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
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?
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.
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.
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
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.
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
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.
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.
but I found this that looks similar, what do you think? Also, you just using a voltage display in parallel? Or what is that fancy display. I must have it, lol
The current can be gotten/assumed through ohms law based on voltage and resistance alone. I forget what it was in that instance but a lot and not even warm. @janpom.
yep, same thing there … not sure the quality of that one but I’m sure it’s probably okay. if it works on the bench, just make sure to pot the coils and mosfets and stuff… I used glops of e6000 to pot mine.
For voltage I just got a voltage display, two wires hooked up to either the input or output (or both) depending on what voltage you want to see… I did the output since I mostly used power tool batteries and they would cut off when dead anyway and had their own voltage displays, and added on some low voltage beepers to other packs I put together to know when to pull them.
I did something similar one time being lazy as hell, I was putting together a gigantic 1P pack for an experiment, half the cells were from a drained pack at like 3.2V and half of them were at 4.1V. Being lazy as hell I decided I’m just going to start slapping nickel on with no care to balance them. The 3.2v cells got noticeably warm, but never hot. Holding them was not uncomfortable at all, like lukewarm water hot. Never measured anything but it worked just fine.
Great design! I’m curious why you made the battery mount in a separate piece than the boost converter housing. I’m thinking of making a similar design in 1 piece which would eliminate the external wires.
I also see you used two different monitors. Is there one you prefer?
Lastly, if you put a battery monitor on the output side would you be able to get a read on the onboard battery status?
Thinking about experimenting with two 6S lipos as an extender to a main 12S4P pack with 18650 cells. In this case I would be connecting them in parallel.
I would not recommend mixing different cell chemistries.
Also keep in mind you discharge lipos to 3.4-3.6V per cell max and LiIon to 2.8-3.2V per cell.
To not over discharge the lipos you would need to cut off at 3.5V per cell which will cut range from you LiIon pack than.
All in all it doesn’t seem to make too much sense for me.
Why not just swap the packs out?
Drive your LiIon pack till it is empty, than change to the lipo pack and drive till it’s empty.
Why you need to wire them in parallel?
