if you could have equal resistance from all 3 going to the vesc, yes
right but that’s energy waste
you’re losing capacity due to internal cell resistance and continuing to charge it which continues to dissipate energy to heat
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I like that wiring. Simple and elegant. I’d still prefer to connect the packs in parallel so that I don’t have to worry about switching between them. That would probably lead to discharging the builtin pack very low before I switch to the extension, which is sub-optimal. Thanks for the idea though. 
How’s that different from 10Ah inside and 20Ah outside though? Bear in mind that 10Ah + 10Ah in parallel is simply 20Ah.
why not just an external backpack with battery?
what happens when you discharge 10Ah?
is it 5Ah,15Ah or 0Ah,20Ah or 10Ah,10Ah
it could be anywhere in there depending on the resistances
it’s possible but so many variables, I don’t think it’s reasonable
We’re assuming that the wiring is capable enough that the external pack does not have a serious resistance.
You can have a external backpack with battery. If discharge 10Ah then inside pack will be missing 3.3Ah and outside 6.6Ah
The only time you run into an issue is a senario where the outside pack has significantly more resistance than the inside. For example, you have a 5Ah lipo and you ran a 20Ah battery with 20 gauge wire outside. Once current exceeds the capability of the 20 gauge wire, the lipo will start to drain more, and recharged when current demand is gone. In any reasonably thought out parallal build, you can run any external Ah to internal Ah as long as wire size is thought through.
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I remember someone doing it on the other site, was there any follow up with how it’s doing?
assuming you make 2 packs with similar resistance and the wire not being a bottleneck, yes it’s possible, I just don’t think it’s reliable long-term, pack resistance will drift, I’m talking about behavior 1-2years down the road
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There’s some guy on here with a backpack battery, but the ESC is also in the backpack, mostly to alleviate voltage spikes. I run twin 12s4p in parallel no problem mostly for redundancy.
Well if you’re considering resistance from use, the internal pack would end up seeing more resistance over time as it wears, the external pack that is used less often will have relatively higher performance than the internal, which actually means over time, the extra wire length from the outside pack will have less and less significance.
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I’m clearly just not a fan of all these variables lol
(twins I have less of an issue with)
I’m actually kind of working on a clip on range extender, like a 10s4p with high storage cells, like some NCR18650Bs, on a boost converter that feeds a consistant 50v 4A to the charge-only BMS non-stop until the pack is dead. (Only get switched on once the board battery is partially drained). It would be like if the board was plugged in to the wall as you’re riding. The board battery still does it’s normal job, except it’s getting juiced up when you’re cruising, waiting for a light, etc etc. Just for those times when you wanna go for that extra long ride.
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that I likey
though I’m not sure how the power flow would work being charged and discharged at the same time, or is it charging only when discharge 0?
Well because the extender is going to be CC/CV, during high discharge it will just contribute it’s 4A through the BMS to the ESC. So i guess if you were measuring precisely at the board battery, it would see about 4A less current. The only concern would be regen, which is why it shouldn’t be used when the battery is still relatively full, but this is going to be up to the operator to understand when to switch it on.
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I’ve done a couple variations of charging on the go. I have yet to try plugging anything in in parallel, but plugging into the charge port with anything through a boost converter is super convenient… power tool batteries, toss a couple in the bag, it beeps when it’s time to swap, or plug in a larger pack to ride for a very long time… I’d end group rides at 90%, everyone else crawling or dead… which to me means I could ride as far as I like as long as I have enough spares. Swapping and carrying as much extra juice as you like, modularity as in use any power source you cobble together, these beat out parallel packs all the way imo.
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When accelerating and using more battery amps than your charge current the battery is discharging. When cruising using less battery amps than charging, it’s like you are Regen braking but from the BMS instead of the ESCs- the small charge current just flows as part of the discharge current when accelerating and acts like Regen when not accelerating.
When charging while riding you should lower your Regen braking to account for the additional charge current being fed into the pack while riding.
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This is neat!
My module that I’m drafting up is trying to go off a 4S pack. Likely a 4s10p, using USB-C power delivery for recharging (60W module), which will also allow it to work as a massive powerbank with USB-C out for accessories. Boost converter to perhaps up to 50v 4A (not sure yet). Printed ABS enclosure etc.
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1 - Not a problem at all, the lower internal resistance will provide most of the current, even if it drains a bit faster, as soon as the current reduces they will equalized again
2 - Depend on the packs, calculate the lowest theoretical internal resistance of each pack, and see how much difference you can have without a too big charge current flowing in either of them
3 - I have this idea too, my plan is to have an Arduíno on the extender pack that gets powered on when you connect the external pack to the board, it them checks both packs voltage and if the difference is bellow a certain value calculate in (1), it closes a relay or a contactor to conenct both packs, this makes it fool proof, you can also have just a charge only on the secondary pack for ease of charging
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I have done just this… it works excellent for my needs… 10s3p in the board and I carry a 10s5p in my backpack… I just strap it on the board with ROK straps
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Good idea! I just realized the charge/discharge BMS would only protect the external pack but not the builtin pack. For example, if the discharge current threshold is set at 80A and the difference between the voltage levels would be such that 70A flows from the external to the internal pack, the BMS would allow that since it’s OK for the external pack to discharge that much. Yet it would be too much of a charge current for the internal pack. I would need charge/discharge BMS on both packs to be safe.
Thanks for the other responses as well. I’ll look into the internal resistance and charge current calculations.
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