Cheap chinese cells (How bad are they really) ?

[Nealh]

A good question me thinks.
Not all china cells are bad in fact some are pretty damn good, if one sticks to well known brand ones.

But ....... the ones to watch out for are the real supect ones with dodgy mah ratings and dodgy names like surefire or ultrafire and are really bad.
Recenty I needed a new zoom torch so bought a pair of the T6 type now made from ultra light ali and come with two 18650 cells and yes an amazing 7800mah rating on them, to which we know is totally false.

So I drained both these cells to 2.79v and then charged both to 4.2v or as near as, the results tbh didn't surprise me.
Cell #1 accepted 840mah and cell #2 accepted 784mah.

 

[soundwave]

same as those fake cree lights then just take a zero off the end lol

 

[Sturmey]

9. Safety......
9.1 Overcharge Test......
9.2 External Short-circuit Test.....
9.3 Reverse Charge Test......
9.4 Heating Test.......
Criteria: No fire, and no explosion.(in all cases above)

I think there is also a safety issue in using unknown cells
If you look at Samsung cell spec, although misuse of cell is not recommended, nevertheless, they are tested to withstand certain fault conditions as above.

A combination of cheap untested cell from an unreliable seller and a similar unknown (and perhaps dodgy) BMS could be a bad combination. For example, many BMS's for sale on ebay have no thermal/temperature sensors (which is for example a requirement under the older EN15194).

EN 15194:2009 (E) Page 10
An appropriate overheating and short circuit protection device shall be fitted.

 

[Bikes4two]

I was looking for something of around a couple of amp-hours capacity to carry as an 'emergency tank' for long rides where my normal 10Ah bottle battery might not suffice. If I could get another10 miles from the 'emergency tank' that would be fine.

For the fun of it (as in I knew the given ratings were a lie but I wanted to see what you got for your money) I paid £24 for the battery pack below.

It took 2 months to arrive (maybe it takes a while to collect rejects and make them into a battery pack ).

A couple of charge/discharge tested revealed a capacity of around 2.4Ah - hah, I wasn't suprised!

However, the battery pack fits neatly into the side pocket of my Carradice saddle bag and via a couple of XT60 connectors and an in-line fuse I can disconnect the main battery and button this one in and it gives my another 15 miles on ECO (TSDZ2, 250w) which I'm more than happy with.

I'm fully aware of the safety considerations so I only charge it at 4 amps and on the garage floor and don't leave it plugged into the charger and most times I've been in attendance anyway (doesn't take that long for 2.4Ah).

When riding in ECO the typical current draw is 4A or less. As a test I put the motor into the highest power setting and the voltage sag really was considerable (can't remember the exact amount but I don't use the motor in that mode anyway).

 

[Bonzo Banana]

I bought some fake individual Daweikala cells years ago and was fully refunded by aliexpress when I supplied images of actual capacity of the cells. This is a rare event in Aliexpress customer service. Basically they were sold as 9900mAh which I knew was fake when I ordered admittedly but their actual capacity is 1200-1400mAh, some a little more some a little less. Probably the same cells re-wrapped as in your battery pack although yours are probably re-wrapped as 3400mAh if they bother to re-wrap them at all in a battery pack.

So you got 20x1200mAh cells, a BMS, a battery pack frame, some cabling, plastic wrap sealing and a XT60 connector and of course final assembly. Doesn't seem too bad. I tried to look on aliexpress to see if there were any honest sellers selling a 36V 2.4Ah battery pack but couldn't find one.

I don't know how to measure full capacity in battery packs. I basically use the charging time from fully discharged compared to a known good battery pack but I realise the BMS will protect the battery pack from being fully discharged and this could vary at what voltage the BMS prevents discharging the cells, the higher the voltage the the more reserved capacity and the safer the battery pack but less capacity. I remember reading something like 20% capacity reserved for power tool li-ion battery packs and I think I may have seen similar for some ebike battery packs.

Looking at this aliexpress advert they are claiming 15000mAh capacity per cell (10S2P) and at a ridiculously low price.

Page Not Found - Aliexpress.com

They are saying 10A maximum discharge rate for the pack i.e. 5A per cell but are claiming each cell is 15A capacity so the discharge rate is only 1/3rd of the claimed capacity. I'm thinking 5A is likely a true figure. What capacity 18650 cell has a 5A maximum discharge rate? Is this another 1200mAh cell? However at this price could it be even lower capacity despite the fake capacity claims?

 

[egroover]

the name of a battery cell called 'surefire' rings warning (fire engine) bells in my mind

 

[Nealh]

The better quality cells like lishen and BAK china brand 21700 cells have prooved to be good performers, on ES they have been given a 1k charge /discharge testing by member padja in the czech republic.
Lishen SD model new 4885mah /0.2c, 4211 /1c, 3981/2c.
1000 cycles 4593/0.2c, 3983/1c, 3641/2c .

BAK CG model new 4850mah/0.2c, 4239/1c , 4056/2c.
1000 cycles 4539/0.2c, 4002/1c, 3688/2c.
Not to shabby compared with a known branded cell like the LG m50lt.
LG m50 new 4838/0.2c, 4480/1c, 4378/2c.
1000 cycles 4452/0.2c, 4042/1c, 3926/2c.

 

[Nealh]

I was looking for something of around a couple of amp-hours capacity to carry as an 'emergency tank' for long rides where my normal 10Ah bottle battery might not suffice. If I could get another10 miles from the 'emergency tank' that would be fine.

For the fun of it (as in I knew the given ratings were a lie but I wanted to see what you got for your money) I paid £24 for the battery pack below.

It took 2 months to arrive (maybe it takes a while to collect rejects and make them into a battery pack ).

A couple of charge/discharge tested revealed a capacity of around 2.4Ah - hah, I wasn't suprised!

However, the battery pack fits neatly into the side pocket of my Carradice saddle bag and via a couple of XT60 connectors and an in-line fuse I can disconnect the main battery and button this one in and it gives my another 15 miles on ECO (TSDZ2, 250w) which I'm more than happy with.

I'm fully aware of the safety considerations so I only charge it at 4 amps and on the garage floor and don't leave it plugged into the charger and most times I've been in attendance anyway (doesn't take that long for 2.4Ah).

When riding in ECO the typical current draw is 4A or less. As a test I put the motor into the highest power setting and the voltage sag really was considerable (can't remember the exact amount but I don't use the motor in that mode anyway).

View attachment 49191

One suspects the panny 18650b is a genuine cell but likely they are second or third rate cells not suitable for top rated sellers or as one may suspect are used cells.
Probably roughly tested and cobbled togethe rfor a poor pack but sold with the original 3400mah rating per cell.
It would be alot of effort to recreate the wrap as it looks pretty much an original panny marked wrap.
As stated one has to go in with theirs eyes wide open and expect to get what they pay for, expecting A grade for stupid money is never going to happen form china.

 

[AndyBike]

(How bad are they really) ?

Think- 3am house fire

 

[Bikes4two]

@Bonzo Banana

I don't know how to measure full capacity in battery packs.

I knocked up a tester on a piece of ply as per the schematic below. I also rigged up a similar watt meter that I can temporarily afix to the bike to also measure battery capacity - both very handy tools.