Since it is only 12 or 36V, you could easily find a MOSFET that can do
the work of the contactor and draw zero current, it just needs its Gate
input to be held at the correct voltage to enable it. Presuming that you
are not drawing hundreds of Amps?

Another alternative is a sense resistor that allows the contactor to
drop out as soon as there is no current draw and re-enable the contactor
when there is current draw again. But that could mean that there are
voltage drops while the contactor switches.
Another alternative is a latching relay, which does need a pulse to
switch state, but no power to either stay on or off.
Many choices...

Cor van de Water
Chief Scientist
Proxim Wireless Corporation
Email: Private:
Skype: cor_van_de_water Tel: +1 408 383 7626

-----Original Message-----
From: EV [] On Behalf Of Steve Clunn via
Sent: Tuesday, September 16, 2014 7:32 AM
Subject: [EVDL] 4 to 12 Cell Lithium BMS

I am working on some Golf Carts and portable 12V power packs that get
sporadically and may sit for months without use.

I have contactor on one 12V and 36V that draws about 1 amp.  The problem
is, if everything is happy, the Mini BMS Circuit keeps the contactor ON.
Which draws enough current, that at some point, will drain the battery

I am trying to come up with a way that the owner can walk away from this
system, and come back to it and have it ready to go without having to
switch switches OFF and ON.

One thought I have had is to set it up with a transistor that controls
contactor.  And after so many hours, will turn OFF the contactor.  My
thought was a Mosfet with a large Capacitor and a Momentary switch to
charge the capacitor which would keep the contactor ON for maybe 10
while the Capacitor slowly drained down.

Of course, the operator will have to engage this each time he uses the
battery, but at least it will turn its self off automatically.

I am wondering if anyone has any other ideas... that doesn't involve a
micro processor.

I am noticing that the smaller the parts that I am working on....the
it seems to take !
Steve Clunn
Merging the best of the past with
the best of the future.
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