UPS – shorted battery terminals - Page 1
Forum section: electronics / repair

author: antDolejs
author_rank: Newbie
timestamp: July 31, 2021, 09:32:21 pm
content: Hello,I was trying to test a UPS using my bench PSU, the UPS was working but the 3A of my supply were not enough for any load on the output so I wanted to connect the second channel in parallel to make the PSU capable of 6A. But I was rushing and I connected the channels in series... So 48V got on the 24V battery terminals. It was current limited to 3A and the OCP of the PSU turned the channels off immediately. When I realized my stupid mistake the battery terminals were shorted, so something probably released it's magic smokeTL;DR – the battery terminals are now shorted... What could gone wrong? Could that brief 48V be enough to damage the transistors on the heatsinks? I can't see anything obviously blown. I was hoping I've just blown some protection diode, but I tried (it's super cramped) measuring some diodes near the terminals (in circuit) and they seem fine. The fuses are ok. What would be your bet? I unfortunately wasn't able to find any schematic to referenceIt's APC smart ups SC 1500.Some photos to reference:(didn't work – ERR_HTTP2_PING_FAILED)

author: TheMG
author_rank: Frequent Contributor
timestamp: August 02, 2021, 02:37:28 pm
content: Could very well have caused the MOSFETs to fail. I forget exactly what part they use but they use MOSFETs optimized for high current and low RDSon that have a relatively small Vds rating.

author: antDolejs
author_rank: Newbie
timestamp: August 07, 2021, 12:14:17 am
content: Thanks for your reply! I managed to disassemble it to have a look at the MOSFET and it looks like it's IRF3205 which according to a datasheet I found online has VDDSof 55V, so they should have survived it, shouldn't they?I'm quite surprised that it's such a common and cheap MOSFET, I thought they'd use some weird, hard to get part... I could replace this quite easily...Anyway, what is the other most likely part to die? Probably some diode?EDIT: Found out that the two big 3300uF capacitors are directly across the battery terminals and are rated only for 35V, could they be the issue? They are not bulged or visibly damaged in any way. But they are quite easily accessible so I might try to replace them anyway. It's probably worth the shot as I can't find anything else. The only other thing that seems to be shorted is a big ER5D diode, but it's supposed to withstand 200V (140V reverse) so I can't think of an reason why it should fail...But I'm gonna leave it till the morning, so I don't make another silly mistake...

author: BradC
author_rank: Super Contributor
timestamp: August 07, 2021, 01:38:03 am
content: Remote fault finding by crytsal ball is hard.The MOSFET might have a Vdss of 50V, but I guarantee the Vgs won't be that high. If the circuit was designed for ~24V the gate protection will also be designed to accommodate, you might have fried it.Unless you left the 48V power supply on it for minutes to hours, I can pretty much guarantee it won't be the battery decoupling caps, but I can almost completely guarantee that any cap you replace them with will have inferior ripple current ratings and will got hot and die early in use. Don't do that.You can't do this sort of fault finding by speculation. You need to get the meter and soldering iron out and start testing parts. If it's not obvious in-circuit, then pull it out and test it.