24v from two 12v batteries

Bump · Jan 19, 2014

Leading on from a discussion with a chap on FB... can anyone think of a way round the issue that if you have 2 x 12v batteries they share a common earth therefore not allowing you to get 24v AND 12v from the batteries by running them in series?

I kind of feel there must be a way round it (which doesn't involve running a 24v setup and a 24-12v dc:dc converter, or a 12-24v dc:dc converter) but, can't think of one... - oh and that's 12 and 24 at the same time, I could think of a way to have one or the other...

the_wolf · Jan 19, 2014

Why not connect them up in series and have the connecting positive run through a relay which is operated by a switch. I might be wrong but shouldn't this then mean that when you operate the switch it will open it up to allow 24v

Deasy · Jan 19, 2014

the_wolf said:
Why not connect them up in series and have the connecting positive run through a relay which is operated by a switch. I might be wrong but shouldn't this then mean that when you operate the switch it will open it up to allow 24v

Yes but will change from a full 12v system to a full 24v system which will blow everything that's 12v

Deasy · Jan 19, 2014

It's possible to do with 3 batts. But not 2

SteNova · Jan 19, 2014

Not if all the 12v stuff is wired to one battery

Bump · Jan 19, 2014

no, because both earth's are connected together in any twin battery vehicle system... so, if you connect battery 1 neg to battery 2 pos then measure across battery 1 pos and battery 2 neg you still only get 12v as the battery 2 neg is connected directly to the battery 1 neg... so it's just the same as measuring across one battery...

Bump · Jan 19, 2014

SteNova said:
Not if all the 12v stuff is wired to one battery

you wouldn't have any way of charging battery two without it sharing an earth though...

Bump · Jan 19, 2014

Deasy said:
It's possible to do with 3 batts. But not 2

what and have them all charging off the alternator / split charger too? wouldn't they all still share a common earth?

top-cat · Jan 19, 2014

i used to do it with mine mike when i had the 24v winch motor

oxides · Jan 19, 2014

Separate 24v battery and alternator?

nrgserv · Jan 19, 2014

Bump said:
no, because both earth's are connected together in any twin battery vehicle system... so, if you connect battery 1 neg to battery 2 pos then measure across battery 1 pos and battery 2 neg you still only get 12v as the battery 2 neg is connected directly to the battery 1 neg... so it's just the same as measuring across one battery...

that'll give yer 24v.

that's just shorted battery 2 out

what you tryin to do

presume its vehicle related, and everything needs to share a common earth?

the_wolf · Jan 19, 2014

Deasy said:
Yes but will change from a full 12v system to a full 24v system which will blow everything that's 12v

Not if the 12v system was isolated when the relay was activated

top-cat · Jan 19, 2014

the first battery was wired as per normal and the second battery was wired through 2x barrel solenoids, so in non energised state the second battery would be + to 1st battery + and neg To second battery Neg.when energised the second battery neg would be switched from first battery neg to pos and the pos on second battery removed thus giving 24v between second battery + and chassis. When de energised the two batteries are efectively jumped so both charge off the standard landy set up. I also fitted battery isolators to each battery so i could use alternate batteries when camping ans use the other for starting in the morning.
Im actually thinking of reinstalling it on the new 90 to give 2 speed to the winch, 12 and 24v as good winch do with their set up on bow motors

pressbrake · Jan 19, 2014

Use a bosch parallel start relay allows 12v charging and 24v starting or winch, completely interlocked and safe from high amperage shorting.
Ive been building challenge motors for a long time and its a proven method

Bump · Jan 20, 2014

pressbrake said:
Use a bosch parallel start relay allows 12v charging and 24v starting or winch, completely interlocked and safe from high amperage shorting.
Ive been building challenge motors for a long time and its a proven method

Edit: Found them now.

They do look like they'd do the job! Thanks,

SteNova · Jan 20, 2014

Bump said:
They do look like they'd do the job! Thanks,

what is the job your trying to do?

timtheenchanteruk · Jan 20, 2014

would this not work??

if not, can some please explain why not, ive used it on LED lighting systems, where dimming is done on the -ve leg, and used differing supply voltages (in my case was 2 different PSUs.)

Thor1950 · Jan 20, 2014

Bump said:
Leading on from a discussion with a chap on FB... can anyone think of a way round the issue that if you have 2 x 12v batteries they share a common earth therefore not allowing you to get 24v AND 12v from the batteries by running them in series?

I kind of feel there must be a way round it (which doesn't involve running a 24v setup and a 24-12v dc:dc converter, or a 12-24v dc:dc converter) but, can't think of one... - oh and that's 12 and 24 at the same time, I could think of a way to have one or the other...

Look at ford battery system on there Diesel Pickups might help. I got 2- 12v battery's hooked the negative ground. but the stater is 24 volts. All the other electrics in it are 12v (2005 6.2 liter)

Bump · Jan 20, 2014

SteNova said:
what is the job your trying to do?

It's hypothetical, guy on FB wanted to run a 24v winch on a 12v landy, was trying to help him out but concluded buying the very cheap 24v winch he has on offer, selling it on ebay then buying a 12v winch was a much better idea for him!

timtheenchanteruk said:
would this not work??

if not, can some please explain why not, ive used it on LED lighting systems, where dimming is done on the -ve leg, and used differing supply voltages (in my case was 2 different PSUs.)

If you had a 24v alternator for the 24v charger circuit, yes, however, that would require an unnecessary upgrade to the alternator.

Thor1950 said:
Look at ford battery system on there Diesel Pickups might help. I got 2- 12v battery's hooked the negative ground. but the stater is 24 volts. All the other electrics in it are 12v (2005 6.2 liter)

Thanks Thor, i knew in theory it would be possible, just, couldn't work it out!

ebbadger · Jan 20, 2014

I 200 tdi'd my 24 volt lightweight, fitted the 24volt generator to charge but ran the 12 volt electrics for the glow plugs n starter by just wiring to the + and - on the first battery... worked perfectly no relays etc needed... could do the same to run a 24 volt winch

cheers steve

24v from two 12v batteries

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