Su-vastika builds compact ESS inverters on a 22V (7-cell, 22.4V) LiFePO4 system — a size between 12V and 24V for compact backup. The two real catches are the continuous trickle charge, the start-up surge the inverter needs from the battery, and waking the pack after a low-voltage cutoff — all solved by a proper retrofit BMS. (Su-vastika retrofit range)
Why 22.4V matches your 22V inverter
Su-vastika created the 22V (7S) lithium system for its compact ESS inverters. At 7 cells in series the pack is 22.4V nominal, charging near 25.2V — a middle ground between a 12V (4S) and a 24V (8S) system.
| Parameter | 22V lead-acid inverter | 22.4V LFP retrofit |
|---|---|---|
| Nominal voltage | 22V | 22.4V (3.2V × 7 cells) |
| Bulk / charge voltage | ~25.2V | 25.2V (3.6V × 7) — same |
| Trickle / float after full | Continuous | Stopped by retrofit BMS |
| Low-voltage cutoff | ~21V (~50% usable) | ~18V (~80% usable) |
Catch 1: the trickle charge after full
Once full, a lead-acid inverter holds the battery on a continuous trickle/float current. Right for lead-acid, wrong for LFP. A retrofit BMS cuts the trickle off once full, keeping the cells healthy while the inverter still sees a normal battery.
Catch 2 (the big one): waking up after low-voltage cutoff
When the pack drains to its 18V cutoff, the BMS disconnects to protect the cells. A generic lithium BMS then goes idle and stops sensing — so when the grid returns it never wakes, and the inverter cannot recharge it. A retrofit BMS keeps sensing and auto-reconnects the instant mains returns.
Catch 3: the inverter's start-up surge
An inverter cold-starts on the battery, not the grid. At first switch-on it pulls a large inrush from the 22V pack to charge its capacitors and start — nothing to do with the load. A tubular battery gives it freely; a generic lithium BMS blocks it, so the inverter never starts. A retrofit BMS sets a high enough surge limit to let it start. Full surge-current explainer →
What the retrofit BMS actually does
For a 22.4V pack the BMS is the brain that makes a normal inverter and lithium work together. It:
- Cuts off the trickle/float charge at 25.2V once full.
- Keeps sensing after the 18V cutoff and auto-reconnects when mains returns.
- Allows the inverter's start-up inrush so it powers on.
- Balances every cell and protects against overload, short circuit and over-temperature.
- Works with any brand of 22V inverter / UPS.
Two indicator quirks to expect (and explain to customers)
Because an inverter’s front-panel LEDs are calibrated for lead-acid voltages, two normal effects are often reported as “faults” after a lithium retrofit. Both are simply the BMS doing its job at the correct lithium voltages:
- The “fully charged” light may not come on. The BMS stops charging once the LFP pack reaches its full (high-cut) voltage — which happens before the inverter sees the higher voltage its lead-acid “charged” LED expects. The battery is full; the inverter just never reaches its old trigger point, so the charged light stays off.
- The “low battery” light or alarm may not come on. The BMS disconnects at the LFP low-voltage cutoff, which it reaches before the inverter’s lead-acid “low battery” threshold. So the inverter switches off (because the BMS opened the circuit) without first flashing the low-battery warning.
Neither is a fault — the BMS protects the cells at lithium voltages, not the lead-acid ones the LEDs were tuned for. Setting the inverter to lithium mode (where available) or using a Type 3 (RS485/CAN) BMS restores accurate charged and low-battery indication. It is worth telling customers these two points up front so they are not mistaken for a fault.
What a 22V retrofit costs in 2026 (and the 10-year maths)
A 22V LFP retrofit pack (22.4V, ~2 kWh) costs about ₹25,000–40,000 (indicative) in 2026 — more upfront than tubular, but tubular lasts only 3–5 years and must be bought again and again, while one LFP pack runs 8–12+ years at 4,000+ cycles. Over a decade the lithium pack is the cheaper path, and (per the C20-vs-C1 point above) it delivers more real backup than the bigger tubular bank it replaces.
| Over 10 years | What you buy | Approx spend |
|---|---|---|
| Tubular replacement cycle | an equivalent lead-acid bank, replaced ~3–4× | 2–3× the lithium spend + maintenance |
| LFP retrofit (one purchase) | 22.4V pack, 4,000+ cycles | ₹25,000–40,000 (indicative) |
Per usable cycle, lithium works out far cheaper — a tubular fades after a few hundred cycles, an LFP pack delivers thousands. Full breakdown in our lithium vs lead-acid price guide.
Why a 100Ah lithium beats a 200Ah tubular
Tubular batteries are rated at the slow C20 rate — discharged gently over 20 hours. But an inverter discharges fast, closer to C1, and at that rate a tubular delivers far less than its label. So a 200Ah tubular gives only a fraction of its rating under inverter load. A lithium (LFP) pack delivers nearly its full capacity even at high discharge — which is why a 100Ah lithium gives more real backup than a 200Ah tubular, at half the rated Ah, far less weight and a much longer life. (Kunwer Sachdev explains the C20-vs-C1 maths in detail in his other articles.)
Choosing the right voltage
Match the pack to your inverter — never mix systems. Other guides: 12V · 24V · 48V. New to this? Start with Can you put a lithium battery in your old inverter?
Why trust this — 30+ years and patented technology
From Kunwer Sachdev, the "Inverter Man of India" — 30+ years in power backup, now mentor at Su-vastika. The retrofit BMS work is backed by patents including Indian Patent No. 436188, "System for Real-Time Monitoring of a Battery Using Battery Management System" (filed by Kunwer Sachdev), an Intelligent Battery Equalizer (No. 411360) and a Battery Charge Equalizer that works irrespective of battery type (No. 432802). See Patents & Certificates.
Go deeper: Full retrofit guide · Retrofit battery range · kunwersachdev.com · Solar Man of India
Frequently asked questions
Yes, with a retrofit 22.4V (7S) LFP pack. The inverter charges at 25.2V, matching the LFP charge voltage, so bulk charging works. A retrofit BMS handles the trickle charge, the start-up surge and the cutoff recovery.
Su-vastika 22V ESS packs use 7 cells in series (7S = 22.4V nominal, ~25.2V charge). It sits between the 12V (4S) and 24V (8S) systems.
A generic pack often will not — its BMS goes idle at cutoff. A retrofit BMS keeps sensing and auto-reconnects when mains returns, so the inverter recharges it normally.
Founder of Kunwwer.ai, mentor at Su-vastika — the "Inverter Man of India." More about Kunwer Sachdev →