When 12V battery completely depleted, should I jump start it and drive away, charging it with Hybrid battery in the process? The reason I ask this question, is because today my other car, the 98' Lexus LS400's almost new battery completely depleted. I forgot to turn off the reading light. I jumped the car and drove it 50 miles, then head to my mechanics. After measuring the voltage on the battery, he told me the voltage is only 12.88V now. It should be between 13.65 and 14.65V. He said the alternator might be gone. The reason is, the alternator should not be used to recharge the battery after it completely depleted. It puts too much load on alternator. I should have charged the battery to full state using an external charger. In normal situation, battery is always in near full charge state. Alternator's role is to maintain the battery's full charge state, while providing the electric demand for all devices in the car, such as lights, fans, relays and other things. He suspect the voltage regulator and diode inside the alternator might be very weak now. Conclusion: the alternator needs to be replaced, although it works when I don't turn on light and a/c. The car will be dead if I turn on heat, headlight and drive for long period of time. So my questions: 1. Does Prius has this problem? That is, should we just jump it and drive away, charging it with hybrid battery and MG1 MG2? 2. For non Hybrid cars, after jump start the car, do we need to charge battery with external charger? Or it is just the case for LS400? 3. If I leave the alternator as is and drive the car gently with no heat/fan/light, will the alternator recover over time? Thanks!
No the Prius DC to DC converter should be protected against any over current that you'd get from merely having a flat 12V battery. A fully flat battery actually has a fairly high resistance and the initial charging current is often not that high at all. If the battery hasn't been destroyed (eg let sit too long and sulfate up) then the current will rise once the charging gets under way, but any modern alternator charging system should have a current limit that is easily good enough to protect itself in this event I'm somewhat doubting your mechanics assessment and think you should get a second opinion before getting the alternator changed out. I suspect that the battery is just not fully charged (or not taking a charge) after the flat event. You may have damaged the battery but drive it around for a while first and see if that charging voltage improves.
Yes I am not sure about his diagnosis. This is actually third times I replaced alternator on my Lexus. The first time, original alternator was completely dead. Consider the 170K miles, I think it is reasonable. So I replaced alternator and a new battery. Then in couple of month, the battery was drained, I jump started it and drive for a mile, then alternator dead. He told me it is because the alternator should not be used to charge a flat battery. So I replaced another alternator (3rd). In next one year I only drive this car every other day or so, adding about 7000 miles on it. Now I made same mistake by driving after jump start. My suspicion is how come the alternator's own voltage regulator and circuit protection can not stop overload problem? I bought those alternators (Bosch) from autopartswarehouse.com. All of them are rebuilt. No new available at the time. And I picked the 100 amp than the 80 amp, knowing the load is an issue. But it still gone bad now. I see nothing special about the Lexus LS400. I don't think it uses more electricity than other cars. If the theory is right, it should apply to all other cars. But I never hear that you can not drive after jump start. I am still puzzled...
BTW, how long it generally takes to fully charge a flat battery by driving the car? 40 minutes? 40 miles?
In my experience just driving the car will never fully restore a depleted battery. I'd look into getting a charger. There's some moderate amperage, "intelligent" chargers on the market. Likely one or two overnight charging sessions will be needed, to get the battery nearly back to what it was. A decent quality digital multimeter is another good tool, something in the $30~40 range. The very cheapest might not be that accurate. If you've charged the car, driven it for a day or two, and then check voltage with the car turned off, in semi-sheltered above freezing temps, the voltage should read around 12.6 or higher. If you check voltage immediately after charging it will read high, but a day or two of driving the car will put it back to normal.
Our Prii use a "Constant Voltage" charging mechanism (at just under 14 volts - a little higher when the system senses a weak battery). This is a very slow, but safe (for the battery) charging method. It takes a loooong time to get a reasonable level of charge into the battery this way which is one reason for the many early deaths of the little 12v battery in Prii (short trips lead to low level of charge which leads to sulfation). JeffD
Just to be clear, this is for your 04 Prius right? Let's leave the Lexus non-hybrid out. The 12v battery recharges from the 201.6v hybrid with a DC to DC voltage converter. The charge is quite slow so you need to keep the car on for awhile. If you drive a lot, don't worry. Have you ever replaced the 7 years old 12v battery? If not, it is time to replace. Optima yellow top is lighter, higher capacity and less damage from deep discharges.
IMO - You definitely should be going somewhere else for your service !!! Your service guy must be using a bad supplier of alternators or he is not being honest with you. There is NO way that alternator should have died in "a couple of months" when you put in a new battery also.
A well designed alternator and regulator should not damage itself recharging the car's battery, no matter how flat the battery. I suppose your Lexus could have an undersized alternator, but I doubt it. As Jeff pointed out above, the charging system in the Prius is very primitive. It will take a long time to fully recharge a flat battery. Better chargers, like those found in cruising boats, will have smart chargers that bring a battery up as fast as safely possible. The Prius doesn't do this as it isn't necessary in normal operation. Normally the Prius 12V battery lives a pretty quiet life. Tom
Just to reiterate, BOTH cars will take some time to -fully- charge the battery. It's usually considered to be about 4-6 hours of driving. Idling won't do it for the Lexus, as most alternators do charge on idle, but just barely. The Prius will take longer than the Lexus to fully charge the 12 V battery. The Prius will charge the same on idle as when driving, as it doesn't use an alternator driven from the engine. If you took the Lexus to the mechanic after only 40 min. to an hour from a dead battery, then yes, it won't be charged much. That is why he saw only 12.66V. Keep driving it for several hours and it will be fine.
1. Unknown. I would say that it probably would not damage the aux battery charging circuit but it may take a very long time to fully charge it. I would jump the battery, not drive it too far and manually charge it and the test the battery. Not worth the gamble on my Prius anyway. 2.Most applications you need to manually charge the battery with a battery charger to fully charge it after a jump start. This all depends on how dead the battery is, how large the capacity the battery is, how tough the alternator is. Most rebuilt alternators that I have seen are usually not as durable as the original one was. Many re-builders use sub par internal components ( internal voltage regulators,etc) to keep the rebuild costs down so that being said rebuilt alternators are not the same as a factory replacement and the cost difference between the two reflects that. 3. Alternators are designed to be more of a battery maintainer than a battery charger so I would say no. The way to test your Lexus would be to fully charge your battery on a battery charger then test the charging system on the car with all the electrical loads on( heater, hi-beams,everything you can think off). I used to put a carbon pile on the battery while the engine is running vehicles and load the alternator to capacity and watch the voltage and output, but some of the newer vehicle alternators are not very tolerant of the excessive heat of that test. My old pickup ( 1980's) you can have the alternator charging and full capacity for a long time (I have) and not hurt the alt. but newer vehicles alternators are not as durable. At least from what I have seen.
12.88V across the terminals with the Engine running at 2000+ rpm makes me very suspicious of the battery negative. The alternator will generate a potential of 13.65-14.65V from the battery positive to chassis ground. Measure the voltage at the posts, and the voltage drop from the negative post to a chassis ground point at 2000 rpm.
Nice lateral thinking to look for some other underlying problem Seilerts. Yes I'm very suspicious that the OP has been sold new alternators when the underlying problem is somewhere else. I've never heard of a alternator dying due to having to charge a flat (but otherwise undamaged) battery. I'd also like to disagree with what some others have said regarding the alternator not being able to adequately charge a battery. The fact is that the alternator in any modern car can supply more than enough power to charge the battery. Furthermore it's designed to charge the battery and definitely should not be harmed in doing so. Driving the car will adequately charge the battery but it will take a bit more than the 40 minutes you gave it. Driving in the day time without too much other electrical load is obviously the best, but even driving at night the battery should charge, albeit more slowly.
Thank you all for your insight. I think I will keep driving the Lexus for a week and measure the voltage again. I don't believe the alternator is so easy to break just by charging a flat battery. I will let you know the result by measuring both the poles and pole vs ground.
I bought my (ex-police) Volvo with a dead battery. 140A alt, 100Ah battery. Jumpstarted it. I drove for more than an hour (100+ km, 60+miles) and could not start the Volvo the next day (it's a diesel, so high startingcurrent needed). I charged the battery with a powersupply (30V, 10A set to 14.4V and no currentlimit) and it has worked for 3 years since. Just to show that an hours drive will not charge your battery. BTW: now that I've bought the Prius, I can live with the battery on the Volvo dying (which it may do this winter) as I can go pick up another battery if needed... On my Ford Mondeo, we had to push the darn thing to a garage 1 mile up the road to have the dead battery tested and a new one fitted
Charged my car overnight with a battery tender http://www.amazon.com/gp/product/B000CITK8S/ref=oh_o02_s00_i00_details Now it work normal again. I measured the voltage. it was 12.6V, now 14V. I am confident my alternator is OK. Will feel back to my mechanics, and share all your opinions. Thanks a lot!
If your mechanic says anything than "I'm sorry for trying to rip you off", GO SOMEWHERE ELSE. Your mechanic is either a liar or extremely incompetent.
14V would be normal with the car ON (in Ready, as though you were going to drive it). If you see 14V on the battery with the car OFF and no charger attached then there is a problem with your voltmeter. I would not waste time talking to that mechanic again except to get a refund for the unnecessary parts and labor he sold you.
