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Author: Subject: Alternator - How Big?
spegru

posted on 1/9/21 at 09:08 AM Reply With Quote
Alternator - How Big?

For my Gilbern Invader / Jag V6 / RX8 Gbox build, I am a little short on space between the lower chassis rails sine I want AC and Electrics. The S-Type version of the AJ30 has AC pump on one side and the Alternator on the other. It *just about* fits. But the Alternator is knackered (cracked casing) so I am v interested in a smaller one. One of those 40A/ Kubota/ Daihatsu / Suzuki devices is *alot* smaller but I am worried about the output power. I read somewhere that hat these little ones only do 30A at idle
Thing is that output power of all Alternators seems closely tied to overall size so If I want more output I need a bigger unit.

So my question is how much current do I really need? I am planing to use LED lighting so that will help alot but how much electrical power does the engine need? Seems clear that more cylinders means more ignition, Injection etc. Somewhere else I read that the fuel pump could take as much as 15A and that the whole thing could be as much as 50A, although not sure if I believe that TBH

I can do the maths on lighting, fans, wipers etc but has anyone actually measured current draw on a running engine with ECU and electric pump?
I guess we could assume it's roughly in proportion to the number of cylinders - ie if there is a 4 cylinder figure avail, simply multiply by 1.5 for a 6 Cyl

One way to measure would be to put large spec Ammeter in series with Alternator and disconnect the battery while running. If mine was running I could measure myself I suppose, but that's a way off...

On the other hand the Denso 40A item is tiny @under 100m diameter vs the Jag (150A?) being about 135mm. Maybe there are other types somewhere in between?

[Edited on 1/9/21 by spegru]

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r1_pete

posted on 1/9/21 at 09:44 AM Reply With Quote
Add up the amps require for all items that have an amps rating. Excl starter motor.
For items rated in watts divide watts buy 12 to get amps at 12v
Add the lot together and that is your maximum drain, i.e. that rainy cold night when you have everything switched on.

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CosKev3

posted on 1/9/21 at 10:41 AM Reply With Quote
I struggled with a Denso 40 amp at idle,with headlights on,10inch fan running,small electric water pump flat out etc mine would not hold above 12volts for long.

I've now fitted a land rover freelander alternator which I think is 110amp and that is spot on,rarely see under 13.6volts even sat idling for ages.
And that's now with a 14inch fan which is stated to draw over 20amps,a Pierburg EWP that's stated to draw 36amps plus fuel pump,coilpack etc

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40inches

posted on 1/9/21 at 10:55 AM Reply With Quote
I fitted the 70amp Denso, it's very little larger than the 40amp.
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spegru

posted on 1/9/21 at 11:58 AM Reply With Quote
Thanks 40" & CosKev it would be great to understand exactly how the body size relates to the output. The jag one is 13.5cm and while it just about fits, any reduction would be welcome. How big would you say the main diameter (excluding mounting brackets) is for yours?

I can do the maths on all the components such as wipers etc but I have no real info on how much an injected engine takes to run itself: ECU Coils & Injectors etc

One of the issues is finding an ammeter that is 'big' enough to measure the necessary current. Most Multimeters only go to 10A, if that.
So I have just ordered a clamp type meter, that you just hook over the necessary wire, *without breaking into it* in some way - which would be necessary otherwise. An Item I just found goes to 400A which should be more than enough

https://www.ebay.co.uk/itm/383864193104?hash=item596016c050:g:drUAAOSwqFRgi9vm

Once Ive done the measurements (or if anyone already has such info) I'll do the maths on the ancillaries such as lighting wipers etc, and see if I can calculate what's needed. Although I don't have a 6 cyl (injected) engine to hand I do have a 4 cyl up and running, and I expect to use a 1.5 multiplier on whatever that figure is.

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cliftyhanger

posted on 1/9/21 at 03:38 PM Reply With Quote
injectors/sparks will use very little, 2-3A??
Fuel pump is worth checking, as are wipers, rad fan and heater fan. I assume no electric seats and so on?

I run a denso 40A on my spitfire, halogen headlights, golf fuel pump etc etc.
It has done many trips, including wet at night. Never an issue but I don't have any worry gauges. With LED lamps, I wonder if a 40A would suffice, but a 70 should be plenty. This also assume a mechanical (not electric) water pump.

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CosKev3

posted on 1/9/21 at 04:05 PM Reply With Quote
The other thing to work out is maximum alternator RPM.

That's another reason why I struggled with idle voltage, as had to be careful not to exceed the max alternator RPM with the engine revving to 7100rpm,so small pulley and small alternator meant idle voltage was a struggle.

On my new engine I've built I've fitted a TTS under drive crank pulley

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big_wasa

posted on 1/9/21 at 04:47 PM Reply With Quote
Iíve have a small 70a Denso style one for mine. If you have the space I would fit what ever was to hand.
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spegru

posted on 1/9/21 at 06:53 PM Reply With Quote
Hi BW. It's been a while. What's the body diameter (excluding mountings) of your 70A Denso?

It seems quite possible that alternators are always over rated, perhaps to allow for wear over 100kmiles or whatever. Our Dutton has a tiny 40A jobbie and it's always been fine It is only a 4Cyl but it does have halogen headlamps - not that it ever gets driven a night!

It will be a few days until my clip on ammeter arrives. Then perhaps I'll be able to test this stuff properly

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40inches

posted on 1/9/21 at 06:54 PM Reply With Quote
quote:
Originally posted by big_wasa
Iíve have a small 70a Denso style one for mine. If you have the space I would fit what ever was to hand.

Wise words. I think that I still have the Jag alternator kicking around if you want it, unless I am mistaken it was manufactured by Denso

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spegru

posted on 1/9/21 at 08:02 PM Reply With Quote
I'll hang fire on deciding which to go for. I'll take all the available space that's available.If a 40A will do, that's what I'll use
On the other hand alternative dimensions are always interesting if you have any.
I def wont be using the original S-Type device as the mountings are now unsuitable since I changed the sump over to an X Type one, and anyway it's broken

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sdh2903

posted on 1/9/21 at 08:44 PM Reply With Quote
I have the aj30 and have a (genuine) Denso 40a alternator and a 74mm pulley. I also have heater, heated seats, wipers, coil on plug, electric water pump and a very heavy duty spal fan, do have led lights all round. Not had a problem. It maintains over 13v apart from when everything is on (absolutely everything) and the fan kicks in and it'll dip to 12.7 ish volts.

I was told by a local alternator specialist that a good genuine denso 40a will kick out more like 50+ but there are a lot of crap copies out there. If and when it does go pop I'll prob fit the 70a version.

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russbost

posted on 2/9/21 at 08:33 AM Reply With Quote
I'd be amazed if you ever had a constant draw of more than 30 - 40A. Fuel pump is the single heaviest constant draw, anything that kicks in & out like cooling fan (typically around 8 - 10A) is fairly irrelevant unless you're planning on sitting in stationary traffic with the engine running for hours, if it's not quite adequate at idle that should be irrelevant as it will just draw a little power from the battery which is replaced as soon as the revs go up & alternator power increases.

No. of cylinders doesn't make a lot of difference as ignition/injector loads are all quite small

If you have the room probably better to have a larger, less heavily loaded unit, but if space is really tight 40A should be fine

[Edited on 2/9/21 by russbost]





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big_wasa

posted on 2/9/21 at 11:30 AM Reply With Quote
Re the size I will have a measure at the weekend but body size is very similar to the 40a and lug size just a little bigger.
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snapper

posted on 2/9/21 at 06:33 PM Reply With Quote
IMHO, you donít need to spec the alternator for the absolute maximum drain as it is very unlikely you will ever be running all the items at the same time, sure an electric water pump, twin fans, heater fan, air con and a huge stereo system could drain it but if you have battery in the boot fit a bigger battery to even out the drain





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CosKev3

posted on 3/9/21 at 05:51 AM Reply With Quote
quote:
Originally posted by snapper
IMHO, you donít need to spec the alternator for the absolute maximum drain as it is very unlikely you will ever be running all the items at the same time, sure an electric water pump, twin fans, heater fan, air con and a huge stereo system could drain it but if you have battery in the boot fit a bigger battery to even out the drain


You shouldn't need to pull power from the battery with the engine running,that's a stupid idea.
If you are at the stage where you have the chance to fit a alternator that is easily strong enough that you don't need to worry about it ever struggling you would be stupid not to!
Nobody plans on getting stuck in traffic,but when it does happen on a boiling hot summers day the last thing you want is to be sitting hoping your alternator is strong enough!

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jeffw

posted on 6/9/21 at 07:36 PM Reply With Quote
You need an alternator which will be energised at idle, survive the max rpm (work out the pulley ratios) and provide enough amps to charge the battery (or at least not discharge it) while you have the fan etc running. A 40amp alternator doesn't produce 40 amps at low rpms.

Have a look at the Brise site, this will give you an idea of current v rpm of the alternator for a 60 amp. https://brise.co.uk/denso-60-amp.html />
[Edited on 6/9/21 by jeffw]

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spegru

posted on 7/9/21 at 04:23 PM Reply With Quote
It turns out that the clamp meter that I bought only measures AC current so that's not much help to find out the base load requirement from the engine itself.
Russbost you may be onto the point.
I'd still like to actually measure the load though. Problem is most ammeters do not go high enough.
It seems likely that a shunt resistor is needed so that an 'indirect' measurement using a volt meter can be made (eg 1 Volt across a 1 Ohm resistor equals 1 Amp) - but the shunt resistor would be in series with the alternator - so it needs to be able to handle the current of ~50A. ie it's a big one!





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russbost

posted on 8/9/21 at 07:25 AM Reply With Quote
Most ammeters will do 10A, if you simply pull fuses from all non essential stuff then you should be able to get a figure for the running engine, you'll need to "bridge" across the ammeter to start the engine with a jump lead, but once running you should be able to remove the extra lead & get a running value.
Then turn engine off & remove the remaining fuses, check you have zero draw, then add each fuse in turn & measure the mnax you can draw on that circuit, ie on lights, sidelights & main beam, on heater blower max speed, wipers, max speed & dry screen etc etc. A few circuits like rad fan you'll need to bridge the temp switch, but you should then be able to build a complete picture of just what the total load might be.

Don't forget that though theoretically total might involve heater fan on high, wipers on high, fan running, main beam etc etc this is a scenario which will probably never happen & if it does the chances of you being idling for any time in traffic is virtually nil (you certainly aren't going to have main beam on & highly unlikely to be using wipers on fast & it won't be on a dry screen!), you only really need to look at realistic likely situations

Be interesting to see the results





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gremlin1234

posted on 8/9/21 at 04:13 PM Reply With Quote
quote:
Originally posted by spegru
It turns out that the clamp meter that I bought only measures AC current so that's not much help to find out the base load requirement from the engine itself.

the clamp meter I have is
https://cpc.farnell.com/tenma/72-7226/clamp-meter-trms-ac-dc-current/dp/IN05111?st=dc%20clamp%20meters

(true rms-ac, and dc current)
£52.74 Inc. VAT

and did you know that with clamp meters, if you wind the wire through twice it halves the scale. (and if you wound it through 10 times, the 40A scale would become a 4A scale!)

edit, note when I got mine the included 9V battery was flat. so budget for a 9V pp3 as well.


[Edited on 8/9/21 by gremlin1234]

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spegru

posted on 9/9/21 at 08:39 AM Reply With Quote
Hi Gremlin, that meter looks to have an identical casing to mine. Is it the same inside I wonder. Your data sheet is a bit better, but even on this, it's not very clear what the clamp meter range is - esp on DC Current. Mine doesn't mention DC current at all.

Anyway I did run a test on a 4 cyl engine by inserting a fairly normal 10A multimeter in ammeter mode, in series with the alternator. I got the car started and then disconnected the battery. The engine kept running purely under Alternator power and the reading was just under 12A.

However if I revved the engine at all it would falter and stall. Not sure if that's because the meter was causing too much resistance at higher loads (after all it is only rated at 10A). I need a higher rated meter really. I suppose I could use two meters in parallel and add the figures together..... Incidentally when I tried using the clamp meter onto the alternator I only got a reading of ~2A. but that could just be AC 'noise' on the DC and not a true reading



Anyway 12A for a 4cyl at idle up to ~1500 rpm. This might indicate 18A for a 6cyl but I imagine much of the 12A is the fuel pump (really need to connect a meter in series with the pump to check that) and it seems unlikely that fuel pump current would scale with number of cylinders.

I'll report back with fuel pump data......

[Edited on 9/9/21 by spegru]





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spegru

posted on 9/9/21 at 02:57 PM Reply With Quote
I've just done some more current load testing, (also discovered that I blew a couple of fuses in the meter whiIe was at it yesterday)

I realised that there is no need to disconnnect the battery if you can interrupt the electric feed to the items concerned. I did this at the fuse box, by puling the fuse and inserting the ammeter into the fuse socket. Just as well for this I have separate fuses for fuel pump and ECU/Coils (shared fuse)

Fuel Pump load varies quire a bit but seemed to peak/average ~14A although I did see a momentary 18A.

BTW The pump system is a Bosch 044 type and one of those facet solid state jobbies feeding a swirl pot tank from the main tank


I also ran a test on the ECU/Coils that therefore includes ignition and injectors?: (I think those ford Zetec coils are earthed by the ECU and fed directly but they share the same fuse on this setup)

This showed about 2A at idle rising to ~8A at high revs

No reason to expect Fuel pump load to scale up with the number of cylinders but ECU/Coils/Injectors probably does

So I'm going to hazard that base load for running the engine itself is ~14A+8A = 22A at max revs (when Alternator output is highest) for a 4Cyl which could equate to 14A + 8x1.5A = 26A for a 6Cyl.

LED lighting aside I can see that Rad Fans/ Wipers etc in addition to this could make the 40A alternators quite marginal - but perhaps could just about do it if in tip top condition

spegru


PS
I noted that the engine did not falter with increase speed as it did with everything running through the alternator like I did last time and also that I got a momentary reading of 18A - which indicates that the meter does not max out at 10A as I expected. Probably running everything through the meter at once as I did yesterday is what was causing that problem, as well as blowing the internal meter fuse.

[Edited on 9/9/21 by spegru]

[Edited on 9/9/21 by spegru]





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gremlin1234

posted on 9/9/21 at 04:49 PM Reply With Quote
Its a bad idea to run an alternator without a battery in circuit, so yes testing individual circuits via their fuses is a better way to do it.

you can also measure the voltage drop across the fuse, and get a good indication of the current.

https://www.meeknet.co.uk/E64/Fuse%20Voltage%20Readings%20To%20Give%20Current%20Readings.pdf

this method is particularly useful for testing parasitic loads, because you don't need to break the circuit, and 'wake' systems from sleep states.

edit, similarly clamp meters have this advantage

[Edited on 9/9/21 by gremlin1234]

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spegru

posted on 9/9/21 at 07:48 PM Reply With Quote
Wow that PDF contains a useful chart Ive never seen before. I had no idea that differently rated fuses have a definite resistance.
I'll use that approach another time....

By the way. Since my measurements today, I've discovered that the alternator had a loose mounting bolt which certainly wont have helped get full output from it. Could well explain the battery charge problems we've been having, and the faltering I got when running without battery connected yesterday. Luckily though, that doesn't invalidate today's measurements on fuel pump, ECU etc - it's just that some of the load was likely coming from the battery rather than the alternator





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gremlin1234

posted on 9/9/21 at 08:01 PM Reply With Quote
quote:
Originally posted by spegru
Wow that PDF contains a useful chart Ive never seen before. I had no idea that differently rated fuses have a definite resistance.
I'll use that approach another time....

By the way. Since my measurements today, I've discovered that the alternator had a loose mounting bolt which certainly wont have helped get full output from it. Could well explain the battery charge problems we've been having, and the faltering I got when running without battery connected yesterday. Luckily though, that doesn't invalidate today's measurements on fuel pump, ECU etc - it's just that some of the load was likely coming from the battery rather than the alternator

yep all we need is no 'load' draining the battery in normal running

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