Fitting two batteries


Asked by Nov 12, 2012 at 02:00 PM about the 2004 BMW X5 4.8is AWD

Question type: Maintenance & Repair

Seeing there are lots of problems with battery drain and low voltage Issues could another battery be added and plug
in to the charging system, or are these cars to sensitive. Ignoring the space it will take up, but it might not
unexpectedly breakdown as it has in the past

14 Answers


finding the source of the drain will be easier than doing this. 2 batteries will just take longer to drain. They WILL still drain. Or just put it on a trickle charger every night.


I agree with you, what I can't find out is how sensitive are the electronics are in the BMW X5, lets say in a car with out to much sophistication the battery is just a bit low but well enough to start the engine the alternator kicks in and starts charging and no further problems, the same scenario with the X5 we are finding within a half a mile warning lights come on apart from the Ign light and we have a dead engine, the battery seems dead as you cannot restart, leave it for half an hour bump start it with a small battery I had and every thing comes to life, the car battery then seems fully charged, thing is if the battery was slightly low to begin with will the computer Sense this and start shutting down like the alternator. This has only happened 3 times in 1 year, Yes the battery and alternator will have to be checked over again like in the past but if they seem ok is it that black magic box and the odd sensor


So the vehicle dies while running and what lights come on exactly while driving battery light? When you try to restart the vehicle does it try to turn over or just make a clicking noise, or doe nothing at all? Have you noticed any other weird things like seat heaters not working or radio intermittently will play then turn off?

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do you know what a parasitic draw test is? I recommend that you can do it.. I am not sure if this is the same q. on the 04 can delay the problem with a 2nd battery, but will not fix the problem. If one dies, 2 will also, you will just over work the alternator trying to maintain 2 batteries not just one. There are heavy duty alternators, (used for the diesel models). but recheck the charging system. any check engine lights, or indicators, (scan it anyway incase it did not illuminate).

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Thanks John, yes I will have a go tracking leaks, so far we have never had a flat battery, but when we have had a electric break down as if the alternator has gone into sleep mode after about 10 minutes drive the battery to begin with was slightly on the low side, but well enough to start the engine. After a good charge up the car is back to normal, and the alternator seems to pump out well. May be after your leak test and checking around the alternator cables something might come clear. I have heard that some X5's have a special connector on the battery post in the avent of an accident that it Cuts the power off but will still alow you to start the engine, I'm thinking if there was a lose connection could this upset the alternator the odd times this has happened. The car is in London as it is my son's so it will be a few days before I can play around, I notice as I read more Forums I my list is getting longer, and the experts I know have a little laugh. But thanks for the tip a good start point


i don't believe that was 100% correct about the emergency switch, not to contradict you, it isn't to start the vehicle, but only to allow the electric devices to operate after the fuel-safety switch is engaged...however if it is at the same time, everytime, (about 10 minutes you say), how long until it starts back up, to try to eliminate a coil pack over heating, or a security system module at fault...surely something is getting too hot while running, will it shut off even at idle, same amount of time?

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Hi Robert, The first time with me at the wheel was last winter, temp around freezing point . about 5 minutes drive from leaving my house the warning lights pop up 4x4, triangle one. I can't be sure now if the brake red light came on as well, but the Ign and oil light were not on, the Scary bit was that dindong sound like you have in aircraft warning you to put your seat belts on quick, I swear I looked up for a sign in the roof, I then began to lose power from the engine and needed a landing Site and lucky for me I just got of the main road with a dead engine the steering was very very heavy , the brakes still worked. Tried to restart but just a clicking sound. Switched the side lights on, dead . The battery was dead flat, while my wife walked back home to get the small jump start battery I disconnected the car battery the post were clean. Thinking that might reset the computer, I had no idea I'm pulling at straws. It was a good half hour before I connected all up and put this small jump start battery on thinking this is a V8 not a 500 cc engine but on the turn of the key she fired up. I didn't know then about turning the steering wheel full left and right to reset the lights but as I had to do a 3 point turn . Less than 5 mins to get home .switched off restarted and the battery seemed fully charged up, tried restarting a few hours later not a problem. Then around 6 moths later it happen again with my son, he left it for a while undid the battery clamp thinking maybe a bad connection but no, and with out a jump start the engine fired up and he got home. Then it happened again last week with him, he got the breakdown to come out and the chap said the alternator is dead when he jump started it, back at home to get the car off the tow truck and up into his driveway the breakdown guy put his jump start battery on to restart the engine, he said that's strange the alternator is charging now. So the long and short of it does seem to point at the alternator, but we only seem to break down with in 5 minutes of driving. Any other time As far as the rest of the electrics they all work fine, I haven't yet seen the car maybe in a few day time will have some fun. Sorry this was a bit long


No problem yes it does seem your having a charging system issue. Parasitic draw test wouldn't really help in this situation due to it's not a problem with starting the vehicle after it sits and the battery draining on its own. Now there are two possible issues that could be causing this one being the alternator. There are two different possible alternators on that engine one water cooled or air cooled I have seen both fail before and give you the described issues. The alternator varies its voltage from a command from the DME (digital motor electronics) over a communication bus system called BSD (bit serial data). There could be a problem with that wiring or the DME itself. The last one that you may want to try first since it's probably the cheapest would be the ignition switch, when things start to loose power randomly then work agian it is usually the ignition switch starting to go bad. Hope that help let me know if have any other questions.

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personally not thinking that route, but go for it, to me something is getting too hot, and malfunctioning....good luck


Hi John, thanks you saved me time about the Emergency switch I can see what you mean about that. And the time factor from starting up and power Failure around say 10 minutes makes you think of some heat up point, of course I dismissed this at first being some 6 month gap but now as you point out the breakdowns are getting closer together, so another hunt is on the list. Cheers.


Hi Robert, this was my grey area about the alternator I don't know at the moment if its water or air cooled, knowing my luck if I have to change it it will be the awkward one. This is where I' am really out of touch, are all the components DME/BSD ect Housed inside the alternator as I have this fear some of the commands bits you mention might be in the computer type box. So if the alternator was faulty could there also be a faulty part outside. Which I guess need to be hooked up to a test monitor . The Ign switch great idea, will check that out Thanks for your info


The DME is the engine control unit that is outside the alternator it controls everything for the engine. It also commands how much the alternator should be putting out for voltage. The BSD is the communication line from the DME to the alternator if the DME can't communicate with the alternator it may cause issues (this would be the least likely issue since if there is a communication issue the alternator will just full field. Yes the air cooled alternator is the easier to replace than the water cooled one. The alternator will house the voltage regulator (this controls the amount of voltage to the battery and other vehicle system.) Also will have the voltage rectifier that doesn't allow AC voltage to the vehicle system or battery. Most of the vehicles electrical systems only work on DC current.

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Hi Robert, I just been going over all the links, and the answer I sent back to you the other day I think went missing some how, you can see now why I'm weary of computers. Me sitting behind one. Any way should you have not got the last one from me I like to say thanks, In just a few lines you put all the jigsaw bits together about the alternator and the link with the computer, now I do feel free that the magic box it not to blame all the time. Thanks again and to all for their input as well


Hi Tunebyear, your welcome glad I could be of assistance to you. I hope what we talked about points you in the right direction of getting the vehicle fixed. Just don't get discouraged about the issue with your vehicle they really are well designed vehicles. Let me know if you have any other questions or need help with anything.

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