Re: My Battery Monitor

[Lee Hart <leeahart@earthlink.net>]

Steve Richardson wrote:
> Surely there is a better choice than Lee's $8 relay
> (Lee, does that include the socket?).

The T92 is a sealed relay, and comes in either PC mount or with mounting
feet and 0.25" fast-on terminals. I used it because I could mount them
in or near the battery box. Only one pair of high-current high-voltage
wires needs to go from the control board's A/D and charger to the
battery box.

You might think of relays as old-fashioned, but they are cheap, simple,
and reliable. Sealed relays are pretty quiet; you can't hear them
clicking unless everything else is totally silent.

A good relay is rated for 100,000 operations at full load. Since I
switch them slowly when doing nothing, and faster as needed, they should
easily last the life of the car.

Remember that each $8 has two contacts; battery + and -.  A solid-state
replacement would have to cost under $4 for each 240v 30amp "contact",
have an essentially zero voltage drop at low currents (so it doesn't
affect voltage measurement accuracy), negligible heating at 30 amps
(unless you want to add heatsinks and cooling), and fail-safe isolation.

I think some form of solid-state switch is practical for low-current
monitoring, but not for carrying charging currents.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen