I don’t know whether the OP is able to set it up with this build but the way I would go with this is to have separate chargers for each pack. I have a build with 4P, 4P, and 10P all wired to discharge in parallel but each pack has its own charger. The nice thing about this is that I can charge at low current and I use just 3A chargers. I have another build I’m working on which will have 2P + 2P + removable 6 or 7P (the 2P + 2P will be sort of like the jump pack).
You might be charging all 3 together with however many chargers you plug in.
Not really much different than 1 giant 18P pack with 3 chargers. Which seems to work.
This is the part that can start fires:
Wasnt talking about the ziptie, was more about never disconnecting to charge which as I said offers no charge protections on the secondary pack
Yes it’s important to always charge to full with the removable pack and not be mixing voltages and yes I believe there may be some cross-charging going with the separate chargers. The lights on the ones attached to the smaller packs will get to green first but I still always leave them all plugged in.
I have a battery switching system on my board shown in the first portion of this post here:
I designed the dual loopkey in such a way that it is physically impossible to plug in both at the same time, that is the only reason why I would consider this setup safe. This is not a parallel pack situation but a pack switching situation. I personally think parallel packs have a bit too much risk involved, but there are ways that they can be done relatively safely.
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Your design is so sweet. If I was still running a double pack setup this is what I would do hands down.
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It does, as there isn’t a chance of connecting them together again at different voltages. It’s a huge risk averted if you simply never disconnect the parallel packs.
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Lots of things are dangerous with user error. My blender is a hazard if I stick my hand in it while it’s running.
Just tie the loopkey to the board. Make it impossible to use another loopkey. It’s not that hard.
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…or just choose to build it a safer way, because it’s easy and cheap to do so.
Ill take risking human error vs the chance yhat at any time pack B gets fucky and then goes boom because there is no charge protections on it.
What you’re recommending is more dangerous than trusting folks to connect packs at the same SOC.
Your hyper focus on one danger has clouded your mind to not weigh the other dangers involved.
You are essentially suggesting to folks that it’s fine to charge without protections, stop it.
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Keeping the packs separate is simpler and safer. If you always have extra loopkeys around and don’t trust yourself not to plug two in at the same time, there are awesome solutions like @DeadLightning came up with, that take the human error potential away 100%.
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I had thought maybe you meant if it was in the wrong mode but that shouldn’t be possible either as it is still not an actual short. The current will still be going through the meter and would prevent damage. At worst, the multi meter should just blew its fuse or potentially self destruct on itself if it is a cheap one. Also that would only happen if the multi meter has two separate current circuits and you’re accidentally on the lower rated one.
I don’t see how my probes could connect the way you mention, but I do think probes with a slightly different design may be able to accidentally touch like that.
Parallel batteries are dumb and unsafe.
I play with batteries every damn day and like to think that I know a thing or two about them.
Even I have done fucky wuckys with parallel packs, at the hands of complacency.
The best option is just to build one pack for one board. Multiple packs, parallel packs, charge and ride packs are all going to offer known risks (and likely several unknown risks) in a number of everyday situations.
Not everybody is 100% on all the time. Its very easy to make stupid mistakes.
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Dammit…will you battery nerds stop with the frakkin’ tech-talk and stick to English for us normal folk! 
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Range extender packs freak me out.
However, they can be made a bit safer by fusing the extender pack’s discharge.
The rider safety downsides of discharge fusing don’t really apply to an extender pack. The main remaining downside is added resistance.
FTFY
it’s not bad to aim for identical pack voltages, but the it’s also not bad to have the packs .1V per cell off of eachother, so long as you plan to either not fully discharge the packs/are going to charge them together before going out
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To add to that, they also need to be closer if they are in the mid-charge range. On the top and bottom of the ranges, they can be slightly further apart, I think.
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So then it would seem to be better to only use dual packs if they each had a common-wire-BMS and they were both wired for discharge.
No bypass, no separate wire BMS.
Notice how I made a nice helpful reply concerning technical details and machines, without any direct insults to a human.
Though the capacity change per voltage change ratio changes at different charge levels I don’t think that affects the level of current flowing between the packs when connected. That would be determined only by the combined resistances and voltage difference (Ohm’s Law).
Doing it at a lower voltage (for both) would, I think, result in the current running for a longer time though. I guess that can be viewed as a negative.
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