Re: My Battery Monitor

[Richard Bebbington <richard@rbebb.demon.co.uk>]

Ron Schroeder <rjs@bnl.gov> writes
>At 01:59 PM 11/29/00 +0000, you wrote:
>>If using relays to connect batteries to a central measuring
>>device, please make sure your circuit CANNOT energise more than one 
>>relay at a time, or you'll short out part or all of your pack!
>> 
>>Also, think of what might happen if the contacts weld closed....
>>...you might have switched off the relay coil, but the contacts
>>are still closed, and then you switch on another relay...
>>...plasma frenzy!!
>>Positively guided contacts would be a good idea, as well as using
>>a multipole relays, so an active relay will lock all the others out.
>>
>>Just a little more fuel for the fire!  ;)
>>-- 
>>Richard Bebbington
>>
>
>You must fuse each sense wire with either relays or solid state switches.
>
>Ron Schroeder

Yep, fuses are a MUST anyway, but bear in mind that they all
have to be rated to break full pack voltage, possibly at
very high current. Standard automotive fuses won't work!

Plus, if you do blow a fuse because your monitor turned on
two relays at once, you've got to open up your battery boxes
to change the blown fuses, and the relays might be damaged
by the short-circuit current that blew the fuse.

I feel it'd be better to avoid this situation by trying
to make it almost impossible for the monitor to turn on
two relays at once - the whole idea of this thing is to
make battery maintenance easy for non-techies, right?

Re: MOSFETs or other solid state devices - remember that
FETs have an intrinsic body diode. If you put reversed
voltage across it, it'll conduct.

Bad ASCII art warning!

  drain    =   positive
    |
    |
    +--+
    |  |
 k  |  +-||
    -     | 
    ^     |
 a  |  +-|+--- gate
    |  |
    +--+
    |
    |
  source   =   negative

a and k are the body diode's anode and cathode.



I have a feeling that this diode will cause a short circuit
if you try to use FETs to connect one common measuring
device up to many different points in a chain of batteries!
Try drawing it out, remembering to draw in the body diodes that
come with the FETs.... as soon as you turn on one FET, 
somewhere there's a diode that'll complete a short circuit....

Lastly, you need to be careful with your gate drive for FETs.
They can turn on very fast, and the gate is effectively just
a capacitor, so if noise gets into your gate drive, the device
could be turned on when you don't want it to be on!

This is why I'd use relays - FETs are great, but they come
with their own problems too.

Regards
-- 
Richard Bebbington