Charging two batteries?

[steamer]

--Curious to know if there are any know downsides to stuffing an
extra 12v battery in a car, hooking it in parallel with the existing one and
expecting the alternator to charge both of them. I expect it would take
twice as long to charge 'em both, but is there a danger of screwing up the
alternator? Thx,

 

[John G]

--Curious to know if there are any know downsides to stuffing an
extra 12v battery in a car, hooking it in parallel with the existing one
and
expecting the alternator to charge both of them. I expect it would take
twice as long to charge 'em both, but is there a danger of screwing up the
alternator? Thx,

--
"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
www.nmpproducts.com
---Decks a-wash in a sea of words---

Why would you do that.
The car designer did a quite reasonable job of providing a battery suitable
to serve the car for the normal life of a battery.

If you have some extra use such as camping then you need to go to proper
separation of main and auxiliary and some of the Recreation news groups
would provide answers.

John G.

 

[default]

--Curious to know if there are any know downsides to stuffing an
extra 12v battery in a car, hooking it in parallel with the existing one and
expecting the alternator to charge both of them. I expect it would take
twice as long to charge 'em both, but is there a danger of screwing up the
alternator? Thx,

People with RVs do it all the time, likewise tow truck operators.
Marine batteries are frequently run that way - through a switch that
allows either or both to start the motor - but while under weigh both
are paralleled so they charge, then one or the other is running the
accessories keeping one in reserve.

The downside, from my boating experience, is when one of the batteries
has been abused it can sit there dragging down the good battery.

No expert on all forms of alternators . . . but the excited field
style tends to be relatively bullet proof. The circuit usually uses a
tickle charge from the battery to get the alternator pumping, but once
it is, the main current for the field winding comes from an extra trio
of diodes - so if the output is being dragged down, the voltage
available to the field is also dragged down - so it tends to be
current limiting.

 

[steamer]

That's why you should use isolation diodes.

--Understood. Yeah the application is a little nonstandard; nothing
to do with an RV or other 'normal' app.

 

[feebo]

--Curious to know if there are any know downsides to stuffing an
extra 12v battery in a car, hooking it in parallel with the existing one and
expecting the alternator to charge both of them. I expect it would take
twice as long to charge 'em both, but is there a danger of screwing up the
alternator? Thx,

so long as both batteries were in good order - you might consider
having a big diode to the +ve of the "spare" just to stop back feeding
- some kit I have used in the past has just this arrangement - a
switch can then be used to short out the diode when the power from the
spare battery is required back in the system - good idea to excercise
the spare every now and then.

 

[terryc]

--Understood. Yeah the application is a little nonstandard; nothing
to do with an RV or other 'normal' app.

Non-standard? Doubt it. double battery installations are fairly standard
these days. Basically, if you have anything you want to run off a 12V
supply whilst mobile or moving regularly, then installing a second battery
is fairly standard in all sorts of vehicles and "trailers".

Diodes are one method, relays are another. Isolation switches are commons.

Umm, jus make sure you get the correct type of battery. If you really just
want to install a second spare car battery, then it doesn't matter.
However if you are say touring and driving between places where you can
for a day or two and need to run a frigerator, medical machine, ham
radio, etc, etc, then make sure you get a proper deep-discharge battery.
Also read the LeadAcid FAQ on how to look after them properly for a long
life. Hint max 50% discharge at C/10 max rate.

AFAIK, "marine" batteries are a compromise between car battery and a
deep-discharge and not a proper deep-discharge battery.


handle boat shock

 

[Michael Robinson]

--Curious to know if there are any know downsides to stuffing an
extra 12v battery in a car, hooking it in parallel with the existing one
and
expecting the alternator to charge both of them. I expect it would take
twice as long to charge 'em both, but is there a danger of screwing up the
alternator? Thx,

--
"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
www.nmpproducts.com
---Decks a-wash in a sea of words---

I had a diesel GMC Suburban that came with two batteries in parallel, no
isolation.

 

[terryc]

"Handle boat shock"? What does this mean? Is this some type of alien command
like "Klaatu barada nikto"?

Sloppy editing. I was going to say that I understood marine batteries to
have heavier plates to handle tinnies/tinnys bouncing around on the waves.

 

[default]

--Understood. Yeah the application is a little nonstandard; nothing
to do with an RV or other 'normal' app.

I don't know if I'd just throw in some isolation diodes and expect
everything to be hunky-dory. A tenth of a volt drop represents a lot
of amps to a battery under charge. But it all depends on the use -
long recharge period and the diode probably matters a lot less.

If the goal is/was running accessories - I think a nice elegant
solution would be a mosfet and active switch to isolate the battery
used for accessories.

I know tow truck operators use two in parallel with no ill effects and
plenty of military vehicles do the same. A relay is good for
isolation too, but it may be impractical to use one on a starting
circuit.

If you are planning on using diodes to isolate both batteries, I'd
bump the voltage regulation up by one diode drop.

 

[terryc]

"Marine" batteries may be either deep-cycle or starting types. The
difference between them and an automotive or other battery is
primarily in the terminal type, and the fact that they often have a
lifting handle, where a similar sized auto battery would not. I don't
think there is any internal difference between a marine starting
battery and an automotive starting battery.

Well, in all I've read, that is the first bit that made it clear that
there are actually two different types of marine battery.