[RE-wrenches] Magnum System - Discharged Battery Bank

[Corey Shalanski]

Wrenches,

I visited a customer today whose grid-tied battery backup system suffered
some sort of catastrophic failure, and now I'm trying to play detective to
figure out what happened and whose equipment/design is to blame.

Here is what I know so far:
- System is ac-coupled with a Sunny Boy 7000 (7.85kW array), Magnum
MS4448PAE (x2), 3 strings of 4 12v batts (705Ah total)
- Site visit was triggered by the Sunny Boy registering zero output via
online monitoring.
- SPST Solid state relay installed on Sunny Boy output, controlled via
Magnum router, serves as secondary overcharge protection to Magnum's
frequency shift feature. Technician who initially responded claims he
measured 240v across the relay's terminals - relay manufacturer claims this
is "100% impossible". We removed the relay from the circuit, and the Sunny
Boy is again operational.
- On arrival I found the Magnum display showing a "Low Battery Charge"
message with the bank measuring ~4Vdc. The individual batteries were in
various states of charge, ranging from high of 6.3v to low of -2.6v. These
measurements were taken at rest, all battery cables disconnected, and yes
three of the batts were registering a negative voltage.
- Customer reports that he was not aware of any recent prolonged power
outages.

The three potential suspects would seem to be:
- SS relay: The customer is convinced that this is the weak link in the
system and somehow triggered this failure. I am not so convinced but would
be interested to hear recommendations for properly testing its
functionality.
- Magnum equipment: Since there is low-battery-cutout protection, why would
this feature not have activated and prevented the batteries from draining
so low?
- Batteries: I am not very familiar with modes of failure, but our
distributor suggested that an internal short could be a possibility - what
might have triggered this?

Clearly I am fishing for potential leads here, so any suggestions are
welcome.

Thanks!
--
Corey Shalanski
Joule Energy
New Orleans, LA
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