Passive Balancing

Perhaps the most critical subsystem within the Li-ion RESS is the BMS design. The BMS is a complex system, including not only the main system monitoring circuit but also balancing circuits, communication, safety circuitry and multiple fuses. Considerations must be made as to whether the BMS will actively or passively balance the cells. 

Figure 7.1. Diagram of the principle behind a passive balancer (superfluous current is diverted and dissipated into resistors)...

Passive balancing is classified into two subtypes (1) resistance balancing, and (2) Zener diode balancing. 

The process of reabsorption depends on the HpophiHc—hydrophiHc balance of the molecule. Charged and ioni2ed molecules are reabsorbed slowly or not at all. Reabsorption of acidic and basic metaboHtes is pH-dependent, an important property in detoxification processes in dmg poisoning. Both passive and active carrier-mediated mechanisms contribute to tubular dmg reabsorption. The process of active tubular secretion handles a number of organic anions and cations, including uric acid, histamine, and choline. Dmg metaboHtes such as glucuronides and organic acids such as penicillin are handled by this process. 

Chromium plating from hexavalent baths is carried out with insoluble lead-lead peroxide anodes, since chromium anodes would be insoluble (passive). There are three main anode reactions oxidation of water, reoxidation of Cr ions (or more probably complex polychromate compounds) produced at the cathode and gradual thickening of the PbOj film. The anode current density must balance the reduction and reoxidation of trivalent chromium so that the concentration reaches a steady state. From time to time the PbOj film is removed as it increases electrical resistance. 

In fact, following oxygen scavenging, the balance of hydrazine then proceeds to reduce any cupric oxide to reform a protective, passivated cuprous oxide layer. This premise is, of course, dependent on effective post-boiler scavenging. 

In order to design such an efficient and effective device, one must understand the mechanisms by which drug is transported in the ocular interior. One issue debated in the literature for some time has been the relative importance of transport by passive diffusion versus that facilitated by the flow of fluid in the vitreous (see, e.g., Ref. 226). To predict the geometric distribution even at steady state of drug released from an implant or an intravitreal injection, one must appreciate which of these mechanisms is at work or, as appropriate, their relative balance. 

Equations (20) and (21) have been used to estimate the oral absorption of cefaclor, cefatrizine, and insulin. The simulated results compare favorably to the reported literature values in humans. The macroscopic mass balance approach provides a quick approximation to the fraction of dose absorbed and degraded for both passively and nonpassively absorbed drugs. 

The CAT model estimates not only the extent of drug absorption, but also the rate of drug absorption that makes it possible to couple the CAT model to pharmacokinetic models to estimate plasma concentration profiles. The CAT model has been used to estimate the rate of absorption for saturable and region-depen-dent drugs, such as cefatrizine [67], In this case, the model simultaneously considers passive diffusion, saturable absorption, GI degradation, and transit. The mass balance equation, Eq. (51), needs to be rewritten to include all these processes ... 

DM Oh, PJ Sinko, GL Amidon. Predicting oral drug absorption in humans A macroscopic mass balance approach for passive and carrier-mediated compounds. In DZ D Argenio, ed. Advanced Methods of Pharmacokinetic and Pharmacodynamic Systems Analysis. New York Plenum Press, 1990, pp 3-11. 

More simply, in the early regions of the tubule (proximal tubule and Loop of Henle), Na+ ions leave the lumen and enter the tubular epithelial cells by way of passive facilitated transport mechanisms. The diffusion of Na+ ions is coupled with organic molecules or with other ions that electrically balance the flux of these positively charged ions. In the latter regions of the tubule (distal tubule and collecting duct), Na+ ions diffuse into the epithelial cells through Na+ channels. 

---