solid state relay on bms/bad idea

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solid state relay on bms/bad idea

Steve Clunn
I know its been said before on the list that the solid state relays do
not live up to there rating and I want to report that even with a heat
sink and small fan I just had another one blow on ( when it blows out
it blows on ) while running at about 1/2 power . Luck that the charger
was set below danger voltage so it just shut down but this could be a
problem with li-ion batteries . The car I'm working on has two levels
of safety the bms turns things off at sell voltage and if that doesn't
work the charger is set to turn off .  The guy I'm working for seems
to know what he's doing so I'm lucky there.  He added some extra cells
to his pack . charger set for 45cells , pack is 48.  He then has me
finish charging the pack with a ( 4 in series ) power supply in series
with 300 watt of light bulbs. Its peak amp out this way is 700 ma.
even with this small amount of power these big 260 ah cells come up
the last part in a short time .. I will tonight find out exactly how
much wh it takes to do this . I have a kil watt meter .  Any one using
Li-ion batteries should make sure it will shut thing down ,by testing
it . We did this with a controlled over charge of one cell after the
pack was fully charged . With a power supply we charged a single cell
and checked to see BMS shut down. All working find here . Except the
e-meter .
stEVe clunn

--
Tomorrows Ride TODAY !
Visit our shop web page at: www.Greenshedconversions.com

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Re: solid state relay on bms/bad idea

Cruisin
If you are having a problem with SS relays, I would suggest you take a look at what you are doing. There is something wrong with your design. You are probably using AC SS relays which are a no no on DC.
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Re: solid state relay on bms/bad idea

Willie McKemie
On Mon, Jun 27, 2011 at 02:27:26PM -0700, Cruisin wrote:
> If you are having a problem with SS relays, I would suggest you take a look
> at what you are doing. There is something wrong with your design. You are
> probably using AC SS relays which are a no no on DC.

It is a 240vac relay rated for 40 amps.  Application is BMS control of
ac current going to chargers.  It was not used at more than 20 amps
and was getting hot at that current.  Steve put a computer CPU cooler
on it but it still failed after only a few hours of use.  We were
fortunate to have avoided overcharging cells.

The EVPower BMS control boxes contain apparent SS relays, rated for 20
amps, that have given me no trouble at around 15 amps.  Hundreds of
hours of use on those.  They seem not to get hot.

We now have a very sturdy looking and cheap mechanical relay on it now
and it is working well at 240 vac.  It is actuated with 120 vac and we
are having trouble getting 120 vac to the chargers.  Suggestions on
simple, reliable, and safe relays solicited.

--
Willie, ONWARD!  Through the fog!
http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime  5 days 10 hours 39 minutes

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Re: solid state relay on bms/bad idea

Lee Hart
Willie McKemie wrote:
> It is a 240vac relay rated for 40 amps.  Application is BMS control of
> ac current going to chargers.  It was not used at more than 20 amps
> and was getting hot at that current.  Steve put a computer CPU cooler
> on it but it still failed after only a few hours of use.  We were
> fortunate to have avoided overcharging cells.

AC solid state relays have a 1.5v to 2v on-state voltage drop. 2v x 20a
= 40 watts. That's going to produce a lot of heat -- probably more than
a CPU cooler can handle.

Remember that they also tend to fail *on* (shorted). Provide a backup
systems that can shut the system down anyway if/when this happens.

> We now have a very sturdy looking and cheap mechanical relay on it now
> and it is working well at 240 vac.  It is actuated with 120 vac and we
> are having trouble getting 120 vac to the chargers.  Suggestions on
> simple, reliable, and safe relays solicited.

So you have a relay with a 120vac coil; but your charger is 240vac, and
you don't have a neutral?

One way is to get a little 120/240vac transformer. One easy source is a
transformer with dual 120vac primaries. Connect the two primaries in
series for 240vac, and use the center tap and one end to get 120vac for
the relay.

Or, my favorite light bulb trick. Measure the current that the relay
coil draws on 120vac. Suppose it's 50ma; then it's a 120v x 0.05a = 6
watt coil. Connect a 6 watt 120v light bulb in series. Now you can power
the relay+bulb at either 120vac or 240vac! On 120vac, the bulb will stay
cold; it barely glows and its resistance is so low that the coil gets
almost all of the 120vac. On 240vac, the bulb lights at normal
intensity, drops half the voltage, and the coil gets 120vac.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Re: solid state relay on bms/bad idea

Bill Dube
In reply to this post by Willie McKemie
Silly question, but a SS relay needs a heat sink and needs thermal
paste coupling it to that heat sink. They are very short-lived
without thermal paste.

At 06:35 PM 6/27/2011, you wrote:

>On Mon, Jun 27, 2011 at 02:27:26PM -0700, Cruisin wrote:
> > If you are having a problem with SS relays, I would suggest you take a look
> > at what you are doing. There is something wrong with your design. You are
> > probably using AC SS relays which are a no no on DC.
>
>It is a 240vac relay rated for 40 amps.  Application is BMS control of
>ac current going to chargers.  It was not used at more than 20 amps
>and was getting hot at that current.  Steve put a computer CPU cooler
>on it but it still failed after only a few hours of use.  We were
>fortunate to have avoided overcharging cells.
>
>The EVPower BMS control boxes contain apparent SS relays, rated for 20
>amps, that have given me no trouble at around 15 amps.  Hundreds of
>hours of use on those.  They seem not to get hot.
>
>We now have a very sturdy looking and cheap mechanical relay on it now
>and it is working well at 240 vac.  It is actuated with 120 vac and we
>are having trouble getting 120 vac to the chargers.  Suggestions on
>simple, reliable, and safe relays solicited.
>
>--
>Willie, ONWARD!  Through the fog!
>http://counter.li.org Linux registered user #228836 since 1995
>Debian3.1/GNU/Linux system uptime  5 days 10 hours 39 minutes
>
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Re: solid state relay on bms/bad idea

tomw
In reply to this post by Lee Hart
"Or, my favorite light bulb trick. Measure the current that the relay
coil draws on 120vac. Suppose it's 50ma; then it's a 120v x 0.05a = 6
watt coil. Connect a 6 watt 120v light bulb in series. Now you can power
the relay+bulb at either 120vac or 240vac! On 120vac, the bulb will stay
cold; it barely glows and its resistance is so low that the coil gets
almost all of the 120vac. On 240vac, the bulb lights at normal
intensity, drops half the voltage, and the coil gets 120vac."

That brought a smile.  Clever by half!