I’m looking at this boost converter on eBay to charge my 12s with bms from my van (my van has 300Ah of battery so I’ve got plenty of juice)
May well use the same converter as a range extender as per this thread…
Can anyone see a problem?
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Yea sounds fine. Just triple check the voltage settings on the boost converter. Sound like a great idea, good luck 
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is your van battery the regular single 12v lead acid? if so, you wont get enough amps out of it after the boost. the bigger the difference in voltage the less amps you get out.
for 12s you would want 24v minimum, from memory i think that gives you around 4-5amps. efficiency was not great but that shouldn’t be a problem
I have two small 15ah 12v deep cycle batteries on the charge now. hoping to put them in the ute, charging off the main battery in parallel with another circuit in series to power the boost converter… whether or not this’ll work though, i dunno …
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My van has 220Ah of AGM deep cycle… I can charge 2 x 6s 10000mAh drone batteries simultaneously at 10A each using 2 hobby chargers so I think I’m good for amps!
So I’d have to parallel 
(Series) 
my van batteries to get the volts?
yea that sounds right. 12v up to 24v is fine, only 12v step up. however, 12v up to 48v is 36v difference.
I think it depends on the boost converters maximum input amps. the one i have has 15a input max and i found a huge difference between boosting 24v to 36v and 15v to 36v. with 24v i could get up to 10a output but with 15v i could only get 1.5a output.
looking at boost converters this morning i found this one with 30a input, which might be enough to charge 12s off 12v … emphasis on ‘might’
and yea that would be series connection to multiply the volts of your car battery. parallel multiplies the capacity.
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How exactly can that converter be used as a range extender? I’m missing something here. Do you plan on having an external loop key battery to bump up to 12S?
The loop key boost was the initial idea. If you look towards the end of the linked thread the discussion turned to using boost converters to act as range extenders with good results. Which seems much more straightforward.
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Yeah this is a pretty bad idea. I wouldn’t advise this AT ALL.
Charging the evolve was easy enough because it went through different charge circuitry to limit output current.
If you just slap on a DC-DC converted in parallel to your main pack here’s what will happen.
The buck converter and the main battery will share the load. So if you pull 40 amps overall, you’ll pull 20 from the main battery and 20 from the buck converted.
But the buck converter is stepping up the voltage 5X, so the tiny battery behind it will have to be supplying 100 amps! That’s incredibly dangerous to pull that constantly or for an extended time.
His went through a BMS charger (i believe) which limits the charging current.
But even so, that’s not good. You will be able to over discharge the small pack because it has no protection. It could run out without you knowing and the buck converter is gonna pull more and more juice out of it till it puffs.
It’s just not a good idea.
Adding an additional 2S in series seems to be the best method for what you want. You just need to make sure that the add-on pack starts at the same voltage as your main battery, and that it’s made of the same cells.
Ok I see your point… maybe this reply should be to the other thread though?!
does having the van running make any difference?
The method I’m describing in that post is for when the board is active. It’s using a smaller battery ON the skateboard. This is not for recharging in a car.
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@moderators could the comment on the range extender be moved to that discussion to avoid confusion?
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lol. yes, that was quite confusing
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so there’s this dedicated thread started about boost converters and such now, not much in it yet but looks like where all the new info will go.
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Look man you’re the one who in this thread specifically asked about this and if it’s a good idea, and I clearly explained how that works and why you shouldn’t do it. I don’t think anything’s out of place.
Oh well I guess I’ll agree to disagree.
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Yes using that boost converter will work just fine as it has an adjustable current limit and voltage. I would be careful with the voltage though, its probably worse than 1% accurate, so set it a little lower than you need and check with a multimeter.
Note that the converter has a maximum input current of 15A, so at 12V you are limited to 180W, ie 12s charging at ~3.5A. Still decent.
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@TowerCrisis, the thing you are missing is that this kind of boost converter DOES contain a current limit. So you could set it to 2A output to use it as a range extender. Similarly, you can set an undervoltage limit on the input side to protect your secondary pack (or use a BMS on that pack). A guy i know does exactly this with a backpack full of batteries and a cable running down his leg and plugging into his onewheel.
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