Lithium-ion batteries are the new kid – motive-power wise – and they are beginning to gain traction in distribution centre operations. The increased focus on speed in omni-channel fulfillment is in part driving the new interest in the technology, which promises faster charging, longer work times, better longevity and lighter weight. All this comes at a cost, of course, and we were curious to see who is making it work.
The researcher: New technology – Thin-plate versus li-ion
Thin plate pure lead batteries (TPPL) are a variant of Absorbed Glass Mat (AGM) batteries. Unlike AGM batteries, however, that use thicker cast lead plates, the TPPL cells use thin, pure lead stampings that are as thin as one millimetre.
The combination of ultra-thin, densely packed plate grids with low resistance reduces the time it takes for current to percolate into and out of inner plate areas while also reducing the heating effect. As a result, the batteries will support higher discharge and recharge rates than conventional batteries. High recharge rates can be sustained up to higher states of charge, reducing the time it takes to get to a full charge. In spite of the thin plates, the batteries have a relatively high cycling capability.
The core benefit of the TPPL technology is this ability to achieve a relatively high cycling capability with thin plates. The thin plates maximize plate surface area, which enables significantly faster charging than with traditional deep-cycle lead-acid (PbA) batteries.
The thin plates also allow the batteries to maintain relatively high voltages under high rate discharges. In situations where charging times are limited, for a given amount of charging time the ability to absorb a relatively high charge rate to relatively high states of charge enables the batteries to be brought to a higher state of charge than with traditional lead-acid batteries, and this in turn reduces the amount of sulfation.
The TPPL batteries are also more efficient at converting charging current into usable battery capacity than traditional wet-cell PbA batteries (the TPPL batteries are 85 percent efficient as compared to as low as 60 percent efficient). In fast charge and discharge applications this translates into significantly less internal heat generation, which is important in terms of battery life expectancy. When stored for long periods of time, the pure lead plate grid structure reduces self-discharge rates.
Fundamentally, a TPPL battery is a high-performance AGM battery. Relative to a traditional wet-cell deep-cycle battery, it has a significantly higher charge acceptance rate, albeit with a similar charge curve (i.e. a rapidly diminishing charge acceptance rate as a full state of charge is approached). If not fully recharged with an extended charge cycle on a regular basis it will suffer a progressive loss of capacity. This capacity is frequently recoverable on a limited number of occasions, but only with specialized equipment capable of delivering a controlled overcharge.
The cycle life is similar to that of other ‘high-end’ AGM batteries – i.e. nominally ~400 cycles to an 80 percent depth of discharge, but this presupposes a full recharge after each cycle. Although the usable capacity is nominally 80 percent for the rated cycle life, in the ‘real world’ this is typically considerably reduced, especially in high-rate discharge applications. The batteries are around 85 percent efficient, with the other 15 percent of input energy converted to heat, which creates thermal management issues in high-rate charging and discharging applications.
The li-ion advantage
In contrast, most lithium-ion chemistries will accept charge rates up to 1C (‘1C’ being the rated capacity, and sometimes multiples of 1C) almost to 100 percent state of charge, enabling batteries to be fully charged in an hour (although it should be noted that some battery manufacturers recommend charge rates as low as 0.3C). The batteries will support high rate discharges with very little voltage sag, which means the effective capacity is very close to the nominal capacity.
These batteries can be operated indefinitely in a partial state of charge, and in fact the life expectancy is often improved by operating in a partial state of charge. The batteries will sustain regular discharges to as low as 10 to 20 percent of remaining capacity, so it is realistic to think in terms of 60 to 80 percent usable capacity at each cycle pretty much for the life of the battery.
Given the greater usable capacity, it is reasonable to think of a lithium-ion battery of a given nominal capacity having at least twice the usable capacity of a TPPL battery with a similar nominal capacity.
For the same effective capacity, the lithium-ion battery will weigh between a half and a quarter as much as the TPPL battery. Depending on the added bulk of the battery management system (BMS) and other components, for the same effective capacity the lithium-ion battery will likely have around half the volume of the TPPL battery. The ‘real world’ cycle life of lithium-ion batteries varies depending on chemistry, duty cycles and many other factors but is typically at least several times that of a TPPL battery.
If you calculate the total number of kilowatt-hours (kWh) of energy that can be charged into, and discharged from, a given battery before it fails, and divide this into the cost of the battery to determine a ‘kWh throughput’ cost, because of the higher cycle life of lithium-ion batteries and the greater effective capacity, even at today’s prices, over the life of a battery lithium-ion is cheaper than TPPL, and in many cases considerably cheaper.
This is without taking into account the improved efficiency of the lithium-ion battery (typically, around 95 percent), which reduces the cost of the energy source needed to charge the battery (if the energy source is an engine-driven alternator or generator being operated primarily for battery charging purposes, over the life of the battery this reduced cost of battery-charging energy will make the lithium-ion battery several times cheaper than the TPPL). These comments do, however, presuppose that the capabilities of the lithium-ion battery are fully utilized, which is often not the case.
Nigel Calder is an author and was the Technical Director of the European Union’s Hybrid Marine (HyMar) research project into the applicability of automotive hybrid technologies to marine applications.