Jump starting dead cranking battery off trolling motor battery

[Stizo]

A buddy told me he fried some electrical components in his motor (115 Evinrude) and was told by a mechanic he probably was to blame because he had previously jumped his drained cranking batery (on a few occasions) with jumper cables off his trolling motor battery. Didn't give it much thought until I had no choice but to do the same thing this past summer when my battery took a dump. My motor is fine but now I'm concerned about what to do if the same thing should ever happen again.

Can anybody offer any insight on this subject.

Thanks,

Stizo

[Capt. H (Ret.)]

I can't tell you why but I can tell you that I bought a 2007 Mercury Opti in June and the dealer took my nearly new crankin/deep cycle battery out of the boat and put in a new crankin battery. I was in a hurry to get the new motor fired up so I didn't ask him why but he said not to use a crankin/deep cycle battery. He also used nylok nuts on the battery terminals and said not to use the wingnuts that he had used on the previous motor/battery. I'll ask him why when I have the motor winterized.

[Surface Tension]

Auto and outboard charging systems are not intended to charge dead batteries. Your outboards battery should dedicated to only the outboard, nothing else. The larger the outboard the more critical that that battery be a fully charged cranking battery. Deep cycles don't have and aren't intended for those quick/large draws. When that can't be delivered because the battery is weak or its a deep cycle, electrical components get hot and blow. A good clean connection is also important. Thus the reason for lock nuts and not wing nuts. Afterall your cars battery cables don't use wingnuts.

If your have a weak battery in your car or boat get rid of it.

Jumping batteries. We know that a weak, dead batteries and bad connections are hard on the system. Jumper cables aren't good connections. It be better to directly connect the good battery to the outboard then jump it.

So, if your outboards battery is bad or weak replace it.

Don't run accessories on a cranking battery. Those low and long draws will reduce the battery life.

[BobT]

Another point to keep in mind.

When a battery is low on power it is unable to maintain the voltage level required by the starter drive. The starter drive will demand the same "power" or wattage no matter what the battery can deliver. If the battery's output voltage is low due to lack of charge, the starter motor will still demand the same power draw and that requires more current (P=VA). The increase in current (amps) is what will heat up the wires and motor windings and this can be damaging.

Bob