A customer with a 10-year-old inverter and two tubular batteries asks whether to "go lithium" with the new solar system. Another wants the cheapest backup for a shop in a town with four hours of power cuts. A third has a flat with no balcony and no place to put a battery that gives off gas. The right chemistry is different for each. This lesson compares what is on the Indian market and how to choose.
The chemistries you will meet
Flooded tubular lead-acid
The familiar "inverter battery". Positive plates are tubes of lead oxide; the electrolyte is liquid sulphuric acid. Strengths: low purchase price, tolerant of rough charging, recyclable through a mature network, available in every town. Weaknesses: needs distilled water topping every few months, gives off hydrogen while charging (needs ventilation), heavy (a 150 Ah, 12 V battery weighs roughly 50–60 kg with acid), about 50% usable DoD for daily cycling, and life falls quickly in heat.
Stationary tubular batteries are covered by Indian standards such as IS 1651 (vented stationary cells and batteries).
Sealed lead-acid: VRLA, SMF and gel
Valve-regulated lead-acid (often sold as "SMF", sealed maintenance-free) uses absorbed glass mat or gelled electrolyte. No topping is needed and gassing is much lower, so they suit indoor UPS rooms. But they are more sensitive to heat and overcharging than flooded tubular batteries, and in daily solar cycling in Indian summers their life is often disappointing. Gel types handle deep cycling better than AGM, at a higher price. IS 15549 covers stationary VRLA batteries.
Lithium iron phosphate (LFP, LiFePO₄)
The default lithium chemistry for home storage. Cell voltage 3.2 V, so a 16-cell pack is 51.2 V. Strengths: 80–90% usable DoD, thousands of cycles, 92–96% round-trip efficiency, no maintenance, no gassing in normal use, about a quarter to a fifth of the weight of lead-acid for the same usable energy, and much better thermal stability than other lithium chemistries. Weaknesses: higher purchase price, needs a battery management system (BMS), and should not be charged below about 0 °C (rarely an issue in India except in the hills).
Nickel manganese cobalt (NMC) and similar
Common in phones, laptops and many EVs because of their high energy density. For a home on the ground floor, weight and size matter less than safety and life. NMC cells reach thermal runaway at a lower temperature than LFP and release more energy when they do. For stationary home storage in Indian heat, LFP is the safer choice, and most home storage products sold in India now use it.
Newer options
- Sodium-ion: no lithium or cobalt, works well in cold and heat, but lower energy density. It is beginning to appear commercially; treat early products with care and ask for test certificates and real warranty terms.
- Flow batteries (vanadium and others): for commercial and utility projects, not homes.
- Second-life EV batteries: cheap but variable; only from suppliers who test, re-certify and warranty them.
Side-by-side comparison
| Factor | Flooded tubular | VRLA / SMF | LFP |
|---|---|---|---|
| Usable DoD (daily) | ~50% | ~50% | 80–90% |
| Typical cycle life | 1,200–1,500 at 80% DoD | often lower in heat | 4,000–6,000 at 80–90% DoD |
| Round-trip efficiency | 75–85% | 80–85% | 92–96% |
| Maintenance | Water topping | None | None |
| Gassing / ventilation | Yes, hydrogen | Low | None in normal use |
| Heat tolerance | Moderate | Poor | Good (still keep cool) |
| Weight per usable kWh | Very high | High | Low |
| Purchase price per kWh | Low | Low–medium | High |
| Cost per kWh delivered | Medium–high | High | Low |
Figures are typical ranges from manufacturer datasheets; always check the specific product.
Weights are approximate and vary by maker; tubular weight includes electrolyte.
Lithium needs a BMS, and the BMS must talk to the inverter
Every LFP battery has a battery management system. It measures each cell's voltage and temperature, balances the cells, and disconnects the battery if anything goes out of limits. A lithium battery without a BMS should never be installed.
Two ways the battery and inverter work together: - Closed loop: the BMS talks to the inverter over CAN or RS485 and tells it the allowed charge and discharge current and the true state of charge. This is the best arrangement. Check that the battery is on the inverter maker's compatibility list. - Open loop: the inverter charges by voltage settings alone, as it would a lead-acid battery. It works, but state of charge is less accurate and the BMS may cut off abruptly, causing a sudden blackout of the backup loads.
Retrofitting LFP onto an old lead-acid inverter is often possible if the inverter has a "lithium" or user-defined charge profile, but get the settings from both makers.
Choosing for the customer
- Backup only, a few outages a month, tight budget: flooded tubular lead-acid is still sensible. It mostly sits fully charged and its low price wins.
- Daily cycling for ToD savings or long daily cuts: LFP. Its lower cost per kWh delivered and higher efficiency pay back the higher price.
- Flat, office or no ventilated space: LFP, or VRLA if budget forces it, but warn about heat and life.
- Very hot store rooms (rooftop cabins, tin sheds): move the battery, whatever the chemistry. If you cannot, LFP tolerates heat better, but its warranty may still exclude sustained high temperatures; read it.
- Existing tubular batteries in good condition: keep them on the old inverter for now; do not mix old lead-acid and new lithium on one DC bus.
Key points
- Flooded tubular lead-acid is cheap and robust but needs topping, ventilation and space, and gives only about 50% usable DoD.
- VRLA/SMF avoids topping but suffers in Indian heat.
- LFP gives 80–90% usable DoD, thousands of cycles and high efficiency, and is the safest lithium chemistry for homes.
- Lithium batteries need a BMS; closed-loop communication with a compatible inverter is best.
- Match the chemistry to the duty: occasional backup vs daily cycling.
Check yourself
- Why is LFP preferred over NMC for home storage?
- A customer has 3 outages a month. Which chemistry would you suggest on a tight budget, and why?
- What does closed-loop communication between battery and inverter give you?
Answers
- LFP is more thermally stable (higher temperature before thermal runaway, less energy released) and usually has longer cycle life; weight and size matter less in a home.
- Flooded tubular lead-acid: it mostly sits on float, so its shorter cycle life matters little and the low purchase price wins.
- The inverter gets the real state of charge and the allowed charge/discharge currents from the BMS, so charging is optimal and shutdowns are controlled.