Here's a discussion on a forum about using the alternator to recharge a dead battery.
https://community.cartalk.com/t/kill...-battery/11806

Although it may not kill an alternator right away, because our Xs are so heavily designed with electrical devices, a large amp spike from the alternator could damage some of the electrical modules in the X. How high?? As high as the output of the alternator so it can be from 80 to 120 amps. Don't believe me that the alternator can put out that kind of amperage, here's a Youtube video made by a DIYer as to why one should not use an alternator to recharge a dead battery.

https://www.youtube.com/watch?v=GA7gDj-4YUg

I don't know if it is the case here but some of us just can't spend extra money for tools or equipment when something goes wrong and should know shade tree engineering approaches we should be careful to keep things in perspective.

IMO trashing the alternator charging a battery is extremely low risk. I can't count the number of times over the years I have jumped vehicles, including my X5, to get them going without any issues with the alternator then or later.

Key here is to get the battery load tested. Do it somewhere that sells batteries. They will charge it to do a load test. Good or bad you can leave with a charged battery and won't have to find out if the alternator dies or not charging a battery.

Just wondering, If it doesn't kill the alternator right away how does anyone know the failure was due to charging a battery. I am running the original alternator at over 118,000 miles. When it fails how will I know if the times I used it to charge the battery contributed to the failure verses the many times I sprayed Gunk on it cleaning the engine.

Good plan, bcredliner, re. getting the load test (and charge) done. Two birds, one stone. :thumbup:

BTW, I didn't mean to say you HAD to spend a lot of money - I was trying to say that some faults are best dealt with properly first time rather than McGyvering them and having further issues. I certainly use "shade tree" methods a lot - when I can get away with it.

And re. the alternator thing, I said "cooked" as in "it will get very hot". Whether or not that then causes an issue depends on many other factors. :dunno:

Are there still chargers out there than don't drop output as the battery charges and go into a maintenance mode after full charge is reached? Part of the reason for smart chargers is to make it impossible to harm a battery regardless of how you screw up using them.

If you look at most battery charger they don't come near the amperage that an alternator can output. Most chargers are around 10-15 amps with a starter battery charger usually around 40-50 amps. At 80-120 amps the alternator will be cooking the battery which will lessen the life of the battery.

If you can't afford to buy a battery charger you shouldn't be fixing cars. Take it in and let a mechanic or tech fix it for you. Can't afford to take it in, you shouldn't be driving a car let alone a BMW. Plenty of people don't drive and just take public transportation like Mrs. Upallnight and my daughter.

The rated output of the alternator has nothing to do with the current the battery will draw when being charged.

The charge current is determined by the internal resistance of the battery (more or less fixed) and the charge voltage. The charge voltage is limited by the regulator, so the charge current will never be enough to cook that battery. Unless the regulator fails and the charge voltage goes to high - then all bets are off.

But to "push" 80-120amps through a 12v battery would require the alternator to put out something like 20 volts... :yikes:

[QUOTE=upallnight;1114341]If you look at most battery charger they don't come near the amperage that an alternator can output. Most chargers are around 10-15 amps with a starter battery charger usually around 40-50 amps. At 80-120 amps the alternator will be cooking the battery which will lessen the life of the battery.

If you can't afford to buy a battery charger you shouldn't be fixing cars. Take it in and let a mechanic or tech fix it for you. Can't afford to take it in, you shouldn't be driving a car let alone a BMW. Plenty of people don't drive and just take public transportation like Mrs. Upallnight and my daughter.[/QUOTE

I expected this was coming again. Don't understand how your opinion about who should DIY, own an X5 or have a drivers license is called for or helpful to anyone. Please enlighten me.

I wish they worked way. I own over 20 vehicles and keeping them charged required charging them from time to time. The old chargers I had actually backed down to practily nothing charge wise when the battery was full. The new charges I bought didn't.

I bought black and decker top end chargers (multiple models).

Schumacher top end models. The ship n shore model I found to be the kindest long term charging wise and it still didn't back down completely.

The problem is the smart chargers (non trickle charger models) put out around 2 amps as their lowest charge rate. 2 amps is enough to evaporate the electrolyte in battery over time.

I got crash course in batteries when I converted my house over to solar.

Before all of this I didn't even know 12 volts in a 12 volt battery was almost dead :)

Anyways you cant just keep pumping power into battery without it eventually cooking it. The smart chargers I found out weren't as smart as Id of hoped they were.

The best chargers I found for long term were the battery tender brand and the pulsetech xtreme chargers. Both were safe for long term charging and the pulsetech actually does desulfate a battery.

I tested several totally dead batteries with visible buildup on the plates that were not effected by the smart chargers I had even on desulfate mode. The pulsetech xtreme actually knocked the buildup off the plates. You could see large sections broke away from the plates and the rest was floating on the surface of the battery electrolyte. So it does work.

If you look at the Youtube video that I posted the DIYer use an inductive amp meter to check out what type of amperage the alternator was putting out while recharging the battery. It was around 70 amps. Yes that type of amp requires that type of voltage. That's is why I always advise people not to jump start an X. Better to recharge a dead battery and start the X. That's is why people will post after jump starting their X that they are having electrical issues. The spike in amperage along with the spike in voltage is sometime enough to kill some of the modules in the X.

Well actually I have proven that it doesn't kill an alternator to charge battery back up. I use 100 amp 24 volt alternators to charge my solar battery bank for my house if the weather is bad for long periods of time. They have no issue with this what so ever and have actually powered the house while the batteries are charging.

The main thing that kills alternators is large loads (overheating) and running them without a load connected (the old take the battery cable off to see if the alternator works test).

So charging the battery back up with the alternator is fine. The problem is you would need to drive the car for a looooong time to actually fully charge the battery back. Im not saying you cant but it takes a good bit of time. For those that jump off the car to get it going and then it cranks back later yes you charged it enough to crank. But you didn't fully charge it. If you ever have to jump one off its a good idea to charge it with a battery charger afterwards.

Also every time you let your battery run down under 11.5 volts you are permanently damaging it. I used to not think twice about if a battery ran down that far myself till I got into the solar stuff.

Each time a 12 volt battery goes under 10.5 (I prefer 11.5) or so its damaged. It will charge back but it will never have the same amp hours it had new.

Every time you do it your lowering the capacity of the battery. Do it enough times and you wont have enough amps to even crank the car when it shows full voltage with the key off.