Another battery charging question

ailsaboat

ailsaboat

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Further to my advanced regulator question, I am now wondering about the effects of a split charge system.
If we presume the afore mentioned gizmo is 'doing what it says on the tin' and banging out loads of previously untapped energy - where does it go??
Option 1,
Do i manually monitor the engine battery level, then when full switch over to the domestic battery bank. If so, what's the best way to tell when the engine battery is full, and can you switch over while the engine is running?
Or
Option 2,
A split diode charge system, as pushed by most of the manufacturers.
What I don't understand about this set up is where the current goes, does the diode block split it 50/50??
If so, will my relatively small engine battery get fried because the regulator is banging out enough to charge the big domestic bank?
 
It will only supply the current that is needed to charge the battery but it will do so morequickly and to a higher percentage of fully charged than a bog standard one will.

You can monitor the battery and swich over when the current falls or when a steady voltage reading is reached.

If the engine is running switch to both batteries ON before switching the first off. Never switch both batteries off when the engine is running. Less important but switch to both off rather than both on if the engine is not running.

you could use a split diode system to avoid the switching business. The battery at the lowest state of charge will get the lion's share of the power initially.

If you use a split diode system you have to give thought to "sensing" as they will impose a 0.7 volt drop. You have to decide which battery to sense usually the one most likely to be the lowest. Some diode splitters have an extra diode and a connection for the sensing

A "voltage sensitive relay" (VSR) overcomes the problems that diodes cause. It will charge one battery usually the engine start battery initially and when the volts rise as that becomes charged it switches on the other one.

thre is some useful reading here
 
Yes a good question. The current into a battery is dependant on the inherent voltage of the battery ie what it has in it when not charged) compared to the regulated voltage of the charging source.

Now the old car type system is regulated at 14 volts. At this charging voltage a lead acid battery will take very little current when it is fully charged. If you have a larger capacity battery that is not fully charged that is in effect paralell via diodes to the charged battery then the discharged one will accept current until it is charged.
Now unfortunately a battery inherent voltage rises more quickly than the actual % of charge so you end up with the charge current being accepted being fairly small for much of the later charging of a battery. ie lots of current accepted at first but the last 30% at a small current hence takes a long time.
If you are charging a battery by engine running just for charging you waste a lot of engine running charging at a low current to get near full charge.
However the choice of 14 volts is such that any lead acid battery will not be overcharged at that voltage.

You fit a smart charger and the device charges at a higher volltage to get more current in quickly and for a longer time until the inherent voltage of the battery indicates it has had enough and then the voltage and current drops to a trickle charge.

You have the potential for an engine battery nearly fully charged but paralelled to a large discharged battery being subject to a higher voltage and having more current pushed in than it wants resulting in overheating and electrolyte boiling.

If the batteries are paralelled by a wire then potentiually you have the well charged battery discharging into the larger discharged battery and then being charged as one and hopefully the voltage will rise as one to eventually cut back the current.
If however they are isolated by diodes then they will not be able t share charge.
Further there must be a sensing of the regulated (charge ) voltage. if sensing is taken form the large discharged (domestic) battery then there is potential to boil the negine battery. If you take sensing from the engine battery then the charger may be cut back when that battery is charged but before the other domestic is fully charged.

If you take sensing from the alternator or charger with allowance made for the voltage drop of the diodes then the sensed voltage will be more like an average of the 2 batteries.

So I would think a Voltage sensing relay is better than diodes for isolation in so much as the 2 batteries are hard wired together when the relay operates forcing the batteries to average one another and the charger to charge to the average state.

Perhaps a safer bet would be to disconnect the engine battery from charging after a short time of charging so the charger can concentrate on the bigger discharged domestic battery. But that is not very autoamatic.
So that is as I see it although to be honest I have no experience with smart chargers just an electronic background. olewill
 
Can't add much more to that but having 2 engines, both with Stirling smart chargers I would say that the voltage sensitive relay versus current splitting diodes debate rages on and there are advantages with both. I have 2 splitter diodes, one on each alternator charge line. As the Sterling is battery sensed on the voltage on the battery side of the diodes it automatically raises the voltage to 14.4 volts at the battery compensating for the voltage drop across the diode so I don't think the voltage sensitive relay route is any better if you do have a smart charger. It is of course better if you dont.
Answering your question about overcharging you won't. With a diode splitter and smart charger the important thing is to sense the battery voltage at the battery that is liable to be discharged the most. The charging current should be automatically diverted to the battery that needs it most without cooking the others. I have a voltmeter and ammeter on each of 3 battery banks and can see what happens all of the time. With mine the charge on the engine cranking battery is high only momentarily after the battery has been discharged. then it falls to no more than a trickle. The service battery for lights instruments and general takes more but the one I sense on is the seperate bank I have for Fridge, Eberspacher, AC and anchor winch which I keep seperate. This takes most punishment and therefore most charge, but it does not cook the others while doing so although with any smart charger on a wet battery you should check your electrolyte levels more often as the batteries will gas more. It's also more important to ensure the batteries are in a safe position so as not to discharge explosive gas into areas you dont want it like the engine bay.
 
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Can't add much more to that but having 2 engines, both with Stirling smart chargers I would say that the voltage sensitive relay versus current splitting diodes debate rages on and there are advantages with both. I have 2 splitter diodes, one on each alternator charge line. As the Sterling is battery sensed on the voltage on the battery side of the diodes it automatically raises the voltage to 14.4 volts at the battery compensating for the voltage drop across the diode so I don't think the voltage sensitive relay route is any better if you do have a smart charger. It is of course better if you dont.
Answering your question about overcharging you won't. With a diode splitter and smart charger the important thing is to sense the battery voltage at the battery that is liable to be discharged the most. The charging current should be automatically diverted to the battery that needs it most without cooking the others. I have a voltmeter and ammeter on each of 3 battery banks and can see what happens all of the time. With mine the charge on the engine cranking battery is high only momentarily after the battery has been discharged. then it falls to no more than a trickle. The service battery for lights instruments and general takes more but the one I sense on is the seperate bank I have for Fridge, Eberspacher, AC and anchor winch which I keep seperate. This takes most punishment and therefore most charge, but it does not cook the others while doing so although with any smart charger on a wet battery you should check your electrolyte levels more often as the batteries will gas more. It's also more important to ensure the batteries are in a safe position so as not to discharge explosive gas into areas you dont want it like the engine bay.

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If using Split Diodes .. which impose the drop in voltage at batterys ... you can do as Alisdair Garrod says in his Excellent book on Boat Electrics ... a PBO Publication by the way ..... That is to "cheat" the alternator by coupling a diode across two terminals to get the alternator itself to compensate. That way no need to play about with smart chargers, battery sensing etc.
 
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That is to "cheat" the alternator by coupling a diode across two terminals to get the alternator itself to compensate

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YOU DO WHAT?????
 
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That is to "cheat" the alternator by coupling a diode across two terminals to get the alternator itself to compensate

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YOU DO WHAT?????

[/ QUOTE ]

Yep ... page 64 .... "Fooling the senses". Alastair Garrod - Electrics Afloat .....

The trick is to fool a Machine sensed alternator into increasing its output voltage by equivalent Diode drop. This by fixing a diode between the Auxiliary terminal and regulator ... in the back of the alternator ...

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And there's the article to prove it ... Now it's no good SHOUTING at me ... Alistair Garrod is a respected "expert" in boat electrics .... recognised by the mags etc. So I can only be guided by what he says .... /forums/images/graemlins/cool.gif
 
An alternator regulator senses or samples the voltage usually at the output terminal of the alternator and compares this to a fixed voltage standard. if the sensed voltage is lower than (typically) 14V it increases the current to the field coils (the rotating coils) so increasing the output voltage. Likewise if the voltagee out is too high it reduces the field current and so reduces output voltage.

The theory of cheating the alternator is quite simple in as much as you feed the sensing of the alternator output through a silicon diode. This reduces the sensed voltage by .7 volt so inducing the regulator to force the alternator to produce 14.7 volts regulated.

The higher voltage can then be fed via the splitter diode which in itself drops .7 volt so leaving you with correct 14 volts at the battery terminal.
This higher voltage (provided there was no isolation diode to lose the .7 volt) could also be used to induce the battery to take more current so faster charging but the downside is that when the batteery is charged the current will still be pushed in to cause boiling and general battery destruction. 14 volts being the voltage which will get a decent charge eventually but will not damage the battery with continuous charging. (as in a motor vehicle)

Now the down side of fitting a diode between the alternator output and the regulator is that we are generally using vehicle alternators which have a regulator mounted in the back of the alternator (opposite end to the pulley) which is encapsulated in resin and carries the brushes to the slip rings hence to the field coils. The actual connection of the sensing wire (alternator output to the regulator is via a contact made when the regulator is screwed in place. ( in various forms)
It is not generally physically easy to break this connection to install a diode. It may in fact be easier to find an alternator similar to yours which does not have a built in regulator but just a brush carrier. Replace the brush carrier and fit an external regulator. You can then fit an external diode in the wiring or in fact fit an adjustable regulator which can be set to 14.7 volt output.

I am describinbg in general terms because alternators vary enormously. This is the biggest problem of smart regulators in describing and actually modifying the alternators to suit the smart regulatorwith all their variations.

So the Voltage sensing relay becomes a simpler option in that 14 volts gets to the batteries and no alternator modification is required.
You still can have a very slow recharge of the engine if you have a very flat service battery.

Being in the mood to waffle.... The concept of a diode droppinng .7 volt is very alien to one who masters ohms law where more current equals more volt drop as in a lamp or resistive load. The diode is like a non return valve for fluid it has in effect a spring which holds a seal to stop fluid flowing backwards. It is the overcoming of the spring to get forward flow that reduces the pressure able to go through it. So with a diode no current can pass in the forward direction until the barrier layer of electrons has been got to move into place this takes about .7 volt The .7 volt then always remains the voltage drop regardless of current. (or nearly so the volt drop does increase with large currents but not like a resistor.

I hope this explains why cheating the alternator is not often used.
good luck olewill
 
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Now it's no good SHOUTING at me

[/ QUOTE ] Yes inserting a diode into the sensing circuit is the way it is done and that is what Alistair Garrods book shows. Some diode splitters actually have an extra diode built in that the sensing can be connected to to acheive just that.

What I was objecting to was the out of context statement that an alternator could be fooled by simply connecting a diode, with no consideration to its rating or its polarity, across two terminals. In most cases the only external terminals will be the main output and the warning light connections. Connecting a diode across those two whatever its rating or polarity will not do what is required. One way round would be the equivalent of sorting across the "ignition" switch and the warning light meaning that the alternator was permanently energised and the warning light rendered inoperative (At least if the diode did not blow). The opposite polarity would do nothing.

However thanks for posting the details from Garrods book I am saving those for future reference.

Thanks also to Will for the detailed explanation of how it works and why it can be difficult to do in practice.
 
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Now it's no good SHOUTING at me

[/ QUOTE ] Yes inserting a diode into the sensing circuit is the way it is done and that is what Alistair Garrods book shows. Some diode splitters actually have an extra diode built in that the sensing can be connected to to acheive just that.

What I was objecting to was the out of context statement that an alternator could be fooled by simply connecting a diode, with no consideration to its rating or its polarity, across two terminals. In most cases the only external terminals will be the main output and the warning light connections. Connecting a diode across those two whatever its rating or polarity will not do what is required. One way round would be the equivalent of sorting across the "ignition" switch and the warning light meaning that the alternator was permanently energised and the warning light rendered inoperative (At least if the diode did not blow). The opposite polarity would do nothing.

However thanks for posting the details from Garrods book I am saving those for future reference.

Thanks also to Will for the detailed explanation of how it works and why it can be difficult to do in practice.

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Ok - I admit my wording was a bit slack .... but I quoted AG's book so that people could read for themselves if they wanted to try it. I purposely didn't try from memory to say which terminals - leaving it vague to prompt reading of the book.

Now onto Will's bit about the reasons and also the bit about damaging a battery .... the reasons it works - AG has already explained that in much less words .... As to damaging a battery - I have to disagree ... completely. The whole idea is to boost the alternator regulator to up the output to 14.7 BEFORE it hits the splitter diodes. They will drop it to 14 which is what you would get without the diodes in place. Ampage is determined by the batterys charge state. So where does damage come from if all you are doing is compensating for loss across the splitter diodes.......

I agree that many alternators now are not so easy to do this with ... but I would also submit that many boats are still running around with older style alternators and this is still a valid method.

IMHO of course !! /forums/images/graemlins/wink.gif
 
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As to damaging a battery - I have to disagree ... completely

[/ QUOTE ] I think Will was speculating that you could stick the diode in the alternator to boost its output by 0.7 volts even in simple (one battery ) installations with no diode splitter purely to increase the charging rate. He goes on to point out the reasons for not doing this, namely overcharging, but I think he is probably wrong in suggesting that 14.7 volts will be enough to cause the battery to boil. (I recently left my unregulated charger on my car battery for longer than I should have done and by the time I got back to it the volts had risen to over 15v but the current had fallen to below 2amps and the battery was not showing any signs of serious overheating. I've know the same charger to get up to 16v which is why I usually keep an eye on it)
 
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As to damaging a battery - I have to disagree ... completely

[/ QUOTE ] I think Will was speculating that you could stick the diode in the alternator to boost its output by 0.7 volts even in simple (one battery ) installations with no diode splitter purely to increase the charging rate. He goes on to point out the reasons for not doing this, namely overcharging, but I think he is probably wrong in suggesting that 14.7 volts will be enough to cause the battery to boil. (I recently left my unregulated charger on my car battery for longer than I should have done and by the time I got back to it the volts had risen to over 15v but the current had fallen to below 2amps and the battery was not showing any signs of serious overheating. I've know the same charger to get up to 16v which is why I usually keep an eye on it)

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Yep - I re-read Wills post a few times ... I have to admit that often Wills posts are l...o...n...g and being close-typed make it harder to read. I now see that he was inferring a non dropped higher voltage ...

But as you say - many times this sort of voltage will be shoved at batterys ...

I respectfully submit that Boost Chargers / Alternator boosters do just that for longer than standard chargers ..

I have long wondered if there is a way to boost the charge voltage say 0.4 or 0.5V from normal to give better charge ... But I'm not an alternator / regulator knowledgeable person ... like most - I read the books and do as they say.

Anyway so far - I have had excellent duty from my standard set-up ... and when available mains charging via Maplins £7 splitter to both batterys. So far haven't needed any boost charging ..
 
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I have long wondered if there is a way to boost the charge voltage say 0.4 or 0.5V from normal to give better charge

[/ QUOTE ] Almost certainly possible to design a little electronic circuit that will do it but I am no more able to do that than your are! Perhaps Will H could.

If I come across one I'll let you know.
 
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I have long wondered if there is a way to boost the charge voltage say 0.4 or 0.5V from normal to give better charge

[/ QUOTE ] Almost certainly possible to design a little electronic circuit that will do it but I am no more able to do that than your are! Perhaps Will H could.

If I come across one I'll let you know.

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If anyone came up with that - it would put a lot of people out of biz I think !! No more need for fancy charging boxs etc. As we know - standard alternators are kept just below that level that gives full or near full charge ... to make sure they don't damage the battery. Boosting that little bit will make up that fraction needed to bring battery to near full.
In one move you have improved your 40 amp bog standard alternator such that possibly that 70 amp job you thought about bolting on is no longer needed !!

Interesting thought ... surely it doesn't need too much a box of tricks to do it ...

Mmmmmmm - I think I'll have a wee word with my pal !!
 
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