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.

 

[dingbat]

This warning has nothing to do with charging the battery, per se.

It’s all about preventing voltage transients (spikes) that could occur in the process that could easily damage sensitive control modules in the ignition system.

Keep in mind that most electronic ignition systems operate off 5 VDC or less and are very sensitive to polarity.

I’ve seen more than a couple of ECU blown by people jumping lawn mowers and tractors

 

[jlh3rd]

I've always been taught, read, (various sources over 51 years) and believe that a battery acts as a surge buffer to any transient voltage spikes. Since an electrical system is designed to operate within the alternator's charging range, no system should be damaged.
I've also understood over these years that an alternator is not designed to handle the work load of charging dead batteries over an extended time...but we do it anyway.

 

[Lou C]

ARCO has some good tech info on their site about alternators, one thing I recall from reading it is that alternators are more of a battery maintainer than a battery charger and as Scott said, they don't charge much at idle. In fact, even with a healthy battery, if it's even just at about 50% charge it will take hours to bring it up to full charge (like 4-6 or even more depending on the battery capacity). Every spring I put the charger on my batteries, if one or both doesn't come back up to full charge I replace it.

ARCO Marine Tech Tip on Charging Systems for marine engines

Note: Alternators are not Battery Chargers, so our Charging Systems Tech Tip page will explain what you can expect from your system setup in the way of proper charging of batteries and what alternators can handle.

arcomarine.com

more on batteries:

https://cdn.shopify.com/s/files/1/0110/9521/2098/files/Arco_TechTip_BatteryFacts.pdf?v=1610991147&_gl=1*1ahjrl*_gcl_au*MTk5ODIyNjE3Ny4xNzg5NjQ3MTYwLi0uLS4xNzg5NjQ3MTU5LjE1Nzc0OTQyNTUuMTc4OTY0NzE2MC4xNzg5NjQ3MzIy*_ga*NjQwOTc2MjAxLjE3ODk2NDcxNjA.*_ga_P5FKNP5J0P*czE3ODk2NDcxNTkkbzEkZzEkdDE3ODk2NDczMjIkajEzJGwwJGgyMzY1NjYyNzQ.

 

[boscoe99]

"An alternator is a compact belt-driven generator that’s installed on an engine to convert mechanical power to electrical power. That electrical power helps recharge a connected battery and, more and more, is being used to directly power onboard systems."

https://www.mercurymarine.com/us/en/lifestyle/dockline/the-mercury-alternator-advantage--more-power--more-fun--

 

[boscoe99]

Is the thinking that boats should not have lights, radios, gyros, pumps, trim tabs, MFD's, etc. installed as this equipment would need to be powered by the alternator of an outboard motor?

 

[Lou C]

No but it means that it has to be sized in terms of amp capacity for what loads will be put on it, or else the batteries will never be fully charged. Standard old school alternators for inboards are like only rated @ 55 Amps! Later models are rated for more but you need to take the whole electrical system into account.

 

[jimmbo]

Myself, I would use a newer Smart Charger, at about 8-10 amps, as these can sense when a Battery has sulfated from a long period of discharge, and will adjust the Charging Voltage to clear up the Plates. If this requires removal of the Battery(s) from the boat, so be it...

 

[Mc Tool]

Myself, I would use a newer Smart Charger, at about 8-10 amps, as these can sense when a Battery has sulfated from a long period of discharge, and will adjust the Charging Voltage to clear up the Plates. If this requires removal of the Battery(s) from the boat, so be it...

I can recommend a Victron Blue IP22 smart charger....should have bought one years ago . My only beef is that the bluetooth range aint that great.