how to switch between 2 batteries?

[Frode Undseth]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

 

[Boris Mohar]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

Go to www.linear.com and search for "dual battery"




Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

 

[James Meyer]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.

Kind regards,
Frode Undseth

Why can they not be paralleled? Is there a technical problem or is it
just a company edict. How will the batteries be charged?

Jim

 

[Andrew Holme]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not
sound like an elegant soulution to me

What are the voltages and currents involved?
How much voltage drop between battery & circuit can you tolerate?
Is there a voltage regulator?
Why can't you combine them with a pair of diodes?
How / when are they charged?
When / why will you switch over to the other battery?

 

[Fred Bloggs]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

What voltages and current levels are involved? There are such things as
analog switches, monolithic and discreet.

 

[Frode Undseth]

hi,

The producer of the batteries told us they cannot be paralleled. I guess
it has something to do with the smart battery system..

Battery: http://www.moltechpower.co.uk/smart_ni2020.htm

The batteries will be mounted into a unit where they have to be charged
with a special battery charger one at the time, or at once using
two chargers.

Our application is for portable industrial use and is using 12V 15Ah
capasity.

Kind regards,
Frode Undseth

 

[Frode Undseth]

hi,

Thanx! looks interesting

Kind regards,
Frode Undseth

 

[Frode Undseth]

hi,

Our application is for portable industrial use and is using 12V 15Ah
capasity. The current we draw from it will vary between 1-1.5A. Since
the unit is to be used for long periods using battery power I need to
first empty one battery, then the other without disturbing the rest of
the equipment.

A voltage drop between battery and circuit is not desireable because
this will waste some of the capasity...

We are using DC-DC converters from Traco to generate 5V and 3.3V.

Charging will take place at any time using two special chargers provided
by the battery producer.

Battery: http://www.moltechpower.co.uk/smart_ni2020.htm

Kind regards,
Frode Undseth

 

[mike]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

The application department of your battery vendor should be able to help
you with this...especially if you're gonna buy a bunch of them.

How long will your system run with NEITHER battery connected?

This is critical to your decision. You can't have both hooked up during
the crossover without some lossy elements in between.
mike

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[Rich Grise]

hi,

Our application is for portable industrial use and is using 12V 15Ah
capasity. The current we draw from it will vary between 1-1.5A. Since
the unit is to be used for long periods using battery power I need to
first empty one battery, then the other without disturbing the rest of
the equipment.

A voltage drop between battery and circuit is not desireable because
this will waste some of the capasity...

We are using DC-DC converters from Traco to generate 5V and 3.3V.

Charging will take place at any time using two special chargers provided
by the battery producer.

Battery: http://www.moltechpower.co.uk/smart_ni2020.htm

Have you looked up "the Smart Battery Data System specification (SBDS)"?
Have you looked up the "Smart Charger Systems (SCS) Level 2" spec?
When they say, "Can be modified to suit customer and application
requirements", have you talked to the vendor about your situation?

I'd think they'd have a lot more information than you can get from a bunch
of people who have never seen one of those things before.

Good Luck!
Rich

 

[Roger Lascelles]

The application department of your battery vendor should be able to help
you with this...especially if you're gonna buy a bunch of them.

How long will your system run with NEITHER battery connected?

This is critical to your decision. You can't have both hooked up during
the crossover without some lossy elements in between.
mike

How about a two input buck regulator - with two input switching MOSFETs- one
going to each battery and you change over the switching drive from one
MOSFET to the other. This can avoid the glitch at switchover. You could
have a pair of these two input buck regulators - one for 5V and once for
3.3V.

Roger

 

[Jim Thompson]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

What is the voltage of the batteries?

...Jim Thompson

 

[James Meyer]

We are using DC-DC converters from Traco to generate 5V and 3.3V.

I would add more DC-DC converters to the system so that each battery has
both 5 and 3 volt converters connected to it. Then I would parallel each
converter's output with its twin. That way, both batteries will supply power to
the system but at half the current of one alone. The "capasity" will still be
double. You will not need to do any switching. Plus, you can remove and
replace either battery at any time without stopping the system.

Jim

 

[Fred Bloggs]

I would add more DC-DC converters to the system so that each battery
has both 5 and 3 volt converters connected to it. Then I would
parallel each converter's output with its twin. That way, both
batteries will supply power to the system but at half the current of
one alone. The "capasity" will still be double. You will not need
to do any switching. Plus, you can remove and replace either battery
at any time without stopping the system.

Actually, at large currents, by using both batteries to supply the load
simultaneously, there will be an apparent 10-20% increase of capacity
over two batteries used sequentially due to the non-linear relationship
between capacity and discharge rate- batteries generally supply more
charge at lower discharge currents. It is not necessary to use two
converters, although that is close, the dual battery discharge would
look something like this- a few components like protection diode, boost
converter for gate drive, clock source, etc not shown, A and B control
waveform shown for load sharing mode, N-channel MOSFETS used for best
efficiency:

View in a fixed-width font such as Courier.

 

[Fred Bloggs]

How about a two input buck regulator - with two input switching MOSFETs- one
going to each battery and you change over the switching drive from one
MOSFET to the other. This can avoid the glitch at switchover. You could
have a pair of these two input buck regulators - one for 5V and once for
3.3V.

Roger

This incurs increased losses at state-art-of-the-art switching
frequencies, compared to other approaches.

 

[Jim Thompson]

hi all,

I have a design where I use two batteries as power source. Each
battery is a smart battery with SMBus, and they cannot be parallelled.
So I need to be able two switch between the two without disturbing or
interrupting the rest of my design. (uController and other..)

Does anyone have some design ideas or know about how this is usually
done? Any ic's etc. that is designed for this kind of switching..? I
suppose a relay is the 'dirty' way of doing it, but that does not sound
like an elegant soulution to me

Any help appreciated.

Kind regards,
Frode Undseth

Two P-channel power MOS devices, connected source-to-source, gates
tied together, resistor from gate to common sources, InOut = Drain1,
OutIn = Drain2, makes a bi-directional blocking switch. Just pull the
gate low to turn on.

...Jim Thompson