Getting high current 12v from 24v systems

[superquag]

Just a quick brain - teaser....

Talking to some Party Bus drivers, and found that the installers of LOUD noise sound systems, don't use 12v converters. Instead, they take the 12v feed from the mid-point of the 24v battery-stack.

Shrt story is... the bottom battery(s) have a short life, so much so that the bus engines need to be run for extra time in order to keep the batteries charged. ( Which the alternator can't do properly as the top/bottom batteries are at differant states of charge.) Even so, its not uncommon to replace the bottom battery(s) every 12 months. Or less....

We're talking full-size and 'bendy' busses with several hundred ear-destroying watts.

I thought a dedicated 12v battery with a 24/12 volt charger would be the way to go...

Any thoughts from the Experts ?

[Homestar]

Half tapping a 24 volt system, as you have mentioned is the best way to kill a battery - particulally if there is large current draw involved.

Why wouldn't you use a 24 to 12 volt converter? They produce a stable 13.6 volts on the output, and are readily available and you can get them in high current versions.

How much current are we talking about here - I used a couple of 40 amp units last week to run the electric legs on a semi trailer where we only had a 24 volt system on the genset that is on the trailer - we were half tapping the 24 volt system for about a year, and killed 2 - N200 batteries in the process. The converters are working fine - can't seem why they wouldn't work for a sound system - even a 'few hundred watt' unit as you have described would only draw 20 or 30 amps, and if it is a real thumper, you can buy much bigger units than 40 amp, and they aren't that expensive - less than a couple of replacement batteries.

Edit - these are the units I used the other week - 24- 12V DC-DC Converter 40A - Jaycar Electronics (http://www.jaycar.com.au/productView.asp?ID=MP3066&form=CAT2&SUBCATID=1002#12)

If your after something bigger - DC/DC Converters, to convert voltage up or down, retail, wholesale, off-the-shelf and custom, DC/DC 5 watts to 1500 watts, voltage from 1.5V to 72V, DC/DC power supplies (http://www.powerstream.com/dcdc.htm)

Still not big enough? You can parallel several identical converters if you set the output voltages on them correctly - most have voltage trim pots. Check out this - Paralleling DC-DC Converter Outputs (http://www.reliabilitypower.com/Web%20Site5/Power%20Products/PROD/APP%20Paralleling%20Converters.htm) write up on how to properly set up and load share multiple units.

If that still isn't big enough, you just about have a set up that Cold Chisel could use live to entertain 10,000 people:p, and there is no way the buses alternator would keep up with a system like this.

Cheers - Gav

[Blknight.aus]

depending on your definition of high current and what you want to do with it.

a DC-DC converter is the way to go for upto 40ish amps. which is more than enough for your average sound system.

over that you're better off with a dedicated 12 V battery and a 24-12v battery charger which is the way I setup for car fridges in trucks

[superquag]

Thanks Dave, I did'nt realize the bigger ones were so reasonably priced. (Have been out of hobby Electronics for years now, so I've lost touch)

The only reason I can see for the divided battery approach is cheapness on the install. Or laziness.
One hopes that 'Incompetance' does'nt make the short-list...:wasntme:

I've passed on all the info to one of the bus operators, who is well into the second set of batteries in 12 months...


cheers!

[Blknight.aus]

urmmm, why not just buy 24v sound gear...

really the best solution by far if you want stupid levels of sound power from 12v off the shelf systems is to mount up a second alternator and set that up as 12v

40A@13.8v is only 552W by the time you fudge that down to audio realistically you're only going to be getting 400w effective worth out of your speakers....

oh and sorry, its lazyness and incompetence all in one.

[drivesafe]

Hi superquag, I deal with a company that specialises in this sort of set up.

The equip large motor homes, enormous horse floats and luxury yachts and I was talking to the owner this afternoon and mentioned what you were trying to do.

He has already done a few of these Party Buses and he has seen them need as much as a few thousand watts at 12v derived from the vehicle’s 24v system.

His suggestion was to fit 2 x 200 amp 6v batteries and run a 24 to 12 BATTERY TO BATTERY CHARGER, not a 13.8v power supply.

Where possible make sure the batteries are fully charged before the bus starts it’s nightly run and then the battery to battery charger helps to keep power going into the two 6v batteries while they supply the power as and when it’s needed.

The Battery to Battery charge he has is 40 amps input from the 24v and the 12v out put will vary depending on the draw from the 12v battery ( 2 x 6v ) but it’s able to produce a lot more than the 24v 40 amp input.

[superquag]

G'day Drive,

I'm not trying to do anything, apart from staying FAR away from the Noise....:p
It was just general conversation between a couple of bored bus-drivers, triggered when he started his bus and ran it for half an hour or so. - Then the story came out as to 'how' it was all hooked up. Not only his, but others he knows of...

My own thoughts, - apart from being a cheap & cheerful short-term solution, is to work on the basis that a battery is a storage device, to warehouse lots of Amperes and dump them out when needed. Generally a bucketload every o.6 seconds... :eek:

So yes, running a bank of ginormous A/h 12v batteries fed by an equally giant 24 to 12v charger sounds (sorry, pun just slipped in...) like the easiest way to go.

The last pair of batteries set him back over $400, so he mumbled...

On the bright side (Lasers, actually:D) his clients are not over-fussed about 'quality', only 'quantity' of noise.
Will be interesting to see what he does with the info.

I'm quite happy with schools and general daytime charter work...:p:p:p

[Blknight.aus]

for the cost of 2 sets of batteries he could have set up twin alternators.

[superquag]

Received a phone call this morning from a REDARC Techie.

I'd queried as to whether their 24/12 chargers' auto sensing circuitry would get confused when charging AND having the sound system operate at the same time. (Charger would see a rapidly varying load in time with the Booms...)
- Answer was 'no'. All would be well. Plus, he endorsed the idea of a 12v battery being fed with one of their 40 AMP chargers.

His suggestion was a nice big 12v battery, and the BCDC 1240 charger. RRP of $695.. Looks like overkill, but you get what you pay for, and in this case that includes a MPPT for a solar array !
- Add a panel or two on the bus roof....and you'd never run out of 'noise'.:p

"...for the cost of 2 sets of batteries he could have set up twin alternators...." - They're businessmen, - not LR owners, so they don't think laterally like us.. .besides, that means farming the work out to ??? at $? - Easier to go 'off the shelf'.

So there we have it. :D

[superquag]

Battery World guy suggested using a 'Charger Balancer' across the 24v stack, with a third connection to the 12v point. The idea is that it will monitor both the 12v batteries and make sure that each gets a balanced diet of Ampere.

- Tapping in at the centre point to run 12v stuff does'nt affect your 24v batteries, just makes the Balancer earn its hefty price-tag.
Apparantly this is a common method for 24v vehicle systems interfacing with 12v loads, such as caravans etc. and most economical as it does'nt need an extra 12v battery.

- And he does himself out of a battery sale...;)

[Blknight.aus]

cept they dont work long term (on high current loads)... they just stop the short term effects like boiling off the charged battery withing the amps range of the charge balancer.

you could do the same thing with a string of lights that match the power demand of the stereo wired across the battery thats on the +'ve side of the bridging cable.

[dswatts]

Found them to be better in theory than practice.. Have a look at a company called AMELEC, they are a WA company and make some really good gear here. 24 to 12volt chargers and converters.

[superquag]

cept they dont work long term (on high current loads)... they just stop the short term effects like boiling off the charged battery withing the amps range of the charge balancer.

you could do the same thing with a string of lights that match the power demand of the stereo wired across the battery thats on the +'ve side of the bridging cable.

It did occur to me to use one amp across the 'top' battery and the other across the bottom one.. LEFT channel on 'top' and RIGHT channels on 'bottom' (balanced load on both batteries) - Which is how I'd do it IF it was my bus, and the budget was limited, AND I felt like bothering with floating earths and isolating the inputs, balanced lines etc etc blah blah....

Using a 2kW / 24v inverter/charger driving his 240vac amplifiers is just too... simple. Only $1500 or so at Jaycar, less for 'Trade'. - And keep the engine running !!!

The consensus seems to favour the 'Hi-Tech' approach for predictable and light loads, (Balancers or 24 to 12 volt supplies) and the belts and braces - biggg 12 v battery banks fed with huge on-board charger(s), or total 12 v battery/alternator approach. for the insanely hungry user....

By now I've totally confused the poor bloke !

[Blknight.aus]

It did occur to me to use one amp across the 'top' battery and the other across the bottom one.. LEFT channel on 'top' and RIGHT channels on 'bottom' (balanced load on both batteries) - Which is how I'd do it IF it was my bus, and the budget was limited, AND I felt like bothering with floating earths and isolating the inputs, balanced lines etc etc blah blah....

Using a 2kW / 24v inverter/charger driving his 240vac amplifiers is just too... simple. Only $1500 or so at Jaycar, less for 'Trade'. - And keep the engine running !!!

The consensus seems to favour the 'Hi-Tech' approach for predictable and light loads, (Balancers or 24 to 12 volt supplies) and the belts and braces - biggg 12 v battery banks fed with huge on-board charger(s), or total 12 v battery/alternator approach. for the insanely hungry user....

By now I've totally confused the poor bloke !

try your first approach and something will go foom.... usually an amp. (dont ask)

[superquag]

"...try your first approach and something will go foom.... usually an amp. (dont ask)


- Why not ? :twisted::twisted::twisted::twisted::twisted::oops 2:

I like ' foom ' ....

[Blknight.aus]

I made the mistake of helping someone else do something similar on a truck...

he'd run a pair of switched and fused power feeds in for me, I metered it and got 12v on the 2 wires, brilliant... hooked up the amps to the feeds hed provided aaaand set fire to one of the amps.

got out to check what was going on...

one amp hooked up and went nicely so I hooked up the other to the second power feed but what I thought was the -ve was actually +12v relative to the vehicle earth and what I thought was +12V was +24v.. hooked up the thing, flipped the switch and it worked for about 20 seconds then the smoke started (and Im talking the kind of smoke that puts fozzy on a cold morning to shame)

on the subject of fusing... a 200A fuse will conduct enough grunt to melt the guts out of an amp.. an unfused cable rated for 200a if hooked from the bridge cable of a 24v system will quite happily reduce the ally casing of an amp to slag if you happen to inadvertently drill the mounting screw through the MDF board into the steel frame.

[superquag]

:Rolling::Rolling::Rolling: