Ebike extended battery

This project is still a work in progress and may never end up completed, or may become something radically different than described.

What / Why

I love cycling. It’s my main way of getting around. I bought myself an ebike for commuting and hauling heavy things around with my bike trailer and absolutely love it.

The bike I bought, the Luna Fixed Stealth has a 20ish mile range. A lot smaller than some other ebikes, but a concession I was happy to make for the other features of the bike (mainly, the internal battery and lighter weight).

I have no complaints about the range of the ebike, I was seeking out a very niche set of requirements and the Luna has fully delivered.

But why can’t I have the best of both worlds.

If I could have an auxillary battery, I could strap it on the bike and go for my large grocery runs with lots of hills very far away and have no issue with range, then pop the auxillary battery off, and I still have my nice stealthy commuter.

So let’s mod the Luna.


The ebike mod (name to be determined) is actually very simple. It’s just an implementation of an ideal diode OR with the LTC4359

Or in (less) technical terms, this circuit will draw current from whichever battery is at a higher voltage (charge). In the case of no auxillary battery, current is provided from the internal battery like normal.

A quick PCB was mocked up that fits within the downtube of the Luna, next to the battery.


As of writing this page (and I don’t update this website regularly so who knows when that was), I still need to send the PCB to production.