Originally Posted By: chrome
Originally Posted By: JHZR2
A worn belt cant necessarily drop voltage - the dynamic is actually slightly different - an inability to generate to meet load can.
Look at it this way - the car has a given power draw. Forget that the guy quoted said amps - the car has some amount of load... Let's say it is 100W.
It doesnt matter if the car puts out 7.29A at 13.7V or 6.89A at 14.5V, the load on the engine is the same (within some fraction of a percent due to efficiency of the alternator). So if a belt is slipping, for a given power draw, when a transient occurs, the system cant keep up and it will be bogged down. Not because of current per se, but because the load has increased and the alternator cant keep up to the load, the battery discharges, and then after the load goes away, the whole system has to catch back up when it can. The load is presented by the DC system in the car, the load is variable and the altenrator doesnt control it, it just generates according to the load. But the generation capability is only as good as the mechanical energy that can get worked on it via the belt. If the belt slips, then as the alternator has more load (harder to turn), the belt can only impart so much energy in, and it slides on the pulley. The alternator puts out what it can, and since the battery is there, it gives the rest, but the system droops under the load.
Is it a viable scenario? Sure. Is it a viable reason why the car felt bogged down? That I cannot say.
I see what you mean. An alternator presented with transient loads will always be playing "catch up", right up to the limit of the belt's ability to transfer mechanical power to the drive pulley. A worn belt will potentially slip when the alternator loads up, and limits the ability of the alternator to "catch up" with the transient load, thus causing a drain on the battery reserve.
I agree the above is viable, but the belt must be really worn and slipping badly. I still don't see how it can cause an engine to bog or lose power though. If the load is greater than the belt power transfer capabilities, the slippage will generate noise and heat, something which will be pretty obvious.
Here are my thoughts.
Perhaps the belt slippage is the worst during the cold start and first few miles. During that time the alternator cannot catch up and the battery drains even further. However once the belt warms up sufficiently it is possible that the slippage stops or is reduced greatly. However by then perhaps the load is not constant, as JHZR2 mentioned, but it increased due to alternators inability to catch up with the load, so now the load is say 120W instead of 100W because the battery took a hit more than normal. This extra load means the alternator is even harder to turn and since the belt regained most of its grip, the car now feels more sluggish.
It is entirely possible that because the new belt eliminated any slippage, the alternator is not overloaded anymore so it creates less parasitic drag on the engine.
