Current flow from good to dead battery

[mjoekingz28]

I was watching some good 'ole tractor pulling some time ago and saw where they would attach the rear tires together thru a hose to make sure the tire psi was absolutely even. The point being, the one with higher pressure would release into the one with less air.

Could this happen when trying to jump start a car via booster cables with the engine off? Would one battery of a higher charge exchange power/amperes/current/watts (whatever) with one of a lesser charge?


Or must the alternator be spinning to accomplish this?

 

[bioburner]

Basically what a jump pack does. Dead battery sucks the life from a good one

 

[BigCahuna]

You have to find out if the alternator in the vehicle with the dead battery is doing it's job. If it is, are the wires clean and tight?. If they are, then the battery needs to be checked and load tested. If the alternator won't charge it, jumping it just supplies the vehicle with enough power to start and run temporarily. A fully charged battery won't transfer all or even part of it's charge to a battery, that no longer has the capacity to hold a full charge.,,

 

[mk378]

The voltage from a resting charged battery is not high enough to push any meaningful charge into a dead one. Batteries are not perfectly efficient, there will be losses of energy. It's not like the situation with air in tires where there is no way for air to escape.

Systems with two or more batteries connected in parallel are common though, and it does work. Realize that the intent of this arrangement is not for one battery to transfer charge to the other one, and it cannot work that way. Instead, any charging or discharging affects both batteries the same, so they will stay at about the same state of charge during use.

 

[Virtus_Probi]

In general, connecting batteries directly in parallel is asking for trouble. Those suckers are wicked low impedance unless completely shot and they will never be at exactly the same voltage, so you'll get a current spike that should scare the crud out of you. This is why you bring the negative cable back to frame of the charging car when giving a jump, puts a little resistance in the path to cut down on the current spike.

The battery chargers (not lead acid) I designed supplied a tightly regulated constant current to a depleted battery that then smoothly approached zero as the target voltage (also very tightly regulated) was achieved. It was nice that the battery effectively looked like a big capacitor to help stabilize that high gain loop.

 

[eljefino]

There's a no man's land between charging and discharging. About 10% of voltage. So if you have a resting battery at 12.5 volts it'll take 13.8 volts or so to push more power into it. This is why dual battery systems need both batteries to be exactly the same-- otherwise the "lazy" one will sit out.

If you have a 40 amp alt making all 40 amps and you turn on another load, you'll very quickly see system volts drop from 13's into the 12's.

This is why cop cars have heavy duty batteries; they carry the lights and radio at idle.

 

[JHZR2]

Of course it will.

The delta in voltage will result in a current flow determined by the impedances of the source, load, and conductor.

The bad battery impedance will determine the overpotential required to get current to flow. Vt=Voc+IR for charging, so the open circuit, which is the unloaded electrochemical voltage requires a terminal voltage determined by the resistance. Vt and I are linked by the R of the battery...

I did a test one time, linked up a semi discharged (12.15VDC, IIRC) to a just topped off large battery. Not a huge voltage delta, but the topped battery had a surface charge so was showing 13V. Using 2ga jumpers, I recall the highest current flow I saw was about 4A...

 

[wrcsixeight]

Originally Posted By: JHZR2


I did a test one time, linked up a semi discharged (12.15VDC, IIRC) to a just topped off large battery. Not a huge voltage delta, but the topped battery had a surface charge so was showing 13V. Using 2ga jumpers, I recall the highest current flow I saw was about 4A...


I've done similar with similar results.

Instantly about 15 amps flowed, within 30 seconds this was down to less than 2 amps.

These numbers will vary on the level of depletion, battery health, and the circuit between the two.
Once good battery at 12.8 has fed bad battery at 12.0v for a little while, there is little voltage Delta and little current flow between the two.