Recharging Ni-MH batteries

NutmegCT · Apr 27, 2018

I have a small shop vacuum on the workbench, rechargeable Ni-MH batteries built in. Royal Gator 9.6V.

The "fully charged" vac seems to run down too fast for me. (I'm picky, what can I say ...)

Today I noticed the vac is labeled "9.6 v" - but the wall plug charger is labeled "Output: 7.25v".

The vac says "use only with Royal 9.6 VE charger 440004454". The charger is clearly labeled 440004454 - but there's no "9.6 VE" on the charger.

Does that make sense? Shouldn't the charger output be at least 9.6v (not 7.25v)? Is there some high-tech magic that lets a 7.25v charger charge a 9.6 Ni-MH battery pack?

I can't figure out how to measure the actual charger output, as there's no output unless it detects a load (when plugged into the vac charging port).

Thoughts? Brickbats? Condolences?

Thanks.
Tom M.

Gliderman8 · Apr 27, 2018

No batteries required

Can you measure the actual voltage of the batteries? Possibly they are not 9.6 v.

NutmegCT · Apr 27, 2018

Thanks Elliot. Measuring NiMH batteries when they're soldered together can be a pain, especially if you don't know whether they're fully charged. After taking apart the vac, you could only measure one battery at a time - but you still don't know if they're correctly charged or not.

I just checked the wall charger again - it's a "pulse" charger, so I still can't tell what the output is. Seems to pulse between 0 and 2 volts when attached to my VOM.

Sure seems strange that the charger would be marked (embossed) Output 7.25v, when the vac (and manual) both say 9.6v.

Weird-a-mundo.

Brooklands · Apr 27, 2018

This is why I try not to read too many labels. But I do use a number of wall warts to power items on my slot car layout...

NutmegCT · Apr 27, 2018

As Alice said, this is becoming "curiouser and curiouser".

I called tech support at Royal. Tech support said that it doesn't matter what voltage comes out of the charger, as long as you leave the charger connected until the "charging" light goes off.

huh?

So I said - does that mean I could use a two volt charger to charge a 12 volt battery?

Tech said - "that's what they all do."

I asked "How can a 7 volt charger fully charge a 10 volt battery?" I now got transferred to "elevated support".

Elevated support says "it's just like the trickle charger on my motorcycle. Just leave it plugged in and it keeps the battery from running down."

I asked: "What voltage is your battery?" A: "I don't know".
"What voltage output is your trickle charger?" A: "I never looked."

So now, elevated support has taken down the details, and will have "Advanced elevated support" answer my question, when he/she gets back to the office next Monday.

grumble grumble

Just doesn't seem possible that the 7.25 v "pulse charger" could fully charge the 9.6 v vacuum battery pack.

yeesh

TR3driver · Apr 27, 2018

NutmegCT said:
Thanks Elliot. Measuring NiMH batteries when they're soldered together can be a pain, especially if you don't know whether they're fully charged. After taking apart the vac, you could only measure one battery at a time - but you still don't know if they're correctly charged or not.

Can you see the individual cells? They're nominal 1.2 volts/cell, so a 9.6v pack would be 8 cells; 7.2 only 6 cells.

I can't quite envision how the wiring would not be accessible if you are talking about cells soldered together; but if all else fails, I would use insulation-piercing probes (aka phono needle probes) to push through the insulation on the charger cord, while it is connected to the vac/batteries. That will show you how high the charger is actually taking the batteries. It should be holding them around 1.3v per cell, with a pulse of current when voltage drops below that point. So that would be around 10.4v for a "9.6v" pack, or 7.8 volts for a "7.2v" pack.

The probes will do some small damage to the insulation of course. Probably not enough to worry about, but if you want, you could seal them back up with a drop of rubber cement or similar.

PS, NiMH pretty much requires a smart charger, you can't just charge them to a constant voltage like older battery technologies. So it's quite possible your charger will adapt to the batteries it finds. Or not

NutmegCT · Apr 27, 2018

Thanks Randall.

Basic question: could a 7.25 v charger ever fully charge a 9.6 v battery pack?

Thanks.
Tom M.

TR3driver · Apr 27, 2018

Maybe, but only if it was designed to do both.

I'm fairly certain, though, that if you really had a 7.2 volt (only) charger and a 9.6 volt pack; the result would run down almost instantly. More likely, you have matching charger and pack, and the pack is just getting old. They don't last forever.

NutmegCT · Apr 27, 2018

Thanks. I'll share what I hear back from Royal when (if?) they return my call.

When the vac runs down again (usually takes around ten minutes), I'll hook up the charger and check the voltage in the charger output, altho' I figure measuring voltage from a "pulse" charger would be problematic. I've done the "sewing needle through the insulation" many times on various projects.

Thanks.
Tom M.

dklawson · Apr 27, 2018

I have little to add that Randall hasn't already said.

I don't see how any charger that delivers lower voltage than the battery rating can ever charge the depleted cells. OK... if you have a 9.6V pack and you run it down to totally flat, the 7.2V charger should be able to charge back up to near 7V... but not 9.6V. I think that in this case "elevated support" doesn't mean engineering or technical support.

With that rant over, I do have one suggestion. If you are not satisfied with the support you get from Royal, and you are willing to experiment, buy a 9.6V RC car charger and splice it to your Royal charger's plug. Tyco and other RC car makers commonly use 9.6V Ni-MH battery packs and chargers. Prices should run from about $10 on up. eBay and Amazon are obvious sources.

NutmegCT · Apr 27, 2018

" I think that in this case "elevated support" doesn't mean engineering or technical support. "

Considering how the guy spoke slowly, with lots of "oh yeah" and "coooool", and comments about his motorcycle - maybe in his case "elevated" meant ... uh ... "high"?

:angel:

Gliderman8 · Apr 27, 2018

NutmegCT said:
" I think that in this case "elevated support" doesn't mean engineering or technical support. "

Considering how the guy spoke slowly, with lots of "oh yeah" and "coooool", and comments about his motorcycle - maybe in his case "elevated" meant ... uh ... "high"?

:angel:

Tom- last week I called a hardware store and asked the following question to the hardware manager:

ME: "Do you have a 3/8" x 24 (fine thread) x 2 1/2" long FULLY THREADED bolt?"

Manager: "Is that METRIC?"

What was he thinking (or not thinking)?

DrEntropy · Apr 27, 2018

Elliot!! Think "Hubble"!!!

...or Story Musgrave.

:smirk:

Gliderman8 · Apr 27, 2018

DrEntropy said:
Elliot!! Think "Hubble"!!!

...or Story Musgrave.

:smirk:

Got it doc...

TR3driver · Apr 27, 2018

NutmegCT said:
When the vac runs down again (usually takes around ten minutes), I'll hook up the charger and check the voltage in the charger output, altho' I figure measuring voltage from a "pulse" charger would be problematic.

So think of it as checking battery voltage while under charge.

DrEntropy · Apr 27, 2018

Elliot said:
Got it doc...

mehheh. :thumbsup:

Systems need to be ~matched~ now, whether in space or on-planet. An undercharge voltage from a wall wart likely can't fully charge a higher voltage storage device. Pump it all ya want, 7V isn't 10. Through that wart there ain't AMPS to put it up to charged. No matter how 'smart" it is.

I'd scrap the whole thing and get a more recent device. Or an older one with a 110V cord for power... :wink:

NutmegCT · Apr 27, 2018

Gosh darn it! You young whipsnappers want me to toss the rechargeable and go back to using my old vac?

Jeez - The Edison no longer provides the 12 volt DC we used to get here in the neighborhood!

Brooklands · Apr 28, 2018

Maybe this charger for electric car batteries on display at America On Wheels is what you need Tom.

NutmegCT · Apr 28, 2018

Wow - that's quite a contraption. Do you have any photos of the front, or any instructions or history? Interesting photo of the "petite young lady" doing her own refill.

Tom M.

Brooklands · Apr 28, 2018

Tom,

I might have other photos, but I knew that I had this one taken just before the museum opened. I will try to track another down, or take some more for you on my next trip into the museum. I feel like the old Frankenstein movies just put a bunch of these old chargers side by side for the props of the doctor's laboratory.

Recharging Ni-MH batteries

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