mtn boost driver?

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buck91's picture
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Location: Michigan
mtn boost driver?

Is anybody familiar with the mtn-bst2 17mm boost driver from Mountain? I’m working on a P60 bike light (SF L2B) using a 90cri xhp35 and this driver. My currect set up is

Brass P60 pill, b- contact spring if fully collapsed on install
MTN-BST2 almost fully soldered in, brass button contact with the 10.5A in 4.2A out configuration (est 1-1.3A to 12v LED)
stock SF forward clicky with 22awg spring bypass
Solarforce L2B host, P60 is copper wraped for very snug fit

As it is, I seem to be getting great thermal transfer as the light heats up quickly in hand. On full output (100% guppy3drv mode) I’m only seeing about 5.5A draw off my Samsung INR18650-30Q. Now I have two concerns. First, I wonder if the battery draw current is too low indicating excessive resistance in the host (I’m thinking most likely the switch) or is it just excessive resistance from the DMM leads? Second, I am experiencing the light cutting out after 2-3 minutes on 100% output. It certainly does warm up but the driver is well connected to the pill both with a tight friction fit and ~80% of the perimeter is soldered. I’m thinking of two solutions. First, once I’m done and the build is finalized I am going to pot this setup with 832HD (thermal conductivity of 0.27W/(m*K)) which I would think will help slightly. Second, mtn is recommending swapping the “sense resistors” out with either 2× 0.5 ohm resistors + 1× 1 ohm resistor; or 5× 1 ohm resistors. Now I have a bit of a basic understanding of electronics but I guess I don’t know much about what these resistors do on this driver. He suggests this will result in approx 70% max output which is totally fine by me. My goal, as this is a bicycle light application, is to have a simple interface with the first mode to run for about 90min though a little less is OK. Right now I’m limited to the guppy3drv group of 7/30% as anything more aggressive has a full output mode which causes the driver to has the thermal cutout I mentioned…

So I guess I’m just wondering if anybody has any other specific suggestions? How do these resistors work in this driver and what values would give me a battery draw of about 1.5-3A on 50% mode with my 12v XHP35?