When Boosting another Vehicle is this correct.

Somehow I'm thinking that that's not a good idea if you are the one that owns the charged battery. Correct me if I'm wrong :-)

Reply to
Jimmy
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Yep, you're wrong :-)

Nope, try it and see. Its an experiment anyone can perform in his own garage. That's also why you're always supposed to replace D-cells in your flashlight TOGETHER, because the new ones won't charge the bad ones, and the bad ones will go into reversal and leak.

Well, lets keep the discussion to lead-acid starting or deep-cycle batteries to keep it simple. A lead-acid battery has a current vs. voltage curve for each cell, and therefore for the total battery. Normal or "discharging" current can be defined as current delivered to a load by the battery, and reverse or "charging" current can be defined as current flowing into the positive terminal due to being connected to a voltage source. Open-circuit voltage (no load across the terminals) for a fully charged battery isn't 12.0 volts, its more like 12.8 to 13.0 volts. A very heavily drained (almost dead) battery will still have an open-circuit voltage of about 11 volts, but it drops quickly when a load is applied.

Starting with a charged battery and applying a load, you get a pretty linear chart of discharging current going to the load versus decreasing voltage at the battery terminals as the load is increased. At some point, of course, the battery starts becoming discharged and then both current and voltage begin to drop for a fixed load.

The funny thing is that if you then turn things around apply an external voltage to a partially discharged battery and simultaneously monitor the current going into the battery, you won't see much current going into the dead battery, even if you raise its terminal voltage to 12.8 volts (which is what would happen if you paralleled it with a fully charged battery of the same type.). The curve is not linear as you push the battery's terminal voltage above its normal open-circuit voltage for the first few volts. The reason is that in order to CHARGE the battery, you have to reverse the chemical reaction that produces voltage inside the battery in the first place, and to do THAT you have to push the terminal voltage up to around 13.5 to 13.8 volts. Try it and see- hook up a variable voltage supply to a discharged battery with an ammeter in line. Current will remain in the milliamp to a couple of amps range as you increase through 13 volts, but starting at around 13.5 the current will spike up sharply, and the battery will begin charging.

That's also why large banks of batteries connected together (used for many years as backup power in submarines, telephone exchanges, etc.) have to undergo an "equalizing charge" periodically- because they DON'T equalize with each other, and as they age their charging threshholds can begin to differ a little so that some cells are doing less work than others and are never really taking a charge. An equalizing charge pushes the terminal voltage well above the threshhold of all batteries, and brings them back into a similar state of charge.

If you go into a marine supply shop and look at high-end battery chargers you'll find that they have different modes such as "float" (keep the voltage above the natural voltage of the battery, but limit current so that you don't boil it dry- best for long-term preservation of an unused battery), "trickle" (charge at around 13.5 volts with the current limited to an amp or two to minimize electrolyte loss inside the battery), "normal" (charge at about 13.5 volts with no current limit other than the power supply of the charger), and "equalize" (charge at

15 volts or thereabout for a limited time to equalize cells- should only be done to fillable batteries as it boils away electrolyte).

Similar things happen with other types of batteries that are being used in everything from laptop computers to autonomous deep-ocean vehicles. Batteries such as silver-zinc, Li-SOCl2, Li-ion, and other exotics, but the behavior is very different in detail, sometimes involving "rapid disassembly with collateral damage" of the battery (that means she done blowed up) if the cells aren't kept properly equalized...

Reply to
Steve

Doesnt matter if it's a good idea, thats still the way it works, and "Steve" still doesnt know what hes talking about.

Reply to
bill

I suggest you do a little homework before you make another ASSumption.

You can verify my statements in your own kitchen (equipment required: two D-cell rechargeable batteries- one fully charged and one discharged- several pieces of wire, tape to hold the wire to the battery terminals, a voltmeter to verify the difference in open-circuit voltage between the charged and discharged batteries before you hook them together, and an ammeter to measure the current flowing from the charged battery into the discharged one when you hook them in parallel). That's assuming you continue to disbelive published literature on batteries or electrical engineers that work with battery systems for a living.

No skin off my nose either way.

Reply to
Steve

it's actually quite true and is commonly known by anyone with a tiny knowledge of electricity.... no matter whther it's dc as in a car battery, or ac as in a house..... this is why some elec motors are designed to use a higher voltage, so that they draw less amps..... here's a link for you to check out

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Reply to
tnuc

Reply to
Steve

Ed's post has to do with starter motors.. I'm commenting on your Amps/Volts relationship belief.. Amps don't increase because of voltage decrease..

Taking into account Ohm's Law, what has to happen for current to increase if voltage decreases?

Regards,

Jim

Reply to
Jim

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