Help with Bosch 36V powerpack repair

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repair_saves_lives

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I have been given a Bosch battery pack (36V 4AH) that has been used with an electric bike but has failed and no longer charges. The battery output is still good and voltage in very healthy range, so it looks like the thermistor has failed - hence preventing the charger (AL 3620 CV) from charging (it gives an overheat warning).

Simple solution given all the cells are looking good and well balanced, is to replace the NTC thermistor, but I dont know what NTC thermistor I should be using.... 10k, 5k, or some random other one? Does anybody know?

Picture attached for the sake of explanation. Thanks in advance!!

P.s. Why oh why have Bosch glued the PCB into place so thoroughly?! Its horrible to remove and need to be careful about cracking the PCB when unpeeling it from the black casing material.
 

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repair_saves_lives said:
Simple solution given all the cells are looking good and well balanced, is to replace the NTC thermistor, but I dont know what NTC thermistor I should be using.... 10k, 5k, or some random other one? Does anybody know?
Click to expand...

What markings does the part have on it?
 
Sadly nothing. Its just the little black thermistor that pushes into the thermal compound (white) in the battery casing (one leg is free and one leg goes through a little support ring which can just see in the picture).
It measures 26.2k but doesnt change with temperature hence I figured it must be broken, but I have no idea what it should read....
Thanks for your response!
 
Ah. Do you have access to a working pack of the same model? If so, you can measure that one's thermistor at the necessary temperatures and determine it's base resistance and calculate it's beta, and whether it's PTC or NTC.

BTW, if the thermistor is connected to electronics inside the unit, that could greatly affect any reading you might get of it's resistance...but it should still change with temperature enough to tell it had done so. It might be necessary to desolder one leg to disconnect from the PCB to verify it's actual resistance.

The most commonly used types are 10K NTC, but B values used vary widely even with those.


I have not watched the video, but this one
https://www.youtube.com/watch?v=8PS_7ogubkc
might have the info you're after, google says a bad thermistor is discussed after about 8:21.



If you do find out what it is supposed to be, I recommend ordering a new one from a supplier like Mouser, Digikey, Farnell, etc., as I have had fairly terrible luck getting working real parts of this type from amazon, ebay, aliexpress, etc. They are either completely wrong values, wrong betas, or don't even work (dead short or complete open)--a few worked until they were installed and then failed (or failed in the install process). Not counting the ones that never even arrived, I think I bought several dozen so far and exactly two of them did what they need to after installation. :roll: Next time I need one I'll wait till I have a big enough parts list to be worth it and just get them from Mouser. :oops:
 
Thanks again for the response! Sadly I dont have a working one otherwise I would have indeed tried that. It looks like the traces are fairly simple so should work ok to probe - that youtube video was the one I found before so I bought some 5k NTC thermistors (no idea what the B value is as it was just an ebay one - so thanks for that comment, I'll look on a proper site, I order from farnell reasonably often so shouldnt be too hard). Incidentally I tried boot strapping a 5.1k resistor in parallel but it didnt give me the green light so perhaps these thermistors will not help.....

I think I was mainly hoping somebody else had repaired one of these, or had a working one, as there must be hundreds that go bad and no doubt many for superficial reasons....
 
Sadly despite replacing the Thermistor the battery is still not accepted so I think I will have to concede defeat that there might be more going on when charging than just measuring thermistor output...

Without a working device to compare against (and even with one Im shooting in the dark) I think the battery pack needs stripping for working cells, since they are perfectly balanced and reasonably well charged! Sad days!
 
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