Replacing lithiun cells is not normally done as it's very important that the correct cell is used, these constantly changing with development. It would be best to try and get a cell from the supplier of your battery to ensure compatibility and minimise risks of accidents.

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ok if i can i will. if for reasons that you state i cannot repair it can the battery run ok without the cell by reducing the amph from 20 to say 15 ? thats if it is only one bad cell which i have yet to find out and of course working out the configaration.

just found this over on "endless sphere" wish i had seen it it before buying "vpower" Endless-sphere.com • View topic - ebay LiFePO4 sellers to avoid: ep-battery, volgood

Edited November 4, 201015 yr by theskip1

I think you'd seriously lack power with a cell missing since there aren't many series cells of circa 3.7 volts making up the voltage. Also the BMS might malfunction with one cell missing.

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I think you'd seriously lack power with a cell missing since there aren't many series cells of circa 3.7 volts making up the voltage. Also the BMS might malfunction with one cell missing.

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is there anything positive you can say or is it all doom. (throw me a bone )

i had heard so many positive things about lifepo4 " its the way to go " etc, it all seems to have gone a bit sour. i wish i'd stuck with sla's not so expensive when it all goes t**s up.

Edited November 4, 201015 yr by theskip1

Sorry, I sympathise, but lithium can be a bit inflexible compared with older technologies like SLA, NiCad and NiMh.

 

I treat LiFePO4 with some reservation since it is so new and e-bike manufacturers who have been trialling it for up to 4 years are mostly still not satisfied that it's ready to replace Li-polymer compound cathode types.

 

The only ones that seem to have been performing mostly well to date have been supplied by Li Ping who appears to take care with his individual assembly of them and doesn't make extravagant claims.

.

ok have now put the battery under a no load test using my old powabyke on full throttle and after a 21/2 to 3 hour wait

it cut out at 36.1v.

i thought this a rather high cut off voltage so contacted Vpower, they said this was normal as their batteries were set to cut at 35-36v ?.

this kind of contradicts what i have heard up to now .are they pulling my leg or is that right ?

after stripping it i found 12rows of 18cells 3 of which were reading .5v lower than the rest (thats 54 cells! ). having been in touch with Vpower previously when they had kindly offered to supply any cells that i might need at the cost of postage only. i somehow think they may change their mind when they find out how many there are !.

any way that's the state of play at the moment . comments and advice always welcome.

tony.

Edited November 11, 201015 yr by theskip1

Each 18 cells group is the paralleled cells giving the capacity, the 12 cells series giving the working voltage.

 

Since the LiFePO4 cell working voltage range is 3-0 to 3.3 volts, 12 x 3.0 minimum series working voltage is 36 volts, meaning they are correct with the 36 volts cutoff.

 

From an online data sheet:

 

Specifications

• Cell voltage: min. discharge voltage = 2.8 V. Working voltage = 3.0 V – 3.3 V. Max. charge voltage = 3.6 V.

.

Each 18 cells group is the paralleled cells giving the capacity, the 12 cells series giving the working voltage.

 

Since the LiFePO4 cell working voltage range is 3-0 to 3.3 volts, 12 x 3.0 minimum series working voltage is 36 volts, meaning they are correct with the 36 volts cutoff.

 

From an online data sheet:

 

Specifications

• Cell voltage: min. discharge voltage = 2.8 V. Working voltage = 3.0 V – 3.3 V. Max. charge voltage = 3.6 V.

.

 

so do you think the .5v difference for the three banks points to damaged cells ?

so do you think the .5v difference for the three banks points to damaged cells ?

 

I've found a similar problem with a lithium-ion pack I've just contructed.

I've used 3000ma cells, 10 banks of 8 cells in parallel giving a 24ah capacity.

One bank is dropping to cut off voltage long before the rest.

There may only be 1 bad cell in that bank of 8 so I now need to rip it all apart to find the faulty cell/cell's.

I've found a similar problem with a lithium-ion pack I've just contructed.

I've used 3000ma cells, 10 banks of 8 cells in parallel giving a 24ah capacity.

One bank is dropping to cut off voltage long before the rest.

There may only be 1 bad cell in that bank of 8 so I now need to rip it all apart to find the faulty cell/cell's.

 

how are you conecting the cells up ? do you have the proper spot welder or are you soldering them ? does anybody know a reliable method ?

shit.im awaiting vpower cells in next few weeks.with my luck i will be screwed as well.tried the ebay dispute but it cant find/recognise the transaction no. probably due to the long time ive been waiting. just my luck.

shit.im awaiting vpower cells in next few weeks.with my luck i will be screwed as well.tried the ebay dispute but it cant find/recognise the transaction no. probably due to the long time ive been waiting. just my luck.

 

ah the joys of power asisted cycling.

how are you conecting the cells up ? do you have the proper spot welder or are you soldering them ? does anybody know a reliable method ?

 

I'm soldering them which isn't ideal because of heat. I'm using a heavy duty 100watt iron to make the solder process as quick as possible, the heat is only applied momentarily, I also use a large heat sink to track heat away where possible.

The cells I'm using were tabbed but the spot welds were so poor I pulled most of the tabs off with my fingers. I'm not a fan of pin point spot welds and believe soldering gives a much better joint.

I thouhg about trying one of these in my RC days.

Force 4 Cold Soldering Iron - Only £14.95 - Force 4 Chandlery

Not sure how good the joints would be.

I thouhg about trying one of these in my RC days.

Force 4 Cold Soldering Iron - Only £14.95 - Force 4 Chandlery

Not sure how good the joints would be.

 

I've got one and although they're okay for light duty stuff they are awkward to use. They have a forked tip that depends on the wire your soldering shorting out the tip to produce heat. Absolutely useless for this application.

I've got one and although they're okay for light duty stuff they are awkward to use. They have a forked tip that depends on the wire your soldering shorting out the tip to produce heat. Absolutely useless for this application.

 

Glad I never purchased one in the past then.

 

Solder and good battery bars are better than spot welds and tin (like you said earlier)

Less resistance means better performance and less heat generation when discharging.

I'm soldering them which isn't ideal because of heat. I'm using a heavy duty 100watt iron to make the solder process as quick as possible, the heat is only applied momentarily, I also use a large heat sink to track heat away where possible.

The cells I'm using were tabbed but the spot welds were so poor I pulled most of the tabs off with my fingers. I'm not a fan of pin point spot welds and believe soldering gives a much better joint.

 

yes i also believe soldering is best, but i found that even a 140watt iron was not fast enough so i use a large 1"x 2" copper iron that i heat with a blowtorch. this retains a large amount of heat that can be "discharged" into the joint very quickly and so stops the work from " cooking "

I thouhg about trying one of these in my RC days.

Force 4 Cold Soldering Iron - Only £14.95 - Force 4 Chandlery

Not sure how good the joints would be.

 

no where near hot enough i fear

have now tested the input and output of the bms as follows:-

battery fully charged:- input 40.5v output 38v

at point when it cuts out:- input 36v output 32v

does anyone know if these figuers sound right ?

tony

have now tested the input and output of the bms as follows:-

battery fully charged:- input 40.5v output 38v

at point when it cuts out:- input 36v output 32v

does anyone know if these figuers sound right ?

tony

 

40.5v, 3.37v per cell when charged sounds about right.

36v, 3v per cell at cutoff seems a tad higher than needed for lifpo4's but better that than too low.

The bms will cut off if any one of the 12 parallel banks of cells drops to the cutoff point, check the individual banks.

40.5v, 3.37v per cell when charged sounds about right.

36v, 3v per cell at cutoff seems a tad higher than needed for lifpo4's but better that than too low.

The bms will cut off if any one of the 12 parallel banks of cells drops to the cutoff point, check the individual banks.

by individual banks i presume you mean each bank of 18 cells ? in which case i have already done so, see pic page 3,

Edited November 16, 201015 yr by theskip1