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The application of Appropriate Technology

Charge State

There are two main methods for determining the state of charge for lead-acid batteries:

  • Terminal Voltage – The open circuit voltage (no current flowing) of a fully charged cell depends on its type but will be 2.1V to 2.3V (12.6V to 13.8V for a 12V battery). If the voltage is measured with the charging current flowing it will be increased by the voltage drop across the internal resistance. If discharging the measured voltage will drop due to the internal resistance of the cell. Table 1 gives the approximate battery and cell voltages for various states of charge.
  • Specific Gravity – This is the recommended method if the battery is not sealed and a hydrometer can get into the battery. For a flood-type battery in good condition the specific gravity should vary in the region of 1.25 for a fully charged battery to 1.17 for a fully discharged battery. These figures vary slightly depending on the battery type and the temperature: 0.0007 should be added to these values for each degree above 15°C. Table 2 gives the specific gravity values for several lead-acid batteries.
Table 1: The approximate battery and cell voltages for various states of charge.
State of Charge (approx.) 12 Volt Battery Volts per Cell
100% 12.70 2.12
90% 12.50 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.90 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0% 10.50 1.75
Table 2: The approximate specific gravity values for several lead-acid batteries in various states of charge. * SG = specific gravity at 25°C. ** OCV open circuit voltage per 2V cell.
State of Charge (approx) Apex Suncycle PVStar
100% 1.277 2.12 1.240 2.0866 1.225 2.0950
90% 1.258 2.10 1.230 2.077 1.216 2.0775
80% 1.238 2.08 1.220 2.067 1.207 2.0600
70% 1.217 2.06 1.210 2.058 1.198 2.0425
60% 1.195 2.04 1.200 2.048 1.189 2.0250
50% 1.172 2.02 1.190 2.040 1.179 2.0075
40% 1.148 2.00 1.180 2.031 1.171 1.9900
30% 1.124 1.98 1.170 2.022 1.163 1.9725
20% 1.098 1.95 1.160 2.013 1.153 1.9550
10% 1.073 1.93 1.150 2.005 1.145 1.9375
0% 1.048 1.91 1.140 1.996 1.135 1.9200


The charging voltage must be higher than the battery voltage for current to flow into the battery. There are two basic ways to charge a lead-acid battery from an uninterrupted supply (e.g. mains or a generator):

  • Constant-voltage charge – A constant voltage is applied across the battery terminals. As the voltage of the battery increases the charging current tapers off. This method requires simple equipment but it not recommended.
  • Constant-current charge – An adjustable voltage source or a variable resistor maintains a constant current flows into the battery. Thus requires a sophisticated charge controller.

From uninterrupted power supplies lead-acid batteries are normally recharged using the constant-current technique; the manufacturer’s data should be checked to find an appropriate charging rate. A common rule of thumb used to calculate a suitable charging current is that it should be one tenth of the ampere-hour capacity at the 10 hour rate; i.e. 6A for a 60Ah battery at the 10 hour rate. Another estimation of a safe charging current is the “C/8” rate which is the capacity at the 20 hour rate divided by 8, although Trojan batteries recommend 10 to 13% of the 20 hour rate. Gelled cells should not be charged with more than the 5% of their Ah capacity. Note that you should take into account the ampere-hour capacity of the whole battery bank (see the ‘Battery Bank’ section below).

Trickle or Float Charge

Lead-acid batteries can be maintained over long periods of time by replacing charge lost via self discharge. To do this a continual trickle charge current is maintained across the battery terminals. Typically the current is very small, being the value in milliamperes which equals the ampere-hour capacity (at the 10 hour rate) for cells up to 100 Ah i.e. 60mA for a 60Ah battery. For batteries above 100Ah the following equation can be used:

Trickle Charge Current in milliamperes = [70 + (3 x 10 hour capacity)]

The voltage maintained across the battery during trickle charging should not be higher than about 2.25V per cell (13.5V for a 12V battery). Self-discharge will be reduced by keeping the batteries clean and free of dust, particularly between the terminals.

Solar Chargers

When lead-acid batteries are charged from a variable source, such as PV panels, three charging stages are normally provided by the charge controller:

  • Bulk Charge – Current is sent to the batteries of the maximum safe rate they will accept until their voltage rises to about 80 to 90% of their fully charged value. The bulk charging voltage is typically about about 14.8V but may be as high as 15.5V for a 12V system, this may vary so that the maximum possible current in maintained. Gel batteries often have lower recommended voltages in the region of 13.8 to 14.1V..
  • Absorption Charge – The voltage remains constant, typically about 14.2V for a 12V system(depending on temperature) and the current tapers off as the battery reaches 100% charge.
  • Trickle or Float Charge – For a 12V battery bank a voltage of about of about 12.8 to 13.2V is maintained across the batteries to keep them in good condition.

Some charge controllers have pulse width modulation (PWM) which can be used to provide the last bit of charge and maintain a trickle charge. Rather than letting the current taper off a larger current is pulsed into the battery, the length of the pulses reduces as less charge is required.

Equalisation Charging

(vented liquid electrolyte batteries only)

For a bank of batteries to work efficiently they should all have the same voltage at any given time, similarly for a single multi-cell battery all of the cells should have the same voltage at any given time. However, due to slight irregularities from battery to battery constant charging and discharging leads to an imbalance in the specific gravity of individual battery cells. Also, during use the electrolyte may become stratified so that the electrolyte is more concentrated at the bottom of the cell than the top.

These problems can be rectified by applying an equalisation charge that will return all of the cells to the same voltage and eliminate irregularities in the electrolyte concentration. The voltage used during a equalisation charge is normally 1V higher than the bulk charge voltage for 12V systems and 2V higher for 24V systems, although this may be significantly higher at cooler temperatures.

Equalisation charges are normally necessary about once every month for batteries in frequent use and is most effective on fully charged batteries. The equalisation voltage is normally maintained for about two hours. After equalisation the batteries should be checked to see if they need topping up with distilled water since electrolyte may be lost as gas during this process.