Trying to fix a 6/12volt battery maintainer/charger for a friend.

[guitchess]

Hello,

I have my friend's Schumacher 6/12volt charger.  It was doing nothing to charge/maintain his motorcycle battery.  Upon inspection, everything tests fine other than the voltage is too low(12.4v/6.6v).  There is not much of a circuit.  AC in to a discreet full bridge rectifier with an odd over temp fuse with a switch to select 6/12 volts. 

All components test fine.  Sort of.
Questions:
1. The AC output of the transformer is 13.8v and 6.8v.  This seems to low.  Does this indicate a shorted winding?
2.  The diodes all test fairly close with a  drop of around .548v.  Do diodes increase in drop as they age?  1n5404 rectifier diodes. 
3.  There are no smoothing capacitors.  Could the lack of caps be allowing the voltage to drop too low to perform?

I was hoping for a win this time, but when everything tested good, I was disappointed yet again.  Especially, since this is such a simple circuit. 

Thanks for your input. 

[edpalmer42]

I've seen similar trickle chargers.  Since the output is unregulated you shouldn't call it a maintainer.  If you leave it connected for an extended period it can kill the battery.  What model is it?

The fuse is actually a self-reseting circuit breaker.  If the battery draws too much current, the breaker heats up and pops.  When it cools down it resets.  If the battery is deeply discharged, this is likely to happen over and over.  Eventually, after a number of years, the breaker just dies.  The current value should be shown on the unit.  It should be similar to the 3A rating of the 1N5404 diodes.  If there's no continuity through the breaker at room temperature, you have to replace it.  A good auto parts store should have them in whatever value you need.

The voltage output is misleading.  Since the transformer is putting out 13.8V AC, the peak voltage is actually 19.5V.  Subtract a volt or so for the diode drop and you're looking at an unregulated output of about 18.5V.  Normally, no capacitors are needed or used, but you can confirm the peak output voltage by putting an electrolytic capacitor of almost any value across the output.  Watch the voltage rating and polarity of the capacitor!  You should see a voltage of about 18.5V DC.  This is why you can't leave it connected for a long time.  12V batteries aren't impressed by charging voltages of 18.5V!

Since the transformer is putting out voltage and the diodes are good, have you tested the breaker?

Ed

[pqass]

Why do you think the charger isn't working?   Is it because the battery never seems to charge?

You can't rely on what your multimeter is reporting (at the output clamps) since it's not a flat-ish DC voltage. Without a capacitor, it's just pulsed DC (120Hz humps).  The multimeter is telling you the average DC voltage reading.  It's probably more useful to use your multimeter as an ammeter (use the 10A/20A jack in series with the battery and  dial set to DCA) to determine if the charger is pushing energy into the battery and at what rate.  Also, as the battery charges, the current tapers off on these simple chargers.

I can't comment on whether diode Vf decreases with age, however, they DO decrease with higher temperature (−2 mV/°C).  Also, Vf appears lower with low current flowing through them (ie. when checking them with a multimeter).  I don't have a 1N5xxx series with me but a random 1N4003 shows 0.573V; so not unusual.

It's not unusual for there not to be a cap on these simple chargers.

[guitchess]

Thanks for the replies.

Charger is a Schumacher MC-1.  One amp 6/12volt motorcycle battery charger.  This must be an older version.  Newer ones have a different case and appear to be SMPS while this one is a simple linear type.

I classified it as non working because it won't charge to full Pb voltage(13.8 ) and will drain a full battery down to its output voltage(12.4-7v).

The fuse/breaker tests good.  Or, has continuity at room temp.  Haven't try to heat it to see if it opens.

The voltage output is misleading.  Since the transformer is putting out 13.8V AC, the peak voltage is actually 19.5V.  Subtract a volt or so for the diode drop and you're looking at an unregulated output of about 18.5V.  Normally, no capacitors are needed or used, but you can confirm the peak output voltage by putting an electrolytic capacitor of almost any value across the output.  Watch the voltage rating and polarity of the capacitor!  You should see a voltage of about 18.5V DC.  This is why you can't leave it connected for a long time.  12V batteries aren't impressed by charging voltages of 18.5V!

I knew that my voltage reading(RMS) probably wasn't perfectly accurate with no filtering, but that far off of peaks, wow.  I wanted to test peak to peak with my Oscope, but am afraid of blowing it up, as I have no way to isolate it.  And my grasp of theory is tenuous at best, leaving me short in confidence. 

Will check with installed cap asap. 

[bdunham7]

13.8V_RMS = 12.4V_AVG.  Your AC meter is either TRMS or RMS-indicating, while your DC range will simply give you the average.  The diode drop will likely be very low if you are just connecting the voltmeter, under any load that will drop lower.  So nothing unexpected there.

If the charger consists of only a transformer and 4 diodes, then there's really no way for it to 'drain' the battery unless one or more diodes are defective, and it doesn't seem so.  As others have mentioned, you need to connect a current meter and see what the charging current is over time.  If that setup is working properly I would expect there to always be some charging current indicated, and if there is then the charger is doing what it is supposed to.  Perhaps the battery itself is bad?

[guitchess]

Ok.  It seems the consensus is that I may be incorrectly judging the charger. 

So, I will reassemble the unit, and try testing with some of the methods mentioned.

Current meter, Filter cap to measure actual peak DC.   Also, will test with multiple batteries in various states of charge.

Thanks again.  Will post an update later tonight or in the morning.

[pqass]

Since the charger is transformer-based (not a switching supply), you shouldn't blow-up your 'scope.
Add a 470 ohm load to the output (w/o cap). Look for nice humps, none sheared/missing or below ground.

[floobydust]

All that's inside for repairable components is the two 1N5404 diodes and the glass circuit-breaker.

In general, I've found these types of chargers are terrible. Because mains voltage moves around, they can be under or overcharging.
The battery acts like a filter capacitor, so voltage (when a good battery is connected) is higher than a textbook rectifier circuit.
For testing, put a filter capacitor and small load like a resistor or 12V light bulb to get a better view of what the actual charging voltage can be.

I wouldn't use the charger as-is, and would add voltage regulator dialed into 13.8V for float use. It's really easy to boil a motorcycle battery dry.
Just add an LM317 if you have at least 16VDC available at light loads.

[CaptDon]

For some reason motorcycle batteries of conventional type always seem to develop one bad cell and on some batteries you can simply see it as the plates and separators look like they are covered in white cotton. Take the charger off, wait two days and find you have 10.4 volts on your 12 volt battery. Motorcycle batteries don't seem to survive the winter when outdoors even on a 'maintainer'. I have recently found the AGM batteries don't seem to like being outdoors during the winter on a maintainer either. I think bringing the batteries indoors and trickle charging them for 1 day once a month is a far better plan for maximum life!! Everyone's experience is different. My experience came from two Suzuki's, one Yamaha, and several large garden tractors. Some with normal flooded batteries, some with AGM's and some where I tried swapping technologies.
B.T.W. a 12 volt motorcycle battery with one shorted cell will show about 12.6 volts on a maintainer, until you remove the maintainer and let the battery sit for about 2 days where it will then show around 10.4 volts.

 

[guitchess]

Well now I really feel like a complete moron.

After reassembling the unit, I tested again with the new perspective.  Of course, the info you folks supplied was correct.  My TRMS meter was averaging the voltage.  Tested for current draw on  a known good battery that was sitting at 13.1 volts and it showed a 628mA draw. 

The current test leads to the second supporting reason for feeling like a moron.  I forgot to switch the leads back afterwards and popped the fuse when I tested battery voltage. 

I had thought about modifying the circuit so my friend wouldn't have to worry about it frying his battery.  But, after the two proceeding examples of stupidity, I might just give it back as is.   

Thanks again.

P.S.  Is there a way to completely delete a thread?   

Edit:   I also forgot that I had replaced the clamps.  So, it's likely that I fixed the non charging issue when I replaced them and was too stupid to realize it.   

[pqass]

> Well now I really feel like a complete moron.

Nah!  It was a "learning experience".   
Betcha won't blowup another MM fuse again. 
Nor overlook dodgy connections/joints.


> Just a 48 year old carpenter.  No training. No school.

Say, do you still have all of your fingers? 
If not, you might want to stick to low-voltage work.   

[edpalmer42]

> Well now I really feel like a complete moron.

You're never a 'complete' moron.  There's always new moronic things to do.  Guess how I know!   

Seriously, no matter how much you know, there are always new things to learn.

Ed