Advice needed on cell config for a lightweight 48v pack

[nicklouse10]

Hi,

I'm building a lightweight ebike using mainly carbon components.

My daily commute is less than 3 miles so I don't need a high capacity battery. Weight saving is more important to me. I will be building my own battery using 18650 cells.

I was thinking of buying 39 x 3000mah, 3.7V cells, and configuring them into 3X parallel sets of 13 cells each. Am I right in thinking that will give me a 48V battery because 13x 3.7 = 48.1V

What would be the capacity of this battery?

Thanks
Nick

[stodd]

The capacity will be 39*3*3.7 wh = 433wh. For that sort of commute you could save weight with a smaller battery. Unless its significantly hilly or there are other factors I don't know of and with a light cycle it doesn't seem worth electrifying it at all. You'll probably be going above the 15.5mph assist limit most of the time anyway.

Building batteries is non-trivial: care is needed in the way the connections are put together, and the BMS, with significant risk of fire if it is not done right.
Might be worth asking around on Pedalecs too.
https://www.pedelecs.co.uk/forum/forums ... cussion.2/ and other sub-forums.

[hemo]

Capacity will be 432wh or 9ah x 48v nom = 432wh.
What method are you thinking of to construct the cells in to place ?

If only 3 miles you could possibly go 13s 2p but depending on your controller amps output.
If the use is purely for 3 miles each way and may be other low mileage use then you could go 13s 1p with 21700 cells, Samsung 40T would give 192wh.

Lion cells are nom 3.6v, 54.6v is the max voltage and 42v for min voltage and life cycle. For better life cycle only charging to 4.1v using a smart adjustable bms is the way to go, but for most it will not matter as getting 500+ cycles out of a battery is likely more then 10 years.

[billym444]

do you have a spot welder, as said take care as buiding batteries can go bad.

i would use sony 35a draw cells either 13s2p 5.2ah of 13s3p 7.8ah. should be more than enough for 3 miles and also will cope with the draw.

[lowrider]

nicklouse10
I was thinking of buying 39 x 3000mah, 3.7V cells, and configuring them into 3X parallel sets of 13 cells each. Am I right in thinking that will give me a 48V battery because 13x 3.7 = 48.1V

Yes

hemo
If only 3 miles you could possibly go 13s 2p but depending on your controller amps output.
If the use is purely for 3 miles each way and may be other low mileage use then you could go 13s 1p with 21700 cells, Samsung 40T would give 192wh.

I am looking to go 48v myself, the least I heard of or seen for sale of was 3 parallel groups. I was under the impression that a motor could draw more current than the battery could do easily produce, stressing the battery and shortening its life if you went less than 3p. Do you limit the current draw from the motor to do this?

I also thought if you want a really light battery you needed to go 36v or even 24v.

I would really be interested how this is done although I think I would limit it to a 39 cell battery but a 26 cell battery for shorter distances is tempting.

What are 21700 cells where could you source a battery constructed using these?

[nicklouse10]

cheers for the responses. I have a spot welder for assembling battery packs but I don't have any experience of it yet. I'm going to be doing as much research as possible before assembly.

My commute is short but its up some very steep hills and I have to carry the bike up 2 flights of stairs when I get there, hence wanting to go as lightweight as possible. My last electric bike was over 25kgs!

I'll have to do some reading on those 21700 cells. I'd not heard of those before.

[hemo]

20650/20700/21700 are the bigger capacity brothers of 18650, there are a few available on Nkon.nl or via wholesale sites like Fogstar or 18650 warehouse.
Some batteries are slowly filtering through with the larger concept.
The Samsung 40T I mentioned is a 35a rated cell, though it appears 25a is a good continuous rating for it.


All battery BMS have a continuous amp discharge rate though most can deliver higher ampage but this is usually only for a few seconds to prevent the battery cells from damage.

[hemo]

Lion cells never give the full stated capacity unless you discharge to 2.8v but nearly all BMS will shut down at above 3.1v or 3.2v, so you only get to use 3/4's or 4/5th's of the actual capacity. Mostly it is for cell cycle life and also go much lower then 3.2v and bms won't allow a balance charge as it will detect to low voltage. Once you get to 3.1v they then start hitting the cliff and voltage nose dives quickly to unsafe levels.

Both of these independent graphs show the available mah available from cells given a current loading.
Notice how voltage sag is induced by current load.
https://www.thunderheartreviews.com/201 ... -test.html

https://www.e-cigarette-forum.com/attac ... ng.712749/

[billym444]

i use sony 2600 35a draw, that could cope with most things, when i build for 1000w+ bikes i use them and never had an issue, only downside is the capacity is a bit low. but there a great price. ive got a couple of bafand HDs running on them and no issue at all.

i did a 2p with them a while back (48v) 5.2ah but never got any feedback from him either way. also only wanted short trips. so i would think a 13s2p would cope fine and cover 3 miles no bother.

[hemo]

For 3 miles each way and hills any small battery 13s1p or 2p will cope no problem as long as high current discharge cells are used.
My current town/commute bike uses a pair of 36v 14.5ah batteries alternately, they are now into their 6th year of use, eventually I will have to replace them when they deteriorate more maybe in the next 2 or 3 years. I will build my own drinks bottle battery using 21700 cells and will go 13s1p with high current cells and 4 - 5 ah for approx. 180-220wh which will be good for up to 20 miles.
Samsung 40T (4000mah) is one of the best at the mo for high drain current and capacity, there are higher capacity cells available in the format but lack the high current drain and are only 10a.

[billym444]

not be long before theres affordable 5000mAh high drain then we can make some great packs. i use cases that hold 65 cells so 48v 25ah packs.

[hemo]

The issue will be whether larger format 5k + high drain cells have good cycle life.

Currently on ES some cycle testing of 18650 cells at 1A discharge are being carried out independently by batterie enthusiasts and the standout cells are LG MJI, LG M36 and the old favoured Sam 29E, all retaining 93%+ capacity well in to 300 cycles, worst performers were Panny PF/Sanyo GA. The stand out cell is LG M36 95% capacity after 350 cycles and Sam 29E 93%+ at 350 cycles.

[nicklouse10]

I decided to go with 23 X Samsung 40T 21700 cells, giving me a 13s 2p 10AH 48V battery
I'll post my progress as I build it.

Now it seems I need to learn about chargers and charging!

[hemo]

I decided to go with 23 X Samsung 40T 21700 cells, giving me a 13s 2p 10AH 48V battery
I'll post my progress as I build it.

Now it seems I need to learn about chargers and charging!

Are you spot welding ?

If using a dumb BMS then in the most part you will have little choice but to charge to 4.2v per cell or 54.6v for the pack, you could charge to 4.1v/53.3v and then carry out a full balance charge every 6 - 10 cycles to ensure that the individual voltages 13 cell groups don't stray to far apart.
The other option is to use a programmable smart BMS which will balance at the desired max voltage you set parameters can be done via BT or Uart via a PC.

If not wanting to spend the extra on a smart BMS, I would go with a basic dumb BMS and charge to 4.1v with a full balance charge if needed. To make life easier you could wire in an extra sense lead/connector to exit the pack externally to monitor the cell group balances manually. That way you only ever need to fully balance charge if you notice a cell group wandering, which might only be once a month, quarter or once year. The life cycle of the battery will /should be very good.
As long as cells stay with in 0.01 or 0.03v of each other then balancing won't be needed very often.

[nicklouse10]

This is the BMS I'd planned to fit.

https://tinyurl.com/twkz9gz

I've not bought a charger yet and would appreciate a recommendation.

I'd like to have three chargers to keep at different locations I visit frequently, so I'd prefer not to choose one that's prohibitively expensive