Battery Information

Battery Information

Battery Information

Battery Types

Type Electrolyte Plate         Discharge
Auto Liquid Electrolyte Lead/Antimony Plate High Self-discharge       
Auto Liquid Electrolyte Lead/Calcium Plate Maintenance free         
GEL Gelled Electrolyte Lead/Calcium/Tin Plate Lowest Self-discharge    
AGM Absorbed Electrolyte Lead/Calcium/Tin Plate Low Self-discharge    
  • Available capacity is only about 80% of rated capacity

  • Limit discharges to 50% of their rated Amp-hour capacity

  • Limit loads to 25% of battery capacity (25A load for 100AH battery)

  • Shallow discharge is <20% for best battery life and deep discharge is >50% which will shorten the life of the battery. 50% is the best balance.

  • Full Charge is 12.6V to 12.8V  Open Circuit

  • Fully Discharged is 10.5V      Open Circuit

  • Limit charging current to 20% of battery capacity (20A for 100AH Battery)

  • Charge to 14.4 Volts until current falls to 2% of AH rate  68 Deg F

  • Float charge (13.2V Flooded cell) (13.5V AGM/Gell cell)    68 Deg F

Charging AGM Batteries

Charge the AGM battery at 14.4 Volts (68 deg F) with a max current of 20% of the Amp Hour rating on the battery. When the current in Amps is reduced to only 2% of the Amp Hour Rating, the battery is almost fully charged.

To complete the charge, lower the voltage to 13.5 to 13.8 (68 deg F) and float charge the battery. A 100AH battery will slowly drop to less than 0.100A. Some of the 75 Amp Hour batteries will drop to only 10 to 20 MA.

NOTE: To compensate for battery temperature not at 20 deg C (68 F), subtract 0.005 V/cell for each 1 deg C above 20 deg C; add 0.005 V/cell for each 1 deg C under 20 deg C. Apply the compensation to both the charge voltage and the float voltage

Temperature Charge Voltage Range Float Voltage Range
30 C  86 F 14.10 - 14.28 12.90 - 13.50
25 C  77 F 14.25 - 14.43 13.05 - 13.65
20 C  68 F 14.40 - 14.58 13.20 - 13.80
10 C  50 F 14.70 - 14.88 13.50 - 14.10
0 C  32 F 15.00 - 15.18 13.80 - 14.40

The Battery Charge Process

A 12 volt battery is fully charged when the voltage is about 14.4 Volts and current through the battery has declined to less than 2% of the capacity of the battery in Amp-hours ...2 Amps for a 100 Ah battery @ 68 deg F.

The Bulk Charge Step

When a charge source is first applied to a well discharged battery, charge current begins to flow, typically at the maximum rate of the charge source. If a true 40 Amp charger is connected to an 8D battery which is completely discharged, about 40 Amps of charge current would flow for some period of time. Because most of the charge is delivered at the maximum charger rate, the first step of the charge cycle is called the bulk charge step. NOTE: During the bulk step, battery voltage will steadily rise.

The Start of the Absorption Step

At the instant battery voltage has risen to the maximum allowable voltage of the charge source, current through the battery begins to decline. This simultaneous event of reaching maximum voltage and the start of current decline marks the beginning of the absorption step.

For instance, if the 40 Amp charger is set to 14.4 Volts, then when battery voltage has risen to 14.4 Volts, the charger will now hold the voltage constant. Current through the battery will begin to decline. NOTE: The charger, (or alternator), is not limiting the current at this point. The battery is "absorbing" all it can at the voltage setpoint.

The End of the Absorption Step

The absorption step should continue until current through the battery declines to about 2% of battery capacity in Amp-hours as mentioned above. Without knowing what the current is through the battery, you can't know when it's full. Just because that fancy charger, (or inverter/charger), has kicked out to float is no sign that the battery is full ...there is no charger on the market that measures battery current!

It's a given, then, that you need to measure battery current to know when the battery is full. With a battery current meter, you can discover some very interesting details about the charge process. For instance, you can discover that once the charger voltage limit is reached, battery current begins to decline. If the current decline is rapid, either the batteries are nearly full, or they are NO GOOD! If the current decline is slow, then either the charge source has more output than the batteries can reasonably absorb, or the batteries are NO GOOD! Here's where Amp-hour instrumentation is particularly valuable.

Given enough time at the absorption voltage, charge current will decline to a steady-state value, that is, a low current that either stays constant, or declines very little. At the point where charge current has gone as low as it is going to, then the batteries are truly full. While 2% of Ah rating is close, good batteries will reach a steady state current at less than 1% of Ah rating.

The Float Step

Once a battery is full, a lower voltage should be applied that will maintain the full charge. Depending on the type of battery, (liquid, gel), and the age of the battery, 13.2 - 13.5 Volts is appropriate as a float voltage.

Temperature Compensation

The voltage given above are good only at 68F, (20C). For high temperatures, voltage will be less. It is important to charge batteries with temperature compensation. To compensate for battery temperature not at 20 deg C (68 F), subtract 0.005 V/cell for each 1 deg C above 20 deg C; add 0.005 V/cell for each 1 deg C under 20 deg C.

The Overnight Test

One way to evaluate battery health is to fully charge the batteries and then disconnect them so that you know there is no way they can be discharged by sneak loads. After a resting period of 24-hours, measure the voltage across the terminals with a good digital voltmeter. If the batteries aren't holding 12.6 Volts, (12.8 for gel cells), then they are in poor health.

To determine the State of Charge for a Deep Cycle Battery with disconnected load after 6 to 24 Hrs.

To determine the State of Charge for a Deep Cycle Battery use O.C.V. Percent

Charge Electrolyte Voltage
100 1.277 12.73
90 1.258 12.62
80 1.238 12.50
70 1.217 12.37
60 1.195 12.24
50 1.172 12.10
40 1.148 11.96
30 1.124 11.81
20 1.098 11.66
10 1.073 11.51

Battery Life for batteries used in deep cycle mode.  Battery life is much longer if discharge is less than 30%.

Type CPS
Flooded cells 200 CPS
Gell Cell 1100 CPS
AGM Cells 600 CPS

Killing Batteries

There are many ways to kill batteries ...even very expensive batteries. Below are a few ways to ...NOT!!.. treat batteries.

  1. Overcharge the battery by applying a voltage above 13.8 Volts for extended period
  2. Undercharge the battery by never charging it beyond 13.8 Volts.
  3. Discharge the battery and leave it that way for a few days or weeks.
  4. Let the battery sit unattended without charging for 3 weeks or longer.
  5. Repeatedly discharge the battery beyond the optimum 50%.
  6. Slosh the battery around when it is deeply discharged.
  7. Boil enough electrolyte from the battery that the plates are exposed to air.
  8. Periodically add more acid, or impure water.
  9. Sock the battery with a high output alternator that produces more than 40% of the Ah capacity of the battery.
  10. Mount the battery where it regularly gets above  90F.
  11. Charge it hot until you can't even touch the case anymore.
  12. Use a big inverter on a small battery and run the inverter until it cuts out from low voltage.
  13. Freeze the battery in a discharged state.
  14. Use a starter battery in a deep cycle application.

73's Roy WA4BDS