Float charging is the quietest duty a battery can have — and quietly the easiest to get wrong. The battery sits fully charged across its DC bus for years, held by a constant voltage a whisper above its own rest voltage, waiting for the outage that justifies its existence. That whisper — a few tens of millivolts per cell — decides the battery’s corrosion rate, its water loss, its temperature and ultimately its life. This post explains how to set it, why it is a window rather than a ceiling, and what to watch while the years pass.
What float charging is
A float charger holds the battery at a constant potential slightly above its open-circuit voltage, supplying just enough current to cancel self-discharge and keep the battery at genuine full charge. Recall the pocket rule from our charging guide: OCV per cell ≈ specific gravity + 0.84. Float sits above that by a deliberate margin called polarization — float voltage minus OCV. A cell resting at 2.14 V floated at 2.21 V is polarized by 70 mV. The industry’s float-suitability test, IEC 60896-21, has the manufacturer declare its float voltage in the typical range of 2.23–2.30 V per cell and then prove, after six months on float, that the battery still delivers rated capacity. The exact figure is always the datasheet’s — what follows is why that figure is chosen so carefully.
The Goldilocks window
Intuition says lower float = gentler battery. The research says otherwise. Studies of positive-grid corrosion against plate polarization (Som & Szymborski, 1998) found a minimum-corrosion window at roughly 40–120 mV of positive-plate polarization — and the corrosion rate rises on BOTH sides of it. Float too high and you buy gassing, water loss, heat and accelerated corrosion. But float too LOW and corrosion accelerates too, while the battery drifts toward chronic undercharge and sulfation. Float is not a ceiling to stay safely below; it is a window to stay inside. Manufacturer gel-battery data makes the high side concrete: charging just 0.3 V above recommendation costs about 10% of cycle life; 0.7 V above costs 60%. The low side is slower but just as certain.
Temperature: the compensation and the staircase
The correct float voltage moves with temperature — downward as the battery warms, typically by −2 to −3 mV per °C per cell. A charger without temperature compensation floats correctly on one pleasant afternoon a year. And temperature sets the pace of ageing itself: the handbook staircase for VRLA float life runs roughly ten years at 20 °C, five at 30 °C, two-and-a-half at 40 °C — the halving-per-10 °C rule this series keeps meeting, here in its natural habitat. Buy float batteries for the room you actually have, not the room in the brochure.
| Cell temperature | Flooded (calcium) | Gel VRLA | AGM VRLA |
|---|---|---|---|
| 25 °C | ~25 | ~65 | ~150 |
| 40 °C | ~55 | ~160 | ~410 |
| 50 °C | ~90 | ~350 | ~750 |
| 60 °C | ~140 | ~600 | ~1,500 |
Read that table twice. Between 25 °C and 60 °C the flooded battery’s float current rises about six-fold — but the AGM’s rises ten-fold from a base already six times higher. Current makes heat, heat makes current: that feedback loop is the runway to the thermal runaway described in AGM vs gel, and the reason the air-conditioning in a UPS room is part of the battery system.
Why VRLA runs warmer on float
Sealed batteries float differently, and the classic analysis (Berndt) shows how: at the same 2.25 V per cell, a VRLA battery draws roughly three times the float current of its flooded twin — the oxygen-recombination cycle itself consumes current. Worse, the flooded cell exports about two-thirds of its float energy as vented gas, while the VRLA, recombining internally, converts some 95% of it to heat inside the container. Three times the current, nine-ish times the internal heat — from the same float voltage. Add the thermal-mass hierarchy (relative electrolyte volume roughly: flooded 1, gel ~0.9, AGM ~0.6) and the ranking writes itself: flooded runs coolest, gel holds its own, AGM needs the cool room. Gel owners, note your own boundary: gassing in a gel cell begins around 2.35 V per cell — another reason gel chargers carry gel settings, per the OPzV page.
Strings drift — watch the cells, not just the bus
A float charger fixes the STRING voltage; the cells then share it unevenly. Published telecom-string data (Nelson) tracked a 48 V bank floated at 2.28 V per cell: the cell-to-cell spread grew from 80 mV when new to 280 mV within about three months — some cells above 2.4 V and gassing, others below the average and undercharging — before settling; long-term studies suggest cells stabilise within roughly ±2.5% of the mean. That is normal physics, and it is why standards-based maintenance reads individual cells, not just the bus.
The abnormal case is why the habit matters. In a published experiment on a 24-cell 48 V string, cells were deliberately short-circuited so the survivors shared the fixed bus voltage. One shorted cell pushed the rest to 2.4 V — tolerable. Three pushed them to 2.63 V and the string began walking into thermal runaway. Four shorted cells put the survivors at 2.76 V each — thermal runaway within an hour, generating hydrogen sulphide. If your battery room ever smells of rotten eggs, that is not a curiosity; it is the emergency described in why batteries explode. Cell-level monitoring is not audit theatre. It is the early-warning system — the same logbook religion the nuclear battery post calls licensing evidence.
Float, trickle, boost, absorption — the vocabulary, settled
Float is continuous constant-voltage service: the battery lives connected, always full, current settling to tens of milliamps per hundred ampere-hours. Trickle is periodic maintenance charging — a small current (typically 40–100 mA per 100 Ah for shelf storage) applied to top up, after which the battery is DISCONNECTED; leaving a trickle charger on forever is slow cooking. Boost is deliberate high-rate recovery after an emergency discharge — typically up to ~2.4–2.45 V per cell for VRLA and higher for flooded types, per the datasheet, with the temperature watched throughout. Absorption is simply the constant-voltage stage of a normal CC-CV charge, where the current tapers to saturation. Four words the trade mixes daily; four different jobs.
What doesn’t float
Lithium-ion is not float-charged — its management system terminates the charge, and lithium prefers storage BELOW full charge, refreshed to about 70% every six to twelve months rather than held at 100%. A car battery can live on a well-regulated constant-potential maintainer with a current limit, but the same rule applies as everywhere in this series: the correct voltages are your battery’s datasheet values, not a universal number from the internet.
Float duty is where Microtex stationary batteries spend their lives — the substation banks that wait years for one violent minute, and the 2V AGM cells in UPS halls where float accuracy and air-conditioning decide everything. Terms live in the glossary; a float installation behaving oddly — drifting cells, rising current, a warm corner of the rack — belongs in an enquiry today rather than an incident report next year.