Can an old lead-acid battery have an usually high inrush charging current, such that the alternator belt squeals for the first 30-90 seconds after the engine is started? It is worse in hot weather.

What would be the reason for this?

Is it because the battery voltage dips too much after turning the motor over?

Or is it because the battery internal resistance is too low? This would be consistent with being influenced by temperature. On the other hand, I would expect an old battery to have higher resistance due to sulfation.

The alternator and the belt are new.

I forgot to mention. While the belt is squealing, the engine really struggles, almost to the point of dying, like there is a large load on it. I think this rules out a loose belt. Sorry I forgot to mention this important detail.

  • \$\begingroup\$ It is likely that the belt is also old, and has worn or stretched to slip under heavy load. \$\endgroup\$
    – user16324
    Commented Jul 12, 2020 at 13:10
  • \$\begingroup\$ New belt and alternator. \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 13:11
  • \$\begingroup\$ More than likely the belt is not tensioned correctly. \$\endgroup\$
    – Solar Mike
    Commented Jul 12, 2020 at 13:12
  • \$\begingroup\$ The v-belt has more tension than what I usually use. Any more and and I will be putting wear on the alternator. \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 13:15
  • \$\begingroup\$ How do you know? Has you used a tensioning gauge or is it “best guess”? And, as mechanics, we tested “best guess” against a tensioning gauge, very interesting results. \$\endgroup\$
    – Solar Mike
    Commented Jul 12, 2020 at 13:56

3 Answers 3


To answer my original question, yes it is possible for an old battery to sink large currents. I believe it is caused by plates disintegrating and pieces falling and accumulating on the bottom, eventually creating a short. But this condtion usually persists past the first few minutes of charging, so it can not be called inrush current.

Now that I have thought about it, I have come across a few batteries that drew high current from a charger and never got charged up. I have even fixed a couple of chargers that failed trying to charge such a battery.

But this was not what I had. My problem was indeed corrected by tightening the belt, as most of you have suggested. Although I don't much like how tight it is now.

I did measure the charging current (thanks to mkeith who suggested using a clamp on DC ammeter) to rule out a bad battery, and it was not excessive. In fact, it was under 10 amp.

What really baffled me was that the engine ran really rough while the squealing was happening, almost stalling at times. I thought it was because the alternator load was too large. Which would also mean that the belt cannot be too loose if it can put such a large load on the engine.

However now I believe what was happening instead, a slipping alternator belt caused unstable system power - too much voltage fluctuation. This led to unstable sensor readings to the computer. So the computer was trying to maintain the idle RPM using bad input.

An alternate theory is that the squealing alternator belt presented a rapidly changing load to an idling engine, and the computer struggled to keep up with the changes, resulting in under/over corrections.

Once I tightened the belt, the squealing and the rough performance at start up both went away.

  • \$\begingroup\$ Those ammeters come in very handy for all sorts of things. Glad you solved your problem. \$\endgroup\$
    – user57037
    Commented Jul 30, 2020 at 15:15

You're overthinking this - the belt is probably glazed or the tension is not adequate. Once the belt starts slipping it can quickly develop a slippery surface (glazed) that makes future slipping more likely even when correctly tensioned.

The alternator has current limiting so the torque required will not go above some level even if there is a heavy load on it. The belt should be able to provide the required torque.

Many cars use a multi v groove belt to minimize this issue.

  • \$\begingroup\$ Do you know what sort of current limiting the GM 12SI alternator uses? \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 16:39
  • 1
    \$\begingroup\$ @mcu - most (maybe all) car alternators use the inductance of the stator as the current limiting element. The impedance of this inductance rises with frequency. Since the voltage produced by the alternator action is proportional to alternator RPM and so is the frequency the increased voltage is canceled out by the rising impedance to give a crude current limiting without any explicit circuitry. High power alternators may use more sophisticated means to give more accurate current control. \$\endgroup\$ Commented Jul 12, 2020 at 17:03
  • \$\begingroup\$ please see my updated question. I forgot to mention an important detail. \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 18:55
  • \$\begingroup\$ @mcu - I don't think that changes anything. \$\endgroup\$ Commented Jul 12, 2020 at 19:27

No, you are following a line of reasoning that doesn't really hold up. The inrush current of any new battery will be enormous unless it is limited somehow by the charging source. But a normal car can easily deal with this. If this was an issue, then every car would have this problem every time it starts. Also, if anything an older battery would likely have less inrush current due to higher series resistance.

Also it doesn't make sense that the alternator would be capable of applying so much force to the motor that it could nearly stall it out. At least not if the motor and alternator are working correctly.

I don't know for sure what your problem is, but I absolutely don't suspect that your explanation is correct. The belt noise might not even be the alternator. Maybe it is related to the air conditioning or engine cooling system or power steering system. Maybe it is a bad bearing, not a belt.

There is a good chance that whatever is squealing is also getting hot, so you may consider looking at it with a thermal imaging camera if you have access to one. You can also spray the belts with a lubricant as a diagnostic aid (spray belts one at a time until the sound goes away).

  • \$\begingroup\$ I would have thought so too. But just yesterday in hot weather I removed the heavy wire from the alternator, and it started with no squealing. Although the car did not run very smoothly without it. \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 19:23
  • 1
    \$\begingroup\$ Nowadays there are low-cost DC clamp-on ammeters. If you really want to see what is going on, buy one and clip it around the battery cable. I am sure the charging current changes with time, but it may still give you an idea. The highest setting is 100A, but that should be enough for our purposes. \$\endgroup\$
    – user57037
    Commented Jul 12, 2020 at 19:27
  • 1
    \$\begingroup\$ This is an interesting link. bobistheoilguy.com/forums/ubbthreads.php/topics/1239410/… Apparently, a bad battery (presumably with a shorted cell) can pull large amounts of current and never taper off like it's supposed to..a weak sulfated battery won't pull enough current. \$\endgroup\$
    – mcu
    Commented Jul 12, 2020 at 20:24

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