Retrofitting lithium into a lead-acid site looks like a drop-in swap: same nominal voltage, similar footprint, half the weight. It is not. Four things change, and if you only handle the first one the new battery will fail inside two years — usually without anyone being able to explain why.
The Charger Is the Bottleneck, Not the Battery
A lead-acid charger is designed to do something lithium cannot tolerate: hold a constant float voltage indefinitely. A 48 V lead-acid bank sits at 54.0–54.8 V float forever, and that is fine because lead-acid chemistry tolerates it. A 15-cell LFP pack held at 54.0 V indefinitely is being overcharged every hour of every day.
| Parameter | Lead-acid (48 V bank) | LFP (15S) | What you must change |
|---|---|---|---|
| Float voltage | 54.0–54.8 V continuous | Must drop to 53.5 V or stop charging entirely | Charger profile — not optional |
| Absorption | 58.8–59.2 V, 2–4 hours | 56.0–56.8 V, terminate on current | Voltage setpoint and termination logic |
| Temperature compensation | Required, −3 mV/°C/cell | Not used — BMS handles it | Disable it, or the pack will be undercharged in heat |
| Equalisation | Scheduled 60–64 V | Must be disabled | A single equalise cycle can trip the BMS or damage cells |
| Charge termination | Timer-based | Voltage + current + BMS signal | The charger must be able to read the BMS |
The last row is where most retrofits fail. A charger that cannot talk to a BMS will keep pushing current after the pack is full. If the BMS protects itself by opening the contactor, you get a hard DC break under load — which is worse than overcharging.
A lithium retrofit is a charger project with a battery attached, not a battery project.
What Stays Exactly the Same
Three things genuinely do not change, and knowing them keeps the retrofit cost down.
- Nominal voltage and cabling. A 48 V LFP pack such as a 48V rack battery replaces a 48 V lead-acid bank on the same DC bus, same breakers, same cable gauge. Current draw is similar or lower.
- Physical footprint. LFP gives roughly the same usable energy in 40–60% of the volume and 30–40% of the weight. In most cabinets you gain space rather than lose it.
- The load side. Rectifiers, inverters and DC distribution do not care what chemistry is behind them, as long as the voltage window is respected.
So the mechanical work is small. The electrical configuration work is where the project lives.
The BMS Protocol Question
This is the single decision that determines whether your retrofit is a two-day job or a two-month job. There are three levels of integration and they cost very different amounts.
| Integration level | How it works | What it costs | When to choose it |
|---|---|---|---|
| Standalone (no comms) | BMS protects itself only; charger runs a fixed lithium profile | Lowest | Small sites, single pack, charger with a native LFP mode |
| Voltage/current signalling | BMS drives a dry contact or analogue signal to the charger | Low–medium | Most telecom retrofits — reliable and simple |
| Full CAN/Modbus | BMS reports SOC, SOH, limits; charger and site controller read them | Medium | Multi-pack sites, remote monitoring required, anywhere you want SOC visibility |
If you are retrofitting a telecom site with a site controller already in place, go to full CAN or Modbus. The SOC data alone justifies it: without it your monitoring sees voltage, and voltage on LFP is nearly flat across 30–80% SOC. You will not know whether the site has two hours of backup or eight.
Cold Is the Real Constraint
LFP cannot be charged below 0 °C. Not should not — cannot, without plating lithium onto the anode, which permanently destroys capacity. Lead-acid merely charges slowly when cold. This difference catches out northern and high-altitude sites every winter.
- Discharge is fine down to −20 °C, at reduced capacity (roughly 70% at −10 °C).
- Charge must be blocked below 0 °C. A BMS with a charge-prohibit signal and a heater pad solves it, but the charger has to respect the signal.
- If your site regularly sits below 0 °C and you have no heater, specify a heated pack or keep lead-acid. This is not a place to save money.
What the Ten-Year Maths Looks Like
| Lead-acid (AGM) | LFP | |
|---|---|---|
| Cycle life at 80% DoD | 400–600 | 3,000–6,000 |
| Usable depth of discharge | 50% (to preserve life) | 80–90% |
| Calendar life at 25 °C | 3–5 years | 8–12 years |
| Round-trip efficiency | 75–85% | 92–96% |
| Weight per kWh | 25–30 kg | 8–11 kg |
| Replacement cycles in 10 years | 2–3 | 0–1 |
On paper LFP wins on every line. The reason retrofits still disappoint is not the cells — it is that the surrounding system was never adjusted to let them perform. A pack that is never allowed above 90% SOC because the charger tops out at 53.5 V, and never below 40% because the low-voltage disconnect is set for lead-acid, delivers about 50% of its rated capacity. That is the failure mode, and it looks exactly like a bad battery.
Frequently Asked Questions
Can I keep my existing charger when switching to lithium?
Only if it has a lithium profile and, ideally, a BMS communication input. Most chargers installed before 2018 have neither. Check the manual for a lithium mode and for a CAN or dry-contact input before assuming you can keep it.
Do I need to change the low-voltage disconnect setting?
Yes. Lead-acid LVD is typically set around 42–44 V for a 48 V bank to avoid over-discharge. LFP is happy down to 40–42 V and the BMS will protect itself anyway. Leaving the old LVD setting means you never use the bottom 20% of the pack.
Will a lithium pack work with my existing solar MPPT?
Usually yes, if the MPPT has a user-defined charge profile. The MPPT must be able to stop at a set voltage rather than hold float indefinitely. If it only has preset lead-acid modes, it needs replacing or the profile needs to be editable.
How long does a lead-acid to lithium retrofit take on site?
Two to four days for a single cabinet or small bank: one day to remove and dispose of the old bank, one to install and wire, one to configure and commission, plus a settling day. The lead time is in the charger and BMS configuration, not the physical work.
Send Us Your Existing Configuration
Tell us the bank voltage, the charger make and model, and the site’s lowest winter temperature. We will tell you whether it is a drop-in swap or a charger project before you buy anything.