One for the sparks :)

[8ob]

Some help needed from one of the sparks on board here . I have a small project under way that needs a changeover switch on a battery bank, I would ideally like to be able to draw 12v from two 12v batteries wired in parallel and at the flick of a switch get 24v from the two batteries linked in series. Have bought a 4pdt switch but not sure if its the correct switch or how to wire the thing up as there is no info supplied. https://www.ebay.co.uk/itm/AC-15A-2...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

Anyone?

Bob

 

[skotl]

Couple of potential problems for you, 8ob; that switch is listed with AC specs but you'll need to know what its DC rating is, based on the peak load your batteries might supply - it's easy to burn a switch out if feeding it more DC voltage than it can handle.

Next point is that you'll need a break-before-make switch so that there's no chance it can short the batteries together when switching from parallel to series - that switch doesn't state whether it's b-f-m.

 

[Screwdriver]

Don't do it. While it is perfectly possible to engineer a switching solution which avoids the many potential problems with that setup, I honestly don't think it's worth the effort. Hundreds of amps mean you don't have tiny problems if/when things go wrong. Stuff melts, spanners become fuses, things catch fire, batteries explode...

I recommend you build a plug board and use screw terminals with nice chunky wingnuts. Again, you want to have a good think about how it can be constructed to completely prevent any possibility of a loose terminal contacting the wrong thing.

 

[MattH]

A knife switch might be just the job.

 

[Robotstar5]

A knife switch might be just the job.

 

[Woody.v8]

 

[skotl]

If you really want to do it, then a DPDT should do it, but you need to be absolutely certain that it's make-before-break.

Matt's suggestion of a knife switch is a good one.

But I think there's a lot to be concerned about - if one battery is putting out slightly more voltage then the other then wiring them in parallel will mean it will try to charge its neighbour.

 

[8ob]

Its probably better if I explain what I am trying to achieve. I built the trailer leccy checking box below yonks ago so I can fix/check/faultfind trailer lights/trailerboards in the workshop without the need to have a car attached, I am really pleased with it but would like to build a neater MK2 that can do the 24v truck trailers as well.The new box will have a couple of small motorcycle batteries fitted under the panel

Bob

 

[Luke]

What's the application?

This is a common requirement for off road winches. Off the shelf solutions exist if that's what you're looking for?

Edit: you posted the above at the same time.

Bulbs don't draw a load of current, so what you want can be implemented with relays, but some care needs to be taken to avoid a melted mess....

 

[Robotstar5]

Its probably better if I explain what I am trying to achieve. I built the trailer leccy checking box below yonks ago so I can fix/check/faultfind trailer lights/trailerboards in the workshop without the need to have a car attached, I am really pleased with it but would like to build a neater MK2 that can do the 24v truck trailers as well.The new box will have a couple of small motorcycle batteries fitted under the panel

Bob

I keep meaning to build something like that, but as usual it's a "round tuit"

 

[8ob]

I keep meaning to build something like that, but as usual it's a "round tuit"

Its brilliant, I can sit in the warm and wade through the months smashed up trailer boards

 

[CompoSimmonite]

Get a 12v to 24v step up transformer. Cheap off eBay. I've got an adjustable one so I can charge my mobility scooter in the 12v car and therefore have it set at 28v output. Cost me less than £10.
https://www.ebay.co.uk/itm/DC-3A-12...696931&hash=item1a374ce9f8:g:P3QAAOSwyXhZ3Ilv

 

[Robotstar5]

Its brilliant, I can sit in the warm and wade through the months smashed up trailer boards

Your blokes must be good at their jobs for you to put up with the damage/losses to your equipment

 

[gordon stephenson]

Look into the Land Cruiser 4.2 diesel system 8ob, Mine had the two 12v batteries for general vehicle running but they connected in series to make 24v just for the period the key was turned for starting.

 

[brightspark]

I used to go through loads of trailer boards till I put the lights on my trailer behind a steel angle frame made up under it . now its bomb proof

 

[8ob]

Get a 12v to 24v step up transformer. Cheap off eBay. I've got an adjustable one so I can charge my mobility scooter in the 12v car and therefore have it set at 28v output. Cost me less than £10.

Either way I still need to change from 12v to 24 v via a switch, the flasher relays are different voltages as is the test lead probe which needs to be plugged into the relevant voltage. If it needs a knife switch thats what it will get ( whatever it is ) . I intend to change the end on the probe cable which is currently on crocodile clips and fit a plug to go into a panel socket, the probe is dual voltage so the plug just needs to switch.
https://www.ebay.co.uk/itm/Sealey-A...hash=item19fc2b97d3:m:mWomFxBBlOPJM_fbwuXQC6g

 

[8ob]

I used to go through loads of trailer boards till I put the lights on my trailer behind a steel angle frame made up under it . now its bomb proof

You dont mention idiot proof, bomb proof wont do

Bob

 

[Screwdriver]

You don't really need two 12v batteries in parallel though

Why make it more complex than it needs to be?

 

[brightspark]

You dont mention idiot proof, bomb proof wont do

Bob

take it out of there wages and they will soon learn

 

[8ob]

You don't really need two 12v batteries in parallel though

Why make it more complex than it needs to be?

I realise I can have 12v but by connecting to one of the batteries but I still need to switch between 12 and 24 and if I did this only one battery would get used of the two when working on 12. Just thought if its possible I could use the pair in 12 or 24 with the correct switching.

Bob