current draw for starter motor

[Dan]

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?

 

[Tim Wescott]

Dan 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?


Starters draw very substantial current -- I believe over 100A if the

engine is stalled. That's why the cable from the battery to the starter
is so fat. If I were going to do this I'd see if I could find a remote
starter solenoid from some automotive application; this would give a
device that's already optimized for the application and that I could get
at a parts house when it broke down.

There may be systems like this already; check trailer supply or RV
supply houses. I believe that you can get something that will do charge
management for two batteries; I'm not so sure about the switching.

Final note:

Just buy a new battery. It's cheaper and more dependable.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Paul Burridge]

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?

For a truck? Might be several hundred Amps! Your rely will have to be
pretty beefy, although there *are* solenoids around that can cope with
that kind of current. You might well have to use a series of relays
culminating with your *big* solenoid to handle the job.

--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Tam/WB2TT]

"Dan" <danNOSPAMsteely2001@yahoo.com> wrote in message
news:uAMfd.23892$1f.1407@tornado.tampabay.rr.com...

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?


You could connect the standby battery to the system through a diode and low

value resistor. The diode voltage drop will be very low once the battery is
charged. If the primary battery ever fails, raise the hood and switch the
cable.

Tam

 

[Vlad]

On Wed, 27 Oct 2004 14:44:07 -0700, mike <spamme0@netscape.net> wrote:

Dan 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?



Why do you need a backup battery???
If you want to power something, run it off the backup and leave the
starter battery alone.
If you need to start from the backup, you should use a second
starter solenoid with an "emergency start" pushbutton.
Charge 'em thru a diode isolator.
Trying to switch between batteries has all sorts of issues...like
what does you radio do when it momentarily loses power?
If you intend to routinely "run down" the battery, be sure to get
a battery designed for that purpose. Standard car starting
battery doesn't last long if you deep discharge it."
mike

If you use as suggested, an ordinary starter relay note that those
relays may not be designed for continuous duty.

Vlad

 

[Tam/WB2TT]

"Vlad" <Bla@dot.com> wrote in message
news:5bk0o0d09mulbp4gvvonai3j1jh1qfv0el@4ax.com...

On Wed, 27 Oct 2004 14:44:07 -0700, mike <spamme0@netscape.net> wrote:

Dan 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?



Why do you need a backup battery???
If you want to power something, run it off the backup and leave the
starter battery alone.
If you need to start from the backup, you should use a second
starter solenoid with an "emergency start" pushbutton.
Charge 'em thru a diode isolator.
Trying to switch between batteries has all sorts of issues...like
what does you radio do when it momentarily loses power?
If you intend to routinely "run down" the battery, be sure to get
a battery designed for that purpose. Standard car starting
battery doesn't last long if you deep discharge it."
mike

If you use as suggested, an ordinary starter relay note that those
relays may not be designed for continuous duty.

Vlad

Also, I wonder how much current these relay draw. Possibly enough to run the
battery down in a few hours.

Tam

 

[Gnekker]

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. <snip

This is not a good idea because battery is essential part of the system: it
serves as voltage stabilizer, any interruption between the battery and the
generator can cause voltage surge that may burn engine management system or
any other expensive electronic part inside your truck.

 

[Dan]

"mike" <spamme0@netscape.net> wrote in message
news:418016A7.6080705@netscape.net...

Dan 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?



Why do you need a backup battery???
If you want to power something, run it off the backup and leave the
starter battery alone.
If you need to start from the backup, you should use a second
starter solenoid with an "emergency start" pushbutton.
Charge 'em thru a diode isolator.
Trying to switch between batteries has all sorts of issues...like
what does you radio do when it momentarily loses power?
If you intend to routinely "run down" the battery, be sure to get
a battery designed for that purpose. Standard car starting
battery doesn't last long if you deep discharge it."
mike

--
Return address is VALID.
500MHz Tek DSOscilloscope TDS540 $2200
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/

I was charging a drill battery using an inverter and after about 10 or 15
minutes the engine wouldn't start. I let it set for a while and then it
started. Its a GMC stepvan which I work out of and there is an extra battery
tray on the other side of the engine. I could hook up a volt meter on the
2ndary battery for monitoring and use it for the inverter and then use the
isolation diode to charge it. What kind of isolation diode would you
suggest? Thanks to all for your help.

Dan

 

[Paul Burridge]

On Wed, 27 Oct 2004 21:59:32 -0400, Vlad <Bla@dot.com> wrote:

If you use as suggested, an ordinary starter relay note that those
relays may not be designed for continuous duty.

They're not. They typically burn out after 2 to 3 minutes, IME.
They're only designed for cranking a starter motor for a few seconds,
after all. There are solenoids that can handle the task, however. The
OP may want to look at winch solenoids, which can continuously handle
several hundred Amps and are commonly available in 12 and 24 volt.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Paul Burridge]

On Wed, 27 Oct 2004 23:52:33 -0400, "Tam/WB2TT"
<t-tammaru@c0mca$t.net> wrote:

Also, I wonder how much current these relay draw. Possibly enough to run the
battery down in a few hours.

They are quite current hungry, which is another good reason to avoid
them. The winding resistance of winch solenoids is much higher; they
are efficiently designed for long spells of heavy duty.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[ChrisGibboGibson]

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

 

[Paul Burridge]

On 28 Oct 2004 19:31:42 GMT, chrisgibbogibson@aol.com
(ChrisGibboGibson) wrote:

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.

Not as much difference as when charging Lithium-ions in parallel!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[ChrisGibboGibson]

Paul Burridge wrote:

On 28 Oct 2004 19:31:42 GMT, chrisgibbogibson@aol.com
(ChrisGibboGibson) wrote:

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.

Not as much difference as when charging Lithium-ions in parallel!

Very droll!

Gibbo

 

[Julie]

Dan 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?

Go check out marine options -- very common switch on boat engines where
multiple batteries are the norm. Your local boat shop should have them
available, or you can get them online at many outlets such as
http://www.westmarine.com

http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&productId=91533&catalogId=10001&classNum=297&subdeptNum=207&storeNum=9

There are also devices that you can buy that prevent your battery from being
completely drained (preserve a reserve charge for starting), also a common
marine item.

Note: *NEVER* switch from one battery to another and/or to off while the key is
in the ignition -- doing so is a sure-fire way to smoke a lot of components.

 

[Dan]

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

 

[Rich Grise]

On Thu, 28 Oct 2004 19:31:42 +0000, ChrisGibboGibson wrote:

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.

The battery isolator I've seen at an RV show used diodes for both
batteries, and just picked up the sense for the regulator on the battery
side of the engine battery diode.

Hope This Hepls!
Rich

 

[ChrisGibboGibson]

Rich Grise wrote:

On Thu, 28 Oct 2004 19:31:42 +0000, ChrisGibboGibson wrote:

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.


The battery isolator I've seen at an RV show used diodes for both
batteries, and just picked up the sense for the regulator on the battery
side of the engine battery diode.

Yes that's another common solution. The problem is, it means the existing
alternator wiring has to be changed as it will currently be simply connected to
the starter motor.

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.

Gibbo

 

[ChrisGibboGibson]

"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?

You need to work out what the maximum current through it might be at any one
time with various charge conditions of the batteries and various loads.

Without going through your entire vehicle electrics and equipment there's no
way of knowing. But I'd be looking at much bigger than this. 70 Amp is
*usually* satisfactory in a vehicle but it does depend on the setup for each
individual installation..

Gibbo

 

[Rich Grise]

On Fri, 29 Oct 2004 16:52:21 +0000, ChrisGibboGibson wrote:

Rich Grise wrote:

The battery isolator I've seen at an RV show used diodes for both
batteries, and just picked up the sense for the regulator on the battery
side of the engine battery diode.


Yes that's another common solution. The problem is, it means the existing
alternator wiring has to be changed as it will currently be simply
connected to the starter motor.

True - the one I saw was for a system that was already set up for dual
batteries and remote sense. He was just selling a "better" one that
used four diodes, mounted alternately on the heat sink to spread out
the heat dissipation. (i.e., D1 and D3 to the house battery, D2 and D4
to the motor battery.)

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".

Cheers!
Rich

 

[ChrisGibboGibson]

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 volts
and still drawing quite heavy current. The drop through their diode will be
about 1.2 volts. The engine battery will be drawing substantially less current
and the drop through it's diode will be about 0.6 volts. The engine battery
will therefore be charged at 15 volts. It's too high.

If the engine battery voltage is sensed, then the reverse applies and the
auxillary battery will be charged 0.6 volts too low for a substantial period of
time.

This can drastically increase recharge times which can be critical.

It's a major problem and most marine and RV electric websites discuss it
endlessly.

It is the main reason why there is such a huge move away from diodes and to
other, better systems.

Gibbo