What does depth of discharge mean and what makes it so important?

What does depth of discharge mean and what makes it so important?


Depth of discharge for truck batteries

The depth of discharge states what percentage of the charge capacity can be used without endangering the health of the battery. For conventional truck batteries in the Heavy Duty (HD) and Super Heavy Duty (SHD) categories, the recommended depth of discharge is 20%. With such a depth of discharge 80% of the stored energy remains available. With increasing depth of discharge the state of charge of the battery reduces.

Depth of discharge  State of charge
0% 100 % (fully charged)
20% 80% 
50% 50% 
80%  20% 
100% 0% (fully discharged)

Complete discharge should be avoided. Conventional truck batteries which have been developed as starter batteries cannot meet the increased demands. These changed requirements call for innovative battery solutions such as those which are offered by VARTA® truck batteries with AGM and EFB technologies.

Trucks need electricity – before, during and after the journey

Above all when on the road, batteries are subject to greater stresses than previously because drivers increasingly use the truck for overnight stays. Use of the driver's cab as a hotel involves a larger number of electrical consumers such as refrigerators, coffee machines and entertainment systems. Parking cooler and heating also require power. As well as this, use of free time has changed. Many drivers prefer to stay in their cabs rather than visiting a service station or truck stop. Here they can connect with family and friends online, watch their favourite shows on Sky or Netflix or prepare a snack in the microwave. However, basic functions such as interior lighting also contribute to the load on the battery. Although the lighting only requires between 2 and 5 amperes, at weekends this can quickly add up to 6 hours or about 20 Ah. This alone reduces the available battery capacity by about 10%. A parking cooler is even hungrier for power and drains 10 to 30 amperes from the battery. With the parking cooler switched on, the remaining electrical capacity of the battery is therefore reduced by a further 10% per hour.

Charging cycles and cycle depth

In view of the increasing number of electrical consumers, the importance of depth of discharge is growing. An adequate depth of discharge and long service life can only be achieved with the use of modern battery technology such as AGM and EFB. However, a truck battery must not only be capable of deep discharge, it must also withstand a large number of charging cycles. The charging cycle is the term for discharge and recharging of the battery. Batteries with a large number of charging cycles reduce operating costs and increase service life. The more often a truck battery can be discharged and then recharged, the more overnight stays are possible in the vehicle. With the right battery technology a large cyclic depth of discharge can be achieved in order to supply the large number of electrical consumers during break times. Truck batteries with AGM or EFB technology can supply more energy-hungry consumers over a longer period. This is not only good for operating reliability. A high level of availability of comfort functions also increases driver satisfaction.

VARTA Truck Batteries provide security

In the past, commercial vehicle starter batteries were designed to supply the starter motor, the ignition and the vehicle lights. Conventional starter batteries were able to cope with the operation of electrical consumers when the engine was switched off as long as the addition load was within limits. However, with increased demands and larger numbers of overnight stays, the electrical consumer profile changed. The previous design of starter batteries is no longer sufficient to meet the demands of modern goods transport. VARTA batteries help to reduce total costs (TCO) and assist with economical fleet operation. VARTA truck batteries reliably supply electrical consumers with energy even when the engine is switched off and provide sufficient reserves to start the engine the next day. 

Modern Truck Batteries for heavy demands

The continuous growth in the number of electrical consumers in commercial vehicles over the past decade has meant that battery replacement intervals have become shorter. As a result, fleet operating costs increased. Our engineers were quick to realise that conventional starter batteries would soon reach their limits due to the increased demands of long distance transport. In 2014 VARTA presented the Truck EFB battery with a patented mixing element. The mixing elements in the battery cells address the main cause of failure in wet batteries and ensure considerably better performance in daily use. In this way they also meet the increased demands for comfort in modern trucks. The mixing element utilises the natural movement of the acid when driving and operates entirely without wear. Water consumption is not negatively affected by the mixing element and therefore reduces maintenance to a minimum.

Efficient and high performance Truck Batteries from VARTA

Increasingly stringent environmental regulations are forcing vehicle manufacturers to make greater efforts to reduce CO² and nitrous oxide emissions. Efficient use of fuel is also becoming more important for fleet operators in order to minimise operating costs. On the other hand, parking coolers and heaters in trucks demand more energy. These comfort functions can only be reliably and securely powered by high performance batteries. Our AGM technology batteries are the ultimate level of energy storage units. VARTA is the first manufacturer of AGM truck batteries which meet the requirements of leading European vehicle manufacturers. Due to our many years of experience with AGM batteries in the automotive sector, 80% of all AGM vehicles are now delivered with a VARTA battery. With our Truck AGM battery we are now bringing this know-how into the commercial vehicle market.

AGM batteries with PowerFrame® design

When the vehicle is at a standstill and the engine is switched off, the electrical consumers can only be supplied by a high performance battery. However, the remaining capacity must be sufficient to reliably start the truck the next morning. As well as this, the battery must provide energy to the power-hungry starter – even on frosty winter mornings. This is where VARTA AGM batteries show their strength. The PowerFrame® design generates enough power for excellent starting characteristics even if the charge level is low and the depth of discharge is correspondingly high. Vehicle tests in winter at –20° C show that VARTA Truck AGM batteries enable reliable starting even with a low charge level.

What is the state of the battery?

Many truck drivers have already experienced this: The energy management system of the truck switches off comfort consumers in order to protect the battery so that the vehicle can be reliably started the next day. But how reliable does the energy management system (BMS) actually determine the state of charge (SOC) and the state of health (SOH) of the battery? The problem: With wet batteries, acid stratification often results in a false open circuit voltage. Sulfation of the plates which are responsible for storing energy in the battery is also difficult to determine precisely. Therefore, the BMS tends to switch off some components earlier rather than later. In the long term this involves a reduction of comfort for the driver.

Perfect battery management with VARTA AGM batteries

These problems do not occur with VARTA AGM batteries, because acid stratification and the associated shift in the voltage curve do not exist with this type of battery. In an AGM battery, the acid is bound in a glass fleece separator. The BMS can therefore precisely determine the operating and charging condition of the battery over the entire life of the battery and provide precise energy management. Unrestricted comfort makes drivers feel better and confident that the engine will start reliably the next time it is needed. VARTA AGM technology enables reliable and economical operation over the entire service life of the battery.