14S battery @57.4 V or 15S @ 60V ?

espresso

100 mW
Joined
Mar 8, 2017
Messages
41
I want to build a 18650 battery for my downhill bike with a Bafang BBSHD motor, using Samsung 30Q batteries.
I don't want to fiddle with the controller capacitors, so I need to stick to just under 60 volts.
I pedal the bike a lot and only use the motor for short bursts with the throttle (30 amps for about 30 seconds), or I use just a bit of PAS if I'm going up a really steep hill (about 5 amps).
At the moment I have a 14s7p 21.7 Ahr battery, and this easily gives me 3 good rides, so I feel that it's overkill and heavy.

If I build a new battery I think that I could get away with a 6 or 9 Ahr (2S or 3S) although I'm a bit worried about voltage sag at 30 amps?

Is it safe to use the 30Q at 15A per cell for short bursts like this?
I'll put a temperature sensor and cutout in the pack.

Would I be safer with the 3S configuration that would draw 10A per cell?

Will I achieve anything by running a 15S at just below 60V fully charged (cell voltages 4.0 V), or should I just go for a standard 14S with fully charged cell voltages 4.1 (57.4V pack voltage)?

I know that I will get a better life by running the cells at 4 volt vs 4.1 volt, but will I pack a bit more power in the 15S battery and is it worth the extra hassle?
 
I wouldn't recommend going to 15s. You could never charge it fully and still make the motor run. While it is better to charge to a lower voltage on a routine basis, you need to go to a full 4.2v/cell to make balancing happen, which you might want to do occasionally.

As far as how few parallel cells you can get away with, I don't have enough experience with those cells to make a guess. In general, fewer parallel cells will increase heating which is bad for the cells. 7p might seem like overkill, but unless the weight is a real issue, it keeps the cells happier and you can charge to a lower voltage and not run out of range. I have a 5p pack of GA cells that works fine at 30A.
 
I see now that Luna offer a Mighty Mini battery with the 30Q rated at 6 Ahr, so 14S2P. They say that one can draw 30 A.
https://lunacycle.com/batteries/packs/52v/52v-mighty-mini-cube-samsung-ebike-battery-pack-30q-6ah-3-pounds/
Regarding the 15S,
I suppose I could top up to 4.2 v sometimes to balance, but I would just have to draw down again with a headlight then.
 
I'm sure it will be OK doing short bursts at 30A, but I'm guessing it will sag substantially doing that, and quickly heat up so your bursts better be short and followed by some time at very low power to let them cool off before you go again. Heat kills cells. I would personally consider 3P the minimum for longevity.
 
I totally agree. 3P is better than 2P with 15A Max cells (rating is for cell in free air and ideal lab condition, not in a dense pack with all cells tightly packed togoether). Luna is using the 15A rating to say you can draw up to 30A. Yes you could pull 30A out of the pack for a few seconds without damage just like the manufacturer did in their lab. Just don't expect the cells to last for 300 cycles if you plan to abuse them at 15A/cell most of the time. I`m thinking they`d probably be fine at 8-10A continuous though.... So definitly 3P if that's an option!
 
I'm running 14s2P on my BBS02 and its great with the 30Q cells until about 2/3 rd discharge and then the voltage sag becomes a major problem with the BMS switching off the battery on occasion such is the rapid fall off of voltage. I went 2 x 14S2P batteries for convenience with one on the seat post ( its difficult to find room in a full suspender ) and the spare in my rucksack but I think in hindsight I would have been better to go to 3P as on a 2 hour session I'm just having to switch batteries in the last 1/2 hour. Its a good light weight set up though and convenient with the 2P batteries.
 
Waynemarlow said:
I'm running 14s2P on my BBS02 and its great with the 30Q cells until about 2/3 rd discharge and then the voltage sag becomes a major problem with the BMS switching off the battery on occasion such is the rapid fall off of voltage. I went 2 x 14S2P batteries for convenience with one on the seat post ( its difficult to find room in a full suspender ) and the spare in my rucksack but I think in hindsight I would have been better to go to 3P as on a 2 hour session I'm just having to switch batteries in the last 1/2 hour. Its a good light weight set up though and convenient with the 2P batteries.

And that`s with a 25A BBS02.... The BBSHD pull 5A more at 30A.... Thus a 2P setup with 30Q cells would suffer even more....
Wayemarlow that means you`re having significant sag while still pulling maximum 12.5A/cells for cells rated 15A/cell.
Indeed manufacturer Amp draw rating are to be taken with a grain of salt...
 
Whilst the battery is over 1/2 charge then you can pull full current without a problem, get below 2/3 rd and you get significant sag but if you use a bit of nouse and keep tabs on things then the advantage of lightweight ( there are places I need to lift the bike over styles and through gates etc ) over heavy battery packs attached to the frame does out weigh the disadvantage of a heavy bike ( mine without battery is under 18kgs ), you just need to back of for about 20 minutes of riding or change the pack early.
 
I want to build a 18650 battery for my downhill bike with a Bafang BBSHD motor, using Samsung 30Q batteries.
I don't want to fiddle with the controller capacitors, so I need to stick to just under 60 volts.

Will I achieve anything by running a 15S at just below 60V fully charged (cell voltages 4.0 V), or should I just go for a standard 14S with fully charged cell voltages 4.1 (57.4V pack voltage)?

I know that I will get a better life by running the cells at 4 volt vs 4.1 volt, but will I pack a bit more power in the 15S battery and is it worth the extra hassle?
 
If you don't want your pack getting out of balance you're gonna want to go with less cells and higher voltage. Since the balancing kicks in at 4.2V.

Also, if you don't like to keep your cells at 4.2V there is Bestech BMS. They can configure it to start balancing at 4.2V and balance down all the way to 4.1V.
Since keeping your cells charged at 4.2V is what's degrading them.
 
eee291 said:
If you don't want your pack getting out of balance you're gonna want to go with less cells and higher voltage. Since the balancing kicks in at 4.2V.

Also, if you don't like to keep your cells at 4.2V there is Bestech BMS. They can configure it to start balancing at 4.2V and balance down all the way to 4.1V.
Since keeping your cells charged at 4.2V is what's degrading them.

I can't find this?
One of the Best tech that find is:
http://bestechpower.com/555v15spcmbmspcbforli-ionli-polymerbatterypack/PCB-D167.html
This has a
Over charge detection voltage of between 4.2V~4.35V(Adjustable)
and
Over charge release voltage of between 4.0 - 4.2 (Adjustable).

Wouldn't this have to get to 4.2V for it to start balancing?
I get that it would bring the cells down to 4.0V, but what if I remove the charger at the wrong moment, could it leave the cell voltages at 4.2?
 
The balancing shunts work independently of the over charge detection. You could have the shunts set for 4.1v and still have the over charge detection set for 4.2v or a little higher (to account for reverse sag). If you charge to 4.2v, the cells will take quite a while to drain back down to 4.1v. So if you charge to 4.2v and ride right away, you get the full capacity of the pack. If you don't ride right away, they will bleed down to 4.1 and stay there. Likewise, if you only charge to 4.1v, you can still get the balancing function to work.
 
fechter said:
The balancing shunts work independently of the over charge detection. You could have the shunts set for 4.1v and still have the over charge detection set for 4.2v or a little higher (to account for reverse sag). If you charge to 4.2v, the cells will take quite a while to drain back down to 4.1v. So if you charge to 4.2v and ride right away, you get the full capacity of the pack. If you don't ride right away, they will bleed down to 4.1 and stay there. Likewise, if you only charge to 4.1v, you can still get the balancing function to work.
I can only charge to 4.0 V if I want to stick to the standard Bafang controller. 60 Volts max.
I don't understand what you mean by "The balancing shunts" ?
Are there settings on the BMS for these?
I only see that one can set the
Overcharge Protection - 4.2V~4.35V(Adjustable),
Over discharge Protection - 2.3V~3.0V(Adjustable),
Over discharge release voltage - 2.3V~3.0V(Adjustable)
Over current detection voltage - 0.1V~0.2V(Adjustable)

I don't see any reference on the data sheet for Balancing shunts?
 
There should be specifications for the balancing part (unless it's a really cheap version that doesn't do any balancing). The ones from Bestech Power will have a specification for balance or "equilibrium".

When they say "adjustable", in most cases it is not really adjustable but you can order the boards made within that range.
 
Wel if you want adjustable you could go for one of these.

https://bmsbattery.com/bmspcm/833-smart-bms-10s13s-60a-with-blue-tooth-android-or-ios-app.html#/115-cells_in_series-15/17-battery_type-lithium_ion

I have not tested these yet, so I don't know what the adjustable range is, or if they are even any good yet. But I plan to find out.
Apparently you can even change the balance modes like active or only when the cells reach a certain voltage. Sure as hell sounds good on Paper.
 
There are many reasons I am a fan of 13S / 14S, and one of them is that...in a temporary power outage, they can run a nominal 48V inverter to provide 130V AC...

Also, 15S is very rare, if you decide you truly want that, then you will likely need to build your own pack. Not impossible, but simply a major consideration in the final decision.

Another reason is that...60V is the international standard for electricity to penetrate human dry skin (of course, actual conductivity varies from one person to the next), and of course, a lower voltage is somewhat safer (admittedly, no voltage is completely safe, depending on the scenario)

When a 15S pack is fully charged to 4.2V per cell, it is 58.8V (very close to 60V) but...after the first half minute of running, it immediately drops at least a volt.

I have three 14S packs right now, but...I only charge them to 4.0V per cell for 56V when fully charged (80%).

A 60V nominal pack is usually considered to be 16S, so it would be 68.0V if charged to 4.2V per cell...not worth it for me. It seems the next level cutoff for affordable controllers is roughly 20S. If you want 22S or 24S, the controllers have fewer models to choose from, and are more expensive...
 
spinningmagnets said:
Also, 15S is very rare, if you decide you truly want that, then you will likely need to build your own pack. Not impossible, but simply a major consideration in the final decision.


When a 15S pack is fully charged to 4.2V per cell, it is 58.8V (very close to 60V) but...after the first half minute of running, it immediately drops at least a volt.

I am planning to build the pack myself, so I can build whatever I want.

Your comment in italics above is incorrect. You obviously meant to say
14S at 4.2 = 58.8V
but I get your meaning.

I was just wondering if I had to charge to 3.92V @15S (still sticking to 58.8V), whether I could get a longer life out of the pack, and would I get a tiny bit more juce in the pack?
 
charging to 4.05V is enough. that will considerably exend the lifespan.

so float voltage is 56.7V.
 
Thanks for the correction. I blame the beer...

The Tesla cars and the Chevy Volt charge to 4.05V per cell, and their packs are under warranty, while costing many thousands of dollars. I'm not saying there is anything wrong with charging to 3.9V, but there likely isn't much of a benefit, if any...
 
Well the question still remains...
Will 14S charged at 4.05V per cell
hold or or less power than
15S charged at 4V per cell ?
 
Are you keeping the cell count the same?
 
flippy said:
Are you keeping the cell count the same?

I don't understand your question?

Whatever I do, I will use 3 parallel strings, so
14S 3P at 4.05 V per cell
or
15S 3P at 4.0 V per cell
Which will hold more energy?

I may actually have to hold the 15S at 3.95V to ensure that I am below the magic 60V to not damage my BBSHD controller.

So perhaps I should be asking?

14S 3P at 4.05V per cell
or
15S 3P at 3.95 V per cell
Which will hold more energy?
 
In this case a 15S would store more power.
 
Sd1C4Rk.png

Well i don't have a charge chart so this will have to do.



At 3.9v the cell has about 500mah less than fully charged. and at 4.0v it's only about 100mah. So I think you can do the rest of the math.
 
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