How To Charge the Black Diamond Heated Chalk Bag from USB

Solving an Actual Problem, Imperfectly

On April 1st 2018 Black Diamond posted a new product on their website: The Hot Forge Heated Chalk Bag. But it wasn’t just an April Fool joke, it was a real product you could purchase. A $60 chalk bag may feel like an extravagant misappropriation of gear funds for many people, but for some, myself included, it solves a real problem and is easily worth the price. In cold weather, I have chronic problems with my hands numbing out. What good is friction if your cold fingers have 30% of their normal pulling strength? Cold fingers also increase injury risk. Two years ago I was halfway frozen and tore a lumbrical. The BD Heated Chalk Bag keeps my hands from numbing out during both long stationary belays and also while actively climbing on cold rock.

Unfortunately, this half product, half joke of an item suffers from somewhat slapdash design, an amalgam of several other pre-existing BD materials and technologies. It has a real weekend-project DIY vibe. It reuses down jacket material for the bag itself and the heating element and controller is re-purposed from heated glove and sock products. The battery is a 7.4V 2S Lithium Ion cell charged by an uncommon DC barrel plug size, and nearly impossible to find spares or replacements for. The only option for charging is a clunky wall-wart AC adapter that comes included. And I have already had reliability problems with the heating element, which is made out of a conductive filament that easily fatigues and melts itself to failure through thermal runaway.

This is a product that could only exist or sell in the absence of literally any other competition. Not that I’m not grateful. I love it, merely for the fact that it exists at all, and that it solves a real problem. But its more of a prototype than a finished product.

The Black Diamond Hot Forge Heated Chalk Bag

How to Charge From USB

Probably the most egregious flaw of the BD Heated Chalk Bag is that it requires an AC wall charger. Not only it is bulky, but you need access to an AC outlet to charge the battery. There are a few ways you can go about fixing this drawback. I’m only going to discuss the simplest and easiest one, which is to swap out the charger and leave everything else the same. It requires zero knowledge, skills, or tools.

USB 5V to 8.4V battery charger dongle.

Search Amazon or AliExpress for: “USB 5V to 8.4V Power Charge Cable For Bicycle LED Head Light”

Step 1: Purchase a 5V USB input to 8.4V DC output battery charger dongle

The TLDR: Search for “USB 5V to 8.4V Power Charge Cable For Bicycle LED Head Light”

I bought the one pictured to the left on AliExpress for about $3.50 and can recommend it. You can also find it on Amazon. Search for “USB 5V to 8.4V Power Charge Cable For Bicycle LED Head Light”. The dongle is very small and light, perfect for travel. It has a built-in status light that lets you know if it is actively charging or finished. At 1A max current, it can recharge your battery to full in a few hours.

If you decide to buy a different one, just be very careful to purchase a dongle that is specifically designed for battery charging. Just a basic DC-DC converter is not good enough, it could overcharge the battery and potentially cause it to explode. It must include a current limiter circuit for charging up Lithium Ion batteries.

Note to geeks: although the battery is a 7.4V 2S Lithium Ion cell, the proper way to charge it is with an 8.4V output. I tested the dongle pictured to the left and have confirmed in the lab with a power supply and oscilloscope that it maintains the correct voltage and correctly limits current when the battery nears full.

Find a DC 5.5 x 2.1mm barrel plug adapter kit with a 0.7mm output size.

Search Amazon for: “DC power adapter kit”

Step 2: Purchase a 5.5 x 2.1mm to 2.5 x 0.7mm barrel plug adapter

The TLDR: Buy a 5.5 x 2.1mm female into to 2.5 x 0.7mm male barrel plug adapter. Searching for “DC power adapter kit” will lead you to kits that have one of these in it.

The USB charging dongle from Step 1 outputs 8.4V onto a standard sized “male” 5.5 x 2.1mm barrel plug. The input to the BD battery pack is, from what I could determine, a 2.35 x 0.7mm “female” barrel socket. Thus, an adapter is needed to convert between these differing dimensions. I struggled to find any sellers offering this particular adapter on its own, but did find it included in a DC power plug adapter kit.

Search Amazon for “DC power adapter kit” and you will find a few options in the $12 – $15 price range that include either a 2.5 x 0.7mm or 2.35 x 0.7mm output size. Either of these options will work, since the first number is the barrel length. The important thing is the second number, 0.7mm, which specifies the diameter of the barrel. I found that the 2.1 x 0.5mm adapter size can also interface with the BD battery pack, but it is pretty darn loose and finicky so I wouldn’t recommend it. Go for the 0.7mm adapter width.

Putting it all together: USB power source -> USB 5V to 8.4V charger dongle -> 2.1mm to 0.7mm barrel adapter -> BD battery pack.

Step 3: Plug it in!

With just those two items, you’re ready to charge the chalk bag battery with any USB power source, no voiding of warranty required.

I have been using this method for about 18 months now and have had no issues and am very happy with the upgrade. I don’t need the clunky AC adapter, and I can charge the battery off of just about anything, even solar.

And that’s it!

Converting over to USB charging is incredibly straightforward. The only important thing to really watch out for is that your charger dongle includes the necessary features to safely charge Lithium Ion batteries.

Charging the chalk bag’s battery pack with a USB power bank.