Electric Curiosity

[JOHN OF LONDON]

Hi,

Just working out the wiring for an auxiliary battery and the existing wiring on the HDJ80, which looks original and undisturbed has me puzzled.

The (surprisingly thin) wire from the alternator appears to run direct to the positive terminal of the near (left) side battery. This is odd because its a long way from the alternator and it produces a nominal 24V when the starter works; though I am not sure that the alternator is energised when the starter is running.

It is obvious to me that the simplest way to wire the alternator would be direct to the other battery which only ever delivers 12V; but equally obviously I am missing something as I am sure Toyota will not have run a longer wire without a good reason.

Does anybody have any idea what I have missed in understanding the wiring?

 

[ByronJ]

Hi,

Just working out the wiring for an auxiliary battery and the existing wiring on the HDJ80, which looks original and undisturbed has me puzzled.

The (surprisingly thin) wire from the alternator appears to run direct to the positive terminal of the near (left) side battery. This is odd because its a long way from the alternator and it produces a nominal 24V when the starter works; though I am not sure that the alternator is energised when the starter is running.

It is obvious to me that the simplest way to wire the alternator would be direct to the other battery which only ever delivers 12V; but equally obviously I am missing something as I am sure Toyota will not have run a longer wire without a good reason.

Does anybody have any idea what I have missed in understanding the wiring?

Hi John

The wiring as you describe. But I cannot see a reason why Toyota connected it that way.

Byron

 

[StarCruiser]

It means that the two batteries in parallel charge equally with the negative necessarily connected to the RHS battery and the positive connected to the LHS. During starting the batteries are series connected via the starting series/parallel relay giving 24v to the starter motor only. Once running the batteries are connected in parallel as they are at rest also and charge like this at 12v from the alternator. Only connect to the RHS battery otherwise at starting your charging circuit and therefore your leisure battery will receive 24v. Not good as the smoke then gets out!

 

[ByronJ]

Only connect to the RHS battery otherwise at starting your charging circuit and therefore your leisure battery will receive 24v. Not good as the smoke then gets out!

Only connect what to the RHS battery?

 

[Julian T]

Any accessories, spot lamps to fridges and winches, do not connect anything to the left hand battery.
There is also a power supply point on the RHS near or behind the air filter close to the outside of the engine bay.

 

[uHu]

Sounds like a description of the sense input to the charge regulator. The regulator needs to know the voltage of the battery, i.e. what comes through after the loss in the main cable running from the alternator to the battery.

This is exactly what is missing on most cars, as they only use the output of the alternator as a sense, thereby never charging the battery with more than a handful of amps, however depleted it might be.

 

[chapel gate]

Basically if you connect anything to the ps battery it will have a kick up the arse on start up.. aka, 24 volts..

 

[ByronJ]

Hi,

Just working out the wiring for an auxiliary battery and the existing wiring on the HDJ80, which looks original and undisturbed has me puzzled.

The (surprisingly thin) wire from the alternator appears to run direct to the positive terminal of the near (left) side battery. This is odd because its a long way from the alternator and it produces a nominal 24V when the starter works; though I am not sure that the alternator is energised when the starter is running.

It is obvious to me that the simplest way to wire the alternator would be direct to the other battery which only ever delivers 12V; but equally obviously I am missing something as I am sure Toyota will not have run a longer wire without a good reason.

Does anybody have any idea what I have missed in understanding the wiring?

Hi @JOHN OF LONDON

Your question ‘why did Toyota connect the alternator output to the PS battery’ has been running through my mind all day . Mainly because I puzzled over the same question when I converted the late Mr T to 12v start earlier this year. I did not have an answer then but I think I do now.

Today I have been busy converting Elsie to 12v start and installing a split charge system. This work entailed me staring at the original wiring for a good while and thinking about your question . My conclusion is Toyota connected the alternator output to the PS side battery because it was safe to do so.

I will explain. You clearly understand how the 24v start works otherwise you would not have asked why the alternator output is connected to a source that generates 24v on cranking the engine.

This is my theory of why Toyota connected the alternator output to the PS battery even though the alternator is on the DS side of the engine.

They clearly could not connect anything to the PS battery that would be at risk of over voltage damage (or, if they did so connect, then they would have to introduce some over-voltage protection). So everything 12v is connected to the DS battery which always produces 12v. Now, you will be aware that the positive post of the DS battery is rather crowded - especially with those ‘slow burn’ fuses.

SO, rather than having to create a solution to this crowded terminal Toyota simply ran the alternator output (and the voltage sensor) to the PS battery as this is a simple and cheap solution. This is because the alternator is not at risk from over voltage due to its internal diodes ensuring no current can actually flow towards it. Thus it is impervious to any voltage applied to it - well 24v at least!

Without this diode protection alternators would be driven as motors when the engine stops. In fact a failed diode can drain ones start battery pretty quickly.

Not such a technical answer, just a practical approach, but I reckon I may well be on the right track. Or possibly barking up the wrong tree altogether .

Byron

 

[uHu]

Sounds good.
The OP did say "thin wire", so I didn't think he was talking alternator output.

 

[StarCruiser]

I think Toyota just put the alternator connection where it should be. I can't imagine they would have ducked out of providing a more capable battery terminal if it were the right place to put it.

 

[ByronJ]

Sounds good.
The OP did say "thin wire", so I didn't think he was talking alternator output.

Thanks.
Ah! The sense wire is buried within the loom so I am pretty sure the OP was talking about the alternator output cable . Who knows? Perhaps John will pop back on and let us know .

 

[Dave2000]

With the batteries in parallel the alternator needs to 'see' the voltage from the battery furthest away, this allows for voltage drop between the two batteries.

Mr T could have gone with heavier cables to reduce the drop but a battery sensed alternator was IMO the say to go.

Regards

Dave

 

[JOHN OF LONDON]

Hi Guys,
Not sure that I am any wiser for all your collective wisdom. Yes I was talking about the main output cable from the alternator which I thought was rather thin; I'm looking for a higher output alternator at the moment (any recommendations?) to go with the Lincoln Mk VIII fan which should arrive shortly (Thanks Dave for the advice on that) so a new wire from the alternator will be needed. Where to run this new fatter wire is the question, I would put a terminal post in to connect everything up if I could find a sensible place to put one but I may finish up endorsing Byron's practical answer by running a longer wire across the front even though I don't like the fire safety implications of having another longer than necessary unfused cable very close to where most accidents happen.
I assume the battery voltage sensing wire is one of the three wires that makes up the oval plug on the alternator. God (and Mr T, I guess) knows where the other end is! Given that the batteries are connected together with big thick wires I would be surprised at any voltage difference between them though dirty connections in the relay box might give some resistance, certainly my multimeter can't see any voltage difference. By the by, in electrical terms I think the PS battery is closer to the alternator that the DS battery and if any voltage difference were discernable the PS battery would be higher. As it stands the alternator output is going to the PS Battery, and all the loads are connected to the DS battery and are being fed via the relay box, so I think any voltage drop would be that way round.
I am intending to put my auxililiary battery in the rear and have been puzzling over wire sizes to charge it. Research has provided very little information that's any use and I have failed entirely to come up with an answer I believe in. The puzzle is as follows: Standard caravan wiring through a modern 13 pin socket has a 12v connection included in it which seems to be intended to charge the caravan leisure battery; the wire included in the standard 13 wire cable is pretty thin. Elsewhere, I see advertised split charge relay kits with recommendations to use 16mm 100amp cable. Starting at first principles; we have two largish 12 volt batteries and an alternator capable of delivering "unlimited" amps to the auxiliary battery with only its internal resistance and say 10.5 volts to prevent those amps from flowing. I am intending to use a single 80Ah AGM auxiliary battery to run the fridge etc; does anyone have a reliable figure for the current it will draw?

Cheers

John

 

[ByronJ]

Hi John

Choosing a cable size is not as complicated as it looks. There are two things to consider:

1 - you don't want the cable overheating
2 - you want the voltage drop across it as low as practicable.

All copper cable has a specific resistance which is linear. I.e the resistance of 2 metres of cable is exactly twice that of 1 metre of the same cable. All cables are rated to indicate the maximum current they can carry continuously without overheating.

Let us assume you fit an alternator that can produce 100 amps.

16mm2 cable is rated to carry up to 110A , 25mm2 cable is rated to carry 170A. So either cable will carry your maximum current. The difference is that the 16mm2 cable will put up more resistance per metre than the 25mm2 cable and thus introduce a larger voltage drop which increases with the length of the cable run.

The formula to calculate the voltage drop is "V = IR". In other words the voltage drop is calculated by multiplying the resistance of the wire by the current flowing through it. To calculate this I use one of the many calculators on the internet. This link here gives a much more comprehensive explanation of cable selection and contains a handy calculator:

http://www.12voltplanet.co.uk/cable-sizing-selection.html

Using the above calculator you can work out that:

16mm2, 100A, 5m of cable, voltage drop 0.53v
25mm2, 100A, 5m of cable, voltage drop 0.34v
35mm2, 100A, 5m of cable, voltage drop 0.24v
50mm2, 100A, 5m of cable, voltage drop 0.17v

However it is very unlikely that your battery is going to accept 100A for very long as its own internal voltage will rapidly rise and reduce the current it will accept. Some batteries, such as the Odyssey Extreme range can be recharged at a phenomenal rate but most batteries very quickly reduce the current they will accept while charging.

So if we take a more likely charge rate of 20A the voltage drops improve:

16mm2, 20A, 5m of cable, voltage drop 0.11v
25mm2, 20A, 5m of cable, voltage drop 0.07v
35mm2, 20A, 5m of cable, voltage drop 0.05v
50mm2, 20A, 5m of cable, voltage drop 0.03v

Whatever battery you are buying should have data available showing the rate of charge it will accept over time. So knowing the length of your cable run (assuming your battery earth is run to the chassis as close to the battery as practicable) chose your cable size depending on the voltage drop you are happy with for the current you realistically expect to flow through it.

Hope this helps.

 

[Dave2000]

I done the complete fan/dual batteries/alternator/VSR install and pretty much everything else done to my 80 is in a thread I have had on MUD for around 7 years. Voltage info, running temps, alternator upgrade and so on, I am sure it is all there, if not give me a shout. It also shows my mod to the blank space between the dials on manual transmission 80's, it is a custom display for fan status, engine overheating, oil pressure rad levels, ect.

Very, very important are the fan relays, I purchased a made up kit to save time and TBH it was for want of a better explanation, crap using 4 relays. I eventually make my own usjng three HD units. 12v planet do the correct very HD relays but not the connectors, well not at my last check earlier in the year.

@ByronJ link to 12v planet is excellent, I have used them for many years and did indeed get my cables and plenty of other 'stuff' from them, I also had them correct some info about connectors which was missing, they immediately updated the page, excellent customer response, a really good set up and have been very reliable over the years shipping to Spain.

The aforementioned link:

https://forum.ih8mud.com/threads/my-lc-80-thread.365673/

Regards

Dave

 

[JOHN OF LONDON]

Thanks, Dave and Byron,
Actually I have tried, without success, to find out what battery manufacturers think their batteries will draw and have come to the conclusion that there are too many variables for them to give an meaningful answer which protects them from legal action when little wisps of acrid smoke start to rise. They are happy to tell me what current is optimal for charging, which is great, but I am more than happy to let CTEK take control of that at home. Out on the road a well discharged auxiliary battery will be faced (as my car is currently) with 60 amps or a bit less at 14.4V (the output of the alternator) or a reduced voltage if the current drawn is more and coming from the batteries. Assuming (and I've no idea if it will) the auxiliary battery will accept more than 60 amps it will be at some intermediate voltage between 14.4 and 12.6, less the losses in the cabling. I'm trying to get a handle on these numbers - no success so far! I was hoping someone had fitted an ammeter and actually knew what goes on.
Cheers,
John

PS. Dave I'll go and read your mud thread again re alternators and seek your advice later if necessary. I did the temperature gauge mod and that worked well. I've got a pair of the resistors for this in the garage if anyone else needs a pair.

 

[ByronJ]

Thanks, Dave and Byron,
Actually I have tried, without success, to find out what battery manufacturers think their batteries will draw and have come to the conclusion that there are too many variables for them to give an meaningful answer which protects them from legal action when little wisps of acrid smoke start to rise. They are happy to tell me what current is optimal for charging, which is great, but I am more than happy to let CTEK take control of that at home. Out on the road a well discharged auxiliary battery will be faced (as my car is currently) with 60 amps or a bit less at 14.4V (the output of the alternator) or a reduced voltage if the current drawn is more and coming from the batteries. Assuming (and I've no idea if it will) the auxiliary battery will accept more than 60 amps it will be at some intermediate voltage between 14.4 and 12.6, less the losses in the cabling. I'm trying to get a handle on these numbers - no success so far! I was hoping someone had fitted an ammeter and actually knew what goes on.
Cheers,
John

PS. Dave I'll go and read your mud thread again re alternators and seek your advice later if necessary. I did the temperature gauge mod and that worked well. I've got a pair of the resistors for this in the garage if anyone else needs a pair.

I am not sure exactly what numbers you are looking for John.

The most basic rule for any lead acid battery is that you can charge at ANY current you like until the battery reaches 2.4 volts per cell at 25 deg. C. That's 14.4v for a 12v battery. Below this voltage, essentially 100% of the current goes into charging the battery, so there is negligible heating or gassing. This is the Bulk Charge or Constant Current phase.

The batteries won't reach 2.4v per cell until they are about 70-80% charged. Above 2.4v/cell (14.4v), the batteries start to gas and get hot. So provided your alternator does not raise its voltage above 14.4v you will not damage your battery.

To fully charge a lead acid battery you need a charger like your Ctek to put the remaining 20-30% of charge in over many hours. This is called the Acceptance or Constant Voltage phase, the current flowing will be small at this point. This is why alternators very rarely bring a battery to full charge unless the alternator is running for a long long time. Unless you install a smart controller of course - I have one on my boat.

So to put it another way. Whatever battery you fit will take ALL the amps you throw at it until its internal voltage rises to 14.4v. So make sure you choose cable to handle the maximum current you expect to generate.

 

[JOHN OF LONDON]

Byron,
I think I must be missing the point here somewhere. My alternator, in theory at least, will put out 60 amps at 14.4 volts. Any demand in excess of this must come from the starter batteries, which will deliver 12.5v or so. My assumption is that at this point the voltage in the system will drop, reducing the amps being stuffed into the auxiliary battery to the point where a steady state exists at some intermediate voltage. Actually my expectation would be that the small auxiliary battery would provide enough back voltage and internal resistance to prevent any charge rate even close to 60 amps but... Who knows!

Looking at the caravan world which seems to be quite happy connecting a couple of 110ah leisure batteries to a split charge relay with a thin bit of wire in the middle of a bundle of wires. Given that most modern cars have at least a 100 amp alternator how does that work? Or have I got that all wrong?

John

 

[JOHN OF LONDON]

Byron,
I think I must be missing the point here somewhere. My alternator, in theory at least, will put out 60 amps at 14.4 volts. Any demand in excess of this must come from the starter batteries, which will deliver 12.5v or so. My assumption is that at this point the voltage in the system will drop, reducing the amps being stuffed into the auxiliary battery to the point where a steady state exists at some intermediate voltage. Actually my expectation would be that the small auxiliary battery would provide enough back voltage and internal resistance to prevent any charge rate even close to 60 amps but... Who knows!

Looking at the caravan world which seems to be quite happy connecting a couple of 110ah leisure batteries to a split charge relay with a thin bit of wire in the middle of a bundle of wires. Given that most modern cars have at least a 100 amp alternator how does that work? Or have I got that all wrong?

John

 

[ByronJ]

Byron,
I think I must be missing the point here somewhere. My alternator, in theory at least, will put out 60 amps at 14.4 volts. Any demand in excess of this must come from the starter batteries, which will deliver 12.5v or so. My assumption is that at this point the voltage in the system will drop, reducing the amps being stuffed into the auxiliary battery to the point where a steady state exists at some intermediate voltage. Actually my expectation would be that the small auxiliary battery would provide enough back voltage and internal resistance to prevent any charge rate even close to 60 amps but... Who knows!

Looking at the caravan world which seems to be quite happy connecting a couple of 110ah leisure batteries to a split charge relay with a thin bit of wire in the middle of a bundle of wires. Given that most modern cars have at least a 100 amp alternator how does that work? Or have I got that all wrong?

John

I think what is missing is some detail. If I understand you correctly you are concerned about how much current could potentially flow from your two existing batteries (which are connected in parallel) to your auxiliary battery when you connect the auxiliary battery to them (also in parallel)? This not something you could come up with a figure for as it depends on the state of charge of each of the battery banks at connection time. But potentially the current flow could be huge, much higher than your alternator can produce. But all is not lost as it depends on the method you use to connect them.

On my boat my battery banks are isolated via large diodes so current can flow from the alternator to the batteries but current cannot flow between the batteries. This is simple solution and works well when you have a small start battery and a very large auxiliary bank - as I do. But it also has its difficulties and vehicles rarely use this method.

The most common solution in vehicles is to connect the auxiliary battery to the starter bank via a Voltage Sensing Relay (VSR). I have just fitted one to my car.

The VSR monitors both battery banks and when it detects one of the banks has a voltage > 13.3v it connects the banks. When it detects one of the banks has a voltage of less than 12.8v it disconnects the banks.

So our alternator starts running, the starter bank voltage creeps up over 13.3 volts and the auxiliary bank is switched in. Potentially a very large current begins to flow from the starter bank (mostly from the starter batteries) into the auxiliary bank. But the voltage in the system will immediately drop as the auxiliary bank is switched in (certainly the auxiliary battery would be showing < 12.8v) the VSR detects this and breaks the connection stopping current flow to the auxiliary bank.

The alternator raises the starter bank back up beyond 13.3v and the VSR connects the auxiliary bank back in. Again the VSR senses a voltage of less than 12.8v at one of the banks and so breaks the connection. This can happen quickly causing the connection to made and broken many times a second - referred to as the relay 'chattering'.

To prevent this chattering a time delay is coded in better VSRs. With my VSR it appears to be a fixed 5 seconds between checking the voltage and making a decision. I.e. Once it has connected the banks it waits 5 seconds before checking the voltage of either of the banks. Thus in our scenario above a large current will flow for 5 seconds and then stop. 5 seconds later it will connect them again if either of the banks is above 13.3v. 5 seconds later it will disconnect them if either bank is < 12.8v. And so on.

This may seem a little crude but in practice it appears to work well. I tested my system with a fully charged start battery (80ah) and a flat auxiliary (100ah). At a fast idle the VSR switched the auxiliary in and out every 5 seconds for about 2 minutes then it settled down and the connection began to be made for longer periods. After 10 minutes the connection was permanent and both batteries were showing > 12.8v. I used 25mm2 cable for the connection to the auxiliary battery and it was still cool to touch when the relay stopped switching in and out.

It was worth noting that for the first 2 minutes the fully charged battery bank was pulled down to around 12.6v as the auxiliary was switched in indicating it was being discharged into the auxiliary (though of course the VSR would ignore this for 5 seconds). The alternator took a few seconds to react to this and increase its voltage to compensate.