48V configurations - versatility -

[jaunty]

Greetings,

a newb here and I am looking to build a versatile Li-ion pack from 18650s to power a 48V 1000W rear hub bicycle kit. I would like the capability to easily remove defective cells if necessary. And also to have the packs wired in 12V configurations. Is this a problem for balancing?

The reason why I want 12V configurations is to allow for standard 12V device usage (camping etc, smaller draws).

The issue i am faced with is whether to use 3s or 4s. nominal voltages would yield 11.1V or 14.8V, respectively.

Also, what would be the appropriate way to match voltage to 48V motor? The nominal or full charged voltage? Which would yield 50.4V vs 48.4V for a 12S pack.


Advice is appreciated!

 

[amberwolf]

If you're using "LiPo" / "LiCo" / "LiMn" etc chemistries, 3s would be 12.3-12.6V at full charge, as 4s would be 16.4-16.8V+ dependign on how high you charge them, and may be too high for some of your devices (which can probably work up to 15V or more as you might get that high a voltage on a running car's 12V system). You'd have to test tehm to see if they work without damage at the higher voltage.

If you're using LiFePO4, then 4s would be better than 3s.


Most 48V ebiek stuff is actuallly about 60V full charge (56-60V, depending on # of cells and chemistry). Some only have 63v caps in there, so you'd be pushing their limits if you go too high.

So if you use "12v" sets, it doesn't quite work out.

If they are 12.3v, then you get either 49.2v 4sets or 61.5v 5 sets. If 12.6v, then 50.4 4sets or 63v 5sets.

If they are 16.4v, then you get 49.2v 3sets 65.6v 4sets or 82v 5sets. If 16.8v then 50.4v 3sets 67.2v 4sets or 84v 5sets.


You might want to make sure your controller(s) can handle 100V to be sure they'll work with those sets.

 

[cycborg]

IMO the best way to get 12 V is to use a DC-DC converter. You may find that you want this on your bike anyway for lights etc., while you're riding, simultaneously with your full pack output.

Then, just build a monolithic pack at 13s, which is the typical configuration for this voltage range. 14s would be OK too. Your motor and controller should be able to handle this and you wouldn't suffer from the reduced power of a 12s pack.

For a converter, you can use an AC adapter from an old laptop or something that has a 12 V output and is rated for enough amps to power the devices you're using. You can often get these cheap at thrift stores. You might have to try a couple before you get one that will work since some of them won't power up at lower voltages. And some of them won't work with the wrong polarity input so if it doesn't work on the first try, switch the leads and see if that solves it.

Search "AC adapter" using the Google search option - lots of threads on this.

 

[amberwolf]

If you decide to use a DC-DC or wallwart or ac adapter / ac adaptor for the "12V", bear in mind many of the automotive 12v accessories and what not are meant to run on more like 13-14V, so you could look for ones at that voltage at the rated current.

I use a 15V "battery charger" ac adapter / wallwart on my CrazyBike2 to power all the "12v" lighting except for the headlight and horn (cuz those draw more than it can put out when they start up). I found it at goodwill for a couple bucks or something like that. I wired it's AC power cord to my bike's motor battery via a switch, so I can turn off all the lights if I need to (like when charging).

 

[jaunty]

thanks guys! I'm going to collect as many 18650's as i can and then decide what route to go.

I'm really hoping to segment the pack as much as possible to make it easy to extract bad cells. As it turns out, charging and testing individual cells is pretty time consuming! It will probably be a few months before I can gather enough good cells for a pack.

My plan is to build packs in the 12v range, then add those in series, so we'll see how that goes.


Cheers!

 

[dnmun]

you cannot escape testing all of those cells. if you build a battery with trash then you will spend all your time trying to figure out how to repair it and it is obvious you do not have the skills to do that. you have to charge them up and measure capacity and internal resistance and then combine them so the complete set is balanced for storage capacity on each channel.

to insure the ability to isolate the bad can in the pack you have to build your battery up as a serial pack, 13S1P and then build as many 1P packs as you have cans available that work and use them in the places so that when you combine all of the packs in parallel then each channel has the same identical capacity and internal resistance.

if you connect them in parallel initially then you will never be able to isolate the bad cans.

when you have experience this will become more obvious and you will regret having built it up in parallel first as all these people are now gonna tell you to do.

your serial connection can be small wire since these cans can only produce an A or so at max so even 18-16AWG for the serial link is adequate and you combine them in parallel with the sense wires which can be 26AWG or less since there is no current flowing in parallel except when the BMS is balancing the pack.

 

[jaunty]

you cannot escape testing all of those cells. if you build a battery with trash then you will spend all your time trying to figure out how to repair it and it is obvious you do not have the skills to do that.

Thanks for your input. You're right, I don't have the skills which is why I need to take some extra time and get it right the first time.

I do intend on test the individual cells, I'm just lamenting a bit. I don't really trust the knock off imax b6 i bought for discharge testing so I'm going to devise my own method with a good known load, while measuring the voltage and current.

Basically I am looking for options that would use quick disconnects as to segment the pack as much as possible. I don't want a giant ball of duct tape like I am used to seeing.

 

[arkmundi]

Depends on where you're collecting those cells from. The 18650 cells are a popular format for consumer devices, including laptops. If you're attempting to scavenge old packs, just be careful to match chemistry and IR characteristics. By IR characteristics, I mean the internal resistance of cells increases with age, discharge & use and can be a lot different depending how you're sourcing them. Chemistry should be identical for all cells. Then fairly close IR, meaning you'll need to do that discharge test, cell by cell. Once completed, you'd then need a spot welder to weld cells together. While there are approaches suggested on the forum for mechanical interconnection, I haven't seen any I'd want to use yet.

And then there is still no guarantee your pack will power your e-whatever adequately, as repeated charge/discharge at the amps required will certainly in short order prove how well you selected those cells.

Its why my policy on entry to the battery maker community dictated I'd not go that route. I source new A123 AMP20 cells from OSNPower, their kit and fashion a container to hold the cells in compression. New cells from a decent vendor purchased as a batch will get you closer to battery nirvana. But if battery samsara and suffering is your intent for the moment, then by all means .... I would not want to interfere with a person's karma.

 

[dogman dan]

Hours of fun if you dig that. But yes I agree, it's hard to scavenge a lot of cells identical enough.

One thing I have found works, have a good battery to run the bike. Then pretty cruddy cells can easily handle your lights, recharge your phone, etc. Separate 12v pack works fine for me. 12v 10 ah is tiny, so no big deal to carry it separate.

 

[jaunty]

your serial connection can be small wire since these cans can only produce an A or so at max so even 18-16AWG for the serial link is adequate and you combine them in parallel with the sense wires which can be 26AWG or less since there is no current flowing in parallel except when the BMS is balancing the pack.

So I am planning on building as many 13S-1P packs as I can. I have a 13S BMS coming via aliexpress. Still trying to decide on how to arrange the 13S pack.

So, I combine all the individual series packs sense wires together and then into the BMS?

I also snagged up a 48V - 12V DC converter so we'll see how that goes.

 

[dogman dan]

It's a dilemma. But at least you have the method for getting 12v solved. Now the question is, do I make 13 separate packs, or divide 13 by some other number. Obviously 2s packs leaves a spare, as does 3s, and 4s. If you'd chosen 14s, not a lot better, but at least divides by two.

But 13 detachable plugs is 13 points of failure. I'd say the thing to do is make it in 13 sections, but make the connection between each section from a wire long enough to cut if you need to, and then re connect when you swap in a fresh section. Use hot glue sparingly, then tape or shrink to hold the whole pack together once assembled.

If you have that many extra cells, then it could make sense to have a spare 2s pack, and a spare 1s pack handy.

 

[jaunty]

thanks dan.

yeah i plan on connecting them "permanently" in 13S, so i basically scrapped my original segmented idea (For now at least)..

I snagged up some of those tiny bracket holders for 18650

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So im thinking of piling 13 into series in a brick shape and soldering them together using 3/16 copper braid.

My question is about the balancing leads. Each individual 13S pack will have 14 balance leads sticking out, correct? So i have to combine all those into the bms. I will make those small gauge. I'm planning on having at least 10P

so 13S10P = 130 cells..

 

[jaunty]

to insure the ability to isolate the bad can in the pack you have to build your battery up as a serial pack, 13S1P and then build as many 1P packs as you have cans available that work and use them in the places so that when you combine all of the packs in parallel then each channel has the same identical capacity and internal resistance.

if you connect them in parallel initially then you will never be able to isolate the bad cans.

when you have experience this will become more obvious and you will regret having built it up in parallel first as all these people are now gonna tell you to do.

.

Ok, im reading this again and I can't quite comprehend why building in series makes it easier to isolate cans, especially since I will have balance wires for the BMS running in parallel between the packs. With this, i dont see any benefit to wiring in series.
If one were not planning on balancing at all then I can see the benefit.

I suppose if I were to remove an individual series bank, then I would only have to "snip" the small balancing leads and remove that pack.

 

[dnmun]

you only have to isolate the series from each other for the portion of the battery pack above the self discharging row of cells. but all connections have to be removed above the bad one or the others will keep the self discharge disguised.

 

[Malloot]

were do you guys get the bargain bin and scavenged 18650's from?
And is it really worth it? With 2900 mAh cells going for about 2.40 euro's per cell you should have a much easier time making time making properly controllable packs that will probably outlast the scavenged packs. I understand the idea from a initial investment and environmental point of view but don't know if i would do it myself.

 

[jaunty]

I tried my local recycling center, but they didnt want to give or sell them. I got mine from eBay by searching "laptop battery lot" and they weren't cheap so good luck there. Best deal i found on used cells was 0.8 US dollars, which is 0.71 euros. And that was for

Sanyo UR18650ZT
That was a lot of 20 - 9 cell packs.

So is it worth it? Probably not, unless you like to torture yourself like I do.

 

[999zip999]

You are trying to power a bike with your added weight. This battery is not for a flashlight. It's hard enough to build a ebike battery with new cells. You are going to the dumpster and finding cells good enough for hauling you up a hill. Do you fill lucky or have a lot of time to repair your pack every two weeks. Remember you are going to spend lot's of time sorting matching building and then repairing the new pack.
Better to start with new cells. Then go riding.
Batteries are the heart of your bike, it's the main part to make it work.

 

[jaunty]

everybody has their opinion around here that's for sure!