need to perform surgery on 72 volt triangle battery

[garyal1]

I have a never used 72 volts 24.5AH triangle battery. I purchased it new from China about 8 months ago it is 20S7P and it is 1/2 inch too tall to fit into my Frey HT 100 frame. Currently a 16S8P is installed using a PR. This is totally my fault!



Anyway, I need to preform some surgery and make it physically smaller by 15 mm or more so I want to remove these 7 cells, and then relocate the remaining cell in the second row. This will reduce the dimension I need reduced by about 25mm.

Need advice:
1) How to cut the nickel strips?
a) use a Dremel cut off wheel and a steady hand.
b) use a small side cutter pliers,
c) ??????
d) On the one cell from group 2 that I need to relocate, can I solder the two nickel strips if I overlap them? This will only carry the current from one cell.....? I do not have a welding setup....

2) What do I do with the #1 BMS wire that I will disconnect?
a) leave it disconnected.
b) Attach it to another cell (two on one group.
c) ??????

3) What about the 20S cutoff voltage from the BMS?
a) If only 19 connected will this reduce automatically?
b) I will set the PR / CA3 to correct 30% cutoff for 19S, just wondering if the BMS will be cutting it off based on 20S??

 

[goatman]

im just curious, if you cut those cells off the bottom of the pack, will it still be too tall? the triangle is still the same shape, youre just taking off a tip and that group youre taking off looks like its the first 1s7p so all the bms wires need to be redone

 

[garyal1]

OK...thanks for that advice, that is what I am looking for. I can do that..
I can then leave the wire number 20 disconnected???

As for shape and size. Yes, cutting off that bit makes it fit. I proved this with a cardboard cutout in the "new shape"

 

[goatman]

im not a bms person, but if you post what bms you have, they can help you quicker.

 

[garyal1]

No Name BMS 20S 60 Amp

 

[Vbruun]

Sell it and get another one. This will be just as much work as making a battery from scratch.

 

[garyal1]

This battery was well constructed. So I decided to go ahead with my plan. I already did the removal of one parallel group. I turned it into a 19S from a 20S. Still 7P - 24.5 AH.
As for the BMS, since I know so little; I ordered a 19S 60amp with Bluetooth and phone app for battery status $47...…. Yes, I will have to splice it in, but this way it will take another hour or two and I will have a good new battery. I already verified it fits into the frame. Fully charged state will be 79.8 volts.

What should I call it? a 68 volt battery? A 69 volt battery?

 

[john61ct]

70V is the nominal midpoint

Charge to 77V maybe 79V

 

[garyal1]

Well, I performed the surgery, reduced this 72 volt to a "70 volt" 24.5 AH battery. (lots of free-time with lockdown in effect) With the BMS re-wired 1-19 and # 20 not connected, the BMS worked. I have put about 50 miles on this new battery. 3900 watts. It goes about 10% faster in every gear versus the previous 60 volt battery.

Thermal heat WILL build up in 4-6 KM if you run steady at over 2500 watts on the Bafang Ultra. I have temp sensor protection set to throttle back at 95C. I find 6th gear on my Rolhoff runs at 33 MPH full throttle with watts at about 1800 and no overheating. 8th gear will also not overheat for a long time but eventually it will get to 95C, just takes about 10km on steady full throttle to get there.

I have about 1500 miles on this bike (Frey HT1000 Frame) I did set the throttle ramps to NOT hammer the drive train every time I hammer the throttle. So far nothing has broken and I can get the bike to 43mph without pedaling. I guesstimate that with a fully charge batt (79 volts) my torque is probably more like 250-300NM as opposed to 160NM at full throttle with 48 volt battery. It always amazes me when I am cruising at 30+ then hit the throttle and still feel the torque bump and fast acceleration.