Noob question thread! 2020_Summer

Looking for motor recommendations.

10s12p 35e, 97mm urethanes, also want to go AWD ( just cause ) looking at two spintend escs

That’s not many amps to spread 4 ways. No sense in picking big motors unless you want the extra weight for some reason. 4x 5065s runnin’ 25/40 will still rocket you around.

2 Likes

I put 50A/side on my 12s4p, which is about the limit where these cells start to sag. I would limit yours at 40A/side. It’s way more satisfying to have sag-free power delivery than to have more power and feel it drop when you slam the throttle

3 Likes

A 12s3p won’t handle 120A (60A per side) happily. You either need to dial back your current, or have a bigger battery (4p or even 5p rather than 3). 120A is a BUTT-TON of power.

4 Likes

Yep. Say this more. I struggle to pull even 30/side on the street passing cars. It’s nice to have the power available though

3 Likes

I would recommend max of 25A per parallel group for 40T’s. Which translates to roughly ~35A battery current per vesc (with your 12s3p config). You can pull the max rated discharge of 45A (with 80°C temperature cut) per cell but that comes with increased operating temperature, increased voltage sag and reduced cell lifespan.

As others have mentioned, 120A total battery current is a huge amount and not necessarily an amount you’ll ever draw or need.

If you are still unsure, read over the manufacturer specs and calculate your current settings based on the recommended ratings. Ie, the max manufacture rating of 45A with 80°C temperature cut comes with all of the caveats previously mentioned and is not recommended.

HJK Test of INR21700-40T 4000mAh (& manufacturer specs)

4 Likes

Agreed.
While any cell can be discharged at a level far above its continuous current rating the performance is terrible and the cell ages at a faster rate. At above about 25A the 40T struggles and the Samsung 30T is a better choice. It will run more efficiently and cooler, extending cell life.

4 Likes

Quick confirmation please… I’ve bought a blade fuse cradle and blade fuse to wire between the charge port and battery. I’ve seen it wired into the negative lead in other threads. Is this correct? Also, I will be using a 4amp charger so what size fuse is recommended? 10amp?
Thanks all

There is no “correct”; it can be on the positive or negative charge port lead. Generally it’s a good idea to treat + as “common” and just wire the positive together, and switch and fuse all the negative wires.

3 Likes

Tho this can cause issue with ground back feed so I put loop keys on the + use the ground for the common e.g flex BMS uses the - as the ground for the opto isolate if you connect the can ground then remove the loop key on the - all the current try’s to flow thro the can ground and magic smoke appears

What advantage dose the fuse on the neg give iv seen a lot of BMS common the pos then switch the negs but no idea why

1 Like

The reason for this is because due to the nature of physics, it’s easier and cheaper to make quality MOSFETs that switch the negatives on and off with a common positive

@Gamer43 may be able to give a more detailed answer.

Every BMS I’ve ever encountered had common positive and switched negative(s).

2 Likes

I do this as well, but beware in this circuit you can’t have any internal connections between the lights circuit and the ESC (bypass) circuit

Is it ok to mix batteries with different cycle counts? I’m thinking about cases where if one cell goes bad maybe 500 charges in, can it be replaced with an identical brand new cell? Similarly with LiPo packs, can I connect two otherwise identical packs except for cycle count together in series and/or parallel without any issues? Would it become unbalanced as it gets used and cause issues?

Its better to only mix cells of the same cycle count, and with the same capacity and internal resistance (which you have to use a machine to test for). Its usually less of a headache to just use all new cells.

1 Like

It’d always be better to get a new pack, but what issues can occur? Series should be fine, as that cell would just go out of balance and end with a higher voltage than the rest, but it’s not going to be a limiting factor, and it should just get rebalanced when the pack is charged. With parallel, I’m not sure what the interactions are.

If your p-groups are too out of balance from each other, your BMS may not be able to bring them back into balance. Most BMS’s have a maximum difference between the groups that it will balance, and if its outside of that then it will just lock out the charging and you will have to open up your pack to balance manually.

If one of the cells in your p-group is different than the others then it will be under constant stress. The older cells will want to discharge faster, which will put a larger load on the newer cell. Same thing in reverse when it’s charging.

Perhaps @Battery_Mooch can more elegantly explain why mixing old and new cells in a pack is a bad idea.

Too many scenarios to go through. Some only performance related, some safety related.

But since there could be issues, depending on the cells and the setup, we recommend not mixing old/new cells.

2 Likes

Are there any general guidelines on what constitutes an old cell? It seems like a waste of a pack if less than 50 cycles in, a cell goes bad, and I’d need to get a whole new pack.

Generally, no, not at all.

If it’s done a certain specific way then maybe. But only in that exact way. Without a machine that tests IR the only way is to

Have each P-pack contain the exact same amount of Age A and Age B cells.

So if you have 20 cells that are old and used TOGETHER the same amount, and 30 new cells, you could make a 10S5P with EXACTLY two of the old ones and three of the new ones PER P PACK. But this is inflexible and you cannot deviate from that, at all. Not one cell can be different.

In general, no, you can’t mix and match cells.

Also, none of the cells can be used laptop cells. Ever.

4 Likes

The best way to deal with that (if one cell goes bad, it will kill the whole P-group), is to remove the affected p-group and enough others to result in a number of cells that is evenly divisible by the S-count of your pack, so you can do what Brian said and put an equal number of new cells in each p-group.

You don’t have to scrap the whole pack, but you’re gonna have to tear it all the way down to bare cells anyway so you can shuffle them around.

This is why we put so much emphasis on building for reliability, in the form of proper cells, appropriate connection and construction methods, good BMS, the whole nine yards. Because it’s a huge pain when something goes wrong, and the best solution is to not have things go wrong in the first place.

7 Likes