# Could I get help understanding voltage and current relating to power supply and charger? I have 3 questions at the end

I posted a question regarding a tool and power supply to a few different platforms, but came to the similar confused responses. So I believe I have went wrong somewhere and wanted to start from beginning of how I got to where I am now.

It started from this youtube channel.

First lesson I learned from this guy is that power supply and charger terminology is often used wrong. Stating examples like power adapters and bricks for labtop is a power supply, not a charger. The charger is often inside a device. Like inside the phone and labtop. It looks like a little circuit board usually it seems.

Second lesson I learned is that power supply controls the voltage which I believe "pushes". Charger controls the current which "pulls" and voltage can be lower when charging a discharged battery. So that got me to understand charger should not have higher voltage or current than power supply to be safe. Too much voltage will charge past battery max storage overtime. Too much current can pull too much than the power supply was designed for.

Which leads to the original reason why I learned this. I have a drill and power supply. Picture attached below. I wanted to change the 120vac to DC power supply to 110vac to 250vac to DC because I will be using it in different countries.

This is where I got confused. What I "learned" didn't seem to make sense anymore. Charger in drill voltage and current is higher than the power supply. But that can't be, because it works just fine. Something must be wrong here. I decided to look for other solutions for now.

Finally, this led me to an idea of using a power supply left around the house. I found this.

It accepts 110vac to 240vac. The lower voltage is okay because it is lower than power supply. My understanding is it would charge little over 50% because the max on charger states 8V max and it would stop charging around 5V. I believe the 8V isn't the actual battery capacity, but it must be close to it.

So to sum up, I have 3 questions.

Question 1. Is my basic understanding of power supply, charger, voltage, and current wrong?

Question 2. Why does this power supply and charger work without issues?

Question 3. Can the 5V DC power supply charge the drill?

edit: Thank you everyone. It was difficult to pick what helped the most, but I appreciate all the help. I see where I went wrong.

• You can not charge 8v battery with 5V power supply. Dec 29, 2021 at 13:46
• @uswr263983 Please note that in general this might be true but there are also devices with internal boost converters that can charge a 8V battery from a 5V supply so it does not always apply. For example your laptop may need 20V in and you can purchase a car adapter to charge a 14.4V laptop battery from 12V car battery. Dec 29, 2021 at 14:04
• @justme you right in theory. But practically that cheap screw driver has the simplest charge control inside. Easiest way to OP get some universal power supply same output like original. Dec 29, 2021 at 15:10
• I mistook the drill specs as the charger, when it was the battery. Couldn't find any specs on the charger itself. Ill be going with the universal power supply way. Thank you Dec 29, 2021 at 22:01

Why does this power supply and charger work without issues?

Your drill has an 8 volt (maximum) battery and your charger produces 10 volts DC: -

Inside the drill it converts the 10 volts to a "DC supply" suitable for charging the 8 volt battery.

Can the 5V DC power supply charge the drill?

Not unless Black and Decker have over-burdened their cost with a charging circuit that incorporates a voltage booster (very unlikely).

The charger is often inside a device.

Quite often to very often.

So that got me to understand charger should not have higher voltage or current than power supply to be safe

The charging circuit should be capable of working with the voltages produced by the external power supply and, should not draw currents in excess of what the external power supply can produce at full load. Full load for the external supply is 100 mA.

Too much voltage will charge past battery max storage overtime.

The charging circuit (if designed correctly) will not put too much voltage onto the battery i.e. it knows the battery limits.

Charger in drill voltage and current is higher than the power supply.

I see nothing explicit in the pictures that suggests this but, what can be trickled slowly into the battery (say at 100 mA over many hours) can be withdrawn at much higher values when operating the drill. 12 watt hours suggests 1.5 ampere hours or 1.5 amps for a duration of 1 hour - this doesn't come from the external power supply because it shouldn't be plugged in.

My understanding is it would charge little over 50% because the max on charger states 8V max and it would stop charging around 5V. I believe the 8V isn't the actual battery capacity, but it must be close to it.

It may not actually charge anything into the battery. It's likely that the internal battery will be about 7 volts (8 volts maximum) and 5 volts won't push any charging current into the battery unless B&D have wasted money on some over-engineered voltage-boost circuitry.

A1) Most likely. But these are not easy concepts to explain. And it may not be relevant for many reasons, one being, the charging circuitry for your device is unknown.

And devices can do internal voltage conversions so it might be more confusing how a 3.7V battery inside mobile phone can be charged with a 5V or 9V or 12V or 15V or 20V USB PD power supply.

Also a laptop may have a 14.4V battery, it needs 20V voltage from power supply, and this 20V may come from mains adapter, or a car adapter which takes in 12V and outputs 20V.

A2) They are designed to work together. Don't look at the confusing numbers, as we don't know how this particular battery, device, charger or power supply works. Assuming the device has two lithium batteries in series, the charging circuitry needs more than 8.4V to fully charge the battery to 8.4V and charger may need 10V in to be able to charge the battery to 8.4V, just like mobile phones need roughly 5V to charge the battery to 4.2V.

A3) Unlikely. Don't try it. The device came with 10V supply so it may require much more than 5V to even start charging. Besides even a discharged two cell lithium pack has more than 5V voltage, so would not make sense to connect a 5V supply.

As per your original motivation for these questions : if you need to charge the device in another country with different mains voltage, buy a matching charger from manufacturer that is intended to be used with the device for best results and safety, as mishandled lithium batteries are a fire risk.

• I see. My assumption battery can go 0V was way wrong. As for the psu. I couldn't find one with what I needed so I was thinking of buying an adaptable psu. This was from amazon "Belker 36W 3V 4.5V 5V 6V 7.5V 9V 12V Adjustable Voltage" 9V seemed close at first. Now I'm not sure. I am trying to find one 10V without getting those bigger and expensive ones. Would be better to buy another drill at that point Dec 29, 2021 at 22:06

Question 1. Is my basic understanding of power supply, charger, voltage, and current wrong?

Basically, yes. In the bad old days of linear power supplies and chargers, you could often infer a lot of what was going on from the terminal specifications. These days, with switchmode circuits being used in chargers and power supplies, everything has the capability to be 'smart', and to step voltage up or down cheaply, so you can no longer say things like the supply voltage to the charger must be higher or lower than the battery.

Generally a power supply is a more or less 'dumb' provider of 'power'. It will generally deliver a more or less fixed voltage, and whatever current the load asks for, up to some maximum above which it will shut down nicely, limit nicely, or burst into flames, depending what it has been designed to do.

A charger tends to be a 'smart' device that provides a current, or voltage, or both at different times, whatever the particular battery chemistry requires to charge properly without damaging the battery.

Question 2. Why does this power supply and charger work without issues?

Because the battery charger is designed to work with the voltage the recommended power supply is delivering.

Question 3. Can the 5V DC power supply charge the drill?

Probably not. The drill battery says on it 'use only with xxx3304 power supply', which delivers a nominal 10 V. It's not clear what the 8 V on the battery refers to, but it's given as a maximum, so perhaps refers to the battery voltage (perhaps 4 or 5 NiMHs in series). If the charger is expecting 10 V for a max 8 V battery, it may well be a linear charger or buck regulator, able to reduce voltage only, so will not work from 5 V. It certainly won't 'only charge 50%'.

But it may do. It would be quite possible to design a charger that worked with an input of 5 to 10 V, but it would be more expensive than one that used 10 V only, so it's unlikely. My generic charger works from 8 V to 16 V DC input (designed to be used with 12 V car batteries, I use it with a 15 V brick) and charges 1S to 6S LiPos (3 to 25 V).

A dedicated charger for a power tool does not need that generic capability, so you may rest assured that every corner has been cut to shave the last penny off the price, and assume it really needs those 10 V from the recommended power supply.

Your safest option is to see if the drill's manufacturer does a universal input voltage (110 to 250 V) version of that power supply.

An unsafe option is to use an autotransformer, to provide a 120 V supply in 240 V land. It's unsafe because at 50 Hz, an iron-cored transformer will run at 20% higher flux than 60 Hz. If they've really shaved costs, that may be too much, and overheat the supply.

Another unsafe option would be to use a universal input 10 V DC output power supply, with at least 100 mA capability.

• Thank you. Unfortunately couldn't find a 250vac version of a PSU. Question on last sentence. Why would it be unsafe to use 10v DC with 100mA. Wasn't this what the drill was designed for? I found a PSU with where you can change the voltage and up to 1 amp. Dec 29, 2021 at 21:55
• Chance of connecting it to the drill charger the wrong way. Chance that '10 V' doesn't tell the whole story - tolerances, impedance, ripple. Dec 29, 2021 at 22:59
• I see. Makes sense. Then how do some people mess with this stuff? I'm guessing it would involve lot of testing and I guess that would be difficult for a newbie like me to do. Do you have like a keyword or something where I can search to start learning that? Dec 29, 2021 at 23:19

A short answer: your best bet is to match what the original power supply says. That's what the manufacturer selected, and they know best!

The writing on the drill may refer to the battery, rather than to what the charge circuit expects from the power supply.

The 12Wh (watt-hour) are definitely about the battery. But you can't calculate the charging current, if you don't know the charging time (the 'hour' bit) as well.

• I see. I thought Wh and W was the same thing and calculated Amps accordingly. This would make more sense Dec 29, 2021 at 21:47