building battery pack with 18650 cells in different capacity

[iouhxm]

I try to build a battery pack for my ebike 48v 1000w,
but i have mix 18650 cell from different laptop battery, so they are all different capacity. they are about 1500mah, 1700. 2000, 2200 etc...
can you guys tell me how to pack them in P and S in more details....please.???

can i build it in two 6s3p and then put them in series ??
because i only have the imax b6ac charger.
or do i need to buy a 48v charger??
thank you !!!!

 

[Nobuo]

the important thing is that all the series must be the same capacity. But also the paralleled groups should have cells with similar capacity.

a paralleled group balance their charge between cells continuously (if they are right totally connected), but the series are independent.

If you have 12 series with 6Ah and 1 series with 5Ah, the whole battery is 5Ah, I explain myself?

the battery will stop the charge at 5Ah when the BMS detect that serie is at 4.2V.

You need 13 series, and the paralleled groups could have different number of cells, them important is to have the same "PRACTICAL and REAL" Ah between all the series.

you should buy a 48V charger and a 13s BMS

 

[iouhxm]

can i build it without BMS? i can do battery balance manually with my imax b6 and cheap ebay balancer.

so, i can
group 7 cells for total 12ah
group 6 cells for total 12ah
group 8 cells for total 12ah

then put them all in series, the most important is they are all in the same capacity......right???

 

[Nobuo]

can i build it without BMS? i can do battery balance manually with my imax b6 and cheap ebay balancer.

so, i can
group 7 cells for total 12ah
group 6 cells for total 12ah
group 8 cells for total 12ah

then put them all in series, the most important is they are all in the same capacity......right???

As you will use cells with different brands, capacities and "personalities" if you build it without BMS you have a very high chance to destroy a cell discharging, o making one break itself charging if you don't measure voltages for each cell

 

[iouhxm]

can i build it without BMS? i can do battery balance manually with my imax b6 and cheap ebay balancer.

so, i can
group 7 cells for total 12ah
group 6 cells for total 12ah
group 8 cells for total 12ah

then put them all in series, the most important is they are all in the same capacity......right???

As you will use cells with different brands, capacities and "personalities" if you build it without BMS you have a very high chance to destroy a cell discharging, o making one break itself charging if you don't measure voltages for each cell

so, as you said. a BMS must needed .....right?

 

[Nobuo]

Completely, for avoid a cell destruction. You need to check continuously voltage during discharging, because the series will be gone in a very different times. You need the BMS to know when the first series reach 3V yo avoid destroy them.

The same with charging, the worst series that get gone the first, will get charged the first also and because the 48V chargers compute only overall Voltage it won't stop while that series is getting >4.2Voltage levels.

BMS will save for that both dangerous scenarios.

if you want the BMS to help also maximize the overall capacity you need to make the series with very very close USEFUL Ah (not 2Ah will be 2Ah in real uses if the cells are not the same and have different internal resistance)

 

[iouhxm]

Completely, for avoid a cell destruction. You need to check continuously voltage during discharging, because the series will be gone in a very different times. You need the BMS to know when the first series reach 3V yo avoid destroy them.

The same with charging, the worst series that get gone the first, will get charged the first also and because the 48V chargers compute only overall Voltage it won't stop while that series is getting >4.2Voltage levels.

BMS will save for that both dangerous scenarios.

if you want the BMS to help also maximize the overall capacity you need to make the series with very very close USEFUL Ah (not 2Ah will be 2Ah in real uses if the cells are not the same and have different internal resistance)

thank you very much.

 

[Jonathan in Hiram]

I made a pack in the manner you are talking about and have been using it for over a year now with no significant problems. You do have to keep an eye on your charge level and it's better to make the battery with considerably more capacity than you plan on ever using so that you don't have to run it down close to low voltage cutoff point where you are likely to run into problems. I'm using an iMax B8 to charge and balance my pack and a Cellog 8 to monitor the charge levels.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=58363

Later I updated the construction method with plastic 18650 spacers but still used the same oddball collection of cells most of which I stripped out of old laptop batteries.

I have two 6s12p packs in series for 12s12p and about 22 amp hours total available current, I never use more than 15 Ah so I stay well away from the low voltage cutoff.

If you use less than about 6p the laptop cells aren't going to do you much good, they won't put out enough current to drive a motor other than on a test stand and you are going to hurt your cells and maybe even your motor. 12s12p is really just barely adequate for my 9C DD motor. the geared Bafang on my other bike in a 20" wheel needs at least 6p to run at 12s and more p makes it run better.

Making a pack from laptop cells is doable but it's a lot of work and a critical ongoing chore to make sure the cells stay balanced.

 

[iouhxm]

I made a pack in the manner you are talking about and have been using it for over a year now with no significant problems. You do have to keep an eye on your charge level and it's better to make the battery with considerably more capacity than you plan on ever using so that you don't have to run it down close to low voltage cutoff point where you are likely to run into problems. I'm using an iMax B8 to charge and balance my pack and a Cellog 8 to monitor the charge levels.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=58363

Later I updated the construction method with plastic 18650 spacers but still used the same oddball collection of cells most of which I stripped out of old laptop batteries.

I have two 6s12p packs in series for 12s12p and about 22 amp hours total available current, I never use more than 15 Ah so I stay well away from the low voltage cutoff.

If you use less than about 6p the laptop cells aren't going to do you much good, they won't put out enough current to drive a motor other than on a test stand and you are going to hurt your cells and maybe even your motor. 12s12p is really just barely adequate for my 9C DD motor. the geared Bafang on my other bike in a 20" wheel needs at least 6p to run at 12s and more p makes it run better.

Making a pack from laptop cells is doable but it's a lot of work and a critical ongoing chore to make sure the cells stay balanced.

so should i build my pack as 12s or 13s for my 48v 1000w bike???

 

[riba2233]

Build 14S.

 

[Jonathan in Hiram]

If you are going with a B6 charger you should stick to 12s since with that number you will be charging and balancing 6s at a time, if you go with more than that then charging/balancing is going to be a lot more trouble since you will have to charge any number over 6s separately which means changing wiring around and also changing settings on the charger, that gives more opportunity for problems and human error (programmed the charger/hooked up the wiring wrong).

For laptop cells you want to have as many paralleled cells as you can, the more cells you have paralleled the easier it will be on each cell and the longer your finished battery will last, anything much over 2A continuous per cell on laptop cells gets them hotter than they need to be.

so should i build my pack as 12s or 13s for my 48v 1000w bike???