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'[EE] Basic question on auto batteries'
2006\03\16@145239 by

Hi,

For my autonomous ATV project, I will have two automotive batteries onboard and a automotive alternator generating up to 90A @12-14V. The motors I will use the automate the ATV systems (steering, brakes, etc) work at 12 or 24V. My dillema is: how to connect the batteries, series or parallel?
In series I will have more punch for the motors at 24V, but how do I charge them using the 12V alternator? Can I use diodes to somehow direct the current to charge both batteries at the same at 12V but get 24V out of both? The motor manufacturer said the motors will work at 12V (at half the speed of course) and torque will be the same, since in his words, torque is not related to voltage, but to current.
As the motor was originally designed for 24V, I prefer going that route, but not at the cost of complicating my design on how to charge the batteries... do you have any suggestion?

Cheers

Hi!

> Hi,
>
> For my autonomous ATV project, I will have two automotive batteries onboard and a automotive alternator generating up to 90A @12-14V. The motors I will use the automate the ATV systems (steering, brakes, etc) work at 12 or 24V. My dillema is: how to connect the batteries, series or parallel?
> In series I will have more punch for the motors at 24V, but how do I charge them using the 12V alternator? Can I use diodes to somehow direct the current to charge both batteries at the same at 12V but get 24V out of both? The motor manufacturer said the motors will work at 12V (at half the speed of course) and torque will be the same, since in his words, torque is not related to voltage, but to current.
> As the motor was originally designed for 24V, I prefer going that route, but not at the cost of complicating my design on how to charge the batteries... do you have any suggestion?

Probably opening the bridge between the two batteries with a relay
(contactor) during charge?

Regards

--
He comentado ya que tengo un blog? O:-)

If you have a 12V alternator, you really need to use 12V battery.
The alternator isn't guaranteed to give you a stable output without the
battery present for one thing, and there's no easy way to get from 12V to
24V at that current level.
What is going to power the alternator?    If you use anything powered by
the battery it will be a waste of  power due to friction and resistance.

I had an application that used a 24V surplus air compressor on a truck
with a 12 volt charging system.  I used some starter relays that would
change the battery connections on the extra battery from parallel to
series to run the air compressor.   Years ago a lot of the big semi
trucks had a series/parallel relay.  The starter was 24 volt All other
electrical on the truck was 12 volt. When you would energize the relay
it would change the batteries from series to parallel and run the starter..

I still have 2 of them around for testing 12v/ 24v items.

Bob

{Quote hidden}

It sounds like you might want to look into what RV and long haul
truckers have set up for their dual battery situations.  In them there
are two batteries - one to start the engine and one to run accessories.
They are both charged IRC from the same alternator.  Sounds like your
problem is actually well known and solved in those industries.

Hope that helps.

> {Original Message removed}
----- Original Message -----
From: "William Killian" <william.killianvgt.net>

{Quote hidden}

I'm going to do a search on it, thanks. But I'm afraid my application is a
little bit different. In the RV application, they are probabbly using the
two batteries in parallel and powering the acessories at 12V. While this is
one alternative in my case, what I'm trying to determine if feasible or not
is to connect the two batteries in series, so I can use 24V equipment, but
charging each battery individually with a 12V alternator...

Cheers

From: "David VanHorn" <dvanhornmicrobrix.com>

> If you have a 12V alternator, you really need to use 12V battery.
> The alternator isn't guaranteed to give you a stable output without the
> battery present for one thing, and there's no easy way to get from 12V to
> 24V at that current level.

Well, they are 12V batteries, but I want to connect them in series. Can I
use the output of one of the batteries (12V) to power up the alternator
field?

Cheers

There were recent discussions involving car alternators & at one stage
I came across a site where a guy was using a car alternator to
generate 120V DC.
The only change he claimed IIRC was to modify (drastically) the control circuit.
If your alternator has an accessible controller, it may be feasible to
modify it to produce the 24+ volts required. Insultion requirements
would not be different for the 2 voltages as they are both so low so
it could be pretty straightforward.

If you can locate the part of the circuit that senses the output
voltage, it almost certainly invoves a voltage divider or zener.
Changing components at this point may be all that is required.

At your own risk of course!

RP

{Quote hidden}

> -
I am assuming you are talking about a normal engine/electrical system.
Where you have a battery supplying power to a gas engine that drives an
alternator to charge the battery, and drives you and the vehicle.

Now you want to add a second battery for 24 volt accessories, and keep it
charger via the gas engine driven alternator.

I have a thought but have no idea what will happen to the alternator.  So
please, as I have posted dumb things before, and will in the future, "Don't
try this with my car!"

Just hook the second battery in series with the battery already installed.
When the second battery needs a charge, either by physically swapping
batteries, moving cables or using a large Frankenstein) knife I've created
life" switch, change which one is at ground (frame) potential.

like it might something to think about.

WARNING  When working with batteries, wear eye protection as they do
explode.

{Original Message removed}
> ... 90A @12-14V. The motors I will use the automate the ATV systems
> (steering, brakes, etc) work at 12 or 24V. My dillema is: how to
> connect the batteries, series or parallel?

The solution to meet your want is technically easy but mechanically
annoying.
That's because your want and need aren't fully matched :-)

To meet want:

A series of switches (relays or a rotary cam driven switch)
sequentially switch the batteries.

1.    A+/B+    A-/B-/ground

2.    A+/24+    A-/B+
A sits on B and provides 24V.
B provides 12V.

3.    Back to 1. then
B+/24+        B-/A+
B sits on A and provides 24V.
A provides 12 V.

State 2 & 3 can drive motors.
State A allows 12V battery charging.
Can't have both.

Better is to have 24V alternator and operate usually in mode 2 or 3.
Swap between via 1 now and then to load balance.
12V drain from batteries should be monitored and matched between them.

This is a technically poor solution

_______________

To meet need:

Better.

24V battery system
24V alternator.
24V to 12V down converter (buck etc) charges a smaller 12V battery to
run 12V gear.
No changing, balancing etc. Just works.

RM

On Thu, 16 Mar 2006 15:09:17 -0800, Padu wrote:

>...
> what I'm trying to determine if feasible or not
> is to connect the two batteries in series, so I can use 24V equipment, but
> charging each battery individually with a 12V alternator...

It's not clear if you want all this happening at once (driving along with the alternator charging the
batteries, which are also powering the steering-motors) but if you do mean this, the answer is: No!  To charge
both the batteries at 12V they have to be in parallel, and so they can't also be in series to provide 24V.  By
careful isolation you might be able to charge one battery at a time and swap between them, but this is a

You could use a solid-state up-converter to provide 24v from 12, but this is inefficient.

Really, you need ot decide if you're running a 12 or 24V system and use the right equipment for that.

Cheers,

Howard Winter
St.Albans, England

From: "Howard Winter" <HDRWH2Org.demon.co.uk>
> It's not clear if you want all this happening at once (driving along with
> the alternator charging the
> batteries, which are also powering the steering-motors) but if you do mean
> this, the answer is: No!  To charge
> both the batteries at 12V they have to be in parallel, and so they can't
> also be in series to provide 24V.  By
> careful isolation you might be able to charge one battery at a time and
> swap between them, but this is a
>
> You could use a solid-state up-converter to provide 24v from 12, but this
> is inefficient.
>
> Really, you need ot decide if you're running a 12 or 24V system and use
> the right equipment for that.
>
> Cheers,
>

You guys convinced me. I'll run everything out of 12V and keep it simple.

Thanks

You could alternate the alternator :-)

With relay or FET switching periodically move the alternator connections
back and forth between the two batteries.

Bob Ammerman
RAm Systems

Russell McMahon wrote:

>> ... 90A @12-14V. The motors I will use the automate the ATV systems
>> (steering, brakes, etc) work at 12 or 24V. My dillema is: how to
>> connect the batteries, series or parallel?

> 24V battery system
> 24V alternator.
> 24V to 12V down converter (buck etc) charges a smaller 12V battery to
> run 12V gear.
> No changing, balancing etc. Just works.

Probably not so good if your starter motor is a 12V motor. Another option
would be to add a 24V system (alternator, battery) for your stuff. /That/
would "just work", at least electrically. And might not be too difficult,
mechanically.

You didn't say what size motors you're talking about. The difference is
mainly about copper (thicker wires), probably, and of course beefier FETs
etc. Does it make that much of a difference? One thing to consider is also
that there are plenty of 12V automotive parts around, but not so many for a
24V system. Even if the 12V parts can withstand 24V, they do so because
that's easily possible to happen on a 12V system. I assume that in a 24V
system, the voltages are pretty much doubled, including the possible
spikes.

Gerhard

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