Measuring output voltage of a battery charger

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jacob
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Measuring output voltage of a battery charger

Post by jacob » Sun May 26, 2019 12:39 pm

I have a wall plug battery charger that is listed with a nominal output of 13VDC, but when I measure it directly (voltmeter), it only shows 7.3V!? I presume that some circuitry intended to taper off the voltage when the battery is reaching 100% is involved here (and drops the voltage on me as I measure it) and that this interferes with my measurement, but I don't know. Is that what's happening or is my charger borked?

In the overall scheme of things, I'm trying to debug a neighbor's battery powered weed whacker. The battery charger has a red charging LED and a green ready LED but neither are on when the weed whacker is plugged in. I suspect it's the battery, but I haven't taken weed whacker apart yet to see if there are loose connections further inside the system.

IlliniDave
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Re: Measuring output voltage of a battery charger

Post by IlliniDave » Sun May 26, 2019 2:39 pm

The only reason the charger voltage should be that low (that I can think of) is if you are measuring it with a deeply discharged battery connected and charging. The few I've played around with tend to read ~13-14V "unloaded" then drop to 12-13V when topping off a "low" 12V battery (still w/11V or more remaining charge).

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Re: Measuring output voltage of a battery charger

Post by jacob » Sun May 26, 2019 3:09 pm

The 7.3V charger output is measured when the charger is NOT connected to the battery. I'm measuring it directly off the barrel plug connector.

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Re: Measuring output voltage of a battery charger

Post by Sclass » Mon May 27, 2019 8:34 am

What kind of battery and what is it’s voltage?

Better what is the model and make of the unit?

Could be a flat battery. You power supply sounds too low too but this depends on whether it is just a power supply or if it contains the battery charging controllers as well as you alluded to in the tapering off theory. Li Ion battery chargers do taper off but not when open circuit.

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Re: Measuring output voltage of a battery charger

Post by jacob » Mon May 27, 2019 1:39 pm

It's a Black&Decker CST1000. The battery is a 12V 7Ah sealed lead-acid battery, like this
https://www.ebay.com/itm/Mighty-Max-12V ... 2214467429

The battery shows zero charge and the neutral (black) terminal is corroded. Makes sense, since the unit is ancient (10+ yrs). I'm going to buy a new battery and see if that fixes it. Unfortunately/surprisingly I don't own a 12V PSU, so I can't try running the motor nor attempt to charge the battery directly. Okay, maybe I'll try to clean off the corrosion first and see if that does the trick (hopes are low since the corrosion is bubbling up around the terminal)...Alternatively ask my neighbor/mechanic to borrow his car battery charger to see if that moves the needle. Will report back as we go...

(In case anyone stumbles upon this via google, a 2"+ PH2 bit will reach all the screws required to take the CST1000 apart. Remove _all_ screws to dissemble. The shaft has to come out to free the battery compartment. You can not pull it forward w/o taking everything apart even though it looks like it.)

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Re: Measuring output voltage of a battery charger

Post by bigato » Mon May 27, 2019 2:33 pm

careful, the car battery could be 24v?

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Re: Measuring output voltage of a battery charger

Post by jacob » Mon May 27, 2019 2:35 pm

Negatory, 24V is semi-trucks.

bigato
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Re: Measuring output voltage of a battery charger

Post by bigato » Mon May 27, 2019 3:12 pm

also, responding to a detail to your first post: no, the measurment should not change the reading at all in this case, because it shows as an open circuit for the energy source (very high resistance in the voltimeter and very low current). If you were dealing with a extremely sensitive circuit, it could be a factor in some cases, but definitely not when dealing with this class of devices. But battery charger devices try to be smart sometimes and I am not specially familiar with them. But it is not the measurement.

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Re: Measuring output voltage of a battery charger

Post by ffj » Mon May 27, 2019 6:08 pm

You asking a lot from a ten year old Black and Decker lead acid battery and charger. The fact that any output is occurring is remarkable.

Will the charger allow a cumulative charge at 7.3 until the 12 volts is reached?

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Re: Measuring output voltage of a battery charger

Post by Sclass » Tue May 28, 2019 12:20 pm

That thing is junk. Buy a corded model at Harbor Freight for $30 or take advantage of the revolution in Li Ion/brushless dc tech available today.

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Re: Measuring output voltage of a battery charger

Post by bigato » Tue May 28, 2019 12:53 pm

I asked a nerd friend who knows a lot about rechargable batteries, and he explained me that no lead acid charger would present a lower voltage than the battery's nonimal voltage by design. In fact, he said that for a 12v battery, usually the charger would deliver something around 14 something volts. So we know that the charger is broken.

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Re: Measuring output voltage of a battery charger

Post by jacob » Tue May 28, 2019 1:06 pm

Constitutionally I have a hard time just throwing it out. I have a [bad] habit of taking in broken orphan appliances and fixing them. This is my 4th weedwhacker... one was bought used for $5, the others were free. Unfortunately, I already ordered the battery ... my gut feeling also suggests the charger is sketchy, but OTOH, it's hard to imagine how it would break... especially in a way where it's still showing a voltage.

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Re: Measuring output voltage of a battery charger

Post by bigato » Tue May 28, 2019 1:09 pm

given what i know about ac/dc converters, it seems very likely that it would break exactly like that, delivering *half* the voltage it normally should

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Re: Measuring output voltage of a battery charger

Post by bigato » Tue May 28, 2019 1:11 pm

open it and take a look, you're likely to find some fried component. my bet is a diode.

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Re: Measuring output voltage of a battery charger

Post by jacob » Tue May 28, 2019 1:35 pm

I'd need to do something like this: https://www.youtube.com/watch?v=pvCFhywlVZw Anyone know what he's saying? What's the paste/liquid? Acetone?

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Re: Measuring output voltage of a battery charger

Post by Sclass » Tue May 28, 2019 4:08 pm

Sounds like you’re really in it now. I checked the exploded view here.

https://www.ereplacementparts.com/black ... 11320.html

No electronics inside. Maybe the wall adapter is more than a 12v transformer? Is it something that sees no current flow and backs off on the current? Or is it just a cheap 12v power adapter that happens to be broken?

There are some notes on the description of the adapter piece on the above link. It says weak cells in the battery can damage the charger.

I guess try the new battery and take some measurements with and without the charger. See if connecting the charger marginally increases the potential across the battery.

Good luck.

bigato
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Re: Measuring output voltage of a battery charger

Post by bigato » Tue May 28, 2019 4:17 pm

Only a transformer will only reduce the voltage, but not convert from ac to dc. For that you'd need at minimum one diode. Usually four of them, but you could do it with only one if you were really trying to save on the building.

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Re: Measuring output voltage of a battery charger

Post by Sclass » Tue May 28, 2019 8:39 pm

Oh sorry, I meant to say dc adapter. I assume there is a bridge of diodes and a ripple filter in there at a minimum. I bet it is modern enough to have a switch-mode regulator. I take back what I said about no electronics. Jacob said there was a green and red indicator on the trimmer. I guess the charge controller is on board. Some units will have an AC adapter and do the rectification and regulation on the appliance.

I actually had a mower by black and decker that was similar tech about fifteen years ago. Big wall wart. Two SLA batteries. A green charged light and red charging light. The battery wore out after five years. I remember the transformer was just a 24v (mower was 2x12v) power supply. (But was it 24vac?) I cannot remember if the output was AC or DC. I guess if the output voltage was AC on Jacob’s supply it would read zero on the DC meter, not 7.5v. It would help to know what the writing on the back of the wall supply says.

Jacob’s supply probably states its input and output voltages on the supply. It should probably say 12v and have a symbol indicating whether the connector shield is +/- .

bigato
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Re: Measuring output voltage of a battery charger

Post by bigato » Wed May 29, 2019 3:01 am

Right, good catch! He should try to read it as ac too, in case it is not outputting dc

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Re: Measuring output voltage of a battery charger

Post by jacob » Wed May 29, 2019 7:46 am

Stated output is 13VDC 260mA but it measures 7.5V DC. Stated Input is 60Hz 120VAC 7.5W. It's listed as a Class2 battery charger whatever that is.

All the electronics is in the wall charger (all the whacker contains is a switch at the handle, the motor at the working end, and a bunch of wires). My floor looks like Sclass's diagram above (good find). It would not surprise me if the charger is partially fried(*) (as noted here https://www.ereplacementparts.com/charg ... 96007.html ). It could easily have been plugged in for years. The person I got it from usually doesn't do her own yard work.

(*) I would just have expected it to showing zero voltage instead of the mysterious 7 value.

I'll see if I can get the charger taken apart. Otherwise, I'm tempted to finagle a solar panel and a charge controller and use that to charge it instead. Hopefully, I can get a combo on ebay for under $10. That would be a more interesting project than just replacing the charger.

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