A Lightweight Electric Emergency Bike Pump

Got an ebike? Use the big battery you are already lugging around to power a small portable pump.

Me personally, I like to ride around on ebikes with fat tires, and I have several of them. The most recent addition to The Pacific Fleet is my Surly Big Fat Dummy. Its a monstrous cargo bike that, for me, doubles as a commuter. A few nights ago, on my way home from work – in the dark – I picked up a piece of twisted metal in my back tire. Part of the reinflation process (I use Flatout tire sealant so you need air in the tires as they spin around and seal the hole) requires air in the tires.

Because I wasn’t paying close enough attention, by the time I got the bike pulled over and the metal removed, my 26×4.3″ tire was flat as a pancake. Fortunately, I had a painless solution in my panniers and that solution is the subject of this post.

Now, when it comes to bicycle commuting this ain’t my first rodeo. I have always carried a Lezyne portable fat bike pump and it makes pumping fat bike tires tolerable. But its still far from ideal. Life sucks while you are putting in those 250+ pump strokes. And it ain’t quick by any stretch. If your tire is leaking air while you are pumping, the pump may not be a workable solution. As a backup I used to carry 25g co2 cartridges. Two of those monsters would blast a fat tire up far enough, fast enough, to be able to jump back on the bike and roll a half block or so to let the tire sealant do its job. Followed by another 250 pumps to get the now sealed but mostly-flat-again tire back up to rideable pressure.

But… you can only carry so many single-use co2 cartridges, and they are very pricey at that large size. Some time ago I came across a better way to deal with this issue.

The Portable Pump Solution

Wouldn’t it be nice if you could just connect a hose to a small portable compressor, flick a switch and pump up your tire? And the compressor pumped fast enough to outpace even nail-sized holes in your tube or tire? Well, portable pumps like that have been around for quite a while. Small automotive ones connect to your cigarette lighter plug in your car (I have this one in mine). Unfortunately they run on 12v DC. Your ebike is running at least 36v DC and likely more. So you can’t use those. There are plenty of pumps available that have their own internal battery… but batteries are heavy and so are those pumps. Besides. You already have a great big battery on the bike. Why can’t you use it?

Well, you can but you need a pump that can run on 48v. Fortunately they do exist but you have to look for them. Here is the one I use, purchased from Amazon.

Yeah yeah. I know. China. If you can find a USA-made portable pump that runs directly on a 48v power source, feel free to drop me a line and I’ll add it in here. As it stands, there are only a very few such pumps readily visible on the market and they are all from the Far East. I have used them a half dozen times without incident. Will they last forever? Good question. I do still carry my hand pump just in case.

DIY a Battery Plug

As you may have noticed from the pictures, the pump has an odd plug on it. What you want to be able to do is plug straight into your battery, and to do that you are going to have to get your hands dirty. From here on, I’ll give a step-by-step on how to make this happen.

Step 1: Snip off the plug

View of the cut off plug and spread pump wires

This one is pretty simple. Take a pair of scissors and snip off the plug.

Step 2: spread and strip the wire ends

You can see the wire strippers I used in the picture above. I used the 18ga hole, and I left about twice as much bare wire as I ordinarily need for a crimp connector. These wires are so thin I want to fold it back so the butt-end connector I will use has more material to grab onto.

Step 3: Determine which wire is hot

Yeah thats right. The plug gives us no indication which is the hot wire, so we have to figure that out for ourselves. What I do is bring out a bike battery and connect a bare XT90 pigtail to its output cord. This in turn gives me a bare, hot lead that I had sure better be careful with, and so must you.

View of the bare hot wire setup
Those are bare live wires on the end of that pigtail! Be careful.

So the next move is to bring the bare, stripped pump wires up to the bare battery wires and – after turning on the pump, touch the wires together to see which combination fires up the pump. Getting it wrong will not hurt anything. Just try the other combination if your first try doesn’t work.

View of the marked hot wire next to the live bare wires
When you know which wire is hot, mark it. I used a bit of heatshrink pushed over the hot end.

As soon as you have marked your hot wire, disconnect the pigtail so you don’t have bare hot wires waiting to say hi to the cat.

Step 4: Make the connection

My choice for this job is a combination of the following:
1. Marine adhesive butt-end connectors
2. Adhesive heatshrink over the individual wires on each side
3. Adhesive Heatshrink over the butt-end connectors

I’m looking to make a reinforced and solid connection since the wires on the Chinese side are pretty flimsy. Here’s what it looks like after I have crimped the wires together, but before I have done the final heatshrink of first the connectors, and then the sheathing over them.

overall view of the connection prior to heatshrink activation

Notice the different colored rings on each side of the connections?

Closeup view of the wire connectors

I used ‘step down’ connectors because the pump side wire was so much thinner. 18 gauge if we are being generous and probably 20 gauge if we are being accurate. This is why I folded the pump wires over to double them up. Which will only give more material to the crimp itself. The true strength of the connection comes mostly from the connector ends, plus the adhesive sheathing over top of that.

Step 5: Activate the Heatshrink (last step)

Finally, heat shrink the connectors first, then the individual wire sheaths, and finally the connector sheaths that also go over top of the individual small wires. BE CAREFUL on the pump side as the pump wire is very intolerant of heat and will happily melt on you even with mild heat. I use a heat gun set to low. You could get away with a hair dryer. I wouldn’t want to use an open flame due to the fragility of the pump wires.

View of finished plug
The layered heatshrink is not particularly flexible but it greatly strengthens and stabilizes the connection.

Epilogue

View of finished pump
I have added a Presta adapter to the end of the pump hose. Because I opted for a strong rather than a flexible connection, the battery plug no longer folds into the cubby on the pump.

At this point we just toss our pump into a bag to protect it from everyday rummaging, and that bag into our panniers. We’ll all hope we never have to use it, but we will of course.

View of pump inside carry bag

Author: m@Robertson

I'm responsible for the day-to-day operations at my place of business: Leland-West Insurance Brokers, Inc. We do classic and exotic car insurance all across these United States. I'm also an avid auto enthusiast, a born again cyclist (i.e. an ebiker) and participate in medium and long range CMP and NRA sanctioned rifle competitions.

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