Refrigerated and Frozen Foods Magazine has published an article in which extravagant claims made by TPPL (Thin Plate Pure Lead) batteries (a type of lead-acid battery) are busted by a battery industry expert and user. Read the full text by Nigel Calder below.

In Summary

  • Thin Plate Pure Lead (TPPL) batteries are a variant of Absorbed Glass Mat (AGM) batteries.
  • TPPL batteries perform in a very similar fashion to any other AGM battery, notably requiring regular extended charges at declining charge acceptance rates to bring the batteries to a full state of charge in order to hold sulfation at bay.
  • Compared to most other AGM batteries – at any given state of charge (SoC) – TPPL batteries have the potential to absorb somewhat more charging current, but this is a relatively modest shift in the charge acceptance rate (CAR).
  • Although the time to a full charge can be reduced, the final part of the charge cycle, which is necessary to hold sulfation at bay, will still take an extended period of time at steadily reducing charge acceptance rates.
  • When compared to most lithium-ion chemistries, typically double the nominal capacity is required to achieve the same effective capacity, which translates into two to four times the weight and volume.
  • As with any lead-acid battery, if operated in a permanent partial state of charge TPPL batteries steadily lose capacity, Lithium-ion batteries, by comparison, do not.
  • Although the efficiency at converting charging current into usable battery capacity is higher in TPPL batteries than with most other lead-acid varieties, it is still significantly less than that of lithium-ion.
  • In high-rate charge and discharge situations, the increased inefficiency of TPPL batteries translates into internal heat which reduces battery life expectancy. Even when operated optimally, the cycle life is several times less than that of most lithium-ion systems. Depending on the charging source, the necessity to have a regular extended charge cycle can dramatically drive up the kilowatt-hour ‘throughput’ cost of TPPL as compared to that of lithium-ion.

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First, it should be noted that my experience with thin plate pure lead batteries (TPPL) is limited to Enersys SBS and Odyssey batteries. I have not worked with Northstar and other brands. However, my experience with the Enersys batteries is extensive, and stretches over a decade, including a considerable amount of aggressive ‘real world’ testing.

TPPL batteries are a variant of AGM technology. But whereas AGM batteries (and all other conventional lead-acid batteries) have cast lead plate grids, which conduct current into and out of the battery, and into which the active material is pasted, the TPPL batteries have plates stamped out of a roll of pure lead.

In order to make cast plates strong enough to withstand the physical stresses over time, they have to be relatively thick (a typical AGM plate is 2-4 mm thick) and must contain additives, such as calcium or antimony, to strengthen the lead. The thicker a plate, the longer it takes for the current to percolate into and out of inner plate areas during charges and discharges, while the alloying of the lead in the plate grids results in a certain amount of internal resistance that translates to heat under high recharge and discharge rates. If this heat exceeds a certain threshold, battery plates buckle and short, and other damage occurs. In short, the relatively thick, high resistance plates limit discharge and recharge rates.

The Enersys TPPL plates are stamped out of a 1 mm thick roll of 99.99% pure lead with a very low internal resistance. 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 units. 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 TPPL technology is this ability to achieve a relatively high cycling capability with thin plates. The thin plates maximize surface area which is what enables significantly faster charging than with traditional deep-cycle lead-acid (PbA) batteries. The thin plates also enable 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 versions, 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 (TPPL is ~85% efficient as compared to as low as 60% efficient). In fast charge and discharge applications, this translates into significantly less internal heat generation, which is important in terms of life expectancy. When stored for long periods of time, the pure lead plate grid structure reduces self-discharge rates.

These beneficial characteristics of TPPL batteries have led to some extravagant performance claims*. In particular:

  1. The batteries can be charged at up to the 6C rate (six times their rated capacity). EnerSys has a graph showing that with an initial charge rate of three times a battery’s rated capacity, from a fully discharged state these batteries can be 100% charged in 30 minutes (Enersys Publication No: US-ODY-TM-001 – April 2011). In my experience (based on a considerable amount of testing), in the real world and in a fully discharged state, the batteries may absorb the 3C rate for a limited period of time, but this charge acceptance rate declines to the 1C rate by around 60% to 70% state of charge. Thereafter, although the charge acceptance rate is higher than with traditional deep cycle batteries, it follows the same curve as with any other lead-acid model. To fully recharge these batteries from a fully discharged state, even with unlimited charging sources, is going to take two to three hours. In many marine applications, the batteries do not receive this full charge cycle on a regular basis (i.e. are operated in a partial state of charge)[1].
  2. The batteries are resistant to sulfation. This is at best only partially true. When operated in a partial state of charge, they perform in a very similar fashion to other AGM batteries (of which they are a subset), which is to say that as the partial state of charge cycles accumulate there is a progressive loss of capacity. This capacity can frequently be recovered with a controlled overcharge, but for this to be successful the voltage has to be driven to high levels (sometimes almost to 3v/cell, or 17.8 volts with a 12v battery) at low current levels (3%-5% of a battery’s rated capacity). This requires specialized charging equipment and careful attention. During a high-voltage capacity-recovery charge, some venting of electrolyte will occur. In general, these batteries are manufactured with surplus electrolyte and so they can handle a limited number of these ‘conditioning’ cycles, but over time the electrolyte will dry out and the battery will fail. In a partial state of charge situation, to avoid capacity loss the batteries should be brought to a full state of charge with an extended charge cycle every week, which is not practical in many marine applications.
  3. Up to 80% of the rated capacity is usable for hundreds of cycles. This may be true in the laboratory, but it presupposes a full recharge after each discharge which, as noted above, requires an extended charge cycle. In the marine world, the full recharge often does not occur, in which case the usable capacity is reduced and then, because of the lack of a full recharge, additionally reduces over time. In many applications, the usable capacity is limited to around 50%.
  4. The batteries will support rapid discharges and recharges. This is true up to the 1C level, although on the recharge side as noted above not beyond 60%-70% state of charge and on the discharge side, Peukert’s equation applies as with any other lead-acid unit (although the exponent may be a little different) which is to say the higher the rate of discharge, the less the capacity that can be withdrawn before the voltage crashes. At a 1C discharge rate, the effective capacity is greatly reduced. The other thing to note is that although TPPL is ~85% efficient, at high discharge and recharge rates a considerable amount of internal heat is generated which will reduce battery life if not adequately managed.
  5. If brought to a full state of charge, the batteries can be stored for months without additional charging. This is true, because of the low self-discharge rates.

Fundamentally, TPPL 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 models – i.e. nominally ~400 cycles to an 80% depth of discharge, but this presupposes a full recharge after each cycle. Although the usable capacity is nominally 80% for the rated cycle life, in the ‘real world’ this is typically considerably reduced, especially in high-rate discharge applications. They are around 85% efficient, with the other 15% of input energy converted to heat, which creates thermal management issues in high-rate charging and discharging applications.

By comparison, most lithium-ion chemistries will accept charge rates up to 1C (and sometimes multiples of 1C) almost to 100% state of charge, enabling them to be fully charged in an hour (although it should be noted that some manufacturers recommend charge rates as low as 0.3C). The batteries will support high rate discharges with very little voltage sag – i.e. Peukert’s equation does not apply 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. They will sustain regular discharges to as low as 10%-20% remaining capacity, so it is realistic to think in terms of 60%-80% usable capacity at each cycle pretty much for the life of the unit. 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 one. 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 one. The ‘real world’ cycle life of lithium-ion batteries varies depending on the chemistry, duty cycles, and many other factors but is typically at least several times that of a TPPL unit.

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 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 lithium-ion (typically, around 95%), which reduces the cost of the energy source needed to charge the unit (if the energy source is an engine-driven alternator or generator being operated primarily for charging purposes, over the life of the battery this reduced cost of charging energy will make lithium-ion several times cheaper than TPPL). These comments do, however, presuppose that the capabilities of the lithium-ion battery are fully utilized, which is often not the case.

Author’s bio.

Nigel Calder is best known for his ‘Boatowner’s Mechanical and Electrical Manual’ and ‘Marine Diesel Engines’, both considered definitive works. Nigel was the Technical Director of the European Union’s Hybrid Marine (HyMar) research project into the applicability of automotive hybrid technologies to marine applications. He has been a longtime member of the American Boat and Yacht Council’s Electrical Project Technical Committee (PTC), which writes the electrical standards for recreational boats in the USA.

[1] Odyssey Battery Technical Manual seventh edition