Testing out my jump starter…

circa1971la

circa1971la

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Nov 23, 2018
22
Schock Harbor 25 Los Angeles
Hi I have a portable jump starter (Schumacher SJ1332 Rechargeable AGM Jump Starter for Gas Diesel Vehicles - 1200 Amps). I’d like to test it if will start my Yanmar diesel 2YM15 in case my batteries ever died. To test this can I disconnect the cables from my battery and connect them to the cables of this jump starter? Thank you !
 

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jssailem

jssailem

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Oct 22, 2014
21,113
CAL 35 Cruiser #21 moored EVERETT WA
Sounds like a logical approach. Positive to positive. Negative to negative. Like sticking a new battery into the system. Be sure the connectors of mot cross touch or that in starting the positive slide about and contact the engine.
 
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Davidasailor26

Davidasailor26

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May 17, 2004
5,080
Beneteau Oceanis 37 Havre de Grace
I agree - sounds good. Be sure not to disconnect the jump pack until you shut the engine down, or you risk damaging your alternator depending on how it’s wired.
 
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sailme88

sailme88

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Jan 19, 2010
1,171
Catalina 34 Casco Bay
NO !! you need to keep the batteries connected to the alternator.. After the engine fires the alternator will be producing electricity. With the batteries out of the loop the power will have no place to go. This is like turning off the key before pulling the engine stop...
 
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dLj

dLj

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Mar 23, 2017
3,425
Belliure 41 Sailing back to the Chesapeake
sailme88 said:
NO !! you need to keep the batteries connected to the alternator.. After the engine fires the alternator will be producing electricity. With the batteries out of the loop the power will have no place to go. This is like turning off the key before pulling the engine stop...
Click to expand
As @davidasailor said, just don't take the jump cables off until you shut down the engine. The alternator will be charging the jump start battery.

dj
 
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Davidasailor26

Davidasailor26

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May 17, 2004
5,080
Beneteau Oceanis 37 Havre de Grace
sailme88 said:
NO !! you need to keep the batteries connected to the alternator.. After the engine fires the alternator will be producing electricity. With the batteries out of the loop the power will have no place to go. This is like turning off the key before pulling the engine stop...
Click to expand
The alternator excites its field by sensing the voltage from the battery (or whatever it’s connected to). If it’s connected to the jump pack it’ll sense the jump pack voltage and excite its field enough to bring the voltage up to ~14.3V.

The trouble comes when the alternator is charging and you suddenly disconnect it. In that case the moving electrons suddenly have nowhere to go, and they blow out the diodes.
 
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D

dlochner

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Jan 11, 2014
11,442
Sabre 362 113 Fair Haven, NY
dLj said:
As @davidasailor said, just don't take the jump cables off until you shut down the engine. The alternator will be charging the jump start battery.

dj
Click to expand
Davidasailor26 said:
The alternator excites its field by sensing the voltage from the battery (or whatever it’s connected to). If it’s connected to the jump pack it’ll sense the jump pack voltage and excite its field enough to bring the voltage up to ~14.3V.

The trouble comes when the alternator is charging and you suddenly disconnect it. In that case the moving electrons suddenly have nowhere to go, and they blow out the diodes.
Click to expand
This assumes there is not a blocking diode on the cables that prevents charging the battery from the cables.

If the house and start batteries are isolated and the alternator feeds the house batteries, then disconnecting the start battery and using the jump pack to start the engine will be fine.

Of course prudent battery management in a well designed electrical system with a good system monitor will eliminate the need for a jump pack.
 
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Davidasailor26

Davidasailor26

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May 17, 2004
5,080
Beneteau Oceanis 37 Havre de Grace
dlochner said:
This assumes there is not a blocking diode on the cables that prevents charging the battery from the cables.
Click to expand
If there is a blocking diode on the jump pack cable, and the alternator is configured to use the start battery cable as the sense, then the field will excite just enough to bring the cable up to charging voltage. Still shouldn’t do any harm - the alternator will just be hardly excited, generating just enough power to hold the cable at voltage. Basically like having a fully charged battery there.

I agree that if the alternator is wired to the house bank there’s no risk anyway.
 
kloudie1

kloudie1

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Nov 6, 2006
9,894
Hunter 34 Mandeville Louisiana
If you can see and easily access the large "hot" starter lead, you can connect the jumper positive there and avoid the losses in connections and cabling.. The negative would go to the engine block or an attached bracket..
 
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BarryL

BarryL

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May 21, 2004
1,010
Jeanneau Sun Odyssey 409 Mt. Sinai, NY
Hello,

You don't need to disconnect BOTH cables from the engine battery. I would leave the negative on the engine battery, disconnect the positive from the battery, and use the jump pack cables to connect positive to the positive, and negative to the engine battery negative.

Barry
 
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LloydB

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Jan 15, 2006
821
Macgregor 22 Silverton
" I’d like to test it if will start my Yanmar diesel 2YM15 in case my batteries ever died." it is a booster not a replacer so keep in mind if you have a dead short on your start circuit that will also short out the jumpstart if you connect it.
 
Ward H

Ward H

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Nov 7, 2011
3,651
Catalina 30 Mk II Barnegat, NJ
Consider the engine has a dumb alternator/regulator and just the normal Bat 1, Bat 2, Both, Off switch.
Engine starts and Alt starts pumping volts and amps.
Without a dead battery to absorb this energy, the alt is sending it to the jump pack.
What does that do to the jump pack?
 
Davidasailor26

Davidasailor26

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May 17, 2004
5,080
Beneteau Oceanis 37 Havre de Grace
Ward H said:
Consider the engine has a dumb alternator/regulator and just the normal Bat 1, Bat 2, Both, Off switch.
Engine starts and Alt starts pumping volts and amps.
Without a dead battery to absorb this energy, the alt is sending it to the jump pack.
What does that do to the jump pack?
Click to expand
From the jump pack’s perspective it’ll be connected to a voltage source providing about 14.3 V. That’s no different than being connected to a full battery on a charger.
 
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LaDonaMina

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Mar 7, 2023
64
Hunter 28.5 Savannah
I have the Jeep JumpBooster and use it on the H285 and on my Sea Ray. When I start the engine it stays on till I remove the booster several minutes later with no ill effects to the ignition system or to the jump booster. Not sure what many are saying regarding doing damage to the system/booster. The jump booster I have also has the alternator test indicator that monitors the system for problems and it’s never highlighted any issues. The screen grab is for the Dewalt unit but the Jeep is the rebranded version I have.


IMG_6672.png
 
Davidasailor26

Davidasailor26

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May 17, 2004
5,080
Beneteau Oceanis 37 Havre de Grace
LaDonaMina said:
I have the Jeep JumpBooster and use it on the H285 and on my Sea Ray. When I start the engine it stays on till I remove the booster several minutes later with no ill effects to the ignition system or to the jump booster. Not sure what many are saying regarding doing damage to the system/booster.
Click to expand
If you have a battery connected there’s no harm at all from removing the booster. The battery will continue to absorb the charging current so there won’t be any voltage spike.

The risk comes in if the boost pack is absorbing current recharging itself and then is disconnected with no parallel battery to absorb that current. If the pack is already recharged before disconnecting there shouldn’t be much risk either - in that case there’s no big current being suddenly dumped.
 
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LaDonaMina

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Mar 7, 2023
64
Hunter 28.5 Savannah
This sounds contrary to most car mechanics that told me one way to check the alternator or battery for faults is to start the car and disconnect the negative terminal off the battery and if the car continues to run the alternator if functioning fine. If the car stalls and dies the alternator if faulty and needs either the diodes/regulator replaced and or entire alternator replaced. This should be about the same with the boat alternator although I’ve never done that. Usually just boost it with the jump starter and then remove it after it’s running.
 
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dLj

dLj

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Mar 23, 2017
3,425
Belliure 41 Sailing back to the Chesapeake
LaDonaMina said:
This sounds contrary to most car mechanics that told me one way to check the alternator or battery for faults is to start the car and disconnect the negative terminal off the battery and if the car continues to run the alternator if functioning fine. If the car stalls and dies the alternator if faulty and needs either the diodes/regulator replaced and or entire alternator replaced. This should be about the same with the boat alternator although I’ve never done that. Usually just boost it with the jump starter and then remove it after it’s running.
Click to expand
In a car with a lot of electronics that run the car that may work. If running an older diesel that is not computer controlled it would tell you nothing and could potentially blow your diodes depending upon what current was coming out of the alternator at the moment you pull the negative off.

dj
 
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Scott T-Bird

Scott T-Bird

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Oct 26, 2008
6,085
Catalina 320 Barnegat, NJ
I think these portable jump starters are just the dumbest idea. If you are smart about maintaining your batteries, you will never need it. Don't bother testing it, you should never need it ...
 
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JoeWhite

JoeWhite

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Mar 6, 2008
1,099
Catalina 1999 C36 MKII #1787 Coyote Point Marina, CA.
If this helps, let me tell my story on my M35B Universal engine.
The alternator bracket broke and I replaced it. The new bracket was heavily painted including its mounting hole to the engine. Shortly after I installed it the alternator failed. The reason was that the painted hole provided unreliable ground to the engine cutting off the current and causing damage to the alternator. When I replaced the alternator I also ran a #6 wire from its ground lug to the battery shunt.
So interrupting alternator current will damage it.
 
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