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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Here is a short reply from an electrical engineer:
If the batteries you want to connect both have the same voltage(say for example 40volts) you can connect them in parallel and the load will spread between the two batteries. However one battery may discharge more since it can be a bigger battery(lower discharge rescistance or maybe a lipo battery so higher c-rating). Those two will even out however when you let go off the throttle so it is not an issue.
You should for simplicity sake however use two batteries with the same chemistry since you cant bring a lipo as low in voltage as a li-ion based battery-cell. They work in tandem however but you need to disconnect the lipo pack when the cells reach around 3-3.5v
In short,yes your solution will work. Make damn sure they are the same voltage when connecting them and charge them separately unless the batteries has a discharge/in-use bms
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Good point
One idea I have is just connect the external pack on the charger input of the FlexiBMS, if the external pack is lower it simply won’t engage until the main pack is discharged. If the external pack is higher, it will connect once, if the charge current is greater it will disconnect it, don’t know if that is a good idea, for example, in case the current is too great, does if react fast enough to avoid destroying it self? And depending on the current share, it would disconnect every time it goes over the limit, and no regen would ever go to the external pack
This one you probably can answer, how does it behave in a over current situation? You have to unplug and plug again the “charger” for it to clear the fault?
Correct.
I think there are two problems with that solution:
-
The charge current is highly constrained so unless you’re riding very easily and with a lot of breaks, the internal battery would discharge much faster than the external battery would re-charge it.
-
You would need some way of limiting the current flow from the external pack.
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So I looked a little bit into the internal resistance and I’m actually not sure how to use it. Considering two 10S5P Samsung 30Q packs as an example. My naive calculation goes as follows:
- Samsung 30Q has internal resistance of 0.02 Ohm.
- In a 10S5P battery pack that gives 0.04 Ohm (=0.02 * 10 / 5).
- Max charge current for Samsung 30Q is 4A, so that’s 20A for 5P.
- Max safe voltage difference between the packs is 20*0.04 = 0.8V.
Now, this completely ignores the inductive and capacitive reactance since, to be honest, I don’t really understand those and don’t know if and how they should be taken into account.
Is the maths still more or less correct?
dude. This is brilliant!
I was thinking of doing something like the top mount range extender but this seems a more elegant solution. Besides giving me my deck back, it also looks cool (which is important).
I see a couple different step up converters, do you have any you can recommend? I like the one that looks like it has it’s own case 
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This is the one I use, it’s been working flawless for me after potting all the components to withstand vibrations. I tried two others that ended up failing for other random reasons.
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You also have to calculate the resistance of the external pack, add both together and use that to see the maximum voltage that results in a current lower than either of them can accept
Let’s say you also had another 10S5P made of 30Q, so in this case you could have a 1.6 V difference, probably would be wise to use half the calculated value
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Makes sense. I’m thinking I’ll do some experimenting with some old 18650 cells. Just take two, bring them to a slightly different voltage level, connect in parallel across a multimeter and measure the current flow.
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I think @hummieee did this test a while back with two cells with totally different voltage and than put them together to look how they balance each other and how hot they get.
Not sure if he did measure the current thou.
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