Alternator upgrade

[ghost123uk]

Hey peeps, p.s. = or you could just crank it over, coil LT disconnected (so it won't start) and whilst doing so connect a single jump lead from engine case to a good clean body/chassis earth. If when you do this, the cranking speed can be heard to increase, you have a defective engine to chassis earth. Perhaps an easier and more "real world" test than mucking about with a multimeter

[CovKid]

Good one, and more fun than testing a PP3 battery on your tongue.

[CovKid]

Ghost might know (he's big on DC). I assume that the more you switch things on, the hotter that 6mm dash wire gets and the more current it consumes (now its become a resistor in effect) and the voltage drops. All the same, it was hard to ignore. I'm not a 'volt-watcher' despite having one at the front but I'd grown familiar with the readings I'd get with certain things switched on. It all changed with that new dash cable and I'm on the same alternator. To be honest this is brilliant news as the short runs to work each day barely put a charge in when I was using headlights. I use the DRLs mostly now (unless its actually dark) so seem to get a good 14.5v most of the time but even with them on, it barely drops. I still don't know if 16mm is overkill (suspected 10mm would be OK) but perhaps it does make a difference after all.

That's a good point. In fact it made me think again as to whether my reduced voltage drops are down to just improving the cable run between the alternator and the starter, and nothing to do with the increased amps that the alternator can provide after all.

[ghost123uk]

If I understand what is being asked correctly, then I would say this:- If one had a "draw" of say 35 Amps, and one had a 45 Amp alternator that was in perfect nick, then changing to a 90 Amp one would make no difference at all. Though during all this recent talk of upgrading wiring etc, we have barely covered something, though it has been mentioned. That is the amount of Amps that a somewhat depleted battery draws from the alternator.

In my experience what happens is this. Firstly lets assume a somewhat depleted, say after prolonged cranking or overnight leisure use battery. On first firing the motor up, a fairly hefty current flows into the battery, maybe 20 Amps or more, BUT, this only happens for a very short time indeed, after just a few seconds the current starts to fall.

The reason for this is simple really. A current only flows when there is a Voltage difference. A direct short for example, i'e. nought volts at one end and 12.9 Volts at the other, causes HUGE currents to flow (wire size permitting). Now a "somewhat depleted" battery might be sitting at 11 Volts and the alternator is going to produce 14 Volts. The battery has it's own internal resistance between it's +ve and -ve terminals. So, when the initial ~20 Amps goes into the battery, the Voltage at the +ve terminal rapidly starts to rise, this in turn produces LESS of a Voltage difference between battery and alternator, so the current drops. I have measured the input into a somewhat depleted leisure battery (after about 5 minutes to allow for that initial surge) and it dropped to around 8 Amps, reducing further to a steady ~2 Amps after a run out. That 2 Amps is due to the Voltage difference between a fully charged battery (12.9 Volts) and the alternators design output of ~14 Volts.

It is good that the current drops because your average battery can not take input currents that high for long without damage.

So, your 90 Amp alternator might be slightly better when :- you have "everything on" and have a somewhat depleted battery(s). It will be able to produce more charge during that initial high current phase of battery charging, but once that phase has passed any "headroom" is not being used at all.

Having said all that, if I find a 90 Amp alternator cheap at the scrappy, I would but it and fit it

[CJH]

Thanks Ghost, a good description. It may indeed be the complete answer I was looking for. But...

I leave my van hooked up to the mains on my drive during the winter, and I have a CTEK charging system that ensures all three batteries are always topped up. The two leisure batteries get a continuous float charge (once the charger detects they're full) and the starter battery gets a regular pulse charge. Maybe the 12.6V-12.7V that the starter battery sits at is sufficiently below the 14.5V that the alternator provides that the voltage difference does cause a burst of high current. But what I noticed with the 45A alternator was that it would take a lot of driving (>30 mins) before the leisure battery voltage would creep up to 14.5V, even with nothing else running. This always puzzled me, given that all the batteries were in good condition and nominally full. I'd say that showed that my 45A alternator was a bit marginal in my setup. With the new alternator it reached this level in seconds from a cold start.

What I did notice yesterday while experimenting with the different loads, was that with fridge and lights full on there was a really noticeable drop in engine idling rpm. I wonder just how much more petrol my 90A alternator is going to use over the 45A.

[ghost123uk]

What I did notice yesterday while experimenting with the different loads, was that with fridge and lights full on there was a really noticeable drop in engine idling rpm.

I wonder just how much more petrol my 90A alternator is going to use over the 45A.

Ahh, now you see, = no more because it's just charging it up quicker, then tailing off, instead of slower and tailing off much later. The amount of energy used remains the same. The law of the conservation of energy cannot be broken

By the way, at "full chat" your 90 Amp alternator will use up about 1.5HP

[CJH]

Ahh, now you see, = no more because it's just charging it up quicker, then tailing off, instead of slower and tailing off much later. The amount of energy used remains the same. The law of the conservation of energy cannot be broken

Understood, but only if I drove long enough with the old alternator for it to have tailed off.

By the way, at "full chat" your 90 Amp alternator will use up about 1.5HP

Good job I'm building this 2.1 then.

[CovKid]

So, and I'll try not to cover too much old ground, if cable capacity from battery to dash is increased (and I'm fairly sure the stock 6mm supply is maxed sometimes), we already know it gives a better supply for headlamps but am I right in thinking that the resistance in the stock 6mm cable was reducing available voltage/current generally? The voltmeter would seem to indicate this. If we summise that at times the cable became warm, it must be acting like other ancilleries and stealing power.

[ghost123uk]

am I right in thinking that the resistance in the stock 6mm cable was reducing available voltage/current generally? The voltmeter would seem to indicate this. If we summise that at times the cable became warm, it must be acting like other ancilleries and stealing power.

For sure Ralph, yes, if a cable is getting warm it is certainly using up some of the power. You could work it out quite easily (if you really wanted to ). Measure the current flowing with "every thing on" (everything fed from the dash feed cable that is), Now measure Voltage at the battery, subtract from that the Voltage at the other end.

Figures here "out of a hat" (put your own in) :-

20 Amps flowing to fusebox.
2 Volts difference between battery and fuse box end feed

Using Ohms law - 20A X 2V = 40 Watts being wasted (as heat) which in turn = 3.3 Amps wasted in producing that heat.
Again using Ohms law, 2V divided by 20A = .1 of an Ohm (is the resistance of the cable)

[CovKid]

That accounts for it then John. I must admit I hadn't expected that side effect at all and it came as quite a shock when I saw the higher voltage at the meter which is still fed by the old 6mm run. I should retest using a multimeter I suppose for absolute accuracy but it does prove your point that an alternator upgrade may not be the answer at all - particularly on stock wiring. A 45amp alternator should be plenty really. Before, I'd occasionally see 14.5v on the meter but it wouldn't take too many items at trhe front to pull that down to 12.8v. It now seems reasonably consistent.

If you look at most 30 year old vans, you'll find twisted wires under nuts on the starter, frayed and oxidised alternator cables, badly fitting battery wires, corroded earths, all sorts. None of this helps. It seems logical that sorting those things out and replacing any cable that has a greater load placed on it, will do more for charging and eliminating voltage drop than fitting higher-rated regulators or big alternators. I think as we noted some time ago, trying to force more power through is masking the problem.

These things have become essential lately:

[ghost123uk]

Sure, but you can only "force" more power through the system by increasing the regulator Voltage, not the current capability (I know you knew that). That was covered somewhere and I think most agreed that adjustable regulators are naff and a decent standard 14.5 Volt one is the way to go.

So as not to pee Chris off though, after upgrading his alternator, I have to say (as I did ^^^) that if / when I need another one, or even just if I come across one cheap, I would fit a higher rated one. I can't really say why though It's just nice to have a bit in reserve I suppose.

[ghost123uk]

Propex 2 Amps
Engine 8 Amps
Lights (x2) 10 Amps
Heater blower on full 8 Amps (???)
Rad fan on full 12 Amps (???)
Radio (playing reggae !) 5 Amps
Tail lights/ instruments etc ~5 Amps
Fridge 8 Amps
Trickle charge both batteries 4 Amps
EDITED = Wipers on full 10 Amps (???)

Did I miss anything off ??

= 72 Amps = Chris, you have a point !!!!!!!!!!!!!!!!!!!!!!

[CovKid]

Is it less if you play classical rather than reggae? Less bass = less power.

Will try an encapusulate all this and add into battery to dash WIKI

[ghost123uk]

Check my figures ^^^, they are just off the top of my head, and did I miss anything off the list ?

Of course, my list is "worst case" as it is unlikely you would ever have absolutely everything on at once, such as heater blower and rad fan

Oh, missed off wipers and they use quite a bit don't they.

[CovKid]

Even so, thats pushing the limits of a 45amp alternator. Combination is perfect solution then. 90amp alternator and fat cables = happy camper. Phew - I've got both now.

What a morning - sat here drying freshly-painted headlight pods in front of a fan heater. Good job I'm single