Back-UPS LS 700 flashes "replace battery" and "overload" simultaneously

[BillP]

Hi there,

I got this unit for cheap since it needed a new battery. I got a 12V / 9Ah new batt, but it didn't came to life. Opened up, seen four bulging caps, replaced them with some with the same specs. The manufacturing date matches the period when those faulty caps where installed in many electronics.

Now the unit appears to come to life, but flashes "replace battery" and "overload" at the same time.

Unit doesn't turn on on battery alone, even though it's properly charged. Doesn't turn on on mains alone.

What are the other parts that may need replacement?

[Anonymous user]

APC and Schneider Electric won't tell you. Their official stance is that these units are not user serviceable! This is for good reason: the high energy available in these units can start fires, trigger explosions, burn you or worse. As cheap and common as these and similar models are, the safest thing to do is to simply get another one.

I believe the fault indication points to a welded or sticking relay.

Doing only a partial electrolytic capacitor replacement is not a good idea. Electrolytic capacitors need not be bloated to have gone bad.

Internally these units seem a lot like a low end Smart-UPS (such as the SU420 or 620). They've got line voltage boost capability (though not buck, or reduction capability) without having to use up the battery to cover minor or moderate drops in voltage, and their logic powers down via a relay not long after being unplugged (provided of course the unit is turned off, otherwise it would go to battery and start its inverter). Their output is a modified sine wave.

Again, please carefully consider that the internals of these units, or any other UPS, are very dangerous. If you elect to do anything, you will be doing so ENTIRELY AT YOUR OWN RISK. Be sure that all power sources are disconnected such that no one can restore them and that you discharge the capacitors.

[BillP]

Well at $100 to $150+tax+s&h, this isn't exactly cheap to replace. I had a look at APC's trade-in program, but the rebate is ridiculously low. I would partly agree with them they are not user-serviceable but for a different reason: this unit appears to use a weird solder alloy that doesn't readily melt but instead keeps the consistency of peanut butter.

I also thought about the relays, but all three readily click loudly when powered independently, so I believe they're not welded.

Would you say all the electrolytic caps would have to be replaced around the UPS? Or can the UPS logic fail and prevent the high voltage stages to do their job?

[Anonymous user]

My intention was to recommend a used APC UPS as a replacement. Oftentimes units in need of new batteries are freely available to anyone who will take them.

Yes, I would say that every electrolytic cap is suspicious at this point, and ought to be replaced. Powering the relays externally may have damaged their driver circuits if they were still in circuit at the time. I haven't seen any detailed technical information about the Back-UPS LS product.

The solder you found sounds like the lead free type, and indeed it is not as readily workable as conventional ~60/40 alloys.

[BillP · Accepted solution]

On 12/21/2015 4:22 PM, William said:

My intention was to recommend a used APC UPS as a replacement

Indeed, I got my previous ES 350 in the garbage bin. Just needed a new battery. But on Kijiji, defective UPSes go for no less than 20$, and sellers don't specify what is the defect.

I also use the lead-free alloy for my solders, but apparently it's not the same alloy as APC's. Mine flows easily, though its exact composition isn't mentioned (a shame, but we've only got one shop selling it around here). Replacing all the electrolytics with such an unworkable alloy is sure to be a nightmare...