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Serious designs to improve voltage quality in automobile inevitably have to focus on the cyclical surge in power demand particularly from ignition. Such surge is barely met by most alternator-battery designs at rapid rpm changes and as battery ages. Any effective stabilisation of voltage cyclical transcient will certainly benefit the engine at idling and acceleration, often improves FC and its overall cleanliness. For ignition transcient from 850rpm-3600rpm acceleration in 4-cyclinder induction, Cyclical trancient stabilisation has to be effective up to 120 sparks per sec for 4-stroke = 120 Hz (typical elect capacitor rated ripple frequency) = 8.4ms approx For design of voltage stabiliser, recovery at every cycle for the stabiliser must not take longer than the RC time-constant. This recovery is necessary for the capacitor to be charged up before it can be effective to stabilise the voltage in the next cycle. 60,000uF -10% +75% rated at 105deg C Design assumes C=60,000uF* 0.008ohm ESR rated at 105deg C Design assumes ESR=0.01 ohm Resistance of 1m single-core cable (better than soldered/crimpled) Design estimates Rcable=0.10 ohm Assume 1.5V volt-drop (momentarily unregulated) at 50A (600W) Estimated alternator/battery internal resistance Rsource=1.5/50 =0.03 ohm With Rtotal=0.01+0.10+0.03=0.14ohm RC Time-constant=0.14 ohm x 0.06F = 0.0084s = 8.4ms approx CAPACITANCE.............ALLOWABLE CHARGING RESISTANCE 120,000uF**.............0.069ohm 60,000uF*................0.138ohm 30,000uF***.............0.278ohm 10,000uF***.............0.833ohm * capacitance with longest time-constant for possible effective voltage stabilisation based on the above assumptions. SOME CONTRAINTS IN DESIGN OF VOLTAGE STABILISATION FOR AUTOMOBILE 1. **TOO LARGE CAPACITANCE Large capacitance will not satisfy the RC time-constant from the worst-case cable plus alternator/battery resistance eg 120,000uF would require very low 0.069ohm charging circuit resistance not easily attained without very thick wire, low resistance connecting contact, bigger alternator and battery. Once the time-constant exceeds the required period of cyclical transcient, the capacitor will not have enough time to recover the needed energy to stabilise voltage. 2. ***TOO SMALL CAPACITNACE Smaller capacitance has larger ESR and less than the minimum 20A ripple handling capability. Hence, it may not be effective enough in voltage stabilisation although it has shorter RC time-constant. 3. HIGHER RPM CONSIDERATION At higher rpm, the ignition transcient is shorter. It requires even shorter RC time-constant, hence, smaller capacitor and lower circuit resistance. Lower ripple current and lower circuit resistance becomes design constraints (however, typical street acceleration seldom exceeds 3600rpm). 4. EXPENSIVE LOW ESR 105 deg C low ESR capacitor of greater than 25,000uF are expensive (60,000uF can cost SGD50-100). The usual 100%-500% profit margin will prohibit some designers to use such capacitor eg. http://www.rssingapore.com/cgi-bin/bv/rsww...=sgie&Nr=avl:sg 5. HEAT IN ENGINE BAY Furthermore, the high temperature from the engine bay is never suitable to house electrolytic capacitor. A capacitor may be rated at 125 deg C. Its ripple current handling capability is only a fraction of its rating at elevated temperature. In addition, increase leakage current is detrimental to voltage stabilisation. Worst will be the shortening of useful life when capacitor is subjected to prolonged heat close to its max temperature. Proper thermal insulation is essential to ensure voltage stabiliser is not just useful for the first 1000hrs (12 month). 6. TOO LONG CABLE Routing of capacitor to the cooler cabin is not recommended because of the longer cable which introduces substantial resistance. Otherwise the larger RC time-constant or use of lower capacitance will not be effective for voltage stabilisation. Then, is it necessary for a good voltage stabiliser to cost $90-$900?

Use of electronics has its place in voltage regulation of DC power in modern automobile. The regulation usually handles the rms voltage pretty well. However, its ability to handle quick cyclical transcient changes depends alot on the battery response to the transcient demand under the condition of pulsating DC output from the alternator and its regulation speed. POTENTIAL WEAKNESSES OF AUTOMOBILE VOLTAGE REGULATION 1. Quite unlike typical DC power sources which have large capacitor as power reserve to stabilise cyclical transcient voltages, automobile uses storage battery as its power reserve, instead. Large capacitor is usually better in supplying very quick and sharp repeated demand of power than battery, particularly when the battery is already supplementing power to flatten the alternator's rectified pulsating output. 2. If the battery is no longer new, operating in elevated temperature or not in fully charged condition, its ability to handle cyclical transcient voltages is often compromised. 3. DC loadings simultaneously from powerful fans, external lightings and other high power systems can cripple the stability of battery voltage coping with alternator's pulsating DC, hence, affecting the effective electronic voltage regulation. 4. Under fast changing load, the electronic regulation is always slow in responding due to the lagging control of the alternator's field strength. The control circuit needs the time to respond with additional current to increase the excitor field strength facing the inductively opposing field from the loaded power generating coil. 5. Pulsating ignition can then cause significant sharp voltage dip, particularly at instances when alternator's pulsating voltage close to 0V. Extremely short interval of 1V-4V dip cannot be detected by voltmeter which may still show a healthy 13VDC-13.5VDC (rms). Note: Typical ignition primary draws rms current of 3amp-6amp. Assuming 20% duty cycle for ignition built-up, each ignition effectively draws 5xroot2 i.e 20amp-40amp peak for 20% of the time. This is an important consideration for ripple current in order to achieve effective voltage stabilisation. POSSIBLE ADVERSE EFFECT FROM INSTANCES OF QUICK CYCLICAL VOLTAGE DIP 1. This transcient dip is adverse to performance of engine electricals, particularly the ignition itself and other voltage sensitive systems. 2 Ignition energy will be inadequate or even misfires at instances of significant voltage dip. Perhaps, unstable idling and power-lacking acceleration might be the symptoms of such conditions aggrevated with max cold a/con and full headlights. Any comments?