hand held variable p.s.

Thread Starter

strantor

Joined Oct 3, 2010
6,813
I sent an email to Dewalt, inquiring about the maximum safe discharge rate of their 20V 3.0AH batteries. They don't want to play ball. They dodged my question the first time and put a wall when I pressed further.

I don't have my original message because it was entered in their web-based email app, but it was pretty clear and concise; IIRC it was pretty close to "What is the maximum permissible discharge rate of the DCB101 battery in terms of Amperage and time, and/or "C"?" This was their reply:

The 20 volt will be the peak charge on the battery however it will immediately drop to 18 volt once a load is put on it.

In case this email does not fully answer your question, or you would like to contact us for any reason, simply reply to this email.

Thank you,

DEWALT Customer Care
My reply:
I'm trying to find out how many amps is safe to draw from the battery. It is rated 3.0AH; Normally that means I can draw 3A for one hour, or 1A for 3 hours, but the proportion can go higher; like 30A for 6 min - but I don't know how high it can go. Can it safely go to 30A? 60A? what is the upper limit, and how long can it stay at that limit?


Thank you,
And their reply:
The information that you are requesting is proprietary information.

Our batteries are designed for the use in only our 20V tools and chargers. Our discharge controls built in the system limit current to protect the tools and battery appropriately.

Thank you,
Oh well, I guess that's their prerogative. A simple answer would have been nice.

So anybody have an educated guess what might be safe upper amp limit for these things?

The "fast" charger is rated 4A output.
 

tindel

Joined Sep 16, 2012
937
IDK, seems to me that the person responding to your email may have been giving you a hint - intentionally or not. I wouldn't be a bit surprised if they have current limiting circuitry (i.e. 'discharge controls') internal to their batteries.

I'd do some step load tests to find out... that's not too hard. Maybe you'll find they are current limited, but maybe not. The risk to this of course is that the 'discharge controls' might consist of a fuse that renders your battery useless if cleared.

The other way you can test it is to hook up a 20V supply to your drill and see how much current it takes with your scope.

FYI - Batteries can put out an incredible amount of energy for short periods. I've personally calculated upwards of 300A out of 12V 17Ahr batteries... when starting large motors with large stall currents. I've never actually tested the current, unfortunately - the logistics would be a bit difficult.
 

#12

Joined Nov 30, 2010
18,224
My Makita batteries have an internal limiter based on temperature. They won't charge at all if they have been in the hot truck! However, that suggests an automatic reset function (inside the battery case).
 

THE_RB

Joined Feb 11, 2008
5,438
A few amps should be fine.

If you want better info you need to pull the battery pack casing apart, and see what size cells. Most are a 2C if I remember right. You can get capacity and discharge specs based on the size.

Then check if they are NiCd or NiMH, NiCd have higher current ability and are a much better battery. Recently they tend to use NiMH. And check if there is some circuit between the battery and the pack terminals. :)
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
OK, so I've ordered yet another buck/boost module. This one is just like the swanky one above, but is actually a buck/boost module. It can output up to 25V, even if my battery drops to 18V or less. It's shipping from HK instead of China, and comes as early as 13MAR. I will have some spare buck modules for future use. I'm blowing up all that money I made on eBay last week.
I recieved this module today and tested it out. LIES! It is not a buck/BOOST module. It's the exact same thing as the amazon module. I filed an ebay claim.

"This is advertised as a buck/BOOST unit, and even advertised with supposed efficiency figures for the boost, but it cannot boost. The max output is consistently 2.8V less than input voltage. The ebay listing claims up to 25v output but the spec sheet that came in the box says 20v max output, 23v max input. The unit I recieved appears to be the same plain BUCK (no boost) module that other ebay sellers are offering, not a special one as claimed. I would like my money back or the correct module. The one I recieved is model DP20V2A."
 

THE_RB

Joined Feb 11, 2008
5,438
"Chinese Honesty" (oxymoron) - The art of using whatever english words in the ad that will make the product sell. Often using words taken from other ads from products that sold well.
;)
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
"Chinese Honesty" (oxymoron) - The art of using whatever english words in the ad that will make the product sell. Often using words taken from other ads from products that sold well.
;)
Check it out; Chinese Honesty folloed by Chinese Guilt Trippery:
ebay seller said:
" Dear,Good buyer really sorry for the great inconvenience. How about refunding you 5USD? friend, please donot think it is a little money. it is 32RMB in our currency. I will pay you with my salary if you agree. This deal makes us no margin. Hope you can give me support. and you can keep the item we've sent to you also. Is that ok? or do you have better suggestion? Weather is colder and colder. please take care. May God bless you and your family ms huang "
strantor said:
"Unfortunately this unit is worthless to me if it cannot perform as advertised. If you refund me $5, I have just purchased a $21.99 paper weight. I am still interested in the unit that you advertised; the buck/BOOST module; the one that outputs 25V with <20V input. Does that unit even exist? Can you send that one? Or is it just a fantasy that was created for the sake of advertising? How about this; Instead of you giving ME $5, I will give YOU $5, and you send me PROPER unit that I already paid for. Sounds more than generous to me; what do you say?"
No response yet. I really hope that the description of the buck/boost module was ripped off from another unit and I am able to find that unit. I need it. I really hope it isn't a complete farce.
 

THE_RB

Joined Feb 11, 2008
5,438
You'll be waiting. They don't have a buck/boost one.

It's a buck product, and they copied the "buck/boost" phrase from somewhere that they saw it, maybe thinking it was a more complete english term than "buck".

When buying from China/HK you can get some really good value for money, but you can't believe words and specs on the ads. You have to do some homework and go by the chip which is in the product (read the datasheet!) and the clear pictures they provide of both sides of the PCB.

It's a bit sad that their words lie, but you can get the cheap China price, at the cost of having to do all your homework first. :)
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
"Chinese Honesty" (oxymoron) - The art of using whatever english words in the ad that will make the product sell. Often using words taken from other ads from products that sold well.
;)
You'll be waiting. They don't have a buck/boost one.

It's a buck product, and they copied the "buck/boost" phrase from somewhere that they saw it, maybe thinking it was a more complete english term than "buck".

When buying from China/HK you can get some really good value for money, but you can't believe words and specs on the ads. You have to do some homework and go by the chip which is in the product (read the datasheet!) and the clear pictures they provide of both sides of the PCB.

It's a bit sad that their words lie, but you can get the cheap China price, at the cost of having to do all your homework first. :)
You couldn't be more correct. I took the first line of the ad and googled it; it seemed like a pretty unique identifier with it's misspellings and bad English. Turns up quite a few results; all of them are for this product. They copied the description word-for-word and it's a totally different product.
google this said:
This module isstep-up andstep-downconverter and have voltage display. The performance is more than LM2596 and LM2577.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
Well, after a bit of back and forth, the seller refunded my money. So I used it to buy a simple boost converter/regulator to get me up to 24v. So I will have a fixed 24v supply which should be handy, and a 0-20v supply in the same unit.
 
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THE_RB

Joined Feb 11, 2008
5,438
Did you mean you bought a BOOST converter to "get you up to 24v"?

Or is your battery >24v and the buck can provide output voltages "from 0v up to 24v"?

Sorry but with all the buck/boost up/down confusion I'm getting confused. ;)
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
Did you mean you bought a BOOST converter to "get you up to 24v"?

Or is your battery >24v and the buck can provide output voltages "from 0v up to 24v"?

Sorry but with all the buck/boost up/down confusion I'm getting confused. ;)
DOH! yes, boost.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
I just posted this review to Amazon about the boost modules:
I bought 2; only needed one, but I thought it might be a good idea to get two in case I fried one, after reading the reviews about how finicky the adjustment is. These are 30 turn pots. Upon receiving them, I turned the pot the pot all the way CCW, attached my DMM (and ONLY my DMM) and applied 15v. I got 14 or 15v out. Started adjusting the pot CW at a painfully slow rate of about 1 turn per second. The voltage out remained steady around supply voltage and it was drawing just a few mA up to about 20 turns, at which point it jumped to 60v and then fell to 4V or so, and started drawing 4A from my p/s, within the span of 1/4 turn. I immediately started backing out on the pot but it continued to draw 4A and output very low voltage. Before I could get my hand to the power supply switch, the solder on the LM2577 was boiling.

I set the first one aside and hooked up the second one. This time I turned the pot excruciatingly slow, about 1 turn every 4 or 5 seconds. When I got around 18 turns I slowed down even further. Same thing, around 20 turns, output jumps to 60v then plummets, insane amp draw, smoke from the LM2577. I dissected both boards and found the SS34 Schottky diodes shorted on both boards; caps were still good.

I replaced the diodes and powered the boards up and tried again. This time, instead of jumping to 60v and drawing 4A, they dropped straight to 4V and drew 4A. One of the LM2577s blew up.

I am royally peed off. These things (at least the ones I was sent) are absolutely defective and a waste of time and money.

Either the QA/QC is carried out by drunk apes so some people get good boards and some don't, or someone is coming on here and giving a bunch of B.S. positive reviews so they can offload these pieces of crap.
I doubt they'll publish it, but hey, I got my frustration out.
 

THE_RB

Joined Feb 11, 2008
5,438
Buck converters will go discontinuous at low currents, that's perfectly normal. They may even fire "chaotically".

As long as it makes the required regulated voltage at all load current levels it is fine.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
Here's the Field Service Dewalt 0-20V 0-2A cordless lithium variable power supply. It's been nagging in the back of my mind to finish it so I stayed up until 3AM and just now finished. I don't know if I'll get around to doing a "completed projects" entry for it, but it's pretty simple and I think if you read this thread and look at the pictures that I will post tomorrow, you should have a good idea how to proceed. If any questions, ask away. This is the boost module I ended up using; it is much better quality than the other ones I tried. I made a small undervoltage lockout circuit board to go inside as well, that will open the circuit if the battery gets down to 15V. Will post schematic tomorrow. It has 2 sets of jacks; one for fixed 24V and one for 0-20V. Good mornight.

 

Attachments

THE_RB

Joined Feb 11, 2008
5,438
Here's the photo of the module you used, in case people don't want to follow your link to Amazon (which crashes my browser with its nasty invasive cookies).


:)
 

Thread Starter

strantor

Joined Oct 3, 2010
6,813
As promised, a schematic:



The circuit on the perfboard is to cut power when the battery is drained. I'm not well educated so there's probably a much better solution, but this is what I came up with. It's made from stuff I had laying around, so the parts are pretty random and could be substituted at your discretion. For example, the LM324N is a quad channel opamp; you only need one channel. The IRF064N is way overkill, but with its super_low RDSon, it's probably well suited to running all taped up with no heat sink. The 6.8V zener could probably be subbed for any zener between 9V and 5V; it's just there to give some kind of constant to the opamp. I have tested the operation using my bench power supply and it works great; as soon as I get below 14.0V it opens the MOSFET. But I have not yet had a chance to test it on the battery. I suspect that when connected to battery it will probably act wonky, since as soon as the MOSFET opens and the load is taken off the battery, the battery voltage will spring up above 14V again, and activate the circuit again and then immediately drop again, ad infinitum. So when it starts flickering, I guess that's when it's time to put a fresh battery in, or plug it in.


How bout some pics?

Here's the two boost modules I purchased. DO NOT BUY THE ONE ON THE TOP. It sucks. 2 of them blew up before any load was connected. The one on the bottom is much better, more robust design, and still small enough to fit inside. It is shown with the battery pack for size reference.





Here's the buck module. Pretty cool device. seemed to be very accurate in both volts and amps considering the cheap china that it's made of. Fits perfectly in the little rectangle portion of the charger.





Here's my little LDO circuit. I stole the pot from one of the exploded boost converters.





And here's where I crammed it. Under the battery contact port. Taped up to prevent unintended electron escape.





Here's the cramming location for the boost module:




more pics next page... stinking 6 pic limit .
 
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