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Which battery for a jump start
- Thread starter Chas
- Start date
I may be wrong but I always use the one the alternator is connected to in order to reduce the load on the system.
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I am here: http://maps.google.com/maps?ll=55.906468,-3.241997
The reason I ask is because on the way back from Croatia I needed a few jump starts and I had always thought the one on the drivers side was the one to use, but no matter how long we left the donor vehicle running to hopefully put a bit of electrickery into mine it just would not start, 
but when we tried the passenger side one it fired up instantly 
but when we tried the passenger side one it fired up instantly
Perhaps the pax battery is fully knackered and not holding charge?
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I am here: http://maps.google.com/maps?ll=55.906543,-3.242066
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I am here: http://maps.google.com/maps?ll=55.906543,-3.242066
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It's not safe to jumpstart via the passenger side battery. 
Due to the 12v/24V starter relay, during starting, the passenger battery is connected in series with the drivers battery, which means the negative terminal voltage becomes 12 Volts (instead of 0 Volts or earth) and the positive terminal becomes 24 Volts (only when you're batteries are both in good order, when you've got a failing battery you're obviously not going to see full voltage).
If you connected the negative jumplead to earth, e.g. the engine lifting lug or a point on the chassis, this would be safer but it's still not a good idea, as a minimum you're risking the starter relay & wiring, let alone what you could backfeed to the jumper battery.
Due to the 12v/24V starter relay, during starting, the passenger battery is connected in series with the drivers battery, which means the negative terminal voltage becomes 12 Volts (instead of 0 Volts or earth) and the positive terminal becomes 24 Volts (only when you're batteries are both in good order, when you've got a failing battery you're obviously not going to see full voltage).
If you connected the negative jumplead to earth, e.g. the engine lifting lug or a point on the chassis, this would be safer but it's still not a good idea, as a minimum you're risking the starter relay & wiring, let alone what you could backfeed to the jumper battery.
Brett said:It's not safe to jumpstart via the passenger side battery.
Yeah - I know that NOW!...
Fortunately no harm done...
I can see what you mean, but how do you explain my situation when the car wouldn't start when jumped from the drivers side battery but did from the passenger side and both batteries were then taken out, fully charged and load tested at my local auto LX and both passed OK, so that would preclude one of the batteries being duffBrett said:It's not safe to jumpstart via the passenger side battery.
Due to the 12v/24V starter relay, during starting, the passenger battery is connected in series with the drivers battery, which means the negative terminal voltage becomes 12 Volts (instead of 0 Volts or earth) and the positive terminal becomes 24 Volts (only when you're batteries are both in good order, when you've got a failing battery you're obviously not going to see full voltage).
If you connected the negative jumplead to earth, e.g. the engine lifting lug or a point on the chassis, this would be safer but it's still not a good idea, as a minimum you're risking the starter relay & wiring, let alone what you could backfeed to the jumper battery.
