[RE-wrenches] Breaker tripping question

[Allan Sindelar]

Wrenches,
I am doing a series of corrective equalizations and load tests on a set of
Surrette S460 batteries. I'm using an old Trace DR1524 mod-square inverter
as a charging source from in-shop grid AC power. I am also using the
inverter to power mostly-resistive AC loads: an electric space heater on low
with fan, an incandescent bulb and a CF bulb - the combination being used to
achieve a C/20 discharge rate for load testing. DC metering is handled by a
Tri-Metric meter with 100A (low) shunt; AC by a Kill-a-Watt AC monitor. The
inverter is rated to charge DC at 35A maximum, and consistently charges at
around 32A measured. 35A is a C/20 rate (5A per 100A of rated capacity for
corrective equalization) for these 350 ahr batteries in two parallel
strings.

When I set up this station I used a CBI (Outback-Midnite) DC panel-mount 50A
breaker on the battery cables. The cables are 2/0 or 4/0, being what was
around the shop. The problem is that the breaker tripped while charging.
Thinking it might be a defective breaker, I replaced it with a 60A; same.
It's not a surge load, as it doesn't trip on startup, but 10-30 minutes
later. I understand that these are magnetic-hydraulic breakers, not thermal,
so I don't understand why they are tripping. My hunch is there's a DC ripple
that's messing with the breaker's response curve. Since inverters are
generally installed with heavy cables and matching big breakers or fuses,
this never shows up in normal operation. But in this application there's no
surge capacity to accommodate, so I just sized the breaker to the load plus
reserve: 35A max/50 or 60A breaker. And the OBDC/CBI breakers are rated for
100% duty cycle.

Any ideas?

Thanks,
Allan

Allan Sindelar
allan_(at)_positiveenergysolar.com <mailto:allan at positiveenergysolar.com> 
NABCEP certified solar PV installer
Positive Energy, Inc.
3225A Richards Lane
Santa Fe NM 87507
505 424-1112

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