Follow Up Remote Start Myth

I now think the remote start myth may truly just be a myth. Now I shall tell you what all happened. I think my wife left the key in the accessory position. The battery was dead a couple days later. I boosted with a 20 amp charger until engine would start. Ran the engine briefly and battery was dead a day later. Reated again and a day later the battery was dead again.

My theory is the remaining charge from starting and the brief running was not enough to keep the system that operates with the key off fully discharged the battery.

Now my idea is that if you boost a battery to start the engine continue charging or run long enough to fully charge the battery. Am I right?

If the battery was completely drained for a couple of days it's probably not going to hold a charge. But in general you should fully charge it with a battery charger before starting the car.

I had a reply in 4 minutes!!!!! Thanks

Yes, it takes a good amount of time to fully charge a battery. This is best done with a slow charge, as in a low amp charge with an automatic charger, or an overnight charge on a low setting, say 5 amps or so.

What you don't want to do is try to bring a dead battery back up by jumping it off and running the alternator. That's a good way to burn out an alternator! I know, I'm as guilty as anyone else for doing it, but it's still risky!

You said you charged it enough to crank it with a 20 namp charger. If it truly puts out it rated amps it would take it one hour to put 20 amps in a battery. Most auto's have a 35 amp altenator so if you let it run for one hour then you only put 35 amps into it . Most Car batteries are 725 amps or more . Do the math

You are right. A guy at work managed to totally discharge his battery on a new pickup. We put it on a 10 amp charger for 24 hours. It tested out at 90 percent.

Most alternators these days have way more than 35 Amps,the one wire alternators I buy for tractors have 60 Amps.

(quoted from post at 04:58:55 08/15/16) Most alternators these days have way more than 35 Amps,the one wire alternators I buy for tractors have 60 Amps.

Click to expand...

Yep, 100 amps or more is normal today. Ours is 125.

Rick

If the regulator is right, it shouldn't hurt the alternator to charge up a dead battery. I had a 78 Ford F150 that I used 2 batteries in for boosting. I would put a dead battery in more than once & get it fully charged with no problems.

Fully charge the battery before using the vehicle . Utility electricity is cheaper than smoking the alternator . Particularly in the summer heat. They see a near dead battery as a short circuit and go max output for an extended period of time .

If you have a 50 AH battery, you can take 5 amps out for 10 hours. At least that is what might happen. To charge the battery if it is half discharged will take 5 hours at 5 amps. After all, it is half of 50 amp hours or 25 amp hours. Rate and time... but does the charger really do 5 amps for the entire time?
Some Battery Info

Be aware that "cold cranking amps" have nothing to do with the "Amp-hour" capacity of the battery.

A typical car battery has around 50Ah (Amp-hours) of capacity. To charge it anywhere near capacity from flat dead, you need to put some number of Amps and hours together that multiplies up to make 50. Charging is not 100% efficient, so if you charged at 10 Amps for 5 hours, you'd only be putting 70-80% of the battery's capacity back. But that would be enough to start the car, and the alternator would finish the job as you drove around for a couple hours.

It really doesn't take that much to start a modern car. They fire almost instantly when you turn the key. A few minutes of 20A boosting can put enough of a surface charge on the battery to allow for one start.

The alternator is NOT a battery charger. It is a battery maintainer.
Today's alternators are of large capacity because there is a huge electrical load on them. Every system in the car depends on electricity to run. In years gone by, many of those systems either did not exist or were mechanically operated. Like the fuel system. A mechanical pump fed a carburetor. Now, we have an electric pump in the tank with an electronically controlled fuel injection system operating. Things like ABS, traction control, tire pressure monitors, air bags, and a host of other electronic bells and whistles are continually drawing current from your car's electrical system. That warrants a higher capacity alternator. But it still is not intended to be a battery charger.

A lot of mis-information here.

yes the battery needs to be charged a longer time to help charge it up, and also overcome a weaker cell as the other better cells will take the charge first.

Second.. the alternator is made to put out full current for days on end and has a fan built into it for cooling. So unless you you have a bad diode or a defective alternator, or a shorted cell, it will simply do what its designed to do.

Third.. at a idle or low rpms, it will not put out anywhere near full power. So sitting and idling will only put out less than a third of full current.

Forth.. as the battery voltage comes up... the charge WILL taper to keep the voltage at or near 14.2 to 14.55 volts. So after an 30 minutes or so of higher rpm driving, the regulator cuts the charge back quickly to a lower rate, usually less that half of full current after 20 to 30 minutes.. (unless the battery has bad cells)

Fifth.. old batteries are usually sulfated up,, meaning that they cant absorb high current charge or put out high current anymore, so these batteries cut back on current very very quickly and only a long long charge will have any chance of helping them all though it will be a low current,, usually less than 10 amps. Sometimes a long slow charge will boil off enough sulfate to all the cells to produce the correct amount of current again.

sixth.. if a battery is left discharged more than 24 hours, its possible that the cells were sulfated up and the sulfate became hard.... and its life was reduced or damaged to the point of no longer usable.

seventh.. the battery could simply be at the end of its life anyway and no amount of charging is going to fix it.

I will challenge you that on running a 145amp alternator at 145amps for 24/7 . Particularly in how weather that alternator will prematurely fail .
The 145amp rating is an intermittent rating, not continuous . No different than reading the fine print on a portable 60cycle generator . The capacity stated in large print is usually either "starting capacity " or a max load for 2 or 5 minutes.
Continuous rating is usually in the fine print.

Lot of information that pretty well says what I said the other day. Charge it , charge it , charge it , but at a rate that the battery handles without excess voltage buildup. Easiest is with a slow charge, NOT AUTOMATIC EITHER, and leave it on until voltage is up to normal vehicle charging voltage. And, I have seen a bunch of alternators burned up from installing them onto dead battery machines, combines in particular, without charging batteries at least part way. Way too many and warranty was a real problem on those.

We will have to agree to disagree...
simi trucks run 12 hours at near full load, idle all night and then go again for 12 hours. Have for years with smaller alternators back then and bigger ones now. Although lately the led bulbs are lowering the load. They have run 150 to 200 amp alternators on them... Ambulances run a very very very heavy load... all the time... dual a/c units, inverters, and lots of lights.. They even leave lights on, engine off while at scene.. in the house, treating and loading the patients... then they restart on the second battery and the alternator then has to charge two batteries and run the 100 amp plus load... and in Texas heat.

On surburbans with dual ac units, seat heaters, mirror defrosters can run a 40 amp blower, 15 amp headlight, seat and mirror heaters of another 20 amp load plus engine all day while pulling a 15 amp high power stereo load.

Dont even get into buses with their 200 amp systems and huge triple a/c loads. Manufacturers have gone to two alternators when you start to exceed 200 amps. Its has shown up on fords on the ambulance packages now.

AND yes. alternators are made to charge batteries..
Using bulk mode, then trickle mode... however they dont have the third stage of maintenance or pulse mode ... temperature sensors allow even the charge voltage to change based on temps, that most home chargers dont have. This allows most modern cars to charge to a higher voltage when colder as needed to fully bring the batteries up to full charge. And.. if the battery is good.. max current will not usually exceed 20 minutes as the voltage will build up and the current will taper long before... The vehicle load however may not taper...

We will agree to disagree.

Actually you should charge the battery 24 hrs at a slow rate like 2 amps or so.Then load test it to see if it is actually any good,long life or short life expected.Then recharge if it is good.Then use it.What you did was barely put enough charge in a dead battery to start the engine,and then discharge it again by letting sit in a discharged state.
You still ought to disconnect the negative cable and check for an amperage draw with a multimeter set on the 10amp scale.Leave the meter hooked in series with the cable for a couple of minutes.It will show a draw of 3-5amps or so until all the modules,ECMs.PCMs,ABS,etc power up,then they drop off to less than .5amp (1/2amp). That would be ok.If too high of a draw,then you need to determine what it is and repair the drain on your battery. Mark