Auto Battery Chargers

jerry_njOctober 24, 2005

I've been having some problems with my automatic (shuts down when the battery is charged) auto battery charger - 12VDC. Looking at what's available at WalMart I see they carry both Black and Decker (new one on me) and the old standby Schumacher. Both offer automatic chargers that include digital readouts for voltage and amperage (same readout)and claim to have some diagnostic capabilities, all for about $50 for a 10A max charge. Larger chargers are also available at additional cost.

Any recommendations out on this forum? I could not find any history when I searched this forum for Battery Chargers.

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Both battery chargers you mentioned here are known as trickle chargers. For most peoples use, they are just fine. It's important to understand just how long it really takes to charge a battery fully. A battery is not fully charged until you have matched it's "reserve capacity", often times thats about 80amp/hrs. If your charger is putting out ten amps, it has to do that without dropping below that ten amp number for eight hours. Yes, that also means if it drops to two amps, the full charge could take some 40 hours to achieve.

As far as diagnostic capabilities go claiming any IMO is a significant stretch. VBG...

A real battery charger will run $250.00 and higher. The one I have in my shop was $550.00, eight years ago.... It works fine, but recently I have become aware of a new need in the shop, and it has to do with reflashing computer systems. Most battery chargers, mine included will still pass a level of AC voltage into the system. This AC voltage can be readily seen on an ocilliscope, or with a properlly connected AC meter. The AC voltage can, and sometimes does create problems with software trasnfer so it makes using my charger very risky when trying to keep an electrical system functioning while doing a reflash. The solution up to now was to put the charger away, and use a second freshly charged battery and jumper cables to the car so that both batteries supported the vehicle demand. Well now with ever larger computer systems and longer reflash times, we have been told we need to buy a different style battery charger, one that can be set to provide 30amps, or up to 50 amps current, with no AC ripple. Well,, there goes another $1000.00 I really didn't want to spend :(,,

    Bookmark   October 25, 2005 at 8:37AM
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If any of the chargers are 3-Stage that do Float/Maintanence - you might want to give those some extra brownie points in you purchase consideration.

Here is a link that might be useful:

    Bookmark   October 25, 2005 at 9:57AM
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Thanks, and John G, guess you are up against the old problem: part of the cost of doing business.

Most of my application is charging a boat battery and my garden tractor battery in the spring, maybe once during the winter to keep the electrolite so it doesn't freeze. I do understand your point about AH capacity, and indeed one could consider 10A a trickle charger, I figure anything over 1 A is in the charger category. For a few more $s I can get a higher initial charge rate, don't remember 30A comes to my mind's eye for under $75, I also remember reading too the claim on one that it puts out DC, not a raw rectified AC voltage/current. Do you have a "ripple" limit in mind? Say looking with a "scope" with a 12VDC line what is the maximum peak-to-peak AC ripple the new smart-cars can take? I think it can also be read with a true-rms AC volt meter, but I'll have to give that some thought/study to be sure. One could also use any AC meter behind a large capacitor to block the DC to measure the AC voltage. Hey, this is getting fun, down and dirty, so to speak :-)

I do have two newer vehicles, a 2004 Forester and a 2005 Chevy Colorado and hadn't been aware of a potential problem with charging the battery "on-line", you've given me another reason to buy a new charger; albeit I'm not ready to spend $1K and hope a DC output at low amps is affordable for home use.

Mikie, I'll take a look at the FAQ link you sent, thanks.

    Bookmark   October 25, 2005 at 10:37AM
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Tonight I did take a look at the waveform out of my low cost, don't remember, perhaps I paid $35 8 years ago, charger. It has an automatic charge function which shuts it down from a 10 amp charge to something under 2 amps when the terminal voltage reaches full charge, about 13.8 volts. It is also supposed to be able to put out 50 amps to help start a car.

The waveform as measured across an 8 ohm resistor was a rectified a/c voltage. Thus, there is no attempt in the charge to filter out the 60 Hz "ripple" from the power source. I do not have any data on the subject replacements I was looking at as far as how the output looks. The package talks about a microprocess controlled output, but I think that is to regulate the amplitude of the charge current. The microprocess does have to have a clean d/c voltage supply, but at a low current and could be developed "behind" the output voltage, even separate from it via a separate winding on the secondary of the transformer. Hum. I am worried that what I have may cause problems with my newer autos, should I ever need to attach a charger. I've had to do this in the past only when someone left lights on when they came into the house. That doesn't happen often and the new cars signal you if when you try to exit the car with lights left on. The newest is all automatic, and if I don't throw the manual override the ligts are turned off when I turn the ignition off.

    Bookmark   October 26, 2005 at 11:49PM
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Did you try checking TheCarConnection, MotorWeek or other auto related websites?

I found the following at Goss's Garage at MotorWeek...


Here is a link that might be useful: / Goss Garage / Battery Charging Basics

    Bookmark   October 27, 2005 at 10:15AM
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Jerry, I got the Schumacher charger at Wal-Mart last year. I like it. I got the 60 amp one which I believe cost $77 because it has the " engine start" function. (I used that feature once and it worked great.) I use it on a garden tractor, motorcycle, and cars/trucks.

    Bookmark   October 28, 2005 at 1:04AM
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Thanks for the information. My usual use is for a deepcycle boat battery and garden tractor. Very occasionally it gets used on a car. The car's charging system does a good job of keeping the battery charged. Then there are those times... Clearly having more capacity, will help get a car started when the battery is dead. The current charger I have is a 10 amp charge with a 50 amp starting boost capability. I think it can deliver 50 amps only for a very short period of time and as this is when the battery is "dead" the voltage on the system may be particularly harzardous to any electronic in the car.

As noted earlier I did a resistive load test on the "ripple" of my existing charge, and it wasn't a ripple, it was a recetified Sine Wave. Of course when this is applied to a battery, which asts as a filter, the ripple we'll see is the voltage due to the sine wave current flow across the battery's internal impedance, which is very low. My new concern is the point raised by John G. about electronics in new cars. The word to the wise is, I think, if you car is only a few years old, be careful using a battery charger when the battery is installed in the car's system. I'm not sure having the ignition off protects all of the electronics from the votlage at the battery terminals. It may be best to disconnect the ground terminal while the charger is at work.

    Bookmark   October 29, 2005 at 9:34AM
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One has to wonder about how clean the power is from the vehicles own altenator.

    Bookmark   October 29, 2005 at 11:14AM
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Excellent point Mikie. Most of the time the voltage is pretty steady. But if a diode or stator is failing inside the alternator you can end up with some pretty significant AC ripple, and that has been known to cause issues with certain vehicles.

    Bookmark   October 29, 2005 at 2:55PM
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Yes, excellent point on risks to electronics. All vehicles built in the last, what?, 30 years have altenators and must feed raw a/c rectified current into the battery/electrial d/c system. I'm not sure I understand the point made by John G, I'd assume the system has a full-wave diode bridge to put the full a/c cycle to use. I'd have to give this more thought than I feel up to at the moment, but I'd assume a diode failue would at most open the circuit, but more likely would cut off one half of the rectifier cycle on the d/c side. Still, under normal conditions it appears to me that the alternator's output is turned into and unfiltered polarized alternating current. This would lead me to conclude the battery charger concer isn't. I'm not trying to turn this in an advanced battery charger course, but I do wonder. John G. I'm not disputing your hands-on knowledge and understanding of technical reference material and can say I have seen in one other place the very concern you raised. Still I wonder...

    Bookmark   October 30, 2005 at 10:28AM
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Hi Jerry. Looks like the hook is set.. :)

Trying to understand exactly why certain things happen is what keeps me studying and reading everything I can. It also keeps me asking questions when a further solution isn't really necessary at times as well. Thats why I'm an automotive tech, there is so much to feed the curiosity beast.

Alternators in general use a delta or(triangle) or "Y" shaped stator. The stator is the wires that are visible just inside of the case of the alternator. The ends of the wires that make up the stator are connected to the diode bridge. The bridge typically consists of six diodes which allow the current to flow to the alternator output, and of course from the alternator case. If someone does not really understand how diodes work, they can do some web searching on the subject, or just accept the fact that they act like a one way valve. The diodes are connected to allow electrons to be pulled from the + (positive) side of the circuit, and then pushed onto the - (ground) side. Inside the alternator there is a component called the rotor. Basically it's a coil of wire that has a current flowing through it to create a magnetic field. The rotors ends are designed to pull the magnetic field around the sides of the coil, and into a series of + and - magnets as viewed from the side. The rotor is connected to the pulley and is turned by the belt. This "series of magnets" rotate inside the stator, and the result is electrons are pushed and pulled in the stator creating an AC voltage. This AC voltage is rectified by the diode bridge, and we see it as DC on a volt meter. But looking closer with an ocilliscope we can actually see something else. The DC voltage isn't steady. On a normally operating vehicle with little electrical load (low current flow) there will be small humps as each diode begins and stops conducting. Charging systems also have a capacitor across the output of the alternator. The capacitors job is to slightly charge up higher than the present system voltage, and then discharge when the voltage it stored is higher than the system voltage. This effect is used to smooth out the humps from the alternator. There are two things that happen when a stator or diode fail. The diode failing can either short, or open circuit. If it shorts, generally the result is an alternator that drains the battery and is pretty easy to diagnose. (I'm not going to be real specific here, there isn't enough time but there are other possibilities as well)
When a diode fails open though what you get is a space where instead of overlapping diodes conducting, there is a conductive hump, a period where the capacitor has to discharge longer, and then another conducting hump. I have seen situations under load where the AC voltage on top of the DC output gets up between 2 and 3 volts. (A shorted diode could have an even more dramatic voltage change) Now take a sensor where the output is an analog signal, and allow the system voltage to vary like this and you can get a computer reacting to inputs that are not really occuring. One such example would be false cylinder ID pulses, or incorrect crankshaft speed calculations by the computer. To an inexperienced tech, they pull the car in, it's running erratic, missfiring, maybe an incorrect idle speed. The use a volt meter and see the system voltage around 14v where they expect it to be and they end up running around and testing, and maybe even replacing all kinds of parts trying to fix the problem. It's added a whole dimension to testing when it comes to todays cars. Things like diode suppression when solenoids are operated can be really important to prevent voltage spikes on the system voltage. Getting used to looking closely at voltage waveforms has been really important when it comes to sensors like a wheel speed sensor. Litterally the difference between a sensor that is working, and one that the computer thinks is not can be 1/100th of a volt. With an AC voltmeter, it's impossible to interpret this change in the signal, heck it's hard with a scope by eye. I end up relying on trapping the waveform and using cursers to provide the voltage measurement. (Ahh the luxury of a digital storage ocilliscope) With an alternator I do exactly the same thing. I trigger my scope off of an injector current, or secondary lead if available, and then set the scope to 100mv/div and position the voltage line on the display and look at it between 2ms and 5ms/div. (Sometimes faster, sometimes slower depending on the needs)

Here is a link that might be useful: A tutorial on alternator ripple, not just my rough interpretation

    Bookmark   October 30, 2005 at 12:56PM
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John G.

Well I do like to fish, and I did have unusual luck following the one time trout fall stocking, but that is for another forum.

As soon as I read that the alternator is three-phase, whereas the battery charger is single-phase, says a lot about getting the ripple out. The educational link you provided shows that clearly. I'm not surprised that there is a capacitor on the d/c side to help even the voltage out, but I didn't assume it would be there as it must be very large (I suppose several hundred or even a thousand microfarads) if it is to make any difference in a system running at several ampheres - low impedance.

I do own an oscilloscope, a 5 Mhz dual trace. I just may take it to the garage and see what the waveform looks like with my battery charger on the battery, or safer, on my boat battery that isn't hooked to any electronics when not in the boat.

Thanks, I think all who have a deep interest and some technical knowledge understand your tutorial and appreciate the tine you've given this discussion.

I do not plan to put any more questions on the "table", but I may purchase a new low-cost charger and will use it with caution I never practiced in the past.


    Bookmark   October 30, 2005 at 5:38PM
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John G a friend of mine solved the dc charging problem by making a box to hook accross car battery and connect charger to box, He used 6 500mfd 50 volt capacitors in parralell to make a filter network. This removes the ac ripple in charger output.

    Bookmark   October 30, 2005 at 9:32PM
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Call me a rabble-rouser but I just purchased a combination charger/desulfator. It hasn't arrived yet, but I have an old Interstate battery that has been sitting in my screen porch for 5 years and one Walmart Maxx that is in an LTD that has been sitting in my yard for over 2 years. If I can get those to hold a charge, I'm gonna' buy 3 more of these bad boys to see how many old batteries I can revive. I've read that the only battery it can't save is one with a shorted cell. I'll keep you posted on the progress.

    Bookmark   January 7, 2013 at 5:11AM
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