how to switch between 2 batteries?

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

On Thu, 13 Jan 2005 13:25:34 +0100, Frode Undseth wroth:

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

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?

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.

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:

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

James Meyer wrote:

hi,

Thanx! looks interesting:)

Kind regards, Frode Undseth

Boris Mohar wrote:

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:

Kind regards, Frode Undseth

Andrew Holme wrote:

What is the voltage of the batteries?

...Jim Thompson

On Thu, 13 Jan 2005 14:39:50 +0100, Frode Undseth wroth:

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

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

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

Roger

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.

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