Do I need to unhook all the wires to my car battery before I use an electric battery charger on it?
What do the instructions say that come with the battery or the car manual? If you don’t have those then its safer in my opinion to disconnect both wires, yes.
I don’t see why it would be necessary. The float voltage for a lead acid battery is what? Around 13 to 14? That won’t hurt anything in your car. Besides, your car’s alternator+regulator is a battery charger, and the wires are hooked up when it charges your battery…
No, it’s not necessary to disconnect the wires. Think of it this way: The charged battery is putting a voltage (about 13.8V) across the electrical system already. Connecting a battery charger across the battery doesn’t change that, all it does is force a current through the battery to reverese the chemical reaction that creates electricity.
The answer is it depends.
It depends on how your car is wired, and how your charger works. If you charge the battery wrong the output voltage, on some chargers, may exceed 18 volts. On some cars this could damage electronic circuits (assuming that the ECU’s are live with the key off).
If in doubt, do this. Disconnect the battey negative cable. (always disconnect the negative cable first) Once the negative cable is disconnected you can charge away as the circuit is no longer complete.
On what setting to use on a charger. If you discharged the battery slowly (left the car parked for a long period of time) then charge the battery slowly.
If you ran the battery down quickly (left the lights on) then a higher rate can be used.
In any case if the battery starts to boil, you are charging at too high a rate, and are damaging the battery.
18V? Possibly, but I would find that hard to believe. I mean, I can understand if the charger’s open-circuit voltage is 18, but 18V when it’s connected to the battery?? Does a car battery’s internal resistance ever get high enough to warrant such a high supply voltage? Like I said, I guess it’s possible…
Even if the open-circuit voltage of the charger is 18 volts, it’s going to drop to 13.8 once it’s connected to the battery. Also, the discharge rate has nothing whatsoever to do with the rate you should use to charge it. A dead battery is a dead battery no matter if you used it up quickly or not. Some chargers have do two settings, “normal” and “boost”. Normal is a trickle charge and is best to use, since it doesn’t overheat the battery. Boost gives a high current to get your carr started quickly, but using this too long can heat the battery too much and cause water loss.
There’s no need to disconnect the battery with a properly working charger. If in doubt, monitor the voltage while charging. 16 volts is a maximum safe level, and I’d be uncomfortable if it were much over 15 volts. (The “12 volt” rating is nominal, a fully charged 12v battery at rest shows 12.6v, and the alternator probably charges at around 14.5v.)
Charging the battery at a slow rate is less stressful on it than charging it at a fast rate.
If the battery got severely discharged, be aware that its life has been shortened and its reliability is questionable. Unlike marine batteries, which are designed to be fully discharged in normal use, automotive batteries suffer if that happens.
Crafter_Man Yup the internal resistance does go up on a battery. There is a condition for wet cell batteries called sulphation (sp?). When the battery is in this condition it’s internal resistance is very high and the voltage required to overcome it can go sky high.
There is a test that used to be very common to check for sulphation called a three-minute charge test. The procedure was to charge the battery at a 40-amp rate (or as close to possible) at the end of the three minutes, check the voltage with the charger still running. If the voltage was above 15.5 volts the procedure was to charge at a very low rate (<3A) for a day to try and counter act the sulphation. I have done this test in class and have seen voltages over 18 V. This is the reason for my comment. Also if a battery is close to fully charged, the voltage required to push any more amps into it goes up. So if you connect a battery charger to a almost fully charged battery and set the charger to boost the voltage will skyrocket.
Q.E.D.
No the voltage won’t drop to 13.8 volts, unless the open circuit voltage of the battery is 12.6V. Under normal conditions a battery has an internal resistance of about .2 volts per cell. The charger has to overcome this .2V before any charging occurs. So if the battery is normal the charging current will be 1.2V (.2 X 6) above the open circuit voltage. So lets say you leave the headlights on and run the battery flat. When you first attach the charger the voltage from the charger may only be 3 or 4 volts. As the battery charges the voltage will come up. When the voltage is 13.92V the battery will be fully charged (13.92V-1.2V=12.72V=fully charged) 12.6V or above usually is considered a fully charged battery.
Just for general information here is open circuit voltages and percent of charge
>12.6V 100%
12.4V 75%
12.2V 50%
12.0V 25%
The cool thing about knowing these open circuit voltages is that you can tell how dead a battery is with a voltmeter, and when charging, buy subtracting 1.2V from the running battery charger voltage you get the current state of the charge.
Now the kicker here is that these numbers only work if the battery is being charged at a rate that it will accept. If you try and force too many amps into the battery the resistance will spike and so will the voltage. See my comments above to Crafter_Man.
This is wrong. I was on a task force in my company on reducing battery warranty expense. One of the big problems we were faced with was the problem of cars sitting on lot for extended periods of time and having batteries go dead from parasitic draw. Modern cars have multiple computers, radios, clocks and other items that draw small amounts of electricity. If left parked long enough the battery will go dead. What we found in our investigations was that technicians would quick charge these batteries that had been left alone to go dead, and with in a year the battery would be replaced under warranty. In working with our battery suppliers, we discovered that while the chemical composition of the battery plates does not change between a battery that is quickly discharged and on that went down over time, the surface is in fact different. On a quickly discharged battery the surface is soft and spongy and accepts a charge very easily. On a battery that was drained from parasitic draw over several months, this soft spongy surface had become hard and was very resistant to charging. Fast charging this battery will destroy it. (As our warranty figures showed.)
Anyway to make a long story short, we issued a special show charger to be used when the battery and had been discharged over time, and started teaching the technicians that when the battery had gone down slow, to charge it slow. The net result was a saving of a couple of million dollars in warranty expense. (Yes, I did get an atta boy for my work on that project)
Uhhh, so what are you saying? disconnect it?
I was going to do a slow charge on it anyways. My charger has 2 settings, fast, and slow. My battery slowly discharged over the winter.
Bottom line is this.
There should be no problem with charging the battery with the cable connected.
If you are not comfortable with the cable connected, disconnect the negative cable, it can’t hurt.
Do charge the battey on slow.
If the battery has been totally flat for too long a period of time, don’t expect much.
An alternative, which I use, is to use a battery charger that has a car starting option. I connect it with the battery wires connected, wait three minutes & start the car. The car alternator does the battery charging itself.
If it’s been a long time, you could just get a new battery.
I guess it depends on the design of the charger.
Yesterday I sent emails to the following inquiring about this issue:
http://www.automotive-battery-chargers.com/
http://www.battery-chargers.com
http://www.tractioncharger.co.uk/Leaflets/Traction.htm
I’ve heard back from two so far. Hopefully I’ll hear from the remaining. I’ll let you know the results.
That’s not what alternators are designed for, and it’s not a good idea. The two likely problems are the battery never getting fully charged and the alternator’s life being significantly shortened.
Two notes from experience;
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I always charge my dead batteries on the slow charge. I have tried the fast charge and the trickle charge always tends to get that “finished” voltage between posts the highest. You also prevent accidental boiling.
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The alternator never seems to get the battery back to normal if used as a charging source.
No cite, but I just feel more comfortable removing the battery from the car when charging it.