[RE-wrenches] BMS as OCOD

[Kirk Herander]

Hello William-

I agree with your approach.

I never historically have OCPD'ed lead acids at both ends of the battery
cable run, but do now, esp. with high kwh LFP, where a DC combiner is in
use.

For instance, I had to design a 2000 ADC bus @ a nominal 55 vdc, for a 500
kwh Blue Ion LFP bank. To create enough space for cables and fuses, I
started with -2- 2000A Midnite combiners, using one for positive and one
for negative.

Class T fuses are bolted directly to the DC bus (properly sized copper from
copper.org charts), and the BI BMS breaker protects at each respective
battery cabinet.

Same thing on the  DC to inverter cables, class T's bolted to the bus, to
its respective outback Radian, where the standard 175A breaker resides.

I would never again do a building power system of any type or size without
both ends of a power cable (point of use & point of origin) being
protected, A best practice IMO.



On Tue, Jul 15, 2025 at 1:13 AM William Miller via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Friends:
>
> When I design a lead-acid battery system I decide if I need over-current
> protection right at the battery or not.  My logic is:  if the leads are
> short, don’t go through a wall and are well protected,  I can rely on OCPD
> in the BOS cabinets. If any of these criteria is a no I put appropriately
> rated class T fuses right at the battery.
>
> I’m thinking that since BMS devices have over-current protection baked-in,
> fusing at the battery would not be necessary for BMS equipped batteries in
> any case. Do you agree with that premise?
>
>
> William Miller
> Miller Solar.com
> 805-438-5600
> www.millersolar.com
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-- 


*Kirk Herander / **kirkh at vermont.solar <kirkh at vermont.solar>*

*Owner|Principal, VT Solar, LLC*

*Celebrating our 34st Anniversary 1991-2025!!*

dba Vermont Solar Engineering

802.559.1225
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