Starting engines with jump box & dead house batteries

[deeman7777]

Long story short, I overheard a well respected mechanic tell a group this info.

Story:
The boat had 2 dead batteries since it was in storage for a year. The boat owner was going to use a small jump box connected to one of the dead batteries to start the engine and let the engine sit at idle to charge the batteries. Shore power was not an option.

The mechanic said NOT to do this since the engine alternators would detect a low voltage of the dead batteries and send high amperage to recharge the batteries, which can be dangerous and possible fry some electronics in the process. The mechanic really pushed to have the boat connected to shore power (not convenient at the moment) and to let a battery charger slowly ramp-up the charging amps.

Thoughts on NOT using the engine(s) to charge dead batteries??

 

[gman546]

The warning is valid, but the explanation is a bit off. The alternator isnt going to send dangerous amperage through the electronics, but trying to charge two completely dead batteries can make it run at full output for a long time and overheat or damage it, especially at idle. I would charge and test the batteries first, then let the alternator top them off.

 

[boscoe99]

The mechanic does not understand basic electricity. Sounds like he is regurgitating something he read on the internet.

The world has been jump starting dead batteries and using the alternator to charge them since alternators (and older style generators) were discovered.

 

[Scott Danforth]

if the batteries sat for a long time in a discharged state, they may never take a charge.

best is to pull them and put a charger on them.

alternators at idle do not really put out much current. most require the engine to hit well above 1000 RPM to even start to put out power. if the boat is in the water, then I would get it started and run the motor up to about 1500-1800 RPM. if its on a hose, do not go much over idle.

 

[MikeSchinlaub]

In that Tige I did a little while ago, the panel to the batteries had a sticker that said to disconnect the batteries before charging or electrical damage could occur. So maybe there is some truth to what that mechanic was saying.

I'm not an electrician though, way out of my league.

 

[Mc Tool]

I dont think anything would be damaged but if the batteries are "push it under the door " flat then removing them and using a smart charger in recovery mode is probly the best option

 

[boscoe99]

In that Tige I did a little while ago, the panel to the batteries had a sticker that said to disconnect the batteries before charging or electrical damage could occur. So maybe there is some truth to what that mechanic was saying.

I'm not an electrician though, way out of my league.

Batteries are being charged and recharged every day by most every type of motorized vehicle. Strange sticker.

While at the same time an alternator is powering more and more electrical components in cars, trucks and boats.

 

[boscoe99]

Mechanic sounds like the type that cautions not to store batteries on a concrete floor.