Auto Battery Recharging Time

How long should a car have to run to bring a good battery back up to full
charge after starting? Parents have a year and a half old battery in their
car. For the past 6 months or so, they've had frequent occasions when the
battery hasn't had enough power to start the car, requiring a jump. Took it
to the shop and they said the battery tests good, just needed to be charged,
so they put it on their charger for an hour. According to them the short
hops (3 to 5 miles) my parents typically make gradually run the battery down
to the point that it won't start and that the car needs to run "at least 15
minutes at highway speed" after each start to bring the battery back up to
full charge. Every round trip should be at least 30 miles? Does that sound
reasonable or do I need to have the electrical system looked at?

 

It sounds perfectly reasonable, but not necessarily possible. Frequent
short trips are hard on the engine as well as the battery. Try to
combine trips so the engine has a chance to get fully hot. A
1.5-year-old battery is almost new!

If you can't bundle trips together, resign yourself to the fact that the
engine won't last as long as it could and buy a trickle charger to leave
on the battery between trips.

 

I have the factory original battery in my '00 300m. On Nov 1, the car
(and battery) will be 9 years old.

There is a problem here. Either defective battery, or charging system,
or a current drain in the system.

In my case, 99% of the time the engine is running on my '00 300m for the
past 3 years are for 2 daily round-trips - each being 2.4 miles. Throw
in maybe 5 other round trips per week (each under 10 miles). Maybe once
a month, or once every other month, I'll do a 200 mile round-trip on the
highway.

 

There is no nice tidy number. It essentially depends on how easily the engine starts.

Thats radically overstating it. I mostly do what they do and I dont get the effect they do.

I would.

 

It depends on the car and the alternator. However, a good rule of thumb is
that you must drive a minimum of 8 miles for the alternator to add as much
charge to the battery as the battery LOST when the car was started.

But, this also assumes that the car starts fairly quickly, ON THE FIRST TRY.

In cold weather, or if you have to crank it more, or try several times....
then multiply each time you try to start by 8 miles and you begin to
understand how short trips could kill the battery QUICK.

There is probably nothing wrong with the electrical system in that car. As
someone else suggested, get a trickle charger for the battery, if your
parents continue to do nothing but short trips. Because the alternator will
never charge that battery completely on 3-5 miles. -Dave

 

Erehwon,

If the battery tests good, but is low on a charge, then the charging system
must be tested to determine if it too is good. If the alternator is good,
then I'd suspect some accessory light, etc. and measure the parasitic
current drawn when the car is at rest and everything is off. Short trips
are hard on the engine and the alternator does need to run for some amount
of time to get the battery charge back to replace the starting power that is
consumed, but the trip would need to be very short and/or the starting time
very long for this to happen with any regularity.

Bob

 

I'm with those who say the battery should not be draining because of the
short trips.

Lets' do some simple math: Lets say your starter pulls 75 amps while
starting the car, and it takes 10 seconds of cranking away for it to
start (most cars don't take 10 seconds to start, but let's worst case
this to prove a point). That's 750 amp-seconds pulled out of the
battery. Now - let's assume the alternator is putting back 5 amps, and
that the battery is only 40% efficient on its recharge. Then we would
need to put back 1050 amp-seconds to bring it back to the same level as
it was previous to that last start. That means it would have to run 210
seconds (5 amps x 210 seconds = 1050 amp-seconds). 210 seconds = 3-1/2
minutes.

The numbers I used in the above calculations are in the ball park of
real world - probably even more pessimistic than real world if
everything is working normally - for example, your car probably starts
in less than seconds of cranking - which would cut our necessary drvie
timew to charge it back up to 1/5 of the 210 seconds, or a whopping 42
seconds of driving - less than a minute, less than a mile. So
definitely the 30 mile drive being required to bump the battery back up
after that start is out the window.

Here's a quick test that will tell you if the alternator is doing it's
job: Put the leads of a multimeter in volts mode on the battery
terminals with the engine running. Get the engine speed a little above
idle - 1000-1200 rpm is good. If you're getting at least 13.6 volts,
the alternator is working. Most cars will settle out in the 13.8 to
14.3 volt range.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

How long should a car have to run to bring a good battery back up to full

I think they need to find a new shop.

I drive 1 mile to work, work 8 hours, and drive 1 mile back home... have
been doing this for about 5 years. My battery is close to 10 years old and
has never run down. I live in Minnesota.

I'd suggest to your parents that they take their vehicle to another shop and
have the charging system looked at.

Loren

 

I have a friend who once had his battery prone to not charging well
unless he had a high percentage of his engine on-time running at least
about 2,000 RPM or something like that. Non-highway driving with lots of
stops and lots of coasting would have his battery actually mostly
discharge if he had his headlights on. He had an ammeter to measure net
current to/from his battery for everything except the starter.

It turns out that one of the diodes in the rectifier circuit that was
part of the voltage regulator assembly was blown. This halved the amount
of current delivered by the alternator. Replacing the "voltage regulator"
(actually an assembly including a rectifier circuit, at least in his car)
fixed the problem.

- Don Klipstein ()

 

On re-reading my post, I see that I multiplied the 750 by 1.4 instead of
1.6 - which would have given slightly more amp-seconds and slightly more
run time to make up for the starting. The bottom line is still the
same: It should take no more than 2 to 4 minutes to make up the discharge.

 

That's a good point, Don. My voltage measurement with the engine
running should tell that story, or have the alternator tested by a
competent, honest shop.

 

firstly the answer to your initial question is that there is no accurate
answer because it depends on too many variables, secondly I can tell you
from years of experience with battery powered vehicles that battery
load-testing is a very inexact science. it will easily identify a truly bad
battery but with marginal batteries it gets a bit hit-and-miss. there are
methods for accurately testing a battery but they are too time consuming for
practical use as they involve several drain/recharge cycles and then
applying a known load over an extended time and then checking the specific
gravity of the electrolyte (assuming a non-sealed battery).

but the bottom line is that, yes, you should get the charging system
checked. if it is producing enough voltage then your problem is likely a
weak battery but don't discount the possibility of something like a bad
ground or weak starter (you didn't mention how much cranking is necessary to
get going). it is all too easy to throw a new battery in only to find you
still have the same problems a few weeks later.

also, what about the electrical draw? do your folks have the headlights on,
a/c or heater full blast, radio/cd player running, etc.? this could extend
the time taken to recharge the battery.

 

I had a 1978 VW Scirocco that had a leak in the electrical system. I
had a brand new Diehard in it, but if it sat for a week without running,
the battery would be near dead. I never did figure it out, just sold
that VW! But it wasn't the battery OR the charging system in that case.

 

yes, that can happen. i would think most vehicles draw at least a tiny
amount of power to maintain clocks and receivers for alarms and locking
systems. obviously this shouldn't drain a battery in a week though.

did you actually have the battery tested? it is not unknown for a new
battery to fail.

 

The usual drain with the vehicle off is called IOD (Ignition Off Drain).
As an example, the FSM for the '02 LH cars said that the battery
should hold a charge for at least 21 days. The maximum drain was stated
as something like 35 mA after something like 20 minutes when the TCM
would time out and go to sleep.

 

One more thing to worry about.

My '94 van dies off in about that 3 week timespan if I don't use it whereas
my '83 can sit outside for 3 months and start with no problem.

The price of progress.

 

Yep. Gotta have those memory keep alive voltages.

 

my rule of thumb is 1 miles of travel for each volt

 

That makes no sense. That's not how battery discharge and charge curves
work. You have to define a load to talk about battery voltage at any
given state of charge. With no load at any given point of checking,
your statement makes no sense. IOW, with no load, going from partially
discharged to fully charged is not going to involve multiple volts of
change as you imply.

 

Yeah I had a strange experience concerning my battery at the local NAPA
last fall. My car would start up fine when it was left sitting overnight,
but sometimes when I'd drive it a few miles into town, park it, then try to
start it up again it wouldn't start (wisely I carry a Victor battery
charger around with me in the trunk at all times so I was never stranded).

Did a little research on the Internet and decided on a NAPA 84 series
battery. Some chick clerk tested it with an instrument and said it didn't
appear to be weak (since I bought the car used in Sept. 2004 the battery
had to be at least 4 years old), so I told her I'd "think about it." But
the car kept doing the same thing, so about a month later I took it back to
NAPA. This time a guy tested it with the same instrument and said, "Wow,
this must be the easiest starting car ever -- this battery doesn't have
enough power to start a lawn mower!" So I bought a NAPA 84 series, and he
even installed it for me for no charge.

He never could figure out though why it started up fine in the morning, but
had problems after it was driven a short distance into town. When I first
told him this, he was certain it must have been some other problem than a
weak battery. But a weak battery it was as the problem has disappeared.