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.
- 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.
- 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).
- 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.
- 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%.
- 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.
- 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.