Bosch Powerpack BMS Schottky diode repair

B

Batteryrepair

10 mW
Joined
Oct 7, 2022
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I've noticed that about 50% of the time, the defect of a broken Bosch Powerpack BMS is related to a broken Schottky diode.
Found under the Blue cylinder:
20230927_120312.jpg
These are relatively easy to swap. Use plenty of flux !
You can find a broken diode by measuring the Diode voltage drop: has to be somewhere around 0.2V.
Often they fail short, so 0 ohm both ways.
This is the bad component.
20230927_120418.jpg
Remove the broken diode and prepare the area for new solder (flux and wick)
20230927_120343.jpg

Solder the new diode. On the side of the capacitors (left in pic) it's a bit tricky, but no worries if the solder bridges to the capacitor, they're connected anyways through the pcb.
20230927_120829.jpg
Apply 30V to the terminals where the battery would normally be connected, the LED's should turn on.

20230927_121140.jpg
Then the BMS can be used as replacement:
 
Broken is a poor choice of words when it comes to this problem. Faulty is much better. Faulty diode. As there is clearly no physical damage.

The big question is why it fails? Bad circuit design? It's highly unlikely that the diode is not living up to the specs.
 
Batteryrepair said:
I've noticed that about 50% of the time, the defect of a broken Bosch Powerpack BMS is related to a broken Schottky diode.
Found under the Blue cylinder:
View attachment 340142
These are relatively easy to swap. Use plenty of flux !
You can find a broken diode by measuring the Diode voltage drop: has to be somewhere around 0.2V.
Often they fail short, so 0 ohm both ways.
This is the bad component.
View attachment 340144
Remove the broken diode and prepare the area for new solder (flux and wick)
View attachment 340140

Solder the new diode. On the side of the capacitors (left in pic) it's a bit tricky, but no worries if the solder bridges to the capacitor, they're connected anyways through the pcb.
View attachment 340141
Apply 30V to the terminals where the battery would normally be connected, the LED's should turn on.

View attachment 340143
Then the BMS can be used as replacement:
Click to expand...
This is a great set of pics and sure enough, my BMS schottky diode was shorted. I order replacements from DigiKey per another post diode = SKS36MA-3G but I could not tell from the pics above what the correct polarity of the diode was. I couldn't tell from the original part, even using a magnifier as the lettering/marking was too faint. Fortunately I found another post that very briefly showed the replacement diode and by pausing the video I was able to see the cathode marking. The cathode (bar marking on diode) end should be closest to the battery connection side of the board (on the right in the close-up pics above (pics 1,3 and 4). I had a 50/50 chance and fortunately, when I applied power, the lights came on as described. So far, so good. Before there was no lighting at all. All of the cells in the battery were at about 2.4 V. I used an existing 18650 charger and using clip leads was able to charge each of the 10, 4x18650 in parallel "cells" individually and they all came up to between 3.5 and 4.1 V. I'm going to "equalize them at 3.5 and then reconnect the BMS and see if it will charge. Eventually I'll have to replace the cells but this pack still has some miles left in it. It was the diode failure that killed the BMS. It failed while charging, BTW. I had both my wifes and my batteries on charge after a long ride and hers (bought the bikes together) is still going strong and we've only got about 1500 miles on the bikes. Usually go at least 20 miles between charges so that works out to about 75 charges, well below the 500 the batteries are warranted for (or sadly, two years which we had already passed.) I bought a new batter but had them give me my dead one to explore and it looks like my odds are pretty good for a repair at this point.
 
Last edited:
igeocacher said:
Ordered 10 from DigiKey for a few bucks. Replaced my shorted one on BMS and the lights came on. Thanks!
Click to expand...
I haven't tried any repair yet or run out of bms's but yeah I'll get diods some when I feel to make such a repair and if measurements turns out to be that the diod is faulty.
 
igeocacher said:
This is a great set of pics and sure enough, my BMS schottky diode was shorted. I order replacements from DigiKey per another post diode = SKS36MA-3G but I could not tell from the pics above what the correct polarity of the diode was. I couldn't tell from the original part, even using a magnifier as the lettering/marking was too faint. Fortunately I found another post that very briefly showed the replacement diode and by pausing the video I was able to see the cathode marking. The cathode (bar marking on diode) end should be closest to the battery connection side of the board (on the right in the close-up pics above (pics 1,3 and 4). I had a 50/50 chance and fortunately, when I applied power, the lights came on as described. So far, so good. Before there was no lighting at all. All of the cells in the battery were at about 2.4 V. I used an existing 18650 charger and using clip leads was able to charge each of the 10, 4x18650 in parallel "cells" individually and they all came up to between 3.5 and 4.1 V. I'm going to "equalize them at 3.5 and then reconnect the BMS and see if it will charge. Eventually I'll have to replace the cells but this pack still has some miles left in it. It was the diode failure that killed the BMS. It failed while charging, BTW. I had both my wifes and my batteries on charge after a long ride and hers (bought the bikes together) is still going strong and we've only got about 1500 miles on the bikes. Usually go at least 20 miles between charges so that works out to about 75 charges, well below the 500 the batteries are warranted for (or sadly, two years which we had already passed.) I bought a new batter but had them give me my dead one to explore and it looks like my odds are pretty good for a repair at this point.
Click to expand...
P.S. After balancing the cells to within .02 v of each other, I hooked the BMS back up to the battery pack, soldering the power wires FIRST and then connecting the balancing wire connectors. I temporarily connected it to the case so I could test it with the charger and it charges (as indicated by the blinking top LED)!!. Looks like I've resurrected my pack. I reassembled it an plugged it into my E-Bike and everything came up normally. I'll probably ride it around locally for a while to make sure it can go some distance and recharges as desired but it looks like I've now got a "spare" battery to go with the new one I had to buy to keep riding. I did NOT replace the cells since they all came up fine and I've probably only got about 70-80 charge cycles on them since new. My e-bike guy gave me a couple of other dead packs and I'm going use the connector parts to make up a test jig for future repairs. If I can "harvest" a few good BMS's or repair some with the diode fault (I've got nine more diodes) I should be able to re-cell them for about $200 worth of batteries. You have to watch be careful and buy batteries from a reputable source. It seems there are lots of 18650 scammers out there selling ones that are "fakes" and won't deliver full performance (or not for long.)
 
I swapped out a bms today as it was not litting up any leds at all and the button was broken as well.

Easy peasy.

I think I measured 0 ohms on the bad bms diod so I guess I'll get some diods to try a repair on that unit. This would be the smallest repair I've don't apart from braking an imac 21" rear CPU area of those yellow small capacitors. One came lose and I was trying to put it back :D
 
So I changed a diod on a bms and it lightens up. Allthough this one will have no button as it is broken. Maybe I can replace it but they are tiny. I will for sure make one try

Also I did a second one. When I measure it gives out 5k ohm something and the other side gives 1,5k ohm. When this second bms was broken and still is it had one side at 4,5k ohm, so the same as above on one line and 1,5million ohm on the other side which is alot. The bms was also draining the pack and the heat was mostly next to the button with near or on the rectangular chip. I don
t know but when I feel it also it may be possible the square chip was heating before I removed it from a pack.

I'm using a 60w soldering iron. I have a smaller one I can use which I guess will be a better option. One small battery powered one with (8W) and I guess it should be able to become hot enough... I will try soon.
 
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