This is good info, but i ordered a set of abec 9’s specifically to match my builds color scheme. Lol.
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That works also lmao. I got a bunch of bearings I cleaned that are waiting to be put on a board, luckily I had red and black to go with my build color scheme, Abec 9’s are good
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Yup! Free shipping and coming from just across the state, too. Not bad at all.
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Ok, so, i need a range boost next weekend cuz direct drives arent very efficient lol. So im thinkin i could use my backup battery as a backpack charger/range extender. Im not very knowledgeable on this stuff so i figured id ask here.
Goal: Run power from a pack in my backpack(12s3p, 40t) to board(12s3p p26a), preferably directly via charge port.
Do i need a buck converter or discharge bms at all for this? Or, since theyre both 12s, could i just run straight from say an xt90 to a (x) mm jack? That seems less than ideal. Id prefer to make a small box style device that i could just plug any battery into via xt90, flip a switch to turn it on, and use as a power source.
I’ve been thinking of doing this to a board. having a switch for internal to external battery supply, that would also switch the charger from BMS to VESC so I could just strap a pack to the top of my board and drain it, then swap to my internal battery when that ran out
For that particular use case; 2 loop keys. Never put both in at once. I made a diagram for someone on how that would work once, but im too busy to find it atm.
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could also just use a 3-position latching switch with an appropriate mosfet. loopkeys are fuckin’ ugly and trashy, like using a screwdriver instead of a key for a car.
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There are a few different ways to do this, each with their own pros and cons:
The first way is to just connect both packs in parallel.
Pros:
- very simple
- no limits on power output, assuming your board’s charge port can handle it.
Cons:
- if the two packs aren’t exactly at the same voltage when you connect them, you will damage your shit when you try to connect them together.
- unforgiving of connector/cable ratings and losses. You can easily burn out connectors or blow fuses if you try to pull more power than your charge port or whatever can handle.
- if you lose connection between the two batteries and they get out of voltage sync with each other, you cannot reconnect them until you get them back in sync. See con #1.
The second option is to use a DC-DC converter to act as a charger, rather than having the setup act like a split battery.
Pros:
- nearly foolproof - Once you get the converter set properly, it’s plug and play, with no issues of battery matching or anything like that.
- can be connected and disconnected as needed.
- can use any battery, as long as the converter can handle the input voltage. Got an old 8s lying around? It’ll work!
Cons:
- more complicated/heavy/expensive, since you need a converter.
- the battery will only charge as fast as the converter (and your charge port/wiring) can handle. You might end up draining your board faster than the charger can refill it, so you’d have to stop and let it charge before continuing.
- you lose some energy in the conversion process: DC-DC converters are generally 75-90% efficient, so you will be losing ~10-25% of your extra range.
Option three: Ride battery A dead, then disconnect it and connect battery B.
Pros:
- hard to fuck up
- low additional cost
Cons:
- more invasive - You need some high current switches/loopkeys and connections to allow the full board power to be supplied by the external battery. You can’t just run that through your charge port unless it’s an xt60/90 or something similar.
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Gonna read that better when im not actively running wedding later. Thanks though. Leaning dc-dc.
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I really like option 3. What about having 2 ports externally? One for charging and one for “alternate battery pack” and you could just use an inline switch to turn off power from the internal pack.
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There’s nothing wrong with the theory, but the execution has a bunch of possible pitfalls that could result in Very Bad Things happening.
For one thing, you need some way to absolutely guarantee that battery A and B can never be connected at the same time. A simple switch won’t do that, and also you need a switch that can handle your entire battery current, and good luck finding one small or cheap enough. You either need some kind of mechanical or electrical interlock, or only have one power port, that either A can be plugged into, or B can be plugged into, but only one at a time.
All that stuff means more invasive modifications to the board.
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So I’m gearing up to build my first pack. From what I gather, it’s OK to mix cells if each parallel pack in series has the same capacity. Is it OK to mix 35e and p42a? I’ll have enough to go 12s6p with 3 each of the Samsung and mollicell cells. I intended on doing 12s4-6p of p42a but since they seem hard to source atm.
Ok yeah. Gonna figure out what amperage my charge port can handle, then convert my backpack into a portable charge station with the maximum. I think if im conservative, with direct drives free roll excellence, i should be able to keep consumption relatively low.
I personally wouldn’t recommend mixing cells with such drastically different discharge characteristics. To avoid the possibility of drawing too much current from the 35E cells, you’d have to treat the whole pack as if it were made of 35E cells, which basically negates any benefits the P42A cells would be giving you.
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I’m thinking I just need to be patient and wait till more p42a’s hit the scene. Maybe I’ll sell off the 35e to cover the remaining p42a…
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That would be my recommendation. Mixing cells from different batches or of slightly different ages/cycles (but same type) is much less risky and problematic than mixing high-power cells and high-capacity cells.
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You’re going to make a fire if you try this and reject almost all offered advice like you have been. For that reason I’m not going to spend time commenting further. It’s been discussed, search it out. @MysticalDork gave some good answers
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No. Not under 99.9% of situations.
In fact, even the P42A need to all be new or used in the same battery previously, you don’t want mismatched cells.
If they are mismatched in age or type then they need to be spread evenly through all P packs. So if you have 30 cells A and 42 cells B that are either different types or different ages, you could
- use 2 A and 3 B per P group for a 12S5P battery and have 6 A cells and 6 B cells leftover
- use 5 A and 7 B per P group for a 6S12P battery and have no cells leftover
- use 3 A and 4 B per P group for a 10S7P battery and have 2 B cells leftover
What you should NOT do is
- mix and match the P groups for a 12S6P battery and have no cells leftover
But even if you don’t mismatch the P groups, you are mixing a low-drain cell and a high-drain cell and you’d need to treat the entire battery as a low-drain battery taking like 7A or less per P because of the 35E cells
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I meant 3 each of A and 3 of B in the P groups but I understand what you’re saying. Thanks @b264 and @MysticalDork , I’m going to continue studying pack building then either source more cells for 2 batterys or go all p42a when it’s finally time.
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Sorry I just think loopkeys are ugly. There are much better solutions than that.