If you are seeing a nominal 13.5 to 14 volts with the engine running, you are seeing totally normal automotive electrical system output voltage. You should see about 12.0 volts at the battery terminals with the engine off and no output from the alternator. Always measure the voltage on the posts of the battery, not on the clamps.
If you are seeing the nominal 13.8 volts on the battery with the engine running, and your battery is not holding a charge, there are several possibilities:
(1) your battery terminals are corroded and you are not getting a significant current flow into the battery to charge it through the high contact resistance between the battery posts and the cable clamps. If this is the cause you will have problems cranking the engine even shortly after operating the car for extended periods.
(2) If you can crank the car over after short periods with the engine off, but your battery appears to be losing charge over several days, there are three possibilities -
(a) you have a load draining the battery that you are not aware of (it could be something dumb like a vanity mirror light or glove box light that is not turning off), or
(b) something more serious like a bad diode in your alternator rectifier. If you are going to leave the car sit for more than a few hours, disconnect the battery and see if the charge stays in the battery for extended periods (obviously you will have to reconnect it prior to starting the car the next time). DO NOT disconnect the battery with the engine running, always shut the engine off first. If you remove the battery from across the alternator output with the engine running, the voltage regulator may try to drive the alternator output voltage way too high and damage it, along with other expensive stuff in the car, like the ECU.
(c) The battery itself is defective and is incapable of holding a charge.
If you know how, or have someone who can help you, use an ammeter to measure current drain from the battery with the engine off to see if there is any significant drain going on. There are various things like the radio memory circuit that may draw a few milliamps, I would be worried about anything more than about 50 milliamps continuous. Your fuel pump may cycle once in a while to maintain fuel rail pressure which could cause a current spike if it happens while you are trouble shooting.
If your battery is losing charge in the absence of any significant current draw, or if it does not hold a charge while you keep the leads to it disconnected for long periods, it is defective and may need to be replaced. Before you go for new one, be sure your battery cell electrolyte levels are high enough to keep the plates covered. If you refill the battery, it might again take and hold a charge.
If you have been running one of those battery voltage boosters, you most likely are overcharging the battery and literally boiling it dry. Those voltage boosters work on this principle: they put a solid state diode in series with the alternator voltage reference input from electrical system that puts approximately 0.7 volts of drop into the alternator voltage regulator input so the alternator if fooled into thinking the system is running at that much voltage less than actual, the alternator voltage regulator will than raise its output voltage to compensate for the "lost" 0.7 volts, and your entire electrical system, including the battery, will be subjected to a 0.7 volts higher voltage than normal. The Stage 1 is one diode in series with the sense lead, Stage 2 is two diodes in series for a sensed drop of 1.4 volts, this your raises your system voltage by 1.4 volts , and so on.
BAD BAD BAD for electronics and batteries that are designed to work at nominally 13.8 volts. Also drives the current up out of the alternator and can cause it to exceed its power ratings. If you are trying to power a big sound system the correct way is a higher current output alternator and heavy positive AND negative supply cables to the system DIRECTLY from the battery. DO NOT use the car frame as the negative side of the power to any high current application. A car frame ground path is unpredictable and unreliable across multiple pieces of structure and frame. Fuse BOTH sides of your power supply cables for a sound system right at the battery. If you are tapping power for the sound system off of wiring that serves other circuits, you will be introducing lots of voltage drop into those circuits from the added load of the sound system.
The answer for voltage sag issues is not adding more voltage, it is having less resistive losses in the supply wiring to the loads and a higher power alternator that can supply more current/power at the correct nominal 13.8 volts that the car 's electrical and electronic components are designed to work at.
Those battery boosters are bad science, anyone who thinks they provide any useful function has no real understanding of how a vehicle electrical system works. Take my word on this, I am an electrical engineer with more than 50 years of practical and professional experience working on electrical and electronic systems.
If you are really set on running specific loads at higher than the normal system voltage. there are DC to DC converters that will put out an adjustable tightly regulated DC voltage over a wide range of setpoints above and below the nominal 13.8 V of an operating electrical system over a fairly wide range of system supply voltages. These are useful for equipment that might not perform correctly at the available 12 volts from the battery with the engine off vs. 13.8 V running But they can get pretty pricey depending on the power level you need to be working at.
Originally Posted by stealthee
