When an inverter is switched on for the very first time — at installation — it starts up from the battery, not the grid. To start, it pulls a large initial surge (inrush) current from the battery to energise its internal capacitors and electronics. A tubular battery supplies that surge instantly. A generic 12.8V lithium pack can block it: its BMS treats the inrush as a fault, so the inverter never starts. This has nothing to do with the appliances you connect — it is the inverter itself starting on the battery. A retrofit BMS allows this initial surge so the inverter powers up, which is what Su-vastika and other proper retrofit LFP packs are built to do. (Su-vastika retrofit range)
"Connect a fresh inverter to a tubular battery and it powers up instantly — the battery gives the start-up surge. Connect a normal lithium pack and the BMS blocks that first inrush, so the inverter never even starts. People think the inverter is faulty. It is the BMS limit."
The inverter starts on the battery, not the grid
An inverter does not run off the grid first and charge the battery afterwards — it is the other way round: the inverter cold-starts on the battery. At first switch-on its internal capacitors are empty, so it draws a large momentary inrush current from the battery to charge them and bring up its electronics. Only after it has started does it use or pass through the grid. If the battery cannot deliver that first surge, the inverter will not start at all — even with the grid connected. This is why, on a new installation, an inverter with no battery (or a battery whose BMS blocks the surge) simply stays dead.
Why a tubular battery starts it and a lithium pack may not
A lead-acid / tubular battery has no electronic limiter, so it delivers the switch-on inrush instantly. A lithium pack is protected by its BMS, which has a peak-current limit. If the inverter’s start-up inrush exceeds that limit, a generic BMS disconnects to protect the cells — and the inverter never powers up. The cells are perfectly fine; the BMS surge limit was simply set too low for an inverter’s cold start.
How a retrofit BMS fixes it
A retrofit-grade BMS is engineered with a higher surge / peak current rating and a short-duration tolerance window, so it lets the inverter’s initial inrush pass and the inverter starts normally — then it returns to its protection duties. This is part of the same retrofit BMS design that bans the trickle charge once full, discharges to a deeper cutoff, and auto-recovers after a low-voltage cutoff. (technical retrofit guide)
What to check before you buy
Match the BMS continuous AND peak (surge) current to your inverter so it can both start and run. Ask the seller for the BMS peak current and confirm it comfortably exceeds your inverter’s switch-on inrush. It is the single number that decides whether the inverter will even start on the pack. (See the full buyer checklist in the main retrofit guide.)
The full set of BMS challenges
Start-up surge is one of four reasons a normal lithium pack fails on a normal inverter: (1) the continuous trickle / float charge, (2) the deep low-voltage cutoff, (3) waking up and recharging after that cutoff, and (4) the inverter’s start-up surge current at first switch-on. A proper retrofit BMS solves all four. Start with the overview — Can you put a lithium battery in your old inverter? — then pick your system: 12V · 24V · 48V.
Why trust this — 30+ years and patented technology
This comes from Kunwer Sachdev, the “Inverter Man of India.” He pioneered India’s first plastic-body inverter, and has spent 30+ years building the country’s power-backup industry. He now mentors Su-vastika, where this retrofit lithium technology is engineered.
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), plus 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 technical guide · Retrofit battery range · kunwersachdev.com · Solar Man of India
Frequently asked questions
At first power-on the inverter draws a start-up surge from the battery to charge its capacitors and start. A generic lithium BMS blocks that inrush, so the inverter never starts. A tubular battery supplied it freely. A retrofit BMS with a higher surge limit lets the inverter start.
From the battery. The inverter cold-starts on the battery and only then uses the grid. Without a battery that can deliver the start-up surge, the grid alone cannot start the inverter at a fresh installation.
No. The start-up surge is the inverter energising its own capacitors and electronics at switch-on. It has nothing to do with the load you attach.
No. The BMS still protects against genuine faults. A retrofit BMS simply sets the surge limit so it does not block the inverter’s normal start-up inrush.
Founder of Kunwwer.ai, and mentor at Su-vastika and several other companies — the “Inverter Man of India.” More about Kunwer Sachdev →