What is a traction battery?
In the second in his interactive series Dr Mike explains what features are required to make 2V lead acid traction battery cells for fork lift trucks. He also gives valuable advice on how to choose the best fork lift truck battery for your particular requirements.
What is a traction battery?
According to European standard IEC 60254 – 1 lead acid traction batteries are used as power sources for electric propulsion in applications which include road vehicles, locomotives, industrial trucks and mechanical handling equipment. The batteries can be made of 2 volt cells, or 4, 6, 8 and 12V monoblocs (Fig.1). There is no stipulation on the internal construction of the traction batteries but the external dimensions are defined in standards such as IEC 60254 – 2. The capacity is measured over a defined discharge test from fully charged to 1.7 volts per cell over a period of 5 hours (C5 test).
Traction batteries are comprised of both flooded and VRLA designs, in both 2 volt battery and monobloc constructions. In these designs the positive plates can be both flat plate and tubular designs. For the AGM variant of the VRLA construction, only flat plate versions are suitable due to the requirement of maintaining a uniform compression of the glass fibre mat used for the separator. Tubular battery with positive plate constructions generally give a higher cycle life than the flat plate designs. The tube construction (Fig 2) ensures that the positive active material is held firmly against the conducting lead alloy spine during the deep discharge cycles. The life of the traction battery is defined by the number of standard deep charge-discharge cycles which it can perform until it drops to 80% of the rated or nominal capacity.
The design of traction batteries is critical to providing a long and trouble-free operation in service. In order to achieve this, there are several key aspects of the cell or batteries’ construction which ensure that they are able to stand up to the demands of the cycle duty. The key components are the positive grid alloy, the active material chemistry and the method of separation and plate support. The deep discharge duty requires the battery to be recharged over a long period at a high voltage. This oxidises the positive spine which causes grid growth and eventual failure as the positive conductor becomes completely converted to PbO2. The alloy therefore should be corrosion resistant and also strong enough to resist creep growth. Microtex traction batteries use special alloys which have been developed over decades to give maximum corrosion and creep resistance for their tubular positive plates.
Similarly, other factors such as the active material structure and density are of vital importance in providing the capacity and cycle life required of lead acid traction batteries. The tubular positive plates are dry-filled with a unique lead oxide powder which again has been developed by Microtex over years of experience and laboratory testing. The processes also ensure that the correct, deep cycle form of lead dioxide (alpha PbO2) is formed in the positive tubes. Alongside this, the physical construction of the multitube and the internal support provide a space which collects material shed from the plates during battery cycling. This is important as capacity reduction and failure can occur from short circuit damage due to the shed active material creating a conducting bridge between the plates as the battery ages.
Figure 2 Construction of a 2V lead acid cell for forklift trucks
|Negative Grid||Low SB Lead Alloys - Pb/Ca/Sn/Al alloy||Standard flooded 2v traction cells - VRLA, Gel & low maintenance battery|
|Positive Spine grid||Low Sb lead alloy - Pb/Ca/Sn/Al alloys||Standard flooded 2v traction cells - VRLA, Gel & low maintenance battery|
|Positive active material||PbO2 dry filled 3.6 - 3.8 gms/cc||All types of tubular lead acid 2v cells & batteries|
|Negative active material||Spongy Lead 4.4 gms/cc||All types of lead acid 2v tubular cells & battery|
|Gauntlet||Woven & Non woven - Polyester, PET/PBT/PP||2v batteries & cells - lead acid batteries|
|Separator||Polyethylene, Microporous rubber & PVC||All types of tubular battery, including TGel maintenance free cells|
|Top strap lead alloy||Low SB lead alloy - lead / 2-4% Sn alloy||Flooded 2v cells & monoblocs, VRLA 2v cells & monoblocs|
|Electrolyte|| 1.29 + - 0.1SG H2So4 liquid |
1.29 + - 0.1SG H2So4 Gel/AGM
| Standard flooded 2v cells |
VRLA 2v cells & monoblocs
|Vent cap or vent plug|| Polypropylene open top plugs|
Sealed valve regulated vent plugs
| Standard flooded 2v cells|
Sealed maintenance free batteries 2v cells & monobloc batteries
So far, we have looked at flooded, 2v battery cells. Due to the nature of their charging and operation, this design invariably requires regular topping up with water. 2v vrla battery designs, either the AGM or GEL variant avoid the maintenance required for topping up the battery. This is important if maintenance standards are poor or expensive due to high distilled water or labour costs. However, there is a shorter cycle life associated with the maintenance free designs, the lowest cycle life being the AGM flat plate construction. As a rule of thumb, a 2-volt battery tubular flooded cell will give around 1600 80% DOD cycles at 25’C. The GEL and AGM designs will give around 1200 and 800 cycles respectively. For this reason, Microtex recommend that the tubular flooded and GEL variants should be used for traction battery and electric forklift truck applications
2. How to choose a lead acid traction battery for your battery operated forklift trucks. The majority of traction batteries used in the fork lift truck market are 2-volt cells, of which over 90% are the flooded tubular plate design. These are generally used for pallet and fork lift trucks in multiples of 6 to give 12, 24, 36, etc. up to 60 volt packs with 80 volts breaking the series progression and forming the upper limit for most truck manufacturers. There are standard battery container sizes for trucks from different countries based on their national standards. The majority of trucks in India will use a DIN or BS standard size of cell. This determines the external dimensions, pole arrangement and expected capacity (fig. 3).
Forklift trucks have battery containers which are standard sizes based on multiples of the appropriate cell dimensions. These sizes are also regulated and fig. 3 shows cell and container sizes expected for BS and DIN standards. Considerations when choosing a suitable battery go beyond simply choosing the right capacity, which of course is critical. Other factors which influence battery choice include:
- The make and size of truck
- Length of operation
- Maintenance resources
They will take the details necessary to calculate the size, capacity and type of battery that will fulfil all of your technical and economic requirements. Why take the risk of doing it yourself? Trust in the experience and knowledge of the Microtex technical team and let them do the hard work.
Dr Mike McDonagh CTO Microtex Energy