Charging 2 V AGM cells: float and cycle voltage, temperature compensation and equalisation

Fullriver HGXL 2 V AGM cells float at 2.23–2.25 V per cell and take a cycle (absorption or boost) charge at 2.30–2.35 V per cell, both figures stated at 20–25 °C. Above that temperature the charger should lower its voltage by 3 mV per °C per cell, and charge current is capped at 20 % of the cell’s C10 capacity: 40 A for an HGXL200-2, 200 A for an HGXL1000-2. Multiply the per-cell figures by the number of cells in the string and you have the settings for any bank: a 24-cell 48 V string floats at 53.52–54.00 V and cycles at 55.20–56.40 V, and at 35 °C the float setting comes down to 53.28 V. This guide shows the arithmetic for every common string, explains float against cycle duty, why VRLA cells are not equalised like flooded cells and what to do instead, how to size the charger, and what to check at commissioning.

Where the numbers come from

The HGXL is a single-cell, valve-regulated lead-acid (VRLA) design: flat pasted plates on a lead-calcium-tin grid, electrolyte held in an absorbent glass mat, no free acid, and a one-way valve that lets the cell recombine the gas it produces on charge. Fullriver publishes four charging figures for the series, a float range and a cycle range at 20–25 °C, a coefficient of −3 mV per °C per cell and a maximum charge current of 20 % of C10, and every setting in this guide is one of them multiplied by a cell count or a temperature difference. The voltages are identical from the 50 Ah cell to the 3000 Ah cell; only the current limit changes with capacity. They are windows rather than points: a standby bank in a warm plant room sits best towards the lower end of the float window, a daily-cycled solar bank towards the upper end of both, and we settle the exact point with you when we size the bank and the charger together.

Float and cycle voltage by string

Chargers are programmed in whole-string volts, so the first job is to convert the per-cell figures. The table does that for the five string lengths used in practice: 6 cells for 12 V, 12 for 24 V, 24 for 48 V, 55 for a 110 V DC plant and 110 for 220 V. Each total is the per-cell figure multiplied by the cell count; for the 48 V float lower limit, 24 × 2.23 V = 53.52 V. These are the settings at 20–25 °C, before temperature compensation.

A charger regulates the string total, so individual cells sit slightly above or below the average, which is why the per-cell checks below matter. If your charger offers a fixed “AGM” preset, compare its actual float and absorption voltages against these ranges rather than trusting the label; presets written for 12 V blocks often sit outside the HGXL window. For cell counts, wiring and kWh per cell size, see the 48 V bank guide и калькулятор аккумуляторных батарей.

Float duty and cycle duty

Float: standby, UPS and telecom

Float duty is the life of a UPS string, a telecom DC plant, a switchgear tripping battery or an emergency-lighting supply: fully charged and ready, called on rarely. The charger holds the string at float continuously, supplying only the small current that makes up self-discharge. Fullriver’s design life figure for the HGXL series, 20 years in float service at 25 °C, is a float-duty figure, and the two habits that let a bank approach it are holding the float voltage inside the 2.23–2.25 V window and compensating it for temperature. See the telecom and UPS guide for standby strings in full.

Cycle: solar and hybrid systems

A cycle bank is discharged every night and must take the energy back inside a few hours of good sun; the higher cycle voltage of 2.30–2.35 V per cell drives the charge in quickly enough to finish before dusk. A solar charge controller or inverter-charger works in stages: bulk, delivering as much current as the array allows up to the 20 % limit; absorption, holding the string at the cycle voltage while the current tapers; then a return to float once the current has fallen away or a timer has expired. Once full, float is the bank’s correct resting state, so confirm the charger returns to it rather than holding the cycle voltage on. Hybrid resort and island systems with solar, a generator and an inverter-charger follow the same logic.

Temperature compensation: the −3 mV rule, with the arithmetic

A warm lead-acid cell reaches full charge at a lower voltage than a cool one, so lowering the charger voltage as the cells warm keeps them exactly full in every season rather than a fraction over. Fullriver’s coefficient for the HGXL series is −3 mV per °C per cell, so the compensated voltage is:

V(T) per cell = V(25 °C) − 0.003 × (T − 25)

Worked through for a 48 V string from the upper float setting of 2.25 V: at 30 °C the correction is 0.003 × 5 = 0.015 V, giving 2.235 V per cell and 24 × 2.235 = 53.64 V; at 35 °C it is 0.003 × 10 = 0.030 V, giving 2.220 V per cell and 53.28 V; at 40 °C it is 0.003 × 15 = 0.045 V, giving 2.205 V per cell and 52.92 V. The table applies the same arithmetic to every string.

The table starts from 2.25 V; to work from the lower end of the window, start from 2.23 V and every figure moves down by 0.02 V per cell (0.48 V on a 48 V string). The same coefficient applies to the cycle voltage: at 35 °C the 2.30–2.35 V window becomes 2.27–2.32 V per cell, or 54.48–55.68 V on a 48 V string.

Why the sensor belongs on the battery, especially in Thailand

Battery rooms in Thailand and across South-East Asia commonly run at 30–40 °C, and a cell that has been charging through a hot afternoon is warmer than the air around it. The reliable arrangement is a sensor on a cell in the middle of the string, where it runs warmest, wired back to the charger so the correction tracks the cells themselves hour by hour, rather than the charger’s own internal temperature or a fixed room setting. The KISAE BTS-10K battery temperature sensor does exactly this with the KISAE зарядные устройства для аккумуляторов и inverter-chargers we supply with HGXL strings. Lead-acid manufacturers publish the rule of thumb that service life roughly halves for every 10 °C above 25 °C, which makes correct compensation the highest-value setting on the charger. Fit the sensor and keep the room ventilated, and a bank in a Thai plant room gets far closer to its design life; the tropical climate guide covers battery-room design in detail.

Why VRLA cells are not equalised like flooded cells, and what to do instead

An equalising charge on a flooded (OPzS or similar) cell is a deliberate overcharge: the string is raised well above its cycle voltage for several hours so that every cell gasses, the electrolyte is stirred and any lagging cell catches up. It works because a flooded cell has vent caps, free acid and a technician to top up the water the gassing drives off. A VRLA cell has none of those. Its electrolyte is held in the glass mat, so it cannot stratify and needs no stirring; its valve recombines the gas produced at normal charge voltages; and its water is sealed in for life. Holding an HGXL string at its own cycle voltage, rather than a flooded-cell equalising voltage, keeps that water in the cell and the capacity with it. That is the very reason the cells are maintenance-free, and the routine that replaces equalisation has three parts:

• Build a balanced string. One model of cell throughout, from the same production batch, installed and commissioned together. Keep inter-cell links short and of equal length, torque every connection to the value on the cell and re-check after the first weeks of service. A string that starts balanced tends to stay balanced.
• Check per-cell voltages on float. Record every cell’s voltage at commissioning once the string has settled on float, and repeat on a schedule. Cells should sit close to the string average; one that sits persistently high or low is the one to watch, and an infrared check of its terminals usually shows whether the cause is a connection. On 55- and 110-cell strings a per-cell monitor turns this from a clipboard job into an alarm.
• Refresh with a controlled cycle charge, not an equalise. If the spread grows after a deep discharge or a long spell at high temperature, a temperature-compensated charge at the cycle voltage of 2.30–2.35 V per cell, held for a limited period and then returned to float, brings the cells back into line without forcing gassing. Never programme a separate, higher equalisation voltage into a charger connected to VRLA cells; if the charger has an equalise function, disable it.

Charger sizing: the 20 % of C10 rule

The charge-current ceiling Fullriver publishes for every HGXL cell is 20 % of its C10 capacity: 10 A for the HGXL50-2, 20 A for the HGXL100-2, 40 A for the HGXL200-2, 80 A for the HGXL400-2, 120 A for the HGXL600-2, 200 A for the HGXL1000-2, 400 A for the HGXL2000-2 and 600 A for the HGXL3000-2. A series string carries the same current through every cell, so the limit depends only on the cell size, not the cell count: a 12 V string of six HGXL200-2 cells and a 48 V string of twenty-four share the same 40 A ceiling, and the voltage decides the power.

That gives a quick check on any charger or array. A 48 V string of HGXL1000-2 cells may take up to 200 A, and at the top of the cycle window (56.4 V) that is 200 × 56.4 = 11.28 kW, so an array or charger of that size can run at full output. A 48 V string of HGXL400-2 cells is limited to 80 A, about 4.5 kW, so a larger array needs the controller’s current limit set to 80 A. The 20 % figure is a ceiling, not a target: any charger below it is acceptable, and standby chargers are normally sized on recharge time after a mains failure. For solar, a bank that accepts a high charge current makes better use of a short sunny window, one argument for a larger cell rather than a second parallel string, as the 2 V versus 12 V guide explains. We quote the charger and temperature sensor with every HGXL bank.

Commissioning checks

Much of a bank’s life is decided in its first week. These are the checks we make, or ask the installer to make, when an HGXL string goes into service:

• Before connecting: confirm every cell is the same model and batch; record each cell’s open-circuit voltage and note any that stand apart; inspect cases and terminals; confirm the rack is level and the room ventilated.
• Wiring: check polarity cell by cell before the last link goes on; torque every terminal to the value on the cell; confirm cable and link sizes suit the current; confirm the string voltage equals the sum of the cells.
• Charger settings: set float and cycle voltages from the string table above; set the current limit at or below 20 % of C10; set compensation to −3 mV per °C per cell (chargers that ask for it per 12 V block take −18 mV per °C); disable any equalisation function; confirm the absorption timer returns the bank to float.
• Temperature sensor: fix it to a cell in the middle of the string and check the charger’s reading against a thermometer on the cell.
• First charge: let the string complete a full charge and settle on float, then record every cell voltage as the baseline; where a meter is available, record internal resistance against the datasheet figure (0.20 mΩ for the HGXL1000-2, for example).
• Record and label: keep the commissioning sheet with the bank and label the charger with its settings so nobody resets it to a 12 V preset later.

The full HGXL range is listed on the stationary and solar battery category, the 2 V stationary battery pillar page brings all six guides together, and the comparison guide sets out where AGM sits against OPzV, OPzS, gel and lithium.

Часто задаваемые вопросы

What is the float voltage of a 2 V AGM cell?

For Fullriver HGXL cells the float voltage is 2.23–2.25 V per cell at 20–25 °C. Above 25 °C reduce it by 3 mV per °C per cell, so at 35 °C the window is 2.20–2.22 V per cell. Multiply by the cell count for the string setting.

What voltage should a 48 V AGM bank float at?

A 48 V bank of 24 HGXL cells floats at 53.52–54.00 V at 20–25 °C. With temperature compensation from 2.25 V per cell the setting becomes 53.64 V at 30 °C, 53.28 V at 35 °C and 52.92 V at 40 °C. The cycle (absorption) setting for the same bank is 55.20–56.40 V at 20–25 °C.

Should I equalise VRLA batteries?

Not in the way flooded cells are equalised. A VRLA cell is sealed, its electrolyte cannot stratify and any water driven off by a forced overcharge cannot be replaced, so disable the equalise function on the charger. Keep the string balanced instead: same model and batch, equal links, torqued connections, and regular per-cell voltage checks on float. If cells drift, a controlled, temperature-compensated charge at the cycle voltage of 2.30–2.35 V per cell, followed by a return to float, is the correct remedy.

What is temperature compensation for battery charging?

It is the automatic adjustment of charge voltage to cell temperature. Warm cells reach full charge at a lower voltage, so the charger lowers its output as the battery warms and raises it as the battery cools. For HGXL cells the coefficient is −3 mV per °C per cell, measured by a sensor on the battery such as the KISAE BTS-10K. In a Thai battery room at 30–40 °C it is the single most valuable setting on the charger.

Can I charge AGM batteries with a normal charger?

Yes, provided its float and absorption voltages can be set inside the HGXL window for your cell count, its current can be limited to 20 % of C10, it has no equalisation stage active, and it accepts a battery temperature sensor. Many chargers meet all four conditions once programmed; a fixed-voltage charger or an automotive charger with a desulphation or equalise mode does not. Send us the charger’s specification and we will confirm.

What is the maximum charge current for a 2 V AGM cell?

For HGXL cells it is 20 % of the C10 capacity: 20 A for a 100 Ah cell, 40 A for 200 Ah, 80 A for 400 Ah, 200 A for 1000 Ah and 600 A for 3000 Ah. The limit is the same for a cell on its own or in a series string of any voltage, and any charger at or below it is suitable.

Get the string and the charger sized together

Tell us the system voltage, the daily load or standby duty, and the site, and we will size the HGXL string, the rack and links, and the charging, including the temperature sensor, and quote with lead time. The cells are genuine Fullriver, ordered from the factory for your project and supported from our stock in Trang; we deliver throughout Thailand and freight worldwide at cost. Send us the details, or work the numbers first on the калькулятор аккумуляторных батарей.