Battery charging



Is it a problem to jump start or charge a battery without removing the battery cables.

I have been driving since I was 16 and am now 71 and have been jump starting & charging batteries for a long time. Now I am told that I must remove the battery cables before charging.

P.S. Also, I have a 2 battery system on my boat with an onboard charger that I plug in at the end of the day without unhooking cables. Is there a difference between battery in car/boat?


Yes. The car has terribly expensive junk connected to it. There is virtually nothing on the car which will prevent damage to the expensive stuff. You were told by what source? Is it an owners’ manual or some mechanic. There are lots of things I would recommend when jump starting a car. Eye protection, gloves and a space suit if you have an old one around the garage. The source of your information matters. It has always been best for the battery and the charger to disconnect the cables but it was usually inconvenient. Lots of things change over the years. I get coffee at McDonalds now.


I think the warning is for the protection of the electronic systems that are used in cars today. A lot has changed in the cars since you were a young driver (and me too). The greatest danger in damage to the electronic systems is electrical spikes, like when you first touch a chargers second lead to complete the connection to the battery. You can help keep those pulses down to a minimum by first connecting to the battery and then plugging in the charger to the AC outlet. When making a jump, place the second lead to the ground connection as quickly as you can with minimum sparking.

When I charge a low battery I don’t remove the cables first but the ignition should be OFF at least. I have never had a problem doing it this way. If you want to be absolutely safe though you can remove the ground lead to the battery at least.

I assume the boat charger is just a small trickle charger and won’t over charge the batteries if left on for an extended period of time. Marine batteries are built slightly different from automotive batteries since they have a different service electrically and physically.


The warning is there because people are basically morons and that statement provides some protection. I design electronics and there is no basis for the warnings if done correctly. I have never done it or ever will.


I have read about the risks, but have always taken my chances and jump started with the cables attached. When you are stranded it is too much of a problem to disconnect the cables–and AAA doesn’t do that anyway.


No, you need not disconnect the cables when charging and you MUST NOT disconnect them when getting a jump start. Once your car is started, how would you reconnect the cables? Not a good idea to do it to a running engine.

The advice to disconnect the cables for battery charging is offered because there are a lot of dummies out there who reverse the polarity. While this is not good for the battery, it can devastate some other electronics. So leave the cables in place, but doublecheck before applying outside voltage. This advice applies to either cars or boats.


My wife has a set of jumper cables that are totally idiot-proof. Youn can connect them anyway you want, and they automatically connect the right polarity. Only $35 at Walmart!!


And you lose a few volts because of the diodes.


What’s a few volts between friends?

A typical diode will drop about 0.8 volt - if you have 2 in series (which you do with those cables) you drop 1.6 volts - a properly operating alternator generates about 14.6 volts under load - so you have plenty of power left to charge the other end.


The danger to the car’s electronics is from voltage spikes produced, also. Every time the jumper cable clamps are attached, or adjusted (momentarily losing contact with the battery cable terminals), there are large, damaging, voltage spikes produced. The results, to the car’s electronics, may not be felt for months.

  1. Turn OFF the ignition switches, in BOTH cars.
  2. Connect positive jumper cables to positive battery cables.
    3.Connect the negative cable clamps to clean, unpainted metal, on engine block, transmission block, or fender.
  3. Start donor car, and let it charge the other car a few minuets.
  4. Start the re-charged car. If it’s still too weak, charge a few minuets more.
  5. With the car now started (hopefully), don’t remove jumper cables from EITHER car.
  6. Run the car a few minuets to warm its engine.
  7. Turn OFF both cars.
  8. NOW, you can disconnect the jumper cables from both cars!
  9. Restart both cars, and away you go!

IF the car stalls, re-start from step 1.
This procedure may seem tedious, but, there are people who started having problems with their car some time (days, weeks, months) after giving somebody a jump start. This procedure will lessen your chances of joining that unhappy group.
PS: These pages are downloadable and printable (fyi).


Well, have you ever tried to jump start a car without the battery cables in place? I’d like to see that done! The idea behind removing the battery cables when charging is so that the flow of energy doesn?t have to circulate through the car to charge the battery, and that means that you wont be trying to pass all that charging electricity through the regulator in the alternator.
Your boat is a different matter. The charging system is simple. It doesn?t have to pass through a bunch of circuits to charge the battery.


As far as jump starting, I just don’t do it unless an absolute necessity like wife stuck at airport because trunk light was on. As far as charging, I’ve just never had a problem with the electronics but I always connect the charger first and then plug it in. There are no sparks or voltage spikes that way. Actually I’m more concerned about explosions from the outgasing than the electronics.


Just exactly how large do you suppose those “large, damaging, voltage spikes” can be, since both systems involved are 12 volt?


I just carry a portable battery pack for jumping people and save several steps.


At the time of charging, they can’t both be 12 volts. If they were, one would never recharge the other: since they’re at the same electrical potential. What is missing?


14.5 - 1.6 = 12.9 The battery at best will get a fractional charge. They don’t use diodes.


A manufacturer can’t control what is used for a charger. These can range from really nasty 100A boost chargers to welders. Heck, I even used a kitchen toaster to start a car. Some of these can produce over the 80V car electronics was designed to handle. You are at greater risk from the muffler shop using a Mig welder on your vehicle.


The semiconductors in most electronic circuits operate at 5 volts or less. 12 volts is more than double their working voltage.


The point is, that while one battery may be at a lower potential than the other, both are 12 volt systems. Even with normal charging systems the voltage will never exceed 15 or so volts. Hardly a large damaging spike.

As for the electronics operating normally at 5 volts, and the batteries operating at more than twice that, normal operation would ALWAYS be applying 12 volts to the parts. There’s a serious flaw in that argument.