I have a 24v 1983 Mq Patrol with an sd33 3.3L diesel engine. It runs 24v off 2X 12v batteries running parallel. I want to fit an Mp3 player and a small amp to the audio system, it already has a 24 to 12v converter but I want to run the constants for the amp/player so I cannot tap the converter as if it runs 24/7 the batteries will be flat in no time. I also have been told if I tap into the battery that reads 12v it will give an uneven charge from the alternator and root one battery faster than the other.

I have read to run 12v spotties on the front I can run the spot lights in parallel ( positive of one light connect, negative of the other connected and the left over connections bridged), can I run a 24v line off the constant power supply and run the positive of the amp through this and the negative of the player to the chassis ( part of the 24v system) and then bridge the neg/pos of the player/amp together so they both get 12v and are actually read by the car as one 24v?
The accessory feeds will run through the converter as I am going to run this through the ignition switch and a relay so it only powers the converter when the acc is on.

Part of the game with 24V systems.
If you want to run 12V gear all you can do is hook it up to one battery only.
Our work trucks are hooked up like that and they dont seem to kill one battery quicker then the other.
If you are concerned swap the batteries back and forward every 3 months or so to even the load on them out.

Do some reading on series/parallel setups - you're getting them mixed up.
e.g. the battery setup would have to be in series.

It's imperative you get this right, or you'll do plenty damage.

s**t. I didnt even twig the way it was written when I quickly read through that this morning..........................and Im an electrician......

Do not hook your amp and cd player in series... This sort of works with spot light because it doesn't upset a filament if the voltage is a little under or a little over... Also both lights are the same resistance and hence you get even voltage drop across each lamp (roughly 12V each)...

If your amp has half the internal resistance of the cd player (possible because an amp draws more current than a cd player) than you will have two thirds the voltages on the amp and one third on the cd player... So 16V on the amp and 8 on the cd player... If one has more than twice the internal resistance of the other the difference in voltages will vary... This is why you cannot hook two different things of different impedances together in series... Two identical cd players and two identical amps in series, yeah maybe... Though I still wouldn't recommend it...

As mentioned you can pick up off just the battery of lower potential (the one that has it's earth hooked to the chassis... For all intensive purposes this will work... The issue I have seen first hand at work is we have a portable battery bank which is 120V (10 car batteries in series)... Some gear requires the 120VDC suuply... we also have one site which requires 36VDC (three batteries) so we used to tap off at 3 batteries...

The system had 5-7Amps draw on it... And after one day it would have flattened the bottom three batteries... But the charger was floating and as such it was overcharging the top 7 batteries to compensate for the bottom 3 being flat... In turn it destroyed the whole bank... The top 7 swelled up and the bottom 3 collapsed...

This is why I wouldn't recommend a high current load picking up off one battery... Small current load it can account for... Lots of amps upset the battery charging procedure... One is going flat quicker and the other has to absorb the charge the bottom one can't accept... This is all due to batteries having internal resistance which no one accounts for...

Our fix was to buy big kw DC to DC converters which you mentioned you have... We have two, one for 110VDC to 36VDC and one 110VDC to 48VDC converter... These work fine and flatten the whole bank evenly and keep the charge working evenly...

Its the only way you'll do it... I recommend a big contactor with a 24VDC coil... When you turn the key to accy have it pull the contactor in and supply power through the contacts to the DC to DC converter and then out of that into your 12 volt stuff... Then when you turn the key off the contactor opens up and the DC to DC converter turns off and prevents the batteries from going flat... Big DC to DC converts are not cheap (overs were over $1000 each) but it's the right way of doing it IMO...

Otherwise the cheap option is picking up off one battery and running the risk too bigger current draw will shag both batteries...

Cheers,
Tim

Sorry guys, Series/Parallel.... I knew what I meant but worded it incorrectly. It was a stupid mistake and I should have known better as I work with series/ parallel all the time with the speaker enclosures I make. Sorry
I now understand what you guys meant about series running both the amp and player, makes sense.
Do you guys think tapping into one battery for the players memory ( constant) would be fine then? I cant see the memory ( constant) of the player would use much power at all. Please correct me if I am wrong.
I think running the amp off one of the batteries would definitely kill it. I might see if I can run the amps constant off the inverter. Correct me if I am wrong but its the constant of the amp that supplies the power and the remote feed from the player just tells it when to open the circuit? If this is the case could I not run the amps constant off the inverter? This will mean the constant of the amp will only have power when the ignitions on and will draw from the 24v setup.

Sorry didnt get back earlier, was doing an oil change and fitting a console. Turned out I got sent the wrong oil and fuel filter off ebay, so been trying to get the right ones back as the Mq now has zero oil in it,

Your a sparky Jewman?

Yep.

9 times out of 10 the trigger is just that... The const on the cd player and amp are the main load bearing supply... The trigger wires just trigger them on... I wouldn't think just a cd player running off the 1 12 volt battery would cause much upset... Then the trigger out turns on the amp which runs on your dc to dc converter...

Can only try it anyway... It should work fine...

Cheers...
Tim

The car had in stock form a 12v radio. The only things that were run in 12v were the stereo, interior light and the cigarette lighter, but I think the stereo wire is an accessory feed i.e triggered off the ignition. I thought the constant power supply for a cd player was only to power a memory feed so settings dont get lost when the ignition is off.

The constant power supply is the main supply to run the internal amp in the headunit.
It also does the memory function.

Nope... Some cd players come with a big earth and big yellow wire which is the const, while the red wire is small (trigger)... You'd find out for sure if you put an ammeter in circuit... Also some manufacturers put the fuse in wiring that comes with it... Usually in the yellow wire... Based on those observations the yellow wire powers it and keeps it alive... The red just turns it on...

Cheers,
Tim

s**t hey. Thanks guys. Learn something new every day.
This 24v is proving to be a real pain in the a**.
Might have to be the case of running the constant off the 12v battery for the cd player and the rest off the converter and swap the batteries over every 3 months or so.
I did have to replace both batteries when I got the car because the bloke had left it sitting for 12 months and they both lost charge and would not keep it when I chucked them on the charger. I ended up getting these
http://www.ebay.com.au/itm/321053069751 ... 1439.l2649
The minimum cca for the car I think was 660 and I think these batteries exceed the requirements of the car so hopefully they will be ok as its not cheap to replace them both.

Yes... In short 24vdc systems are a PITA... Shame trucks need it to crank over... Causes nothing but issues... Esp with trailers and the like...

Cheers,
Tim

O.k, been thinking ( yes it hurts)....
If I run the constant for the mp3 player from the 12v battery output and also run a feed to the same constant line on the player from the dc converter so when the car is off the only power the player is drawing from the one battery is to run the memory and then when the player turns on its getting the extra power/ support from the dc converter so its not drawing all its power from the 1 battery?
Im smart with speakers but not too bright with electricity, so excuse me if this is a stupid idea and please set me straight.