Electric Curiosity

[ByronJ]

Hi John

I have been wondering why current flow between main and auxilary banks is not more of an issue and I think it is because, to a large extent, the current flow between the batteries is self limiting.

Essentially the greater the voltage difference between two batteries the more current will flow.

So lets look at what happens when we connect our auxiliary battery to the starter battery (using a switch not a VSR) with 5m of 25mm2 cable (the battery is in the back of the car).

First some numbers:

25mm2 cable is rated to carry a constant current of 170A, but at this current it would generate a voltage drop of 0.58v over 5m.

At 200A this rises to a voltage drop of 0.68V.
At 300A this rises to a voltage drop of 1.03V.

At 20A it generates a drop of just 0.07v

So if we take the alternator out of the picture and assume the starter bank is fully charged at 12.8v and the auxiliary is heavily discharged by 75% and so is at 12v. At the instant we connect the batteries current will flow from the starter to the auxiliary battery as there is a difference between them of 0.8v.

However as the current flow increase so does the voltage drop across the connecting cable. As the current reaches 170A (our cables safe capacity) the voltage drop introduced is now 0.58v thus the difference between the batteries is only 0.22v. Actually the starter battery voltage will also be pulled down by the discharge. I just checked my battery and the voltage at the terminals drops by 0.2v when the ignition is switched. So at 170A there would likely be no voltage difference beween the batteries and thus no current flow.

It would also be impossible for the current to reach 300A as at that point the starter battery will appear to the auxiliary to have a voltage of 11.7v.

In reality I suspect the closed system will balance and a steady current flow be reached almost immediately. What this current would be we don’t know. It could certainly be a fairly large current flow but we can see from the rapid voltage drop introduced as the current reaches 170A that it is very unlikely to exceed 170A and is likely to be much less than that.

Now I just came to this conclusion on my own so I could be completely wrong, but it seems to make sense to me...

 

[uHu]

You are very right Byron. The only problematic scenario with that setup would be if you have a short circuit, or, not uncommon, a shorted battery cell. That's when a battery "dies" and effectively becomes a 10 volt accumulator, which would give a higher voltage difference between the two battery banks. But, with a fuse (fusible link) at each end, next to the battery terminal, you'll be safe. A fuse does introduce an extra resistance, further reducing the current, but that can not be avoided.

 

[ByronJ]

You are very right Byron. The only problematic scenario with that setup would be if you have a short circuit, or, not uncommon, a shorted battery cell. That's when a battery "dies" and effectively becomes a 10 volt accumulator, which would give a higher voltage difference between the two battery banks. But, with a fuse (fusible link) at each end, next to the battery terminal, you'll be safe. A fuse does introduce an extra resistance, further reducing the current, but that can not be avoided.

Thanks uHu, that is good to know . Excellent point about a shorted battery cell. In practice I personally would always connect the banks via a VSR rather than a mechanical switch. I was mainly exploring this manual scenario as John brought up the potential current flow between battery banks as a concern and I had not thought about it before.

 

[Dave2000]

I am good having a shedload of numerical data, but often find in the real world the numbers do not tally with reality, for example I assume that even a new battery has a real world reserve amperage of 85%, only the very best battery brands will be near their stated amperage, so the numbers are already compromised.

Re current moving between batteries. Assuming you have 2, 3, 4 or more batteries each having different states of charge, connect them all together and you will not see huge rushes of current anywhere for any length of time. Within a few moments all batteries (allowing for voltage drop between cables) will read the same, this means they can no longer 'push' current between each other, it is in effect one big battery. As an example, an alternator needs to be 'pushing' 13.8 volts to actually start charging a battery, that's a full 1 volt over the potential of a 12.8 volt battery, so increments below this would balance out within seconds so very little current will flow.

The caravan 'model' is of no use to anyone except.....well caravanners. Caravan plugged in before holiday, 'van' batteries fully charged. A few hours drive to campsite, plug in to mains job done. But what if they stop midway through the journey to get a cuppa. Perhaps use an inverter to power a kettle, watch the tv ect, get back in car and continue journey. Coming from the car perhaps 10 amps? As mentioned earlier this will not bring the battery/s back to full charge any time soon, you need hours and hours to get the battery to the theoretical 100% charge, which rarely happens, this brings us back to my 85% real world figure.

Voltage drop, stick with a maximum 3% @ 12v over cable runs and that will be fine, being a little OCD I worked for under 2% across the entire system.

Feel free to ask anything about my thread, I am not the most creative when trying to explain something a little techie.

Regards

Dave

 

[JOHN OF LONDON]

I think this has been very interesting and I think we, well Byron and Dave, have answered the question. My summarisation, which someone may wish to amplify is that the wire size (my original question) is a matter of free choice in which there are trade offs. The first constraint is that if the connection so thin and short that it will rapidly overheat it will be dangerous, or if properly fused, useless. The definition of too short and thin will depend on the batteries at either end of the wire and their respective internal resistance and voltage drop or rise with current flow. However, the initial current flow will rapidly diminish as the system settles so we are concerned to size the wire for a short term rather than a permanent high current. Beyond this lower limit, because the system is essentially self limiting, the trade off is between wire size and speed of charging.
This brings me to think about what may be a constraint at the other end of the scale with a connecting wire that is too thick and too short. My thinking is that alternators are designed to maintain their output voltage at 14.4 volts and will do their best to do this. My understanding is that it is not uncommon for cars started with a flat battery to suffer alternator burn out shortly afterwards for this reason. Perhaps, having too thick a wire to the auxiliary battery, especially with a little alternator like the 80 may be equally bad.
I now realise that this may be a reason for fitting a long, relatively thin wire from the 80 alternator to the furthest battery! Mr T knows a thing or two.
John

 

[Dave2000]

Your close here John. The stabilisation between batteries takes place very quickly, but when it comes to the alternator it is a different ball game. A battery with low charge will take as much as the alternator can offer, be it 60 amps or 160, and because the alternator voltage will remain high circa 14.4 then the 'push' of amperage is also going to be relatively long term so heavy cables are essential, if you uprate your alternator then the main cable (B+) must follow suit.

A couple of other points, cable should meet the maximum load it is liable to see along with a safety factor, in theory you cannot have too thick a cable, but of course trying to curve a 2" cable within an engine bay is hardly practical.

Re the alternator burning out, assuming it was in good condition prior the battery failure/flat battery it should be fine however, going back many years ago the brushes on cheaper alternators were square where they met the slip rings, this resulted in poor contact prior to running in. The result was overheated and burnt contacts, failure followed soon after. Nowadays brushes are curved so meet the contact surfaces with more surface area.

Sensing battery voltage at the battery furthest away allows for heat affecting alternator output, increases in resistance between battery connections and ageing cables. The difference between 13.8 volts to charge a battery, and the accepted maximum of 14.4 before there is potential for boiling can be the difference between a nearly charged battery and a very nearly charged battery, absolute full charge on a typical journey is a myth.

Regards

Dave

 

[ByronJ]

I have also found this thread interesting and informative. It has made me think about stuff I had not really considered before and I have learnt a lot. So thanks to all who pitched in

 

[JOHN OF LONDON]

Just a final word on this. The last alternator I bought (for the camel trophy land rover many years ago) had a sticker on it warning me to charge the battery before fitting it as charging a flat battery with it would void the warranty. It was explained to me that no alternator was designed to produce it's rated output for more than a few minutes but would run at 70% of its output when being driven in normal service; but at much less at tickover. Demand over this level would result in overheating and risk a burn out.
On this basis if we have concluded that my 80ah AGM battery, run down to 10.5 or 11 volts, will accept 60 amps or more for more than a few minutes the reliability will be compromised (at least!) by having a very high capacity wire to the auxiliary battery.
So my final specification will be a 16mm wire with a 140 amp VSR with a 100 amp circuit breaker at the starter end and a 100 amp fuse at the other end. In addition I'm going to add a voltage limiting cut off to the output side of the auxiliary battery so it switches everything off at 11 volts. I won't get all this fitted until October as I'm going to be away but I will try to get some measurements of voltages and amperages when it's all fitted.

Thanks for all your input chaps.

John

 

[StarCruiser]

John, if you are to be using an AGM battery you need a DC DC charging unit. I've just fitted an Intervolt one from Oz that I found on eBay which is very nicely made, specced and delivers 25A all day long if needed at the correct boosted voltage an AGM needs. Add to this that it has its own voltage and current display, solar input that charges vehicle battery as well and is a very neat, sealed, under bonnet unit that can work in high ambient heat. Using one manages charge and should prevent your alternator being overloaded.

The way the batteries are connected in an 80 means they are charged equally.
The alternator wire is not unfused. It has a fusible link in it adjacent to the battery terminal IIRC. Any wire taken from a battery at less than the starter cable size should be fused unless it can be proven that in a short circuit scenario the batteries would become depleted before serious damage can occur to the cable insulation or surrounding fabric of the vehicle.

 

[Dave2000]

Reliability will not be compromised by having too big a cable, too small on the other hand will be a fire risk. You cannot limit current by cable size....unless of course it has melted.

The cable size must be chosen to carry the worse case scenario for example, an 80 amp alternator would be fine with 16mm cable capable of carrying 110 amps, a nice safe cable choice. A fuse of 100 amps on the other hand will blow around 10% - 15% higher so 110 -115 amps, this could now compromise the cable IMO?

On my 80 the alternator charge wire B+ went direct to the battery and not via the fusible links, in fact IIRC this was the same on the last two 80's I worked on, I could be mistaken?

IIRC one fusible link goes to AM 1 and the other to the glow plugs, neither capable of carry 80 amps, it was awhile back so may be wrong on this? I removed the FL's and fitted fuses.

Regards

Dave

 

[StarCruiser]

I too may be mistaken on this. I'm going from memory with not having actually traced the wiring. There will be good reason either way as disconnecting a running alternator can cause damage to it. In any case increasing the cable size is going to be a positive step and IMO should be done like for like at the same connection points. If changed it will potentially charge the batteries unequally.

 

[ByronJ]

On my 80 the alternator charge wire B+ went direct to the battery and not via the fusible links, in fact IIRC this was the same on the last two 80's I worked on, I could be mistaken?

You are not mistaken. The B+ cable runs directly to the PS battery on every 80 I have ever seen, no fuses or links at all.

 

[JOHN OF LONDON]

Yes I looked at battery to battery chargers and decided on balance that I didn't want to go that way on two grounds. Firstly good quality ones seemed to me to be quite expensive but mostly because they prevent the use of the starter batteries to support the auxiliary battery. Using a voltage sensing split charge relay allows all the three batteries to run the fridge etc for a while until the system voltage falls to the cut off level before running the auxiliary battery down to the minimum level. This has the dual effect of allowing either less stress on the auxiliary battery or a longer "stationary" time and, by charging the three batteries simultaneously making the best use of the power available when moving.
Dave, I'm not convinced of two things and I'm hoping that when I've installed my system I'll be able to take some real world measurements and convince myself at least that some of my assumptions are right. My first expectation is that the auxiliary battery will not accept any level of charge that will come close to making the 16mm charging cable warm. Even if the current hits 100 amps when first connected I shall be amazed if it continues at that level for more than a few seconds bearing in mind that approaching 50 amps of this will be being provided by the starter batteries.
My biggest concern in the new set up is that following a long stop with depleted starter batteries and a flat auxiliary the strain on the existing 60 amp alternator will be to great for it to bear.
I'll return to this thread in October when the system's all wired up and I can get a voltmeter and my Chinese induction ammeter to give me some indication of what is going on.

John

 

[StarCruiser]

AGMs need more voltage than your alternator can produce. There's threads on here that highlight this, hence the DC DC systems. You seem dead set on doing it the way you have highlighted. Good luck with it.

 

[JOHN OF LONDON]

Nothing to report yet on the auxiliary battery set up as it has moved to the back burner on the basis it won’t be needed till summer. New (to me!) Lincoln Mark VIII fan arrived from the USA and fitting it has taken precedence. All seems pretty straightforward and the instalation is complete apart from a couple of outstanding bits that you knowledgeable chaps can possibly help with. Firstly there is a chunk of radiator that is no longer covered by the fan shroud and I am concerned that when stationary, and the prime use of the fan is to cool the condenser rather than the radiator, air will be sucked "backwards" through the uncovered bit of the radiator, into the space in front of it and back through the rest of the radiator without going through the condenser which needs it. Once, a very long time ago, my Orion had a covers with little flaps in that prevented reverse flow. I am wondering if anything similar that would fit might be found at the breakers. Anybody seen such a thing? The second need is for some crimped inserts for the trinary switch so I can wire up the fan to it. The alternative seems to be an expensive pigtail from America and more soldered connections which is much less neat. Anyone know where to buy the Denso crimped connectors and little plastic plugs?

While the electrics are all in chaos I have decided to fit a bigger alternator; which Robson and Francis sourced for me and to look at my wish list for electric modifications.

The list consists of:-

An inertia switch,

A batteries cut off - I've heard about too many fires!

A low oil pressure warning which will also kill the engine if oil pressure disappears

A hidden switch to confuse the ungodly and discourage theft

I've filled many sheets of paper with wiring diagrams trying to achieve all the above and the final (I hope) version is


Some explanation is probably needed to understand what is supposed to be going on. The hidden switch is an OFF ON MOM switch like the ignition and starter switch you had in your kit car. The timer is the adjustable delay relay from 12Volt Planet; the rest are standard bits though the timer demands diode protected relays. I envisage the batteries being cut off by two NO 200amp solenoids with the driver’s side being bridged by a small (20amp?), probably self resetting, circuit breaker.

If I’ve got the wiring right we start off with everything switched off, with just the red LED flashing. The sequence would be to operate the hidden switch, which energises the timer, which connects the batteries. It also operates the fuel relay, but doesn’t energise the fuel solenoid.

Operating the key and starting the engine will then operate as normal provided the engine starts and the oil pressure switch opens before the timer switches off. I think 5 seconds will do this but the process can be prolonged by keeping the momentary switch closed by hand. (I think it would all work fine if an ordinary relay was substituted for the delay timer and the “mom” switch used while starting but I wanted the delay on the oil pressure loss, see below)

If the driver attempts to start without closing the hidden switch and energising the timer he will hear the buzzer and nothing else will happen (the same should apply if it is hotwired behind the dash). Because the NC terminal of the fuel relay is connected to the output side of the thermistor that informs the glow plug ecu I am hoping that this will fool the ecu into thinking that the engine is hot and the glowplugs unnecessary. If I am wrong on this and the glow plugs switch on it will trip the circuit breaker turning everything off until it resets and I’ll be back to the drawing board.

If, at journey’s end the ignition is turned off without operating the hidden switch the buzzer will sound, after 5 seconds, as a reminder. If oil pressure is lost the oil light comes on and the buzzer sounds, and after the timer delay the engine stops. I think this will be enough to cover those occasions off road when the jaunty angle takes the pickup out of the oil in the sump briefly; if not it can be overridden with the momentary switch.

I’d be really interested, if anyone can be bothered, to hear any views on whether this will work or not before I wire it in; or if anyone thinks it is all too complicated to be reliable or sensible and there’s an easier, simpler way of achieving the objects.


John

 

[StarCruiser]

Sounds a bit like you need to mock it up on the bench John. Personally I would go with an aftermarket alarm immobiliser, but it looks like you want to do more than just that. Mock it up is the best advice I can give. You asked earlier about plugs and sockets. If you search sealed vehicle connectors on eBay you should find some.

 

[StarCruiser]

Only connect what to the RHS battery?

Sorry @ByronJ i must have missed this question. The answer is any form of load, including a split charge relay and leisure battery. Why? Because during start the + of anything connected to the LHS battery is elevated to 24v from ground and you really don't want that. If you connect across the LHS battery only then that load will remain supplied at 12v but it's negative will be at 12v above ground (body/chassis) and the positive at 24v.

 

[JOHN OF LONDON]

Hi SC,
The problem with old diesels is that they are easy to start. Jumping the fuel solenoid and the starter relay is usually enough and every teenager knows how to do that.
Apart from parking at full lock with the steering lock on I really want to do something that makes it a bit difficult and not too obvious. The only bit of my wiring diagram that I am not reasonably confident about is trying to fool the glowplug ecu so perhaps I'll give that a try and see if its convinced by 12volts at the thermistor!

My looking for the Denso terminal inserts is a bit obsessive I know but it would be nice to find the OEM ones to make a neat job! You can buy them in 200 unit lots from Alibaba but there are loads of different sizes. I'm trying to work up the fortitude to visit our local Toyota dealer to ask if they have them but I'm not hopeful.

John

 

[StarCruiser]

Hi SC,
The problem with old diesels is that they are easy to start. Jumping the fuel solenoid and the starter relay is usually enough and every teenager knows how to do that.

I expect most teenagers these days would be looking for where to plug the laptop. Steering locks work unless broken off and trying to hotwire a vehicle with an alarm sounder going at 120db should attract at least a raised eyebrow or two, not to mention increase the stress level of whoever is trying to drive it away.

 

[Dave2000]

I used to build custom alarm systems/immobiliser years back. First thing to remember is that police do not look for stolen cars, the stolen car needs to attract attention to it, of course alarms are often ignored as well.

So, unbolt the middle row seats, lift the oval plate, now fit a solenoid fuel shut off controlled by a relay and reed switch.

No fuel will flow when no voltage is applied, apply 12v and fuel will now be able to flow.

Wire your relay to latch, and gets its coil voltage via the ignition switch, glue a magnetic reed switch behind the dashboard or somewhere of your choice, check that your selected magnet works the switch ok.

Supply to the reed is also via the ignition.

So, thief gets in car, breaks steering lock, hotwires fuel pump and starts engjne, jumps in and drives off, a few hundred yards down the road the engine cuts out, your car is now a pain in the arse blocking a road or someone's gateway, police called immediately!

Your use is, jump in car switch on ignition, pass the magnet normally:

Hanging from key ring.
Casually thrown in centre console.
Glued to your favourite ring.
Enter other choice here.

across the area where the reed switch is glued and relay activates, the latch method will keep the relay energised, if you switch off the ignition the relay will release cutting the fuel making red LED flash. Make sure you use a 5 pin relay, 87a can be used to flash the LED warning thieves AND YOU the fuel shut off is active.

Built and fitted them to many expensive vehicles, never a problem, even used timers to ensure the fuel shut off did not energise until the journey from say a remote farm to a town centre had been covered.

Simple, reliable, easy to hide wires in OE loom,

Regards

Dave