Inverter batteries need distilled water: the first top-up 6 months after purchase, then regularly. Everything you need is on this page. It is free and no login is needed.

When does my battery need water?

Find your next top-up date in seconds and save it to your phone calendar. No email or sign-up needed.

Is my battery OK?

Answer these questions by looking at your battery. You will get clear advice on what to do next.

1Is the battery swollen, very hot, leaking, or smelling of rotten eggs?
2Is the top smooth with no caps or indicators (a sealed battery)?
3Do any float indicators show low (usually red or white)?
4Is the backup time shorter than before?
5Is there white or green powder on the terminals?
6Is the battery more than 4 years old?

How to top up inverter battery water

Tubular and flat-plate inverter batteries slowly lose water while charging. If the level drops below the plates, they are damaged for good and your backup time falls. Topping up takes 15 to 20 minutes. This guide shows you exactly how to do it safely.

  • Time15 to 20 minutes
  • DifficultyEasy, with care
  • New batteryFirst top-up after 6 months
Get a free top-up reminder

Why does an inverter battery need water?

The liquid inside a tubular or flat-plate battery is electrolyte: sulphuric acid mixed with water. Every time the battery charges, a little of the water splits into hydrogen and oxygen gas and escapes through the vent caps, and some also evaporates in hot weather. The acid does not escape.

So over time the level drops and the remaining acid gets stronger. Plates that are no longer covered dry out and harden, and too-strong acid corrodes them. Both permanently reduce backup time. Adding pure distilled water brings the level and strength back to normal. You only ever replace the water, never the acid.

Does my battery need water?

Needs water

  • Tubular / Tall tubular
  • Flat plate
  • Has removable vent caps or float indicators on top

Never add water

  • SMF, VRLA, "sealed" or "maintenance-free" batteries
  • Lithium-ion batteries
  • Smooth top with no caps. Never try to open these.

Signs it is time to check the water:

  • Float indicators show LOW (often red or white)
  • Backup time is shorter than before
  • It has been 6 months since purchase (new battery) or since the last top-up
  • Long power cuts, heavy daily use or hot summer months, when water is used up faster

Know your battery

Parts of an inverter battery (cut-away view) + − Vent caps (one per cell) Positive terminal (+) MAX level MIN level Plates: keep covered Float indicator Negative terminal (−) Electrolyte (dilute acid) Cut-away view. Never open or cut a real battery case.
Inside each cell, the lead plates sit in electrolyte. The level must stay between MIN and MAX: above the plates, below the top.

Know your vent caps

Inverter batteries use one of three kinds of cap. Open them by hand only.

Screw capTurn anticlockwise to open, clockwise to close. Hand-tight is enough.
Float indicator capCommon on tubular batteries. Hold the cap body, not the thin rod, and lift it straight out. Pour water through the opening, then push the cap back in.
Strip capOne strip covers 3 cells. Lift gently from one end, then press it back until every plug clicks in.

Reading the water level

Float indicator

FULLLOW
LowAdd water now.
FULLLOW
MiddleOK for now. Check again next month.
FULLLOW
FullStop pouring.

Indicator colours differ between brands (for example red or white for low, green for full). Check the label on your battery.

Inside the cell

MAXMIN
Too lowPlate tops are dry. Exposed plates harden (sulphate) and lose capacity for good.
MAXMIN
CorrectPlates covered, level between MIN and MAX.
MAXMIN
OverfilledAcid bubbles out while charging, corrodes the terminals and floor, and weakens the battery.

Use the right water

Use only

  • Distilled water or de-mineralised (DM) battery water
  • From a sealed can labelled "battery water" or "distilled water"
  • Kept closed and clean between uses

Never use

  • Tap, well or boiled water
  • RO / drinking water or bottled mineral water: these still contain minerals
  • Rain water, AC drain water or ice
  • Acid: only a technician should ever adjust acid

What you need

  • Distilled / battery water (a 5-litre can lasts several top-ups)
  • Rubber or nitrile gloves
  • Safety glasses
  • Clean plastic funnel or battery-filler bottle
  • Torch
  • Two clean cloths
  • Baking soda, old toothbrush and petroleum jelly (for terminals)

Buy distilled water and battery accessories

Step by step

  1. 1Put on your safety gear

    • Wear rubber or nitrile gloves and safety glasses. Battery electrolyte is dilute sulphuric acid and burns skin, eyes and clothes.
    • Take off your watch, rings and metal bracelets. Metal touching both terminals can cause a short circuit and serious burns.
    • No smoking, matches, lighters or sparks nearby. Batteries give off hydrogen gas, which can explode.

  2. 2Switch off the inverter and mains

    • Turn the inverter off with its power button, then switch off and unplug it from the switchboard (or switch off the MCB that feeds it). Make sure the battery is not getting power from any source.
    • You do not need to remove the battery cables to add water. Leave them connected.

  3. 3Top up after a full charge, then wait 30 minutes

    • The best time is right after the battery is fully charged. The liquid rises while charging, so filling a discharged battery to MAX makes it overflow later.
    • After switching off, wait about 30 minutes so the battery cools and the gas clears.

    Exception: if you can see the plates, add just enough water to cover them, charge fully, and then top up to MAX.

  4. 4Wipe the top of the battery

    • Clean dust and dirt off the lid with a dry cloth before opening anything. Dirt that falls into a cell shortens the battery's life.

  5. 5Open the vent caps

    • Open the caps by hand (see "Know your vent caps" above). Do not force them with a screwdriver.
    • Put the caps upside down on a clean cloth, in order, so dirt does not stick to them.
    • Keep your face away from the openings.

  6. 6Check the level in every cell

    • Shine a torch into each opening, or read the float indicator. A 12 V battery has 6 cells, and each one must be checked separately.
    • The liquid must always cover the plates and should sit between the MIN and MAX marks.
    • Never dip a metal object into a cell to check the level.

  7. 7Add distilled water slowly

    • Use a small plastic funnel or a battery-filler bottle with a nozzle. Pour slowly into one cell at a time.
    • Stop when the float indicator reaches FULL or the liquid reaches the MAX mark. Never fill to the brim.
    • Tip: pour a little, then put the float indicator back in the hole to check the level. Repeat in small amounts until it shows green.
    • Use only distilled or de-mineralised battery water. Never add acid, tap water or RO water.

  8. 8Close the caps and wipe up

    • Put every cap back firmly in its own opening, then check that none are loose.
    • Clean the lid and the indicator windows with a soft cotton cloth so you can read them easily next time.
    • Wipe any drops off the lid with a damp cloth. For acid spills, wipe with a cloth dipped in water mixed with a spoon of baking soda, then with clean water. Keep all caps closed while doing this.

  9. 9Check the terminals

    • White or green powder on the terminals is corrosion. With the caps closed, scrub it off using an old toothbrush dipped in baking-soda water, then dry.
    • Apply a thin layer of petroleum jelly to stop it coming back, and make sure the cable clamps are tight.
    • Never touch both terminals at the same time with a metal tool.

  10. 10Switch on and record the top-up

    • Switch on the mains and the inverter and let the battery charge normally.
    • Tap "I have topped it up" in your reminder email, or in My Account, so we schedule your next reminder automatically.

How often to top up

BatteryNormal useCheck sooner if
New battery (any type)First top-up 6 months after purchaseDaily long power cuts
Flat plateAbout every 3 months (90 days)Summer, heavy use, battery older than 3 years
TubularAbout every 6 months (180 days)Summer, heavy use, battery older than 3 years
Tall tubularAbout every 7 months (210 days)Summer, heavy use, battery older than 3 years

A quick look at the float indicators once a month is a good habit. Our free reminder works out your exact dates from your battery type and usage.

Where to keep the battery

  • In an airy place, never inside a closed cupboard
  • Away from direct sunlight, the kitchen stove and water leaks
  • On a battery trolley or stand, with some space around it
  • Out of reach of children and pets

Problems and what to do

What you seeLikely reasonWhat to do
Water finishes much faster than beforeOvercharging, high room temperature or an ageing batteryGet the inverter charging voltage checked by a technician
One cell is always lower than the othersWeak cell or small leakBook a check-up; the cell may need testing
White or green powder on terminalsCorrosion from acid fumes or spillsClean as in step 9 and apply petroleum jelly
Backup still low after topping upPlates damaged earlier, or battery near end of lifeAsk for a battery health test
Swollen case, very hot, hissing, rotten-egg smell or leaking liquidSerious internal faultDo not open or top up. Switch off, open windows, keep people away and call a technician now

Frequently asked questions

Can I use RO or drinking water?

No. RO and bottled water still contain dissolved minerals that coat the plates. Use only distilled or de-mineralised battery water.

I overfilled the battery. What now?

Close the caps and wipe the lid dry. Do not try to pour the liquid out. Ask a technician to remove the extra safely before the next full charge.

Should I add acid when the level is low?

Never. The acid stays in the battery; only water is lost. Adding acid damages the battery. Only a technician should adjust acid after testing it.

Can I top up while the inverter is running?

No. Switch off the inverter and mains first, and wait about 30 minutes.

How much water will it take?

It depends on the battery size and how long it has been. Add slowly until the indicator shows full. Usually only part of a 5-litre can is needed.

My battery has no caps. Does it need water?

No. It is a sealed (SMF / VRLA) or lithium battery. Never try to open it.

Read more

Prefer a technician to do it?

Our technician will top up every cell with distilled water, test each cell, clean and grease the terminals, and check your inverter's charging.

Book home service Set up free reminders

ELCMART, 2 Kalibari Road, Khowai, Tripura - 799202

This guide is general advice for lead-acid inverter batteries. Always follow the instructions from your battery maker. If you are unsure, ask a trained technician.