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How to jump off only a battery

Have a 99 chrysler t&c. The engine seems to be in good shape an starts reliably if the battery is not dead, even at 10f below. The problem is that every once in a while the battery looses the charge for an unknown reason . The car turns over slowly and the solenoid chatters in and out. Happens several times a month; I’ve started carrying a spare battery with a fair amount of reserve capacity and 4 ga. cables. Even then, it is iffy as to whether the car will start. It cranks slowly and the solenoid may still drop out.

This has happened even over a short period of time such a s grocery shopping.

I researched one of those lithium battery packs that can be inserted into the cigarette lighter. My understanding is that the emergency pack has to recharge the main battery for some time. It makes sense that you don’t want to run starting current through the aux. voltage outlet.

Should I be letting the spare battery recharge the main battery for a few or more minutes?

I would try and fix the original problem. Those back up batteries are okay for jump starting.

You probably have an intermittent parasitic draw. Maybe a light in the glove compartment stays on. These cars are known for their BCM (Body control Module) going bad and causing all sorts of wicked problems.

A shop with expertise in car electrics should be able to figure it out.

Don’t use the jump pack batteries that hook through the power port. You are correct, they just can’t deliver the current. The jump packs with great big cable clamps are the best to “jump start” the car.

That said, is may or may not be a battery. If the jump pack is connected and the solenoid still chatters, it may be a starter or the cables from the battery to the starter. If you want to keep driving this car, you need to get this fixed, not just buy another jump pack.

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op here. While I won’t argue that it would be best to find and repair the problem, that wasn’t what I asked.

If I get a jump from another running car, I’ve got plenty of juice to start. If I jump from an isolated auto battery, I am trying to start the engine and charge the car battery at the same time. This is a heavy draw because the car battery is sucking all the current.

I’m thinking that I might be better off to try and bring the car battery charge up by letting the spare recharge it for a while. It is always better to use a power supply sized for the job. If you use, e.g. several batteries in parallel, a battery with a low internal resistance may wind up supplying a large fraction of any current rather than just half.

To explain in another way, you have the spare battery trying to feed a high impedance starter motor and a low impedance battery in parallel. All the current goes through the low impedance battery.

I have a charger with a boost function for this type of situation and it does the job. My situation is a little off the beaten path that people may not have faced before.

Don’t know about newer cars so check out potential issues, but in theory you could disconnect a post from your battery and hook the jumper cable to the post, though depending on how you do it the brains of the car will probably get reset.
I think some of the newer cars prefer a connected battery, there would be a greater chance of sparks and a battery explosion, maybe the ground connection to the car would be a better option.
Just a theory, waiting for persons with more experience to concur or disagree.

I would prefer a fix rather than a band aid, this might be helpful.