48V20AH LiFePO4 Battery - Removing Cells

[HeWhoWas]

Hey Guys,

I've built up my first ebike, a 750W bafang built on a Giant ATX 2015, and managed to pick up a 48V20AH battery off ebay for $180AUD. I've been running it on a daily basis to and from work and it's great, but having a 10kg battery on a rack at the rear makes it handle like a pendulum, so I'd like to mount the battery in the frame.

Cut to a week or two later: I've received my EM3EV frame bag and a quick comparison of the battery size to the bag volume shows it isn't going to fit. Damn. I'm reasonably comfortable around electronics so I decide the best thing to do is to remove some of the cells from the battery to make it small enough to fit. I don't mind losing 4-5AH if it means the bike will handle better.

Currently the battery is in a 16S8P configuration. What I would like to know is if I reduce this so it's 16S6P will I need to change the BMS system to allow for fewer cells in parallel? Or does this depend on the BMS system that it's using? I've attached some picture to this post, but am loath to remove the shrinkwrap from the BMS as I don't have anything to readily replace it with.

Thanks all.

Edit - The pictures are too large to attach. I've uploaded an album to imgur here: http://imgur.com/o0870pi,WAehriw,ZAc52LY,X7EkmGa

 

[docnjoj]

Hey Guys,

I've built up my first ebike, a 750W bafang built on a Giant ATX 2015, and managed to pick up a 48V20AH battery off ebay for $180AUD. I've been running it on a daily basis to and from work and it's great, but having a 10kg battery on a rack at the rear makes it handle like a pendulum, so I'd like to mount the battery in the frame.

Cut to a week or two later: I've received my EM3EV frame bag and a quick comparison of the battery size to the bag volume shows it isn't going to fit. Damn. I'm reasonably comfortable around electronics so I decide the best thing to do is to remove some of the cells from the battery to make it small enough to fit. I don't mind losing 4-5AH if it means the bike will handle better.

Currently the battery is in a 16S8P configuration. What I would like to know is if I reduce this so it's 16S6P will I need to change the BMS system to allow for fewer cells in parallel? Or does this depend on the BMS system that it's using? I've attached some picture to this post, but am loath to remove the shrinkwrap from the BMS as I don't have anything to readily replace it with.

Thanks all.

Edit - The pictures are too large to attach. I've uploaded an album to imgur here: http://imgur.com/o0870pi,WAehriw,ZAc52LY,X7EkmGa

Gee Paul really does assemble nice batteries and it is a shame to have to take it apart. That said, if you only remove cells that are in parallel, you will only reduce the capacity, not the voltage and the BMS should be fine.
otherDoc

 

[HeWhoWas]

The battery is a $180 ebay special. I actually kind of regret the purchase, but hey as long as it works for 6 months or so I'm not too much out of pocket.

 

[dnmun]

there is no picture of the battery and you refuse to name the manufacturer so there is not much help we can provide. there have been several fires caused by the Vpower packs that shorted out. it appears you are unfamiliar with batteries so you may be best served by using the battery as intended.

 

[999zip999]

Don't do it. Just use it. You can only make a Big mess of things. That being said. You would need to take 2 cells out of each parallel strings. In doing so it would be very very easy to short a buss tab on the positive end of the cell. Just because it looks peaceful sitting there it has a lot of stored energy not easy to understand till you short something.
It's like gasoline looks like water till you get a spark.
Just use it till it dies. Then think of getting a good working battery that better fits your needs. Ebikes are a learning experience that takes time and money.
Does it fit in the triangle ? If so make a way to lock it in and go ride. Then save your money for a good em3ev battery and learn from your 180.00 ebay battery. Cheap.

 

[docnjoj]

Sorry that I misread and thought that the battery was from EM3EV. I think if you want to change it's size and configuration you should since it is a cheap battery. When it dies then do get your next one from a reputable company like EM3EV, Ping, Ebikes.ca or even Tummich in Europe for bare batteries.
otherDoc

 

[dnmun]

you can download VGA format pictures here. but it is clear from what you said that it must be a Vpower pack. not sure where the 117 count came from but if it is 16S then you should not have an odd total number of cans. something shoulda clicked.

 

[HeWhoWas]

there is no picture of the battery and you refuse to name the manufacturer

I guess no one can see the link in the OP to the imgur album? Pictures were far too large to attach here and I didn't want to resize them in case some detail got lost. Also, I'm reasonably sure in order to refuse something you have to be asked and say "No".

Don't do it. Just use it. You can only make a Big mess of things.

To thost who are advising me to not do this at all, while I appreciate your concern that wasn't really the question I was asking. I more than understand the risks involved and potential dangers. Having watched one of my friends get a little lax around a 48v battery was a good learning experience.

That being said. You would need to take 2 cells out of each parallel strings. In doing so it would be very very easy to short a buss tab on the positive end of the cell. Just because it looks peaceful sitting there it has a lot of stored energy not easy to understand till you short something.

I think you may have misunderstood my question. From what I understand batteries are tied into series to provide a certain voltage. In this instance 16 cells are in series to provide 48V @ 2.5AH. Then there are 8 cells in parallel, providing a total of 20AH. I don't want to reduce the voltage, just the AH...so why on earth would I need to be removing cells from the strings?

 

[999zip999]

From the parallel string of 8p remove 2p for 6p or 15ah. With lower available amp draw from battery.

 

[Gregory]

Yes as 999zip999 said you can remove two parallel strings. You may want to reduce your current draw to compensate for the loss of capacity.
No you don't need to modify the BMS. There are some risks with shorts, damage etc if you fudge it up.

 

[dnmun]

duh#1

 

[dnmun]

duhduh#2

 

[dnmun]

i have always found it nearly impossible to cut through the nickel strap where the cells are tight against each other. just no way to get the scissors into the gap because there is no gap.

you could also cut off the top end of the pack, say from channel 12 through 16 and put that in another location so the first 12 would fit in the bag you have decided is the only thing you can use. then put 12-16 in another location and connect them with some large cable. then you can retain the entire pack.

it looks like Vpower has changed their design and using 2500mAh cans now instead of the old 18650 cans. this is much improved and they even are cutting back the corners of the paralleling strap where it overhangs the case on the anode end of the can.

 

[999zip999]

It's a 8p ( 20ah~ 3.6v charged ) then x 16s ( 57.6v charged ) battery. The only way that bms can be hooked up.
Charger voltage 58.6v or so.
Go see if it fits in your triangle ? Answer.

 

[dogman dan]

Another vote for don't from me. But if you do, I have a very vague memory of some guy being good enough with a dremel to use that to cut straps. Damn good! I'd end up cutting cells if I tried it.

Cut so that if you do, you ruin only the cell you are removing, and nick only the outer shell, not the inner, + part.

Good luck!

 

[dnmun]

when the blade cuts through the strap on the anode end it will end up contacting the case underneath so he will have a huge short and it will cause the blade to melt and jam in the slot as the entire parallel array drains all of their stored charge into the short.

make sure you make a utube video for these people when you do this.

we finally get Vpower to change their ways and now somebody will out do them for shorts.

 

[arkmundi]

Am seconding the motion to simply NOT. Unless a battery pack is constructed so as to latter by de-constructed, it'd be a mess of an operation with the most likely consequence you'll ruin it. Chalk it up as a lesson, and get a battery so as to mount the way you'd like. And keep this battery on reserve or try to sell it.

 

[dnmun]

i think my idea is the only safe way to cut it down in size because you can fold the strap open by pulling the pack apart at the other end and that allows you to cut the entire length of the strap with the scissors and can get the scissors between the two rows of cans that way. that is how i always cut the Vpower packs apart.

if you cut it into two identical sections like most people do along the top of 8S then you can make it fit in two panniers on each side in back which is how most have done this before.