Every hour a 400-amp alternator is charging (not freewheeling) wastes almost 1 gallon of fuel. 

Alternator torque requires engines to produce one horsepower (HP) for every 25-amps of charging current produced. A 200-amp alternator requires about 8 HP, and a 400-amp alternator will require 16 HP from the engine. 

Diesel engines require, on average, .06 Gal (.21Li) or .40 lbs (.18Kgs) of fuel per hour to generate 1HP. Alternators in technology-rich vehicles will typically only charge the bank up to 90% due to the vehicle’s charge voltage regulation. The efficiency of standard alternators at medium speed is limited to 70-80% (at 77°F/25°C) by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. This efficiency reduces dramatically at higher temperatures and at high speeds, mainly due to fan resistance. Combined, even by working as hard as it can, it is almost impossible for the alternator alone to fully charge batteries to eliminate Partial State of Charge (PSOC) conditions.

Avoid Prolonged Use of added Electrical loads during “Engine-off” periods. Battery performance and service life are dramatically affected by the frequency and depth of battery discharge.

Here are some common electrical loads for popular devices used in commercial vehicle fleets to support driver health and comfort:

• Refrigerator at five amps
• Entertainment systems at four amps
• Heating/AC at 19 amps
• Sleep assist systems
• CPAP at three amps
• Interior lighting at four amps

If these additional loads are used together for a period of 6 hours while your vehicle is parked, they will collectively consume 210Ah (35-amps per hour x 6 hours = 210 Amp hours) from the battery pack.

Avoid premature battery failure by choosing the proper battery technology to match the frequency and duration of key-off or anti-idle loads.

Every cycle, the battery will only recover to “very near” its original capacity if adequately charged. Even if properly charged, a lead-acid battery incrementally loses some of its original capacity with each successive discharge event. A standard starting battery can only be regularly discharged to <3% before it experiences negative consequences. A high-capacity (high Reserve Capacity - RC, or Amp hour- Ah) heavy-duty battery with a deep-cycle design can only regularly use a maximum of 30% of its rated capacity before causing incremental and permanent damage. Discharging a starting battery to 50% or a lower depth of discharge will dramatically reduce battery life.

The battery you choose needs to have adequate specifications to support the job it's being asked to do. Your maintenance supervisors and operators should be trained in best maintenance practices to protect this investment.

Choose battery technology that includes: 

• Countermeasures against acid stratification by introducing acid mixing or immobilization technology. 
• Deep and micro-cycle capability through enhanced active material ratios, densities and alloys (less of a starting battery and more of a cycling battery). 
• Fibre dividers reinforce active material against shedding caused by acid stratification and vibration. 
• Enhanced negative plate performance through increased carbon and/or other additives. 
• Element bonding that protects against positive grid growth and vibration. 
• Anchor bonding that protects against shock and vibration. 
• Higher lead content using thicker or more plates may be sufficient to secure the increased positive and negative plate active material for increased capacity (high reserve capacity or amp hour) and cycle life while maintaining enough surface area to produce adequate cold cranking performance (CCA).

A few words about acid mixing and acid immobilization...

Acid mixing technology uses components that mix the acid to equalize the acid’s specific gravity evenly throughout the battery’s cells. Acid immobilization technology uses absorbent glass-mat material to suspend acid to slow stratification in the battery’s cell (“AGM” batteries). While the same electrochemical reactions take place, the negative consequences of acid stratification are delayed in acid mixing and AGM batteries because acid mixing technology defeats acid stratification and acid immobilization technology slows (but does not entirely stop) the stratifying effect of gravity on battery acid.

If the battery's temperature, or electrolyte in flooded types, is above 110° F (43.3° C), allow it to cool. To determine the battery's state of charge with the battery's temperature at 80° F (26.7° C), use the following tables. Table 1 assumes that a 12.65 voltage reading or a 1.265 specific gravity reading represents a fully charged battery. For other battery or electrolyte temperatures, use the Temperature Compensation tables below to adjust the Open Circuit Voltage for VRLA DRY CELL batteries or Specific Gravity readings for flooded types.

Table 6 - Electrolyte Freezing Point

Table 7 - Hydrometer Temperature Compensation

Table 8 - Voltmeter Temperature Compensation

A specific time is difficult to determine because recharging depends on so many variables:

• Depth of discharge
• Temperature
• Size and efficiency of the charger
• Age and condition of the battery

The initial charging current with a Discover battery should be 15% to 30% of the battery's C20-hour capacity rating.

It will take about 60% of the total charge time to bring a VRLA DRY CELL AGM or GEL battery from 0% SOC to 95% SOC. It will take 40% of the total charging time to put the last 10-20% of the charge back into the battery.

The charge is a quantity of electricity equal to the rate of flow (Amperes) multiplied by time (hours) and is usually expressed in Ampere-hours (Ah). Once the charger has been turned on for 1 to 2 minutes, the charge rate in amps will indicate the approximate charge time in hours. A battery with a 0% state of charge is defined as having been discharged to a point when the terminal voltage is equal to or less than 1.75 volts per cell (10.50 Volts for a 12-volt battery) measured under a steady load at the battery's 20-hour rate at 80˚F. The 20-hour rate is the battery's capacity divided by 20 hours.

Typically, the total charge (capacity of the recharge) that must be returned to a VRLA DRY CELL AGM or GEL battery to achieve a 100% state of charge is from 104% to 112% of the charge removed. For comparison purposes, the returned charge for flooded electrolyte batteries must be between 115% and 130% of the charge removed.

NOTE: Variables such as the rate of charging current, ambient temperature during the charge cycle and the control of the voltage during the charging cycle will impact the ability of the battery to be properly replenished and the ongoing performance of the battery.

How can you tell if a battery is fully charged?

The only true way to tell if a VRLA DRY CELL AGM or GEL battery is fully charged is by using a good voltmeter to determine the open circuit voltage (OCV) without any load applied to the battery. Accessible flooded-type batteries can also use a hydrometer.

Table 5 - State of Charge vs. OCV Charge

Divide the above values in half for 6-volt batteries or by six to determine cell voltage. The TRUE OCV can ONLY be measured after the battery has been removed from the charge or discharge load for 24 hours.

How can undercharge harm my Discover battery?

In many respects, undercharging is as harmful as overcharging. Keeping a battery undercharged or continually undercharging allows the positive grids to corrode, lead sulphate to build up, and plates to shed, which can dramatically shorten life. Also, an undercharged battery must work harder than a fully charged battery, which also contributes to shortened life. A constantly undercharged battery has a significantly reduced capacity because of the effects of acid stratification. It will readily be inadvertently over-discharged and eventually damaged.

How can you tell if a battery has been damaged by under or overcharging?

The only way is with a load test. Use the same procedure for VRLA DRY CELL batteries that you would use with a flooded cell battery:

1. Recharge if the open circuit voltage is below 75%.
2. If adjustable, set the load at three times the 20-hour rate.
3. Apply the load for 15 seconds. The voltage should stabilize above 9.6 volts while on load.
4. If the battery has a CCA rating, you can apply a load equal to ½ the rating for 15 seconds. The voltage should stabilize above 9.6 volts while on load. To apply a more determined test, you may apply a load equal to 100% of the rated CCA for 30 seconds or a load of 5 to 7 times the AH rating for 30 seconds. The voltage should stabilize above 7.2 volts while on load.
5. If below 9.6 volts (7.2 volts for the 100% CCA test), recharge and repeat the test. If below 9.6 volts (7.2 volts for the 100% CCA test) a second time, replace the battery.