The Guide to Forklift Battery Charging Methods

Posted by Chris Parkinson on

Why do I need a guide?

Not so long ago setting up a charging system for a forklift battery amounted to a charger for each forklift and a battery for each shift. Operators would simply charge the battery on the forklift overnight ready for work tomorrow. Because forklifts are expensive charge stations additional ‘shift’ batteries are used in operations where the batteries didn’t last the day to ensure the forklift could stay in operation. The daily cycle also included a period of time for the battery to cool down at the end of charging.

 Battery longevity and charge frequency depends on:

  • Whether the battery has been abused, through over discharge, drying out, opportunity charging or running hot.
  • The weight of the loads and the height and distance the forklifts travel and lift to.
  • The capacity and efficiency of the forklift and battery.
  • The time spent working versus resting.

For example, a large grocery distribution centre running 24/7 can expect to change the reach truck batteries at least once a day, but a pallet mover in the same facility will run for two or three days without needing a battery change.

Operations that use a spare battery need a charging area and battery change equipment. Hoists and cranes were once the only option and the management system, if any, was ‘flat’ or ‘charged’ on boards and signs. This, of course, was labour intensive and often meant batteries were not properly rotated. Shift changing, the practise of all operators changing their battery at the end of their shift, exacerbated the problem causing lines of forklifts outside the battery room.

Over the years the process of changing batteries has significantly improved:

  • Forklift companies provide rollers in the battery compartment for safe and quick side extraction by purpose built, powered, battery changers.
  • Management systems clearly identify the best battery to take each time so the battery assets are rotated properly, avoiding opportunity charging and overheating.
  • Data from chargers and management systems provides information to ensure lean battery operations; reducing change times and inventory while also extending battery life.
  • Awareness of poor practises, like shift changing and opportunity charging, is improving and operations now run their batteries until they are flat before going on charge and then fully recharging.
  • Modern chargers are efficient and versatile and can help minimise charge time, prevent battery damage and reduce power consumption.
  • Duplicating chargers halves the number required and reduces the use of space and power required.
  • A number of manufacturers have developed specific chargers for fast and opportunity charging as alternatives to battery changing.

What’s the difference?

The difference between conventional, fast and opportunity charging comes down to the rate the battery is charged, expressed in amps per 100ah. Fast and opportunity chargers charge a battery at a higher rate than conventional charging so that batteries can be ‘topped up’ to last the day. Fast chargers have a higher starting rate than opportunity chargers.

Charging is most efficient when the battery is between 30 and 80% state of charge as acceptance is higher. Approximately 13% of battery capacity is returned in an hour of charging when the battery is between 25% and 75% discharged at the start of charging, but that drops to 9% when a battery is at 85% capacity. 

All batteries must regularly charge to the level that gassing occurs to circulate the acid, and they also need an ‘equalise’ charge to level out the cell voltages. The gassing phase in a charge starts at around 85% SOC, over this rate is also when most of the heat in the battery is generated. High temperatures kill batteries and for every 10º over 40° the battery operates at, on average, its cycle life is halved.

Conventional charger operations are designed to charge the battery to 100% SOC every time, and have a regular equalise charge set on a timer and/or a manual switch, with an inbuilt cool down period so the battery is not used at a high temperature.

With opportunity charging operators are always putting the battery on charge – regardless of the SOC, but an opportunity charger is set to stop charge to 80% SOC to avoid heat generation. The battery still needs a full charge at night and an equalise charge on a day when the forklifts are not in use. For opportunity charging periods of around an hour are best as short charges have little or no effect on battery capacity.   

Charging Method

Charger starting rate ‑ amps per 100 AH

Fast

31 - 60

Opportunity

21 – 30

Conventional

16 – 20 

 

It should be noted that battery companies have their own recommendations on start rates for their batteries. The high starting rates in fast charging mean special cables, connectors, packing and trays are required to manage the high currents and resultant temperature. Furthermore BIMs are used to identify the battery to the charger and regulate the temperature so charging stops if temperatures escalate excessively.

So what’s best for my operation?

Conventional Charging

WHY?

Using spare batteries ensures the forklift keeps working at all times and that the operation can respond to variations in the workload.

OPERATIONAL CONSIDERATIONS
Shifts

Any and all. When spare batteries are required battery management systems can optimise the number of batteries for the operation; there should be enough to ensure the batteries have a cool down after charging and not so many that they are unused for long periods. An easy rule of thumb for planning spare battery requirements is to count the changes in a 24 hour period.

Charging

When the battery is discharged the forklift returns to a central battery room, exchanges the flat battery for a fully charged battery and returns to work. In modern change systems the battery is removed with a powerful magnet or vacuum so there is no handling and the carriage is supplied with appropriate safety materials such as stops and gates so there are no risks. 

The battery is charged on a stand so it is well ventilated and exposure to ambient temperature means cooling is quicker than if it were in a forklift. Multi-level battery stands minimise the footprint of the charger area and are an efficient use of space, going 6 levels high 90 batteries can be charged in just a 15m space.

Forklift out of operation for charging

Using spare batteries the forklift is out of operation while it travels to and from the battery room plus the change time, around 2 to 5 minutes, depending on the activity in the room, the type of changer and the management systems. 

Cold and freezer storage

Additional spare batteries are required due to reduced capacity.

MANAGEMENT CONSIDERATIONS:

  • Safe work practises and risk assessments apply to the battery charging operation.
  • Managers should review discharge levels of the batteries at change to ensure maximum uptime, as well as to guard against shift changing and opportunity charging. Battery management systems provide reports to support this activity.
  • If the workload increases additional batteries may be required, again management systems can help managers identify this. Modular and adjustable racking can easily accommodate any extra batteries.

POWER CONSIDERATIONS:

Operations require standard single and three phase power points in one area, in addition, it is good practise to select a central battery charging area to minimise forklift travel time.

Fast Charging (FC) and Opportunity Charging (OC)

WHY?

Perceived value in a reduction of equipment and space by ‘top up’ charging the batteries throughout the day instead of changing the battery for a spare.    

OPERATIONAL CONSIDERATIONS

Shifts

FC: 2 shifts of 8 to 10 hours or 3 shifts of medium duty (2 to 4 hour run times per shift).

OC: Extended single shift of 10 to 12 hours, or 2 shifts with a light to medium duty.

Charge process

Requires operators to plug the forklift in to chargers at any extended opportunity such as breaks or shift changes, remote charging areas need to be located near entrances or break areas.

24 hour battery power requirement

  • FC: Cannot exceed 1.6 times the 80% rated capacity of the battery. For example; 36 volt 1000 AH x .8 = 800 AH x 1.6 = 1280 AH maximum.
  • OC: Cannot exceed 1.25 times the 80% rated capacity of the battery. For example; 36 volt 1000 AH x .8 = 800 AH x 1.25 = 1000 AH maximum.

 Forklift out of operation for charging

  • FC: Minimum of 6 hours every 24 hours for a complete charge, and once a week for a full charge, condition charge and equalize charge.
  • OC: Minimum of 12 hours every 24 hours for a complete charge and once a week for a full charge, condition charge and equalize charge.

Cold and freezer storage

Batteries operating below 25°C (77°F) lose capacity as the temperature decreases and this needs to be factored into the calculations when assessing a facility. Extra caution should be applied to cold storage where battery capacity can decrease between 20 and 60%.

MANAGEMENT CONSIDERATIONS

  • As the forklifts will be inoperable if the battery is not topped up management needs to ensure operators are plugging in the forklifts whenever possible. In existing operations with poor charging processes or a lack of maintenance the probability of issues is high. Skipping charges is not an option!
  • Capital expenditure processes for new batteries are increased as they are purchased more frequently.
  • If the comparable battery change operation is sizeable enough to warrant a battery change operative this role is not required with fast or opportunity charge, however, battery maintenance and general repairs still need to be included in the operations maintenance tasks.
  • Changes in the operation, such as increasing shifts, need to take into consideration the limitations imposed by the battery charging method.
  • Safe work practises and risk assessments apply to the battery charging operation.

POWER CONSIDERATIONS

  • Fast chargers require significantly more power than conventional chargers and the power needs to be distributed to multiple remote charging stations around the facility. Existing facilities are rarely ‘power ready’ and will usually require expensive upgrades to their power.
  • Compared with older Ferro resonant chargers new high frequency chargers will show a reduction in power cost due to improved efficiency.
  • Charging is done at peak periods throughout the day when energy costs are higher.

OTHER CONSIDERATIONS

  • FC: Batteries need to be supplied for the chargers, with terminals, cables and connectors designed to withstand the higher currents and temperatures.
  • FC: Specialised ‘BIM’s are required to communicate with the charger about the condition of the battery to reduce temperature damage, however this also restricts the power returned.
  • Independent battery monitors should be specified to provide information on battery use and abuse.

Charger splitting

Evidence of charger utilisation gained from iBOS battery management reports shows that chargers are often in use for shorter time periods than operations managers expect, rarely charging for more than 20% of the time. It is increasingly common in many forklift markets to utilise one charger for 2 battery positions, this practise is known as ‘charger splitting’. 

Charger splitting, with battery management, offers many financial and operational benefits, it:

  • Halves the cost of chargers.
  • Halves the power requirement for the battery room.
  • Maximises asset utilisation.
  • Halves the space requirement for chargers.
  • Provides the perfect operational conditions for batteries with cool down periods, asset rotation and no opportunity charging.

The practise involves having two dedicated battery positions for each charger and swapping the chargers to waiting batteries when the connected battery is charged.

The iBOS charger splitting module streamlines this process by:

  • Providing an overwatch to ensure no batteries are lost.
  • Using the ‘cool down period’ to charge the second battery.
  • Reporting on battery availability, opportunity charging, discharge levels and charger use.

Charger splitting with iBOS shows charger utilization often increases to 40% or more.

More costs: battery quantities, maintenance, service and charge v change times.

Battery life is significantly reduced during fast or opportunity charging, which means operations need to buy new batteries more often.

For example: A 750 AH battery has 600 useable amps per cycle and is guaranteed for 1500 cycles, therefore, it will supply 900,000 useable amps over its life, which is 600 amps a day for 6 years. Opportunity charging during the day replaces an extra 25% Amps.

As a budgetary guide this means with conventional charging 15 to 20% of the fleet will need to be replaced per year over the 6 years but with opportunity charging the replacement rate will be 40% or more after the first 3 years.

Furthermore, run times are reduced and maintenance increases as a battery nears the end of its life, and this is accentuated if it has experienced high temperatures, over discharging and uncontrolled opportunity charging.

Also, due to the heat generated by fast chargers the batteries need constant watering. Auto-water is often offered as the solution to this challenge but comes with many risks, such as unattended water leaks around batteries and chargers.   

When comparing labour costs for battery change times versus charge times it is also important to consider the time it takes the operator to take the forklift to the charge area, park, connect the charger, walk to the break area or entrance, return to the charging forklift, disconnect the charger and repeat the forklift safety check, and that this process occurs multiple times a day.

Fleet Rotation Systems - FRS

The patented Fleet Rotation System (FRS) combines any high rate charging methodology with the iBOS, battery management system to reduce the risks involved in not having back up batteries.  Operators drive into the charging area, plug their forklift into a spare charger and are directed to the best forklift to take based on battery charge level. This ensures batteries with the most charge are used and reduces the risk of running out of power. Furthermore, it means all the forklifts are used evenly, something that rarely happens in conventional park and charge where fully charged forklifts, are often overlooked by operators. Hot batteries also have an opportunity to cool down.

The iBOS also complements a park and charge system by providing management with information on how the chargers are being used and whether there is the right number of assets for the job.

In summary

Although there are more options for battery charging these days there is certainly no ‘magic bullet’ solution. The idea of liberating forklifts from the battery room can be appealing but it’s safe to say that for operations that go down that route the problems they are trying to eradicate just return in a different form.

It is very difficult to return to a conventional charge system from opportunity or fast charge, therefore, it is vital for operations to examine every cost and the true value of any perceived benefit, before committing. Many facilities have ideas about how their battery operations work that often turn out to be inaccurate when battery monitoring reports are reviewed.

Some of the best run operations - facilities with a solid understanding of their operational requirements, employ a hybrid system, which uses different methods of charging and changing to suit each truck type and its energy demands. Like transport networks where diesel trucks, electric buses, petrol cars and pedal power all have their place, our forklift battery operations will evolve to a place where, fast, rapid, high rate, rotation and change all co-habit.

Speaking the language

Ampere-Hour Throughput: Accumulated ampere-hour recorded during discharge and/or charge over time.

Amps/100 AH: Charger current output measured in amperes per 100Ah rating of the battery being charged.

B.I.M.: Battery Information Module, a device on the battery which tells the charger it’s specification, current capacity and temperature so the charger can adjust its’ input accordingly.

B.P.T: Batteries Per Truck, the number of batteries per forklift expressed as a ratio i.e. 1.75 BPT for 10 pallet jacks would be 18.

Gassing Voltage: Batteries gas at 2.4V/Cell at 25°, approximately 80% SOC, gassing is essential for battery longevity however it creates heat and gives off hydrogen. 

Equalise: (At least) three hours of the finish current charge after the battery reaches charge complete.  It ensures the battery cells have equal charge and SGs. This should be provided for weekly either by day of the week or cycle count.

SOC – State Of Charge 0-100% relates to the capacity available in the battery. Less than 20% is over discharged, the optimal minimum for opportunity charging is 30%.

Video Resources

https://vimeo.com/180987593 - Triple Level Power Changer Operation Video.

https://vimeo.com/180993122 - iBOS Battery Organisation System Animation.

https://vimeo.com/202875879 - FRS Fleet Rotation System Animation.

https://vimeo.com/202878815 - Attach-A-Puller Operation Video.

Sources

BCI Standards (Battery Council International)

Eternity Batteries – Opportunity charging guidelines.

Warehouse IQ November 25th 2012, Battery and Charger Guide.

ForkliftAction, November 17th 2016, A Battery Built for Cold Storage Applications.

Hawker, June 13 2016, Fast, Opportunity & Conventional Charging, V 2.0

www.phlsci.com

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