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Battery + Regulator

Optimizing Battery Performance

Background

Lead-acid storage batteries store energy by the conversion of lead sulphate into sulfuric acid per the following reversible chemical reaction:

PbO2 + Pb + 2H2SO4 <====> 2PbSO4 + 2H2O
Charge Discharge

First, looking at the discharge side of the equation, it is apparent that a fully discharged battery contains no sulfuric acid in its electrolyte, only water (sp. gr. 1.00). Thus, a discharging battery is progressively more susceptible to catastrophic freeze damage, a cracked case, as the concentration of H2SO4 decreases. A fully charged battery (sp. gr. 1.26) will not freeze at -60F. Further, the concentration of H2SO4 is an indication of the state of charge of the battery and can be measured as specific gravity (sp. gr.) with a hydrometer. Also, batteries self discharge (no external current flow) at a rate of about 10% per month. Storage in a discharged condition affects the crystalline structure of the lead sulphate on the plates, rendering the reaction irreversible. Conclusion; batteries should be maintained in a fully charged state for both these reasons and the obvious maximizing of energy storage.

Next, the charge side of the equation. Modern automotive storage batteries will readily accept high charging rates, hundreds of amps, well in excess of any onboard charging capability. The critical problem is determining the point at which the reaction is complete, i.e. a fully charged battery. Charging current forced through the battery beyond this point is most destructive! This excess energy decomposes the water in the acid-rich electrolyte, producing both internal heating and the emission of a dangerously explosive mixture of hydrogen and oxygen. This loss of water further concentrates the acid solution which drastically shortens battery life. The point of full charge, commonly called the gassing point, is very much dependent upon battery temperature. Figure 1, a typical Gassing curve, shows this nonlinear temperature dependency of battery terminal voltage at full charge.

Thus, optimum battery performance, as measured by both maximum stored energy availability and maximum battery life, requires a regulated system that will charge the battery at the maximum current capacity of the alternator, up to, but never above the gassing point voltage, and then shut down until battery terminal voltage again falls below the gassing point, as determined by actual battery temperature.

Regulators

Common regulators are the old, remotely mounted, electro-mechanical relay type and the newer electronic types that are integrally mounted on the alternator. Both employ some crude temperature compensation, but this compensation senses regulator and/or alternator temperature, not battery temperature! Thus, the regulator and the battery are thermally out of phase most of the time for several reasons. First, the thermal time constant of the massive battery is orders of magnitude longer than that of the regulator's light thermal mass. Additionally, the alternator may well be supplying large and variable currents to other electrical loads, e.g. headlights, air conditioning, and fans, thus it could be running at a high temperature rise above its under the hood ambient. When these regulators are a temperatures higher than that of the battery they will tend to undercharge the battery, and vice a versa. To minimize the potential for overcharging and further protect the battery's life potential, these charging systems use a tapered charging profile, in which the battery is very slowly or never brought up to full charge and maximum energy storage, this being the lesser of the two evils, but therefore requiring a much larger and more expensive battery. Thus, these regulators are a poor compromise even in the common engine compartment configuration in which the battery is located in the same thermal environment as the regulator, but they are a mild disaster in remote battery locations situations.

A State Of The Art Regulating System

An American integrated circuit manufacturer has developed a charge regulating system that meets the above stated criteria for optimum battery performance (Analog Devices, Inc. Semiconductor Div., Automotive Sensors Group, 804 Woburn Street, Wilmington, MA 01887-3462, (617) 937-2637, Product #AD22180/22181 {BMC/ACC}). Initial interest is from a major German automobile manufacturer (you expected American auto interest?). It is a two-chip set, a sensor and a regulator, and requires no additional components.

The sensor, the Battery Monitor Circuit (BMC), a voltage comparator with a temperature dependent threshold, is packaged in a three lead, T092 plastic transistor case. Physically mounted on, and thus in thermal contact with the battery, this sensor measures both the battery terminal voltage and temperature, and outputs either a charge (high, B+), or a don't charge (low, B-) command signal based upon the Gassing Curve compensation integrated onto the chip.

The companion Alternator Control Circuit (ACC), a hybrid charge control regulator, is packaged in a four terminal metallic power device package and is intended to be physically mounted on and in thermal contact with the alternator. In response to the BMC charge/don't charge command signal, the ACC will drive the alternator to deliver full or zero output current respectively, subject to the following additional control functions designed into the ACC.

  1. Fixed maximum battery voltage limit of 14.4 volts to protect other electrical devices from over voltage at low battery temperatures.
  2. A second maximum voltage limit is controlled by a remote sense line input to the ACC. This input is intended to be taken directly from the headlamp bulb connector and thus it will limit voltage at the headlamp to 13.8 volts, a maximum value for optimum halogen lamp performance.
  3. Over temperature protection for both the ACC and the alternator. When the ACC internal temperature reaches 110C it begins to limit alternator output current linearly until shutdown is complete at 160C.

Implementation As An After-Market Upgrade

This regulator system has been easily integrated into the author's '77 Lancia Scorpion with the expected superior performance and no problems. Using adhesive transfer tape, the BMC and the three pin header connector were mounted on a 1" square aluminum sheet which, in turn was mounted, using the same adhesive, to the side of the (remotely located) battery. Using a Bosch alternator with integral regulator, the original regulator package was removed from the brush holder assembly and replaced with the ACC regulator package, the two new sense inputs being brought out to an in-line connector for the interconnect wiring. These simple mods are shown in the accompanying photograph. If this article generates sufficient interest, the author will offer this and similar upgrade kits for a variety of classic cars.

This project has been a belt tightening experience, but my battery loves the charge!


This article was originally written by Bernie Benz for and first published in the March 1992 (Issue 33) Lanciana, the journal of the American Lancia Club.

 
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Last updated: 2002-09-01