Jump Starting

[reeac]

Most articles about jump starting refer to the use of jump cables in a car to car situation. I've done a bit of that myself in the past. I'm a bit puzzled about the use of a battery pack though as although I owned one as a precautionary measure for some years I never had to use it and finally it refused to accept charge and so I junked it - a replacement battery seemed to cost as much as a complete pack. I see from reading the user instructions for a Clarke 5 in 1 model that you need to wait two minutes after connecting the pack in parallel before trying to start the engine. To me this is counter-intuitive as I fear that the 18 Ah jump battery would lose all its charge into the flattened 180 Ah boat batteries. If this is likely to happen then my instinct would be to operate the starter immediately after connecting the battery while the jump battery has most of its charge available. The only justification for the two minute delay might be if there is an immediate creation of surface charge within the flattened boat batteries which could give a useful boost to their output voltage. Anyone any practical experience of this situation? Is such a delay advantageous? Would it be better to boost one of the two boat batteries?

 

[VicS]

Most articles about jump starting refer to the use of jump cables in a car to car situation. I've done a bit of that myself in the past. I'm a bit puzzled about the use of a battery pack though as although I owned one as a precautionary measure for some years I never had to use it and finally it refused to accept charge and so I junked it - a replacement battery seemed to cost as much as a complete pack. I see from reading the user instructions for a Clarke 5 in 1 model that you need to wait two minutes after connecting the pack in parallel before trying to start the engine. To me this is counter-intuitive as I fear that the 18 Ah jump battery would lose all its charge into the flattened 180 Ah boat batteries. If this is likely to happen then my instinct would be to operate the starter immediately after connecting the battery while the jump battery has most of its charge available. The only justification for the two minute delay might be if there is an immediate creation of surface charge within the flattened boat batteries which could give a useful boost to their output voltage. Anyone any practical experience of this situation? Is such a delay advantageous? Would it be better to boost one of the two boat batteries?

The intructions say, "2. Switch the vehicle ignition ON, and leave in this condition FOR APPROX
TWO MINUTES. (This will provide the vehicle battery with a short ‘boost’
charge to allow for easier starting)."

Perhaps the reason is to provide the vehicle battery with a short boost charge to allow for easier starting

 

[pvb]

Why would it be necessary to turn the ignition on? The boost charge would start as soon as the pack was connected.

 

[theoldsalt]

.....or perhaps connect the emergency pack directly across the starter motor that way the boat batteries will not drain the emergency pack.

 

[Jcorstorphine]

Deleted my last post due to finger trouble. I recently jump started my sons 2 litre diesel. While we were waiting for the two mins, the cables got pretty hot and I would guess the current was in the order of 50 to 70 amps. After 2 mins we tried to start his car but without sucess. We waited a further 5 mins and got it started. Point being, the two mins gives time (with the engine running) for the dead battery to recover. Other point is the high current during the two min wait will lift the temperature of the battery acid which will increase the state of charge due to the fact that chemical reactions double for every 10 degrees.

 

[reeac]

.....or perhaps connect the emergency pack directly across the starter motor that way the boat batteries will not drain the emergency pack.

You'd need the panel switch (mustn't call it ignition) on so that the alternator was connected to the boat batteries when the engine started.

 

[VicS]

You'd need the panel switch (mustn't call it ignition) on so that the alternator was connected to the boat batteries when the engine started.

The "ignition" or panel switch would not normally disconnect the alternator from the battery. Only the battery isolator should do that.

It has the be on to provide power for the starting circuit, preheater circuit ( although both of these may involve relays as well) instruments and warning lights but like PVB I cannot see the point of switching it on for 2 minutes .

 

[Avocet]

Presumably, the total amount of energy in the "system" (i.e. the two batteries connected together) remains the same? In which case, I don't suppose it matters too much how long the two are connected before you attempt to crank. I guess if the leads are a bit on the weedy side, it's better to leave it for longer so that the current flowing through them when the starter is operated is reduced by the amount of energy that has flowed into the flat battery?

 

[reeac]

The "ignition" or panel switch would not normally disconnect the alternator from the battery. Only the battery isolator should do that. It has the be on to provide power for the starting circuit, preheater circuit ( although both of these may involve relays as well) instruments and warning lights but like PVB I cannot see the point of switching it on for 2 minutes .

I always understood that you should never switch this switch off when the engine is running because it would isolate the alternator from the batteries . Some marine engines and presumably diesel cars have a solenoid operated fuel valve which stops the engine when this switch is operated so no problem but my VP 2002 and my Kubota tractor have a manually operated fuel shut off.
Re. The point about total charge energy, I've wondered about this ..if the 18Ah jump battery is capable of starting the engine then there should be even more chance if it's paralleled with the boat batteries but I don't know whether total charge energy In the battery array is the relevant criterion.

 

[[70521]]

I have a handle to do that. Must get round to trying it one day.

 

[reeac]

I have a handle to do that. Must get round to trying it one day.

Good luck with that! I sheared off the projecting pins that the handle engages with when I tried. Haven't managed to remove the remaining bit so can't install a new pin. I believe that VP can supply a relevant tool but at considerable cost. My two Elecsol batteries managed 10 seasons before failing so can't grumble.

 

[VicS]

I always understood that you should never switch this switch off when the engine is running because it would isolate the alternator from the batteries . Some marine engines and presumably diesel cars have a solenoid operated fuel valve which stops the engine when this switch is operated so no problem .

And you will often find that the engine manual tells you not to switch off at the key/ "ignition" switch while the engine is running. However if you turn to the back of the book and study the electrical diagram you'll find that it does not in fact disconnect the battery from the alternator.
With simple yacht engines where they are stopped by the operation of a solenoid valve I think you will find that is usually an "energise to close" valve, which is why you have to hold the key in the "stop"position against a return spring not merely turning to off or keep a stop button depressed until the engine has stopped turning. This is unlike diesel cars where the solenoid valve is energised to open and closes as soon as the key is turned to the off position.

Large and modern electronically controlled diesel engines may well operate differently but I'd still not expect to find a situation in which switching the "ignition" switch to off actually disconnects the battery from the alternator.

 

[Jcorstorphine]

Totally agree with VicS. When my sons car battery went flat we took it to the main dealer who checked the charging rate which was initially 70 amps so doubt if the ignition switch would cope with 70 amps let alone the wiring.

Going back to OP question about the two mins. The initial current you can draw from a battery is dependent on the Potential Difference, the resistance of the motor and the internal resistance of the battery. When the standby unit is connected to the dead battery, there will be a heavy draw on the standby battery trying to lift the voltage of the dead battery thus limiting the current available to start the engine due to (a) the drop in voltage due to the current drain (b) the internal resistance of the standby battery. In other words, if you try to start the engine without waiting, you will have two parallel loads on the poor little battery naimly, the starter and the battery.

 

[William_H]

Firstly the "ignition" switch in usual style of diesel engine provides power to the starter solenoid the instruments and also the initial excitation of the alternator. Once running and charging an alternator can self excite while output is permanently connected to the battery. (via a battery isolation switch)
Re jump starting using one of those battery pack jump starters. Firstly much depends on why the original battery won't strt the engine. If it is flattened by discharge but is a good battery then yes the jump start battery could provide enough charge in waiting 2 mins for most of the start current to come from the recharged main battery. However if the main battery is simply tired and unable to provide the start current (a common failure mode) then it is probably charged as much as possible. In this case the jump battery will add more current for the starting. The amount depending on the capability of the little jump battery and the size of the jump start leads. In this case start as soon as possible after connection of the jump battery.
If the main battery is dead from a dead cell then it is likely you would be better off with the main battery disconnected. (perhaps not so common)
What is important is to be wary of sparks when connecting and of the possibility of Hydrogen explosion. Especially if the battery has been on charge. Connect the leads to the main battery then to the jump battery so sparking is remote from the big battery.
IMHO for a boat those jump start boxes are more a gimic than anything else. The battery is obviously small and you would be better off having a proper alternative (dual) battery arrangement of decent sized batteries. Always used, always charged. But always start on only one battery. Keeping the other in reserve.
good luck olewill

 

[reeac]

Thanks, everyone, for your contributions. There seems to be some mystery regarding the 2 minute pause, the role of the switch and also whether total charge energy in the batteries is the important criterion. Another factor regarding the usefulness of a jump start pack is whether it might be useful in a car starting role. We have three cars with batteries aged nearly 13, 10 and approx. 4 years so one of them' going to pack up sometime soon. Fine if it's during the boating close season so the pack is at home .... . The problems of a consumerist society!

 

[lw395]

Only time I've tried using a jump pack to start a car, it failed miserably. 2 litre petrol Mondeo, flat battery due to courtesy light left on. It was cold to be fair, the light was left on due to the door being frozen in the morning and not shutting properly. In the evening, a properly flat but probably low impedance battery?
I can only guess that letting the batteries share the charge hopes to get a lower impedance and thus more start current. sounds like a compromise against the lost charge due to inefficiency and lower volts, although the effect of warming the battery may help in cold weather.
Some BMW bike owners used to swear by insulating the battery, an attempt at starting would heat the battery, leave it a minute or two and it would go.