One for the sparks :)

[Robotstar5]

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

Doesn't do the parallel bit, but a simple switch will give you 12/24V

 

[8ob]

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.

Thanks for the info, two more questions though. Would a diode fitted on each battery live stop one robbing charge from another and exactly what knife switch do I need ?

Bob

 

[Screwdriver]

TBH now that you've said what it is, clearly it doesn't need such a robust solution. I'd still avoid connecting lead acid batteries in parallel though. You could leave it permanently wired as 24v, then switch to take 12v off either battery. You could even use batt A as positive earth and batt B as negative. Just in case some bright spark has fitted an LED bulb...

You don't need a knife switch for testing trailer boards.

 

[8ob]

Quite a lot of my 4x4`s run two batteries in parallel , why would this be any different ? Do I need to change the battery type ?

Bob

 

[Jlg]

Have a look at this it can do what you want https://m.ebay.co.uk/itm/4-Position...Operation-UK/202050958784?hash=item2f0b2cf5c0

 

[Screwdriver]

It's not the parallel I'm so worried about as the swapping over and charging in situ etc. when you would need to fit a diode. Easy to make a mistake when changing out the batteries and if the circuit can be switched from series to parallel, get that wrong and someone might end up exploding a battery.

When might anyone connect up two car batteries incorrectly? Typically when they are trying to run them in series (I note people often assume red to red, black to black) so if someone else borrows a battery then puts it back wrong, they could end up easily connecting two batteries backwards.

 

[bigfellayork]

May not be of much benefit but hey ho.
Sealey model 125v4 boost pack is switchable from 12/24 volts. They made have a parts or wiring diagram available.
This unit uses two 12v batteries.

 

[knighty]

how are you going to charge them ?

why not just leave them in series and charge at 24v ?

then your switch just needs to pick where it draws power from, 12v or 24v

a charge should last for an age anyway ?


EDIT: oops others already posted the same point above, I missed the posts

 

[rtbcomp]

And don't forget the fuses.

 

[rtbcomp]

If you just want it for testing trailer boards:



Small intelligent chargers can be had for peanuts off Ebay, you can have people testing 12v and 24v boards at the same time and each battery will be charged correctly irrespective of which gets used the most.

 

[8ob]

May not be of much benefit but hey ho.
Sealey model 125v4 boost pack is switchable from 12/24 volts. They made have a parts or wiring diagram available.
This unit uses two 12v batteries.

I have the ring3000 12/24 booster box, that has a rotary switch on the front for voltage changeover. @rtbcomp that only charges 12v so I assume the one charger is wired to the switch that has the two batteries in parallel. It needs the switch at the 12v position to charge, red lights on when charging and the same light changes green when done, nice touch to incorporate if poss. Fuses are going to be the re settable ones this time

Bob

 

[rtbcomp]

In my diagram the batterires are in series and there is a charger connected to each battery, so each battery is charged independently according to its needs, both chargers are switched on at the same time.

 

[armalites]

You do know that a 12v source will light a 24v bulb?

It'll just be a bit dim.

But for testing purposes...

I thought read somewhere that 12v bulbs were tested to 24v as well so it would work the other way round too. All of the LED trailer lights I have seen have been multi voltage too.

 

[8ob]

You do know that a 12v source will light a 24v bulb?

It'll just be a bit dim.

But for testing purposes...

True but on the truck trailers there are a lot of 24v led markers etc and they wont fire up on 12v, I am happy enough with the 12/24 set up but am going to change the flick switch to a rotary. I dont think 12v leds will like 24v shoved into them if the flick switch gets accidentally knocked, the rotary is going to be safer. I will follow @skotl diagram, just need to work out how many poles I actually need

Bob

 

[8ob]

I thought read somewhere that 12v bulbs were tested to 24v as well so it would work the other way round too. All of the LED trailer lights I have seen have been multi voltage too.

Some are dual voltage but there are a lot that are not, the two lamp units going on the test rig are dual voltage which will simplify things but I know there are a lot of single voltage units on my trailers.

Bob

 

[Ruffian]

Doesn't do the parallel bit, but a simple switch will give you 12/24V

View attachment 131116

If you did it this way you could use 2 fia kill switches and only one key, this way couldn't have them both working at the same time and know it will be disconnected before the other is connected

 

[daleyd]

If it’s only for testing lights etc could you not get hold of a couple of cheap 12/24 volt power supplies, be safer in the event of a fault and without the faff of charging etc??

 

[armalites]

If it’s only for testing lights etc could you not get hold of a couple of cheap 12/24 volt power supplies, be safer in the event of a fault and without the faff of charging etc??

I guess issue there is it wouldn't be any good for test trailers buried in the back of the yard though

 

[daleyd]

I guess issue there is it wouldn't be any good for test trailers buried in the back of the yard though

He said it was for use in the workshop

If portability is needed then it’s not the answer but if it’s going to be attached to a wall somewhere then it’d be fine.

 

[armalites]

He said it was for use in the workshop

You're right he did mention workshop