Any battery/alternator ee gurus in the house

Status
Not open for further replies.
C

chefwong

Joined
Jul 17, 2003
Messages
931
Location
New York
How long does it generally take in terms of driving hrs/time in relation to charging a discharged battery ?

I was working on the car and the battery drained to the point where I needed a jump to start the car.

Fast forward 2 weeks later, I decided to *top off* the battery figuring the alt would not have charged it to it's max. I put a battery tender on it at 8 amps and I just came back from checking on it - after 45 minutes and it's already in float mode !

Dunno how well the type of battery plays into the factor but it's a Odyssey AGM battery
 
I'm no guru, but an alternator was not designed to charge a low charged battery. The charging system consists of a alt. voltage reg. and a starting battery. The charging system has to maintain the battery and power the electrical devices in your car ex. wipers,heater,radio etec. So to get to your question there are a lot of variables involved here. The condition of the battery and size of it,condition of the alternator and whether its working as it should or its marginal at best. The best way to handle this situation would to run you car with no accessories running for a least 5-6 hrs. Otherwise charge a dead battery with an 8amp charger takes any where from 8-10 hrs
 
Alternators charge at 14 Volts + or - a little, usually determined by the OAT. The difference between this and the actual terminal voltage will determine the charging rate.

If the Alternator is rated for 60 Amps @ 14 Volts, and your battery is dead flat, it will charge at the full 60 amps, minus the current used by the car systems, until the terminal voltage is at 14 Volts. Notice I said terminal voltage, not battery voltage. As the battery voltage rises, the current required to maintain 14 terminal volts will taper off, until the battery is fully charged.

If the battery has a charge indicator, (sealed batteries usually do), you need to check it to see if it is holding a full charge. If not, your battery has been damaged and will need to be replaced. Most autoparts stores will do a load test for you to see if the battery can still maintain a normal charge.

If your battery has removable caps, open them and look for low water in one or 2 cells, if so, the battery may have been damaged to some degree. In my experiance, a severely damaged battery will have low water in the cell next to the negative terminal.

An easy test to see it the battery is still good, is to measure the voltage, then start the car and immediately shut it off. The voltage will recover to the first measurement in less then a minute. If it doesn't, it's days are numbered.

I've seen batteries that dropped to 11 volts, and recovered in 5-10 minutes. The car started and ran fine, but the battery died in a matter of weeks.

Hope this helps some.

Wayne
 
Depends mainly on the alternator. The alternator in my Ford Mustang is a 130-amp unit that can put out 80 amps at idle. It's 130 amps at around 2000RPM or so, as I recall.

Given that the typical car battery is around 60 amp-hours, in theory, that alternator could charge that battery in an hour at idle assuming the rest of the car only needs 20 amps to run. In practice the battery will probably not accept that high of a charge rate without using a higher voltage than the regulator in an alternator is set to, so it will take longer to charge.

The original 75-amp alternator on this Mustang had pretty poor output at idle. I think something like 20-30 amps.
 
offtopic.gif

This is kind of that, but I had a question based on the site you gave to describe AGM battery construction. One of the don'ts he listed for batteries is NOT to add electrolyte back into the battery. That kind of goes against what I've been told will help extend the life of a battery - keeping acid covering the plates so as to avoid sulfation occuring at a faster rate. Does this not work?

Now, back to the topic...I agree with several of the posts above, an alternator is designed to run the car while in operation, not charge a severely discharged battery. I have fried my share of alternators doing this. The more that you take out of an alternator at all times, the shorter its life span is going to be. Even with the high amperage ones that we have today, it will still shorten the life, and still will not take care of a very discharged battery properly.

If the battery can be saved, the best way to save it is a nice, SLOW, and very LONG charging. Usually I use the 2 amp 12 volt setting on my automatic charger for this, but if you are sort of "in a rush", you can use the 10 amp setting, but no more. The faster you try to charge a battery, the more superficial the charge, and the faster it will discharge again.
cheers.gif
 
Quote:

One of the don'ts he listed for batteries is NOT to add electrolyte back into the battery. That kind of goes against what I've been told will help extend the life of a battery - keeping acid covering the plates so as to avoid sulfation occuring at a faster rate. Does this not work?

Click to expand...


Don't add electrolyte (acid). Add distilled water only.

It works. Not for AGM batteries though, but you aren't going to be adding water to an AGM battery without using a drill. AGM batteries are sealed except for a one-way vent.

For non-sealed batteries, yes, of course, add distilled water as needed.
 
Folks, keep in mind that when you jump start a dead battery, the act of jumping the battery will start the charging process. Of course, if the battery is shorted internally, you risk the alternator as well as some wiring.

Also, when you actually start the car, it will be idling and the alternator won't be putting out its rated current.

Probably not a good idea to race the motor right after a jump start. Give it a few minutes after a jump start before driving off.

YMMV....
 
ive got a vw sirocco alternator hooked up to a 5hp honda motor. i used it to charge dead batterys! dont anyone try and say an alternator cant do it, because i do it all the time with my golf car's 6 6v deep cycle batteries. sometimes once a day even. i also use it for charging a dead car battery, such as forgetting to close the door and the dome light kills the battery.

in the few years ive had it, it hasnt failed me.
 
Early on I learned avoid the high current 'fast charge' as it killed a new battery, and instead to use a low current 'trickle charge'. My old Sears battery charger is rarely used, but when it is it's for an overnight trickle charge, at something like 2 amps.
 
Float mode is the mode of a charger where is keeps the battery exactly fully charges and can be left connected this way forever. Sometimes called a battery maintainer. A trickle charger is a low amp charger but it is NOT a float charger. You can get float chargers by them self that really only keep a battery charged, not made to charge a battery, and some bigger charges have a float mode. Some of the float chargers also have a desulfurification circuit. Many boaters keep their boat batteries on a float charger for the winter.
 
Float mode typically puts about 13.8V into the battery.

The problem with trickle chargers is that they are often unregulated, and one of the interesting characteristics of an unregulated power supply is that as the current draw goes down, the voltage goes up, and vice-versa

So as the battery nears full charge, it draws less current, and the voltage into the battery goes up, sometimes to as much as 15V. NOT what you want. The excess voltage will cause gassing in the battery, and if it's an AGM or gel cell battery, it can be ruined.

I've used a regulated 13.8V power supply as a float charger before. In fact I used one to charge the battery in my car when it didn't have enough left to turn over the engine.
 
Good read guys.

My trickle charger has a profile for AGM batteries.
It actually has profiles for AGM, flood and gel at 2/4/8 amps.
I use it on the winter stored cars. It works great on the stored cars.....and was actually expecting the battery to need a longer charge but I guess between the 2 weeks, it was pretty much close to full.
 
My friend sells batteries, nothing but batteries. He says it takes an alternator an hour to recover the battery from starting the car.
 
Really? Let's do the math on that.

Let's say it takes 3 seconds of cranking to start a vehicle and it takes 600 amps to do so.

That's 1800 amp-seconds.

To recharge the battery, we must put 1800 amp-seconds back into it plus a little more, since the charge/discharge process isn't 100% efficient.

How many amp-hours is 1800 amp-seconds? One amp-hour is 3600 amp-seconds.

Therefore 1800 amp-seconds is 1/2 amp-hour.

That means that it would indeed take the alternator slightly more than an hour to recover the battery from starting the car IF it only charged it with 1/2 amp of current.

But it doesn't. It can put much more current than that into the battery.

If it charged the battery with 1 amp it would take half an hour.

2 amps, 15 minutes.
4 amps, 7.5 minutes.
8 amps, 3.75 minutes.
16 amps, 1.875 minutes.
32 amps, .09375 minutes.

I would venture to guess that most alternators will put at least 16 amps of charge current into the battery even at idle, so you're looking at roughly 2 minutes to recover the battery from a start, assuming it took 3 seconds and 600 amps. Which incidentally is high--it takes nowhere near 3 seconds or 600 amps to turn over a properly tuned engine in good weather.
 
Status
Not open for further replies.
Back
Top Bottom