Battery capacity drop

[Deleted member 4366]

Like this please. All 10 cells with the two connectors unconnected, so 10 results.

 

[Khumarahn]

Here they are:
>0.3 MOhm
>0.3 MOhm
>0.3 MOhm
>0.3 MOhm
>0.3 MOhm
>0.3 MOhm
~80 KOhm
~56 KOhm
>0.3 MOhm
>0.3 MOhm

 

[Deleted member 4366]

Clearly something is wrong, but not the result I'd have expected. There's no solder bridging those pins at all is there, where you unsoldered the daughter board?

One last thing to try before passing judgement. Charge it up and do your discharge test again but use the B- as the negative rather than the normal negative (P-). You'll need to keep checking the cell voltages to make sure none go below 3v.

If it won't charge with the connectors in, see if you can charge between the positive and B- rather than C-. Likewise, you have to check that nothing goes over 4.2v.

The basic idea is to check the cell-pack capacity before deciding on a new BMS.

 

[trex]

where can he get a BMS for bottle batteries?

 

[Deleted member 4366]

From a bottle battery BMS shop:

http://www.bmsbattery.com/bmspcm/391-10s-lithium-ion-bms-for-bottle-battery-case.html

 

[Khumarahn]

There is no leftover solder joining the connector pins...

I'll try to check the cells.

I sent a message to bmsbattery, from whom I bought everything, but they didn't reply yet.

 

[Khumarahn]

I ran a discharge test, drawing power from B-. I followed voltages on cells, and looks like 10th cell is lagging behind. Below are my measurements taken at several points in time:
* 9 cells at 3.85, 10th at 3.65
* 9 cells at 3.80, 10th at 3.60
* 9 cells at 3.70, 10th at 3.50
* 9 cells at 3.67, 10th at 3.37

I stopped right after voltage on 10th fell below 3, I have taken around 5Ah from the battery by that time.

 

[Deleted member 4366]

Your root cause then is a a bad cell or two. Repair is very tricky. Here's what's involved:

http://www.pedelecs.co.uk/forum/threads/bottle-battery-repair.15289/

 

[Khumarahn]

Many thanks for the guidance

Which cells is a good replacement option? And maybe it makes sense the container to something like this?

 

[Deleted member 4366]

Maybe you stick with branded cells like this one:
http://www.bmsbattery.com/36v/682-bottle-ebike-battery.html
or if you want lightweight, see if you can get a bottle battery from Kudos or Woosh. At least you'll have some sort of guarantee.

 

[Khumarahn]

I am going to try to repair my battery. If it doesn't work, I will buy something decent.

I tried to look up 18650 cells on ebay, and got confused. Do I need protected cells? Can I buy "UltraFire 18650 5600mAh" for 10 pounds and mix them with what I have?

 

[trex]

assuming you have identified the bad cells. You need to buy new cells with the same capacity. Your battery is probably made with 40 x 2600mAH cells.
Just post what is written on the cells, members should be able to tell you where to get compatible cells.
I do not try to put you off, replacing bad cells does not guarantee success. You could throw good money at it for no result. If I were in your situation, I'd cut my losses and auction the 5AH battery on ebay.

 

[Khumarahn]

I see. What will happen if I have 2600mAh and I replace bad cells with 3000mAh?

 

[Khumarahn]

This is what my cells look like. I don't think there's anything written on them except this one line:

 

[trex]

if you replace 2600mAH cells with 3000mAH cells, they will still work.

 

[Deleted member 4366]

There's 40 cells in your battery in 10 groups of four. One group is down, so you need four cells. I think some of the other groups might be not far behind. Ideally, you need to charge up just that low group and continue your test to see how quickly the other nine drop off before deciding what to replace.

Unfortunately, you can't rely on Ebay 18650s because there's so many fakes that are as good as useless - particularly the Ultrafire cells. Another thing, you need cells with tabs welded on so that you can solder them.

If you had a spare BMS, and some good cells, it would be worth repairing, but if you've got to start buying stuff, it's a harder decision. It's going to cost about £50 to fix it, and then the next cells might fail soon after.

I think replacement is a better option.

 

[Deleted member 4366]

You need to match the capacity of old and new cells otherwise the BMS might struggle to keep it in balance.

 

[Khumarahn]

Does there exist a serviceable version of a battery? By service I mean, that it is straightforward to replace bms or any of the cells...

I will look in the direction of the small saddle bag batteries, but this would imply packing a bms and controller too...

 

[trex]

batteries with prismatic cells are much easier to fix. Fewer cells, cells have solder tabs, BMSes have standard wiring.

 

[Khumarahn]

would you recommend any particular prismatic cells, and where to get them? And BMS? I am looking for a 36V 10Ah battery.