That Cap Fits Me Right Fine

[Jetaway]

Folks,

Car is 1972 240Z with stock alternator and voltage regulator (mechanical).

Yesterday morning I knew the only hope I had of finding a parking spot was in the semi-reserved faculty lot on the top of a ghastly, horribly obsolete, parking garage. Since this involved four complete circumnavigations of the active parking areas with 100s of stressed -out hormonally charged neophyte drivers, I decided to turn on my headlights for safety.

It worked, and worked well as when I returned to my car 11 hours later it was dent-free. Not enough juice left in the battery to light up the LED dome light, but at least it didn't get a dent.

Got a jump and headed home. As expected, the ammeter pegged out at the right side for the first couple miles, then settled down to a position indicating, if the gauge is linear, charging at about 15 amp hours. (Normal position given the electrical load is one pointer width on the positive side.) So far, so good. It stayed in that position all the way home, 70 miles, with the RPMs hanging right around 3000. The ammeter was in the same, or damn close to, position when I got home. I had hoped for a return to the more normal level, but, hell, I had drained that battery pretty good.

This morning, about 11 hours later, I start up the car and start on some errands. The ammeter went all the way to +45 then quickly settled down to its normal position at idle. However, once the engine gets above 1500 rpm or so, the pointer swings to the right and stays up in the 30 -- 45 amp hour range until a return to idle.

A couple of questions:

Theoretically, if I had a battery rated at 400 amp / hours and charged it at a rate of 10 amps / hour, it would take 40 hours for a full charge. True?

I can understand why even an hour of freeway driving wouldn't be enough to fully charge a depleted battery, but why would the charging fall to the 10-15 amp/hour range last night but increase to 30 - 40 amp/hours this morning?

Did I do something bad, really bad, to the battery by draining damn near all the charge out of it, and then recharging it by driving?

Chris

[EScanlon]

Charge your battery with a proper battery charger. The alternator will recharge the battery from the nominal amounts used to start, run, and operate the car. When you discharge the battery steadily and to the extent you did (11 hours of headlight drain), the alternator will constantly be trying to catch up to a stored amperage it can't meet quickly (it uses the voltage/amperage capacity from the battery to in turn generate more). It WILL do it, eventually, but only if you're driving every day for an extended period of time.

Charge the battery and you'll essentially "refill" the battery.

FWIW

E

[Jetaway]

I think I can borrow a charger from somebody. If not, I guess I'll have to come up with an excuse for a long drive in the Z. Not that briar patch!

Thanks.

Chris

[LeonV]

Buy a charger, I think I got my Sears plug-in charger for $50 or so. It has modes for starting, and different rates of charging, including a deep-cycle. You can probably get them cheaper but my unit has been very useful.

[ajmcforester]

I'd charge the battery you can even bring it in to an auto part store most will charge a battery. A large discharge like this can be hard on the alternator and regulators and can cause them and the battery to fail or have a shorter life. Charging the battery will help. Also a battery tender for $35 is a good investment they help make things last longer, also if you drain the battery low they will slowly recharge the battery.

[240260280z]

As a rule of thumb; lead-acid batteries should not be drained beyond 1/2 charge if one wants them to last for a long time.

Car batteries are designed (plates sizes, spacing, and geometry) for short burst of high current output not long periods of low current output. Which what leaving one's lights on does.

In charging a battery, there are also electrochemical related inefficiencies and heat losses,

Good read: http://en.wikipedia.org/wiki/Lead–acid_battery

Edited September 2, 2011 by Blue

[240260280z]

From the wiki link:

Fast and slow charge and discharge

The capacity of a lead-acid battery is not a fixed quantity but varies according to how quickly it is discharged. An empirical relationship exists between discharge rate and capacity, known as Peukert's law.

When a battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by diffusion of these chemicals throughout the volume of the active material.

If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start the car.

On the other hand, if the battery is given a slow charge, which takes longer, then the battery will become more fully charged. During a slow charge the interface charge has time to redistribute to the volume of the electrodes and electrolyte, while being replenished by the charger. The battery voltage remains below the charger voltage throughout this process allowing charge to flow into the battery.

Similarly, if a battery is subject to a fast discharge (such as starting a car, a current draw of more than 100 amps) for a few minutes, it will appear to go dead, exhibiting reduced voltage and power. However, it may have only lost its interface charge. If the discharge is halted for a few minutes the battery may resume normal operation at the appropriate voltage and power for its state of discharge. On the other hand, if a battery is subject to a slow, deep discharge (such as leaving the car lights on, a current draw of less than 7 amps) for hours, then any observed reduction in battery performance is likely permanent.

[FastWoman]

Another vote for a trickle charger (battery tender)! This is the tortoise approach to charging your battery, but it will definitely get you there.

Regarding your battery, a starting battery has very porous, sponge-like plates. That gives it more surface area and therefore more capacity for high current. When you substantially discharge the battery, your spongy lead erodes away until it crumbles. When you recharge the battery, much of your original plate material will still be sitting at the bottom of your battery, and your plates will be smaller. Thus your battery is permanently impaired.

[Jetaway]

Thanks all,

Blue's Wiki link explains, or at least I think it explains, why after charging at an ammeter estimated 7-10 amps for an hour, the next morning it would shoot up to a charge level in the mid 20 amp range. The fast charging of the dead battery (the gauge was pretty much pegged to the right) for the first 10 - 15 minutes saturated the interface and led to the drop to a more moderate level of charge (the 7-10 amps). After sitting a night, the charge in the interface diffused, and the battery could take a higher rate of charge.

No charger located, but I drove the Z for an hour and a quarter, hour and a half, at around 2500 rpm, twice today. A country drive rather than a commute. Charging rate seems normal now, but the general direction of the posts is with my somewhat pessimistic sentiment. I did things I shall not name by name to the pooch of a battery. Is there a test that an auto parts store would do that could be fairly definitive in a pronouncement of battery death in the short term (say three months) or should I just stay attuned to the usual clues of a dying battery?

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<=== Put my head here.

Anyone remember ASCII art?

Chris

[Jetaway]

+

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<=== Put my head here.

Anyone remember ASCII art?

Chris

Now I'm really confused because my composition was correct but when it posted all the spaces were cleared out, resulting in, well, something that made even less sense, but when I quoted for this reply, it is correct again.

Oh, technology!

This will probably end up striped of spaces again. But hell, I 'm curious.

Chris

[EScanlon]

Harbor Freight sells an inexpensive charger (regular list under $35) that will give you both trickle, charge and engine start. I found it to be equivalent to the higher priced Sears Craftsman model at $40.

Personally, I'd rather buy the Sears model (USA and all that) BUT for something that on sale will be priced down to $25 or so, and you only use occasionaly.... Sadly, my pocketbook has to come first.

E

[ajmcforester]

You know if you dump the old acid and put new acid in the battery will be nearly at full charge. Charging the car just resets the chemical reaction, the harsher the reaction the more desegregation of the metal. If you think you got 3 months till death better get your self a new battery it is one car part I never like to push because as it starts to ware out it puts strain on other parts. I actually test on my cars the cranking amps every 3 months, and in cold moths monthly. I got in the habit of this because I'd pop the top off and check the plates if they were good and the amps went down I'd replace the acid, if worn out I'd replace the plates also.