current draw for starter motor

[Rich Grise]

On Sat, 30 Oct 2004 10:05:35 +0000, ChrisGibboGibson wrote:

Rich Grise wrote:

[snip]

The other problem is the voltage drop across the diodes is dependent
upon the charge current so the fully charged battery gets over charged
or the flat battery gets undercharged. There's nothing you can do about
this.

This is a non-problem because Lead-Acid cells can sit at a constant-
voltage "float" voltage indefinitely. The topped-up one will simply not
charge any more, while the other one "catches up".


It is *far* from a non problem.

Sit at *float* voltage indefinitely, yes, sit at *acceptance* voltage, no.

Assume the regulator senses the auxillary battery and they are at 14.4

If you do this, then it's a different scenario than I've outlined. I
did not say, "Take the sense voltage from the auxiliary battery", so, yes,
if you assume that it's set up differently than I said, of course it's a
problem.

Thanks,
Rich

 

[ChrisGibboGibson]

Rich Grise wrote:

On Sat, 30 Oct 2004 10:05:35 +0000, ChrisGibboGibson wrote:

Rich Grise wrote:

[snip]

The other problem is the voltage drop across the diodes is dependent
upon the charge current so the fully charged battery gets over charged
or the flat battery gets undercharged. There's nothing you can do about
this.

This is a non-problem because Lead-Acid cells can sit at a constant-
voltage "float" voltage indefinitely. The topped-up one will simply not
charge any more, while the other one "catches up".


It is *far* from a non problem.

Sit at *float* voltage indefinitely, yes, sit at *acceptance* voltage, no.

Assume the regulator senses the auxillary battery and they are at 14.4

If you do this, then it's a different scenario than I've outlined. I
did not say, "Take the sense voltage from the auxiliary battery", so, yes,
if you assume that it's set up differently than I said, of course it's a
problem.

And further down I wrote "if you sense the voltage from the engine
battery....." and detailed the other problem scenario.

A part of the post that I note, with interest, you snipped.

Whichever one you sense, it's a problem.

Gibbo

 

[Tam/WB2TT]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041028153142.01302.00003096@mb-m04.aol.com...

If your car *isn't* very recent (like within the last 2 years in which
case
this *might* not work) do this.

Install your secondary battery and run your auxillary loads from that.

Look at your alternator, there will be a terminal marked D+ or "ind". This
will
provide a 12 volt power feed only when the alternator is producing power.

Use this to energise a heavy duty relay which connects both batteries
together.

When you're charging, you will charge both batteries. As soon as you shut
your
engine off, the batteries will be separated.

It's a very common system. Probably millions of them operating
satisfactorily
in RVs and boats all over the world.

Don't try the diode idea. It's a waste of time. The volt drop across the
diode
means your secondary battery never gets even close to being fully charged.
0.6
volts makes a *huge* difference when it comes to lead acid battery
charging.

Gibbo

Once the battery is almost charged, the voltage drop on an MBR1035 at 50 ma
and 50C is only about 0.2V. Whether the spare battery wire is connected to
the main battery, or the alternator might make a bigger difference.

There are still a problems, though. If he connects the inverter to the spare
battery, he will discharge both batteris. Likewise, he can't have a relay
that stays on indefinately, because any power relay would run down the
battery eventually. Turning off all power when the key is off is no good for
reasons people have pointed out. Simplest solution is to keep the engine
running when using the charger.
Tam

 

[Ken]

On Wed, 27 Oct 2004 12:33:30 GMT, "Dan" <danNOSPAMsteely2001@yahoo.com>
wrote:

I want to put a backup 12v battery in my truck and I was thinking of using
(2) battery cut-off switches to switch each battery in and out of the
system. If I could use relays, then I could do some wiring logic or use
the N.O. of one relay and the N.C. of the other. What kind of relays
could I use in place of switches? What amp draw does the starter require?

I depends, but 100-300 Amps!

 

[Terry]

"Ken" <___ken3@telia.com> wrote in message
news:rakpo0tjcsse4mu97oavcusmm4gj80fek1@4ax.com...

On Wed, 27 Oct 2004 12:33:30 GMT, "Dan" <danNOSPAMsteely2001@yahoo.com
wrote:

I want to put a backup 12v battery in my truck and I was thinking of
using
(2) battery cut-off switches to switch each battery in and out of the
system. If I could use relays, then I could do some wiring logic or use
the N.O. of one relay and the N.C. of the other. What kind of relays
could I use in place of switches? What amp draw does the starter
require?

I depends, but 100-300 Amps!

Agree.

It does depend on how big the starter is cos that depends on whether you are
trying to start a V8 diesel or a three cylinder Suzuki!
But if the starter is, say, two horsepower that will require, at 12 volts at
least 150-200 amps for the few moments it normally takes to start the
engine. More in cold weather!

 

[Terry Given]

Dan wrote:

T'will work fine. There'll be a wire on the D+ terminal already that feeds

the

ignition warning light. Parallel it up to a feed to the coil for your new

relay

(other side of the coil to ground).

All will work wonderfully.



So how heavy a relay should I use? 30A? Or more?

Dan

FWIW I once did some tests on a 12V 30A T9D style pcb mount "automotive"
relay (I wanted to use it to bypass a DC bus soft-charge resistor). At
30A dc, the relay got so hot the plastic case melted. I used 2 in
parallel (kinda - I had several soft-charge resistors, so split the
circuit up to ensure the relays shared current).

Cheers
Terry

 

[ChrisGibboGibson]

Terry Given wrote:

Dan wrote:
T'will work fine. There'll be a wire on the D+ terminal already that feeds

the

ignition warning light. Parallel it up to a feed to the coil for your new

relay

(other side of the coil to ground).

All will work wonderfully.



So how heavy a relay should I use? 30A? Or more?

Dan



FWIW I once did some tests on a 12V 30A T9D style pcb mount "automotive"
relay (I wanted to use it to bypass a DC bus soft-charge resistor). At
30A dc, the relay got so hot the plastic case melted. I used 2 in
parallel (kinda - I had several soft-charge resistors, so split the
circuit up to ensure the relays shared current).

Indeed. The typical "30 amp" automotive relay self destructs at 20 amps after
about 15 minutes.

The Lucas 33RA "70 amp" automotive relay self destructs at 40 amps after about
20 minutes.

They're a joke.

For continuous ratings we now insist on *real* contactors. ie where the
manufacturer gives *full* specs.

Our preferred units are made in the UK by Albright. If they say it's 100 amp,
it is indeed 100 amp, probably more.

Thing is, they're not even expensive, probably cheaper than the imported crap
that appears on the market.

Gibbo