Running Car Without Battery DONT DO THIS!!!

Post by **MrAl** » 24 Oct 2017, 06:15

Hello there,

There are some people that suggest that it is just hype that you can not run a car without the battery in place, that is, connected normally. In other words, start the car, disconnect the battery, and it should be OK to let run like that.

Now i dont agree with this in any way and i wont do it no matter what anybody says, but is there any definitive proof that this is harmful to the car or electronics in the car?

Thanks a bunch.

Post by **jimmy57** » 24 Oct 2017, 06:48

Once the engine starts the battery is used for 2 things: to stabilize voltage level and to provide current flow when the alternator is not capable. A car can run w/o harm with battery disconnected but if some load is applied, like cooling fan coming on, The voltage will fall and the voltage regulator in alt will increase current to rotor to increase voltage (and amperage) and with no battery the voltage may go up to a level high enough for damage. Yes, it can cause a failure. I know of a 2004 XC70 that accidentally had the ground lead disconnected while vehicle was running when spare was removed from rear storage. The vehicle ran for several minutes and then it was shut off. A bit later it would not start and nothing worked so the ground lead was found laying next to battery and re-connected. When it was started the instrument cluster (DIM) and the CCM were dead. Those two parts had to be replaced. There were overvoltage DTCs in many other modules. Battery is accidentally disconnected a lot w/o damage when the cable is immediately put back. A faulty alternator can run a car without battery connected. A totally failed one that does not charge at all will not keep it running. Most failing alternators still produce some current so it is not a valid test.

Post by **Clemens** » 24 Oct 2017, 07:23

From a feeling I have (based on my 2002 Audi A6) this is more of an issue with CANBUS vehicles. They are very sensitive to anything electrical. My instrument cluster (among other things) died after jumpstarting it a few times after it sat for a while.

Post by **misha** » 24 Oct 2017, 08:10

Don't even think about running it without battery or with battery disconnected!

Post by **wizechatmgr** » 24 Oct 2017, 12:18

Don't do this... On old carbureted vehicles it wasn't too big of an issue. Same thing with old diesels that are mechanical only.

When you get into computer modules I've seen equipment that flakes out with as little as +-0.05V difference. Alternators produce dirty DC power - the battery essentially cleans it up instead of requiring a filter capacitor bank.

It "may" run, but I'm unsure if it will for long and what equipment it may take with it. Suppose this would make a good video for a vehicle on its way to the scrap yard.

Post by **abscate** » 24 Oct 2017, 12:19

I think the percent who think the moon landing was a hoax is still 5% and actually climbing. There probably decent intersection with the run a modern car without battery crowd

Post by **shaker_chi** » 24 Oct 2017, 13:34

Back in the day, a good way to test your alternator was to jump start the car and then disconnect the battery while it was running. If the car shut off, the alternator was bad. Wouldn't do that with anything built after 1990 though.

Post by **MrAl** » 26 Oct 2017, 09:03

Hi,

Thanks for the replies here.

I also realized that if we take our car to a mechanic for a new battery replacement, they do not simply start the car and then take out the battery and put a new one in. Instead they energize the system with a secondary 12v power source, then disconnect the battery, then put the new one in, then disconnect the secondary source. It would be nuts i think to try to do it by just keeping the car running with no battery even for a few seconds.

I cant understand how anyone would recommend this, but there are people who do. They dont take any responsibility though for what they are advising. And yes, it may come from experience with older cars.

Post by **misha** » 26 Oct 2017, 09:47

Older cars without computers and modules yes...that is one of the proper ways to test the alternator.
For everything with ecu and whole bunch of modules....NO!

Our old carbureted 244 could do this without problems untill a fuel injected 244 arrived.

Post by **jimmy57** » 26 Oct 2017, 14:57

CAN itself is not what makes a modern car sensitive to stupid tricks. It is the electronics fitted everywhere. Old clusters with rotating speedo cable and heated wire gauge movements had no electronics. When we wanted MPG trip computers and auto up/down windows and all the various gadgetry that brings microprocessor electronics, CAN just made comm between the modules easier with much more data and reduced wires.
I have had cars run with batt disconnected years ago when the alternator light was on and a rectifying diode was bad. If it runs w/batt disconnected that means the alternator has enough capacity for a few things. On a car with a perfectly working alternator and engine idling if you turn on enough things to draw more current than alt is providing it dies. It is the wet your finger to sense wind version of alternator evaluation.
There is a BIG reason to not do this: airbags. The SRS module has extensive safeguards but what if something is wrong? Doing silly things on airbag fitted cars is equivalent to walking across in front of a close speeding oncoming car because the law says he is supposed to stop for you. YOU PAY FOR THE FAILURE.

Too bad this question came up too late. I could have done a mythbusting exercise on a car I had to scrap for legal reasons 3 months back. It had operating systems and I could have abused it in many ways to see what happened.