly and producing a transient the machine electronics were never designed to absorb. Confirm the controller's input capacitance too, and whether a pre-charge circuit is needed, because closing a contactor into a large capacitor bank welds contacts.
Weight Is a Feature in Some Machines, Not a Penalty
Everyone sells lithium on weight savings. In a counterbalance forklift, weight savings is the problem.
A LiFePO4 pack typically weighs between a third and a half of the lead-acid battery it replaces. On a sit-down counterbalance truck, a reach truck or a multidirectional truck, that battery mass is part of the stability calculation printed on the data plate. Remove several hundred kilograms from the rear of the machine and the center of gravity moves, the load chart stops describing reality, and you have created a safety problem no amount of BMS sophistication fixes.
Two legitimate paths exist: integrated steel ballast inside the pack to bring installed weight close to the original battery mass, or a revised data plate reflecting the new rated capacity, done through the proper channel for that machine. Anything else is improvisation.
While you are measuring, measure everything. Compartment dimensions, tray standard, terminal position and polarity, cable exit direction, restraint. A pack smaller than the compartment moves under acceleration and braking unless it is mechanically located, and a pack sliding around a steel box eventually finds a terminal or a harness. For outdoor or washdown machines, specify the ingress protection target explicitly; our guide to IP-rated and ruggedized battery packs for field equipment covers what those ratings actually mean.
LiFePO4 Low Temperature Charging Is the One Real Regression
I will say plainly what most lithium marketing skips. There is one operating condition where lead-acid is more forgiving, and it is cold charging.
Charging LiFePO4 below roughly 0 °C drives lithium plating on the anode instead of normal intercalation. The damage accumulates and does not reverse on the next discharge. Lower charge current slows the process but does not eliminate the mechanism; temperature is the controlling variable. This is why a competent BMS blocks charging below a threshold near 0 °C while still permitting discharge down closer to -20 °C. That behavior is protection, not a defect, and operators should hear it before winter rather than after the first support call.
If your equipment charges in an unheated space, you have three options and each carries a cost. An integrated heater works, but it consumes energy, needs a power source before charging begins, and adds warm-up delay to the shift. Low-temperature cell formulations extend the charge window, but availability, price and supply continuity are all narrower. Or you condition the charging area, which is often the cheapest answer in a fixed facility and impossible in a mobile one.
Two warnings. Ambient air temperature is not cell core temperature; a cold pack in a warming room stays cold inside for a long time. And every charging source must respect the low-temperature limit, including solar, alternator and generator inputs. One unprotected path is enough to damage the pack.
What Changes Inside the Building
The infrastructure story is where the finance case actually gets made, and it is stronger than most engineers expect.
Flooded lead-acid gases hydrogen during charging, which is why regulators care about battery rooms. OSHA 29 CFR 1910.178(g) requires charging areas for powered industrial trucks to provide for flushing and neutralizing spilled electrolyte and adequate ventilation for gassing fumes. Alongside that sits the recurring labor of watering, equalizing, cleaning corroded terminals, and the handling gear for battery swaps.
Sealed LiFePO4 packs remove the hydrogen and the acid. They do not remove all obligations. Fire codes, insurer requirements and lithium storage practice are their own subject, and anyone who says the switch eliminates compliance work is selling, not engineering. What it does eliminate is a specific and expensive set of daily tasks.
Shift structure matters more. Lead-acid motive batteries typically need six to eight hours of charging plus a cooldown before reuse, which is why multi-shift operations keep spare batteries, changing stations and floor space to hold them. LiFePO4 charges in roughly one to two hours and tolerates opportunity charging during breaks, so one pack often covers multiple shifts. That is where the real total cost of ownership case lives, and it beats any cycle-life table.
Check the electrical service while you are at it. Faster charging means higher instantaneous draw, and across a fleet charging at the same break you may find the panel, not the battery, is the constraint.
Certification and Documentation Buyers Should Demand
This is where supplier answers get vague, so ask precisely.
For motive industrial equipment, UL 2580 is the standard forklift packs are generally evaluated against, with UL 2271 sometimes applying to lighter equipment depending on energy level. Stationary and auxiliary systems fall under UL 1973. Internationally, IEC 62619 covers lithium cells and batteries in industrial applications including forklifts, AGVs and stationary storage, its 2022 edition adding BMS functional safety and thermal propagation testing. For transport, UN 38.3 plus an SDS and Class 9 handling apply to essentially every lithium shipment.
The question separating serious suppliers from resellers: is the certification at cell level or pack level? Certified cells inside an uncertified pack tell you little about how that pack behaves under abuse. Ask for the certificate, its scope, and the models it covers.
Selling into Europe adds a layer. Industrial batteries above 2 kWh face a digital battery passport requirement from 18 February 2027, and carbon footprint declaration obligations extended to that category in February 2026. Confirm current status before committing, since guidance keeps moving. Detail in our article on the EU Battery Regulation and the digital battery passport.
The Retrofit Spec Sheet Worth Sending With Your RFQ
Vague RFQs get padded quotes. That is not cynicism, it is risk pricing. If a supplier cannot tell whether your application needs 150 A continuous or 400 A, they quote for 400 A and you pay for it.
Send this with your first enquiry:
- Machine make, model and year, plus the motor controller part number, its low-voltage cutoff and maximum input voltage tolerance.
- Existing battery: nominal voltage, rated capacity and the rate it was rated at, external dimensions, tray standard, terminal type and position, total weight.
- Duty cycle: continuous current, peak current and its duration, energy per shift, cycles per day, shift pattern.
- Charger: make, model, available profiles, and whether it can be reprogrammed or must be replaced.
- Temperature range at charge and at discharge, stated separately, and whether the charging location is heated.
- Whether battery mass contributes to machine stability, and the installed weight the replacement must reach.
- Communication: CAN, RS485, display integration, fault logging, and what the operator needs to see.
- Destination market and required certification listings, specified at pack level.
- Environment: vibration, ingress protection target, washdown exposure, mounting and restraint.
- Documentation expected at delivery: test reports, UN 38.3 summary, SDS and packing configuration.
Keep that as a standing template. It is also the fastest way to find out whether your supplier is an engineering partner or a catalogue.
At PackForge Energy, most lead-acid replacement projects start exactly there: reviewing the machine, the charger and the duty cycle before anyone talks about cell selection. Our LiFePO4 battery pack and engineering and battery pack design pages describe how that runs, and our guide on how to evaluate a custom lithium battery pack manufacturer covers what to ask any supplier before you commit.
Over to You
For those of you who have run a lead-acid to LiFePO4 conversion on real equipment: what actually bit you? The charger, the controller calibration, the counterweight, cold weather, or something nobody warned you about? And for those still evaluating, what is holding up the decision?
Leave it in the comments. The failure modes people share are usually more useful than any spec sheet.