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24 volt system, Can I tap 12v off the middle of the battery pack?


Steve 90

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Will shortly be going 24v for the winch's on my truck but not keen to be adding an extra battery for the vehicle electrics. I was hoping to only run 24v alternators (no 12v at all) then hook my two original batteries in series for the 24v. I can then tap 12v for the vehicle electrics off just one of the batteries in the 24v pack. My question is will the alternator sense the batteries properly and charge them both correctly? As six of the cells will have more drain on them than the other six will it cause problems or will they just balance out? Ive spoken to a few people who have given different points of view but no real sure answer. So Can anyone who knows a thing or two about electrical principles cast some light on the subject? (electrical principles was always on a Thursday morning following student night so its all a blur to me :wacko: ) I want to keep the system simple and not add too much complication so it seems the perfect fix assuming it will work. Otherwise I'll have to run a 12v and 24v alternator and a 3rd battery which i'm not keen on.

Many thanks.

Steve.

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If you are drawing 12v from a 24v system , one battery will be draining at a greater rate than the other and this will mess up the charging from the 24v alt ,

Dont ask me the physics of it , we used to bunk off science and go scrumping

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If you are drawing 12v from a 24v system , one battery will be draining at a greater rate than the other and this will mess up the charging from the 24v alt ,

Dont ask me the physics of it , we used to bunk off science and go scrumping

Yep, Know it will drain the one quicker than the other but was wondering if it would balance out across the batteries somehow and the alt would cope or not? I wasn't convinced either way, you obviously think not! A local chap who does a lot with electrics/electronics says its not uncommon practice to tap different voltage feeds off at various points along packs of cells and its fine ass they draw the overall voltage down, The alt will charge and push it back up. Spoke to another chap who says (as you do) that the alt wont charge the lower cells properly! So I'm kinda looking for a definite answer with some reasoning/science to back it up.

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Best illustrated by example.

Say you have 2 12V batteries in series that are 100% charged and giving 24V

If you drain both batteries to 80% charge and then recharge them, they are both at 100% again. No problems there.

If you drain one battery to 80% using a 12 volt feed the other one is still at 100%. This means you can either put up with one battery lower than the other, or try and bring it up - but as the same amount of juice is flowing through both, you are trying to make one battery 100% and the other one 120%! This results in much boiling, hydrogen and general distress on the part of the overcharged battery.

It would work if you were taking out a small amount and recharging the batteries independently on 12V chargers at regular intervals, and/or only taking a small amount out of one of them but I don't think it would work well in the long term. From what I can gather on here the boys that run 24V challenge winches mostly have 2 batteries for the 24V system and a third one for the 12V system running off two separate alternators, as you described.

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Hmmm, sounds logical but in practice its not an issue.

As soon as the terminal voltage of one of the battery's falls below that of the other battery current will flow from the higher voltage to the lower voltage to effectively even out the difference. If you take on board this fundamental fact of electronic principles (that for a current to flow there must be a potential difference) then regardless of whether you are tapping into one of the batteries current will be drawn from both.

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Hmmm, sounds logical but in practice its not an issue.

As soon as the terminal voltage of one of the battery's falls below that of the other battery current will flow from the higher voltage to the lower voltage to effectively even out the difference. If you take on board this fundamental fact of electronic principles (that for a current to flow there must be a potential difference) then regardless of whether you are tapping into one of the batteries current will be drawn from both.

Thats the way the Gordon (first bloke I spoke about) put it. He said its common practice and the difference in level of batteries will just equalize!

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Hmmm, sounds logical but in practice its not an issue.

As soon as the terminal voltage of one of the battery's falls below that of the other battery current will flow from the higher voltage to the lower voltage to effectively even out the difference. If you take on board this fundamental fact of electronic principles (that for a current to flow there must be a potential difference) then regardless of whether you are tapping into one of the batteries current will be drawn from both.

doesnt that only really apply when batteries are connected in paralel?? which would not be the case here

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Best illustrated by example.

Say you have 2 12V batteries in series that are 100% charged and giving 24V

If you drain both batteries to 80% charge and then recharge them, they are both at 100% again. No problems there.

If you drain one battery to 80% using a 12 volt feed the other one is still at 100%. This means you can either put up with one battery lower than the other, or try and bring it up - but as the same amount of juice is flowing through both, you are trying to make one battery 100% and the other one 120%! This results in much boiling, hydrogen and general distress on the part of the overcharged battery.

It would work if you were taking out a small amount and recharging the batteries independently on 12V chargers at regular intervals, and/or only taking a small amount out of one of them but I don't think it would work well in the long term. From what I can gather on here the boys that run 24V challenge winches mostly have 2 batteries for the 24V system and a third one for the 12V system running off two separate alternators, as you described.

Yep, Makes sense. Not sure if I should give it a try and see if I can get away with it or not or just go with a third battery which I really didn't want to do.

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It would be an expensive task to change the truck to 24v , so a huge dropper or a third battery and split system may be the only choices .

Well you may have to shed weight in other areas , Passenger seat , heater ,cans of carlsberg .

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It would be an expensive task to change the truck to 24v , so a huge dropper or a third battery and split system may be the only choices .

Well you may have to shed weight in other areas , Passenger seat , heater ,cans of carlsberg .

Passenger seat and heater maybe but I'll stick with 12v before I get rid of the carlsberg! I carry it instead of spanners as an emergency kit in the event of breakdowns!

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Guest noggy

wtf!! 60 quid for a 24 to 12 volt dropper, thats ridiculous!

wait until landymanluke gets here, he will tell you how to do it for a couple of quid!

just a couple of resisters i think!

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You have to look at the commercial vehicle aspect , 24v system , 12v radio , they always use droppers never do they tap into one battery to supply the radio , and they only demand a small supply , When you start running starters , lights wipers ,etc on a 12v tap there is going to be problems

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wtf!! 60 quid for a 24 to 12 volt dropper, thats ridiculous!

wait until landymanluke gets here, he will tell you how to do it for a couple of quid!

just a couple of resisters i think!

That's a 45A version, that is...... HUGE... more than you would ever need.

A couple of resistors can give you 12V, but they need to be rated to the current they are supplying, too high and you will pop them very quickly, not to mention the voltage variance when applying different loads....

I pointed that one out as the top end of the scale, a much smaller 5A for £13... http://www.roadking.co.uk/products.asp?recnumber=201

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The batteries will equalise - but over a long period. The current flow to the more discharged battery will be limited by the fully charged one. BogMonster is correct that if the difference in the charge state is too great, the current will be forced through both batteries and can damage the fully charged one.

In practice it's best not to do it if you can avoid it - you will damage one battery.

My advice is to run the starter and heater on 24v - they will wizz, but should be OK. Replace the lights with 24v versions and if you must have heated seats, connect the driver and passanger side in series. OK they'll both come on together - but that's better than your bottom catching fire. Run everything else from 12v through a dropper. You can get droppers designed for car audio and CB's for little money. You can of course connect several cheap droppers in parallel which is usually cheaper than one big one. This does require a bit of re-wiring - but fortunately each of the circuits is separate and available at the fuse box.

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Ignore this bit if you don't like electrickery

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There are a couple of other options which may be worth considering.

Some alternators including many fitted to 300tdi and Td5 are what is known as 'Battery Sensed'. This means that the regulator in the alternator sets the output voltage to achieve 12v (actually closer to 14v) on another input it uses to measure the actual battery voltage.

Many of these can be tricked in to charging at 12v or 24v. If you connect the charge lead to the 24v terminal and the sense lead to the joint between the batteries (which sits at 12v), the only way it can achieve 12v at the junction, is to output double that voltage - and charge both batteries.

An option would be to use a small battery (like a Hawker PC680 bike battery) which only weighs a few Kg to run your 12v systems. then use a couple of high current relays to switch the alternator charge lead between the 24v set and the 12v battery. Use a low current relay to change the sense lead from the junction of the 24v set to the +ve of the small battery. Then use a Voltage sensitive switch to detect when the small battery is low and use this to effect the changeover.

This way, the 12v battery has charging priority. If it is at full voltage, the 24v set is charged. On most challenge trucks, there is little requirement for the 12v circuit - so the alternator will spend most of it's time charging 24v.

The biggest single advantage of doing this is that an alternator has a specified current output - say 120A. It will produce 120A at 24v just as well as at 12v (though it may get a bit hot if there is insufficient cooling). 120A 12v alternators are a lot more common than 120A 24v - and a lot cheaper too!

Another option which does not need a third battery at all is to use a voltage divider (two resistors) connected between 24v and earth. The junction of the resistors will sit half way between the two batteries (normally 12v) You need to build a simple comparitor which detects a difference between your potential divider reference and the junction of the two batteries. If both batteries are similarly charged, the reference will be at the same voltage as the 12v junction. In this state the alternator charge lead is connected to the 24v terminal. If the difference is more than a couple of volts, the comparitor switches the chang-over contacts on the relay which switches the charge lead from the alternator from the +ve of the 24v set to the 12v junction of the batteries. You connect your 12v circuits to the junction of the two batteries.

This way, the 12v junction is charged only when that battery voltage has fallen below that of the other battery - and it needs boosting. Otherwise, both batteries are charged in series at 24v.

I've not decided which I'm going to use yet - but the simplest and lightest weight is to run high current stuff off the 24v circuit and everything else from a dropper (I conveniently have a pair of 30A droppers). I'll certainly be using my 12v alternator at 24v though - and I might even add some trickery to make it spit out 230v mains, and 36v for welding!

==================================================

Si

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You probably are very aware of this; but you dont need remote sensed alternators to "trick" them to other output voltages. As long as one can control the amount of electricity flowing in the field windings, nearly any output voltage can be achieved. Plenty of free schmatics for such "home-made" voltage regulators on the web.

How are you going to achieve 50Hz on the 230 volt setup? I would think it would be easier to run a 230V alternator from a separate pulley, than trick a 12 volt alternator to produce usable 230volt...

Are the welding current going to be straight DC or high frequency AC?

I dont think you are going to get a cooling problem - alternators usually dont mind beeing run at other voltages.

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As for the original question on 12 volt from 24 volt - it is normally considered bad practice, because of the reasons already mentioned. Modern day car batteries are pretty resillient though, so as long as the current draw is low enough you might get away with it.

If you draw high currents from a single battery in such an arrangment you are actually killing both batteries. One is overcharged, and one is left undercharged.

If I remember correctly it think I have seen some landcruisers with a similar arrangement - 24 system, but 12 volt headlights. Weird, but factory provided setup - the headlights were balanced between both of the batteries.

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