Charging in Depth: CC-CV, Chargers and Fast Charging
The charge profile, why a lead-acid charger destroys a lithium pack, what fast charging actually costs, and the practices that add years.
Battery Fundamentals · Part 5 — Pack Design and Charging · Chapter 13 · 13 min read
More lithium packs are destroyed by their chargers than by their duty cycles. The two mistakes that do it — a lead-acid charger on a lithium pack, and charging below freezing — are both easy to make, both invisible on the day, and both entirely preventable with equipment that already exists.
73.0 V
Correct 20S LFP charger
73.5 V
A 60 V lead-acid charger
0.02–0.05C
Termination threshold
Weekly
Full charge for balancing
13.1 — The CC-CV profile
Lithium charging has two phases and, for a deeply discharged cell, a third that comes first.
13.1.1 — Trickle / pre-charge
For cells below roughly 2.5 V, a low current of about 0.05C until the cell is safe to charge normally. Skipping this on a deeply discharged cell is how a recovery attempt becomes a thermal event.
13.1.2 — CC — constant current
The charger holds current constant and the cell voltage rises. This delivers roughly 80 to 90 per cent of the capacity and occupies most of the time.
13.1.3 — CV — constant voltage
Once the cell reaches its cut-off — 3.65 V for LFP, 4.2 V for NMC — the charger holds that voltage and current tapers away. Charging ends when current falls to a termination threshold, typically 0.02 to 0.05C.
Important
The charger must stop, not float. Lead-acid chargers hold a float voltage indefinitely because lead-acid needs it. LFP does not, and holding a lithium pack at high state of charge accelerates SEI growth and calendar ageing — the mechanism in Chapter 3. A charger that floats is slowly destroying the pack it is connected to.
13.2 — Choosing a charger
- •Voltage must match the S count and the chemistry. A 20S LFP pack needs a 73.0 V charger — 20 × 3.65 V. Not 72 V, not 73.5 V.
- •Current rating sets charge time and thermal load. 0.2 to 0.5C is the range that trades acceptable time against long life. On a 100 Ah pack that is 20 to 50 A.
- •Communication changes what is possible. A smart charger talking to the BMS over CAN can be commanded to reduce current, stop, or refuse to start when the pack is below 0°C. A dumb charger cannot be told anything, so every protection has to live in the BMS and be enforced by disconnecting.
- •Termination behaviour is a specification. Ask what the charger does when current reaches the taper threshold. “It goes green” is not an answer; ask whether output actually ceases.
13.3 — The lead-acid charger trap
This is the single most common and most damaging field error in the Indian three-wheeler market, and it happens because the numbers look close enough to work.
A 60 V lead-acid charger against a 20S LFP pack
| Lead-acid charger | What LFP needs | |
|---|---|---|
| Peak voltage | ~73.5 V | 73.0 V |
| Equalisation stage | Deliberately overcharges to stir electrolyte and desulphate plates | None. LFP has no equalisation phase and no tolerance for one. |
| Float stage | Holds voltage indefinitely to offset self-discharge | None. Float holds the pack at high SOC and accelerates calendar ageing. |
| Termination | Does not terminate — transitions to float | Must terminate at 0.02–0.05C taper current. |
Important
The peak voltages differ by only half a volt, so it appears to work. What does the damage is everything after the peak: an equalisation stage that overcharges cells LFP cannot tolerate, and a float stage that never lets go. A pack charged this way for a season will show unexplained capacity loss and, in the worst case, swelling.
In plain English
Practically: when you sell or fit a lithium pack, the old charger must leave with the old battery. Log the charger serial number against the pack. A customer who keeps the lead-acid charger “as a spare” will eventually use it, and the resulting failure will arrive as a warranty claim with no obvious cause.
13.4 — Fast charging and the trade-off
Higher charge C-rate means more heat, more polarisation, and — near the top of charge and at low temperature — lithium plating. There is no way to fast-charge without paying in one of those currencies.
Technical framing
The compromise used in cars is tapering: high current at low state of charge, reducing progressively as SOC rises, because the plating risk grows as the anode fills. This is why a car charges from 10 to 80 per cent far faster than from 80 to 100.
For a three-wheeler on a depot schedule, fast charging is usually solving a problem you do not have. Slow overnight charging at 0.2C is better for cycle life, cheaper in equipment, lower risk, and it fits the vehicle’s actual idle window. Fast charging earns its cost only where the vehicle’s revenue depends on turning round mid-shift.
13.5 — Charging practices that extend life
- •Charge to 100 per cent at least weekly so the BMS can balance — but do not leave it sitting there for days. Most passive BMS boards only balance above a threshold around 3.40 V/cell and only while charging, so a fleet that habitually stops at 80 per cent never balances at all. The imbalance accumulates silently for months and then presents as sudden capacity loss. A weekly full charge is a genuine maintenance instruction, not a nicety.
- •Avoid habitually discharging below 10 to 15 per cent. Shallower cycling buys disproportionately more cycles — the DOD relationship is in Chapter 5.
- •Never charge below 0°C. Non-negotiable in a North Indian winter, and it must be enforced by the BMS rather than by instructions to riders. See Chapter 6.
- •Let a hot pack cool before charging. A pack coming off a summer afternoon shift is already above its comfortable range; adding charge heat on top compounds the ageing.
- •Use the matched charger, and record its serial number against the pack. This is how you resolve a warranty dispute in five minutes instead of five weeks.
- •Store at 40 to 60 per cent SOC if the pack will be idle for weeks. Inventory and off-season vehicles both count.
Quick check: test yourself
1.A customer says the new lithium pack “works fine” on their old 60 V lead-acid charger. Is there a problem?
Show answer
2.A fleet charges to 80 % every night to “be kind to the battery”. Six months later capacity drops sharply. Why?
Show answer
3.Why does a charger need to stop rather than float?
Show answer
4.Is fast charging worth it for a depot-based three-wheeler fleet?
Show answer
Chapter summary
- ✓Lithium charging is CC then CV, preceded by a 0.05C pre-charge for cells below about 2.5 V. Charging ends at a termination threshold of 0.02 to 0.05C.
- ✓The charger must stop, not float. Float holds the pack at high SOC and accelerates calendar ageing — a floating charger is slowly destroying the pack.
- ✓A 20S LFP pack needs a 73.0 V charger. A 60 V lead-acid charger peaks at about 73.5 V, which is why it appears to work, and then runs equalisation and float stages LFP cannot tolerate.
- ✓When fitting a lithium pack, the old charger leaves with the old battery. Log the charger serial number against the pack.
- ✓Fast charging trades cycle life for turnaround. For depot-based vehicles with an overnight window, 0.2C is better in every respect.
- ✓Charge fully at least weekly so the BMS can balance — passive boards typically balance only above ~3.40 V/cell and only while charging, so an 80 % ceiling means never balancing.
- ✓Never charge below 0 °C, let hot packs cool first, avoid habitual discharge below 10 to 15 per cent, and store idle packs at 40 to 60 per cent SOC.
Frequently asked questions
Can I use my old lead-acid charger on a new lithium pack?+
No, and it is the most damaging common error in this market. The peak voltages look close — about 73.5V against the 73.0V a 20S LFP pack needs — which is why it appears to work. The damage comes after the peak: a lead-acid charger runs an equalisation stage that deliberately overcharges, then floats indefinitely. LFP has no equalisation phase and no tolerance for float. The old charger should leave with the old battery.
Why must a lithium charger stop rather than float?+
Because holding a cell at high state of charge is one of the two conditions that accelerate SEI growth, the other being heat. Lead-acid needs float to offset its much higher self-discharge; LFP loses only 2 to 3 per cent a month and does not. A charger that floats keeps the pack in its worst storage condition continuously, producing calendar ageing the customer reads as premature failure.
Should I charge my battery to 100 per cent or stop at 80?+
Charge fully at least weekly, then avoid sitting at 100 per cent for days. Most passive BMS boards only balance above a threshold around 3.40V per cell and only while charging, so a fleet that always stops at 80 per cent never balances at all. Imbalance then accumulates silently for months and presents as sudden capacity loss. A weekly full charge is genuine maintenance, not a compromise.
Is fast charging bad for a battery?+
It costs cycle life through heat and polarisation, and near the top of charge or at low temperature it risks lithium plating. Cars manage this by tapering — high current at low state of charge, reducing as the anode fills, which is why 10 to 80 per cent is far faster than 80 to 100. For a depot-based three-wheeler with an overnight window, 0.2C charging is better in every respect and cheaper in equipment.
What voltage charger does a 20S LFP pack need?+
73.0V — twenty cells at 3.65V each. Not 72V, and not the 73.5V a 60V lead-acid charger delivers. Current rating of 0.2 to 0.5C trades acceptable charge time against long life, which on a 100Ah pack is 20 to 50A. A charger that communicates with the BMS over CAN can additionally be commanded to reduce current, stop, or refuse to start when the pack is cold.
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Battery Fundamentals is an original educational series on lithium battery technology. Threshold tables, cycle-life curves and worked examples use representative values drawn from published product specifications and widely-observed industry patterns, not measured data from a specific product. Always consult the current manufacturer datasheet before making design, purchasing, warranty or certification decisions.