Battery bank A to battery bank B - auto switching

[David HK]

First of all greetings from a wonderfully warm Hong Kong where we have had clear blue skies and temperatures up to 24 C each day for the past ten days. More good news is that the days are getting longer and summer is on the way.

The nature of my enquiry is to ascertain if anyone has developed a way of automatic switching from battery bank A, to battery bank B to battery bank C using some form of auto switching.

Ghurd's LVD circuit seems a logical start so that when 11.7 volts is detected the required switching takes place.

Any ideas?

Having switch through the battery banks how does one arrange it to go in the opposite direction?

Programmable logic?

I have two battery banks - one is SLA and the other is wet lead acid.

Regards,

David

[ghurd]

Are the battery negatives connected together?

My first thought is the LVD circuit could output to a power P-fet instead of a power N-fet.
The P-fet operates a DT relay coil, which energizes to switch the load to the other battery.
Not elegant.  2:30AM.  I think it would work.
G-

[David HK]

Thanks Glen,

Yes, there is every reason to connect the battery negatives - if you can dream up a circuit i will give it a try.

I am getting ready foir the summer season hence the question.

Regards,

Dave

[TomW]

Just a couple questions I had while reading this. No help to offer just curious:

What kind of voltage & amperage is your target switching rating?

I assume you need to break before make on the connection to avoid massive current surges from one bank to the other and also avoid false sensor readings?

Are you switching sources and loads together or just sources or just loads?

Sure seems that G's circuit will do the voltage sensing?

LVD and Dump modes would fit the voltage sensing but some logic would be needed to determine when to switch "back".  I seem to recall seeing a solenoid operated DC transfer switch on Feebay awhile back so that may be the off the shelf  "switch" you need?

Anyway, interesting project

Tom

[madlabs]

The control part seems easy but finding a relay that can handle the amps that doesn't cost an arm and a leg seems harder. Anyone got a good source?

Jonathan

[ghurd]

Tom and Jonathan have valid points about the load.

This circuit would be workable for DC lighting, gecko traps, car stereo, etc.

This is Not suitable for power inverters or large loads.

The logic of when to switch back to the primary battery is built into the LVD.
Heavy loads could cause the relay to chatter.

I am sure there are 100 ways to do it better, but this is what I came up with.
G-

[zap]

A ups does this type of switching between grid and battery in fractions of a second.  Could a ups (or two) or it's electronic guts be configured to accomplish the task?

[joestue]

a ups switches the ac side with a relay..

you're looking to switch the dc side with a relay.. not the same.

if a 10-30 millisecond delay is appropriate/usable, just use relays.
48 volts won't kill them, and the arc will extinguish as soon as it 'makes' contact with the other terminal.

if you want to use fets to be able to switch the batteries in 10's of nano seconds for zero current interruption, an inductive turn on snubber is what you will have to build, and a 9 volt battery to power a flipflop 24/7...

[SparWeb]

A barrier diode would allow you to use source A, B, C or all together depending on their relative availability.
Is that a resonable option for you?
You can get 100A and higher barrier diodes (obviously costly) and at high current levels they have to be cooled.
Not a perfect solution, but it does offer 100% availability.  Instead of making switching necessary, it can be "optional", allowing you to shut off one bank without having to do something to the other bank first.

[David HK]

Thanks to all for the contributions.

My battery banks do not supply heavy loads to inverters and the like on a continuous basis. At present the load is very small and supplies 12V DC to temperature gauges that monitor my solar hot water system and display outside my kitchen window with separate repeaters in my car port.

Between now (February) and the end of June I use a 12 Volt oscillating fan to move the air in my bedroom whilst an open window is protected by a mosquito net. In July and August its 220 Volts air conditioner cooling when night and day temperatures are in the 33 C range. I shall probably add another 12 V fan (total 2) this year to make the batteries work harder.

The reason I now have two battery banks is that people know of my interest in renewable energy and occasionally give me batteries  which are taken out of service at their work locations. Recently they have been SLA 12 V @ 17 AH.

I am sure many people have thought about this option and no doubt this article will be of interest to new people in the forum.

Apart from the automatic option I have already designed in my mind a simple mechanical YES break device where a copper bar is unbolted from Bank A and re-fastened to Bank B while no loads are present.

Regards,

David in Hong Kong

[Madscientist267]

If you went outside the box far enough, this could go solid state...

Some MOSFETs driven in parallel in common source configuration instead of the relay contacts to switch the banks on and off?

IRFZ44N (or better yet, IRFZ44V) comes to mind; The 'V's are good for upwards of 70A continuous each (not that I would personally push a single tranny that hard).

If you drive the gates hard enough, they should dissipate minimal power and so heat shouldn't be a problem. I'd think a set of four in parallel would handle almost anything that I could personally throw at them, with ease... 100A or so @ 24V... ?

Sure, you'd be switching the negative side, but it should work.

Or, if you're hell-bent on keeping your grounds firmly attached to one another, dump some buckage into it and change over to P-Channel to switch the positive side instead...

With the addition of a bit of 'hardening' at the 'output' with some caps, harsh loads such as inverters shouldn't be too much of a problem either, I would think.

Just a thought...

Steve