Negative plate rechargeability

On the other hand. AGM separators offer little control over the oxygen transport rate or the recombination process. The arrival of too much oxygen to the negative plate could result in overheating, hindrance of the battery s ability to recharge, or even a loss of capacity. Furthermore. AGM separators exhibit low puncture resistance. 

Clearly, this is unacceptable. This is what must be dealt with, however, in properly charging VRLA products. If this thermodynamic fact is not addressed, VRLA products will fail prematurely, not due to dry-out or grid corrosion, but due to insufficient recharge of the negative plate. 

There have been a small number of reports published recently that describe the development of purpose-built batteries for RAPS systems. A VRLA battery for use in PV power systems has been described by Shiomi et al. [27]. The negative plates contain a high level of carbon, but the particle size and concentration of the additive are not described. The recommended level of carbon is simply given as ten times normal levels . The type of VRLA battery used in the experiments is also unclear. Batteries with and without the additional carbon were operated under simulated PV duty for extended periods. Increasing the carbon by ten-fold was found to extend the cycle-life of the batteries from 400 to 1000 cycles. This improvement was attributed to the formation of a conductive network of carbon around the peripheries of lead sulfate crystals. The subsequent increase in conductivity was claimed to improve the rechargeability of the negative plates and, thereby, to suppress the accumulation of lead sulfate. 

Figure 9.21 Nickel-cadmium rechargeable battery, (a) Cover, (b) positive contact, (c) resealable vent mechanism (in the event of a pressure buUd up), (d) positive tab welded to positive contact, (e) insulating seal ring, (f) negative plate, (g) separator, (h) positive plate, (i) negative tab, and (j) nickel-plated steel case. (Reproduced with permission from Ref. [25], 1985, WUey-VCH.)...

A further example of a rechargeable battery is the nickel-iron cell. In the discharged state the negative plate of this cell is iron with hydrated ferrous oxide, and the positive plate is nickel with hydrated nickel oxide. When charged, the ferrous oxide is reduced to iron, and the nickel oxide is oxidized to a hydrated peroxide. The cell reaction may thus be represented by... 

The positive plate in a lead-acid cell accepts a charge less efficiently than the negative plate. Therefore, O2 and H2 are evolved non-stoichiometrically during the recharge of a lead-acid battery with O2 evolution occurring... 

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