Amperage to expect from Alternator

[jono2.0]

Hello,

I've owned my very first narrowboat for exactly a month so far so am a real newbie.  I have:

Sailaway 60'
4x 110Ah Lead Acid wet leisure batteries
Canal Line 42 engine with 175A Alternator and a 50A for engine battery
Victron Multiplus 12/3000/120
Victron BMV-700

I've wired up the batteries so that everything is via the shunt with nothing connected the battery side neg.  

My question is what sort of amperage should I expect to get from the alternator?  I realise you should take every reading from Victron BMV-700 with a pinch of salt but am getting the following readings:

Currently, Battery at 91%
Battery 13.78v
28.10A is being put in from the alternator.  I've seen the amps as high as 57A but never higher.  I'm trying not to let the batteries get too empty as they are only a month old.
 

I have set the BMV-700 up as best as I can using the Bluetooth settings tab so it's not at default settings.

Just curious really as I'd have thought a 175A Alternator would pump more out.  I realise the more already charged the battery bank is the less amps needed to fully charge - that could well be it.

Thanks Jono

 

[Robbo]

Batteries will take what amps they want, if they are are nearly fully (above 80%) charged they can't take as many amps as they can when only 50% charged.  You will also need to give the engine some revs, tick over won't spin the alternator at the revs required for max amp output.

[WotEver]

A few points. 

If you rev the engine a bit does that voltage rise higher than 13.8? That's way too low a voltage for batteries still in bulk. 

How are you estimating the 91% charged? Don't trust the BMV, they tell lies. At 14.8V charging (a whole volt higher than at present) the batteries won't be charged until the current falls to around 4 or 5 amps. At 13.8V they will take weeks to fully charge. 

To answer your original question you'll only see a high current from the big alternator when the engine is running at highish revs and the batteries are very discharged. Even then, not for long. 

[MtB]

Jono,

Broadly speaking a generic alternator isn't going to charge your batteries to 100% in less than a dozen or three hours. Possibly a whole lot more, if ever. Their output fades away as the battery charge rises, just as you've found.

This is the reason firms sell external alternator regulators. To artificially lower the voltage presented to the alternator to fool it into delivering its max current (for a while at least).

[larryjc]

Plenty more expert than me will reply but:

The batteries will only take what current they can due to build up of internal resistance as they charge up.  So if they are well charged then they won't take much.  When 'fully' charged they will probably only take less than a few amps irrespective of the size of the alternator.

13.78 volts is very low - if that is the charging voltage you are getting with the engine running - but if the engine is only at tickover the alternator may not be properly working.  Try it with the revs at least 1200-1400 rpm.  If its still that low I suggest you do have an alternator problem.

My last boat had a 120 Amp alternator and I never let the batteries go below 75% and never saw more than 50 amps at engine start and that soon dropped to less than 30. When it was less than 5, I was happy - but at 14.4 charging volts.

 

Bugger - 3 people can type faster than me!!!

 

 

Edited March 3, 2017 by larryjc

[nicknorman]

We have probably the same alternator on a Beta 43. The "morning after" on starting the engine with say 75% SoC, I see up to 160A or so, but this falls off within a minute or two to 120-130A, and then further decreases over time. The Beta 43 can do this just above idle, say 900rpm, though I try to keep the revs a bit higher to start with, for alternator cooling and to keep the belt loads down.

If you are only getting 13.78v at 28A, something is wrong.

Check the voltage output direct on the alternator terminals using a handheld multimeter / DVM. I would expect it to be 14.4 or so at 28A. If the alternator voltage is not much over 13.8 at 28A despite it spinning fast, it is broken. Be careful of the belts.

If the voltage is up around 14.4v then:

Since fairly high currents are involved with these alternators, any weak link in the connections chain ie thin cable, poor connections, will drop a lot of voltage and reduce the current substantially. So use the handheld multimeter / DVM to measure the voltage between the alternator +ve terminal and the battery +ve terminal with the engine running (be careful of the belts). There should be no more than 0.1 or 0.2v voltage difference at high currents, much less at 28A. Then repeat measuring the voltage between the alternator negative (probably the casing) and the battery -ve. Again, no more than 0.1 or 0.2v difference at high currents, less at 28A.

If you find significant voltage drop, try to follow the cable and measure the voltage drop between the alternator and any point where there is a connection. Battery isolator switches are a good place to start looking! When you find the source of the voltage drop (if any) you need to fix it!

 

[jono2.0]

wow, loads of answers already - thanks.

 

OK, I've just been out in the rain (cruiser stern) and monitored the BMV-700 via Bluetooth on iPhone and I revved the engine way over 1500rpm and the max voltage was still below 14v.  So I think either something isn't connected well enough or I'm doing something wrong with cabling.  I'll check the alternator directly with a multimeter when it's not dark, cold and wet! (thanks nicknorman your reply came in whilst writing above)

For now I've got my Honda EU20i, which works really well on LPG I may add, plugged into shore line and MultiPlus is very happy.  Volts now 14.47 and amps 38.9A.

I'll leave it on for the rest of the eve to allow it to go into float etc.
 

Bound to me my dodgy newbie wiring! lol

[Richard10002]

Unlikely with a 175A alternator, but he might have an old alternator which only outputs 13.8V. On my seagoing boat built in 1995, the alternator was old and only output 13.6V - pretty much useless, but I didnt understand things in those days, (2006-2010).

[jono2.0]

2 minutes ago, Richard10002 said:

Unlikely with a 175A alternator, but he might have an old alternator which only outputs 13.8V. On my seagoing boat built in 1995, the alternator was old and only output 13.6V - pretty much useless, but I didnt understand things in those days, (2006-2010).

 

Thanks but I hope this isn't the issue.  The engine is exactly 1 month old today and straight from Canal Line.  I'll do some multimeter work tomorrow.  Thanks

[ditchcrawler]

Still running the genny ?

[jono2.0]

 

24 minutes ago, nicknorman said:

Since fairly high currents are involved with these alternators, any weak link in the connections chain ie thin cable,

 

I have 35mm cables from alternator to battery bank and then 95mm cables between each battery

6 minutes ago, ditchcrawler said:

Still running the genny ?

I am indeed.

[Alan de Enfield]

8 minutes ago, ditchcrawler said:

Still running the genny ?

 

1 minute ago, jono2.0 said:

 

I am indeed.

 

Naughty, naughty.

(40 minutes after 'switch-off time')

[jono2.0]

 

3 minutes ago, Alan de Enfield said:

Naughty, naughty.

(40 minutes after 'switch-off time')

 

 

Oh I see what you mean (got to remember the rules).  The funny thing is I am so out in the countryside I'm miles from any house, pitch black everywhere and not seen a boat in days let alone one being near me or even within a good mile or so.

So apart from a few sheep looking at me from over a field, is it ok still?

[Alan de Enfield]

No one to get upset ?

Carry on.

[Robbo]

10 minutes ago, jono2.0 said:

 

I have 35mm cables from alternator to battery bank and then 95mm cables between each battery

I am indeed.

 

That is too thin really, what's the length?

[WotEver]

24 minutes ago, jono2.0 said:

I'll leave it on for the rest of the eve to allow it to go into float etc.

Let's pretend you didn't say that and you're running it during the hours of 8 to 8 tomorrow...

Do not fall into the trap of thinking "It's gone into float, good, the batteries are charged."  They almost certainly won't be. Turn the charger off and back on and you'll probably see it go into absorption again. When your charging current drops to around 4 to 5 amps at absorption voltage (14.4 or whatever) only then are you approaching fully charged. 

When the current (at absorption voltage) doesn't drop over a 45 minute period you've finally achieved 99.9% charged. 

[jono2.0]

Just now, Robbo said:

 

That is too thin really, what's the length?

 

1.6m with a fuse in the line too.

1 minute ago, WotEver said:

Let's pretend you didn't say that and you're running it during the hours of 8 to 8 tomorrow...

Do not fall into the trap of thinking "It's gone into float, good, the batteries are charged."  They almost certainly won't be. Turn the charger off and back on and you'll probably see it go into absorption again. When your charging current drops to around 4 to 5 amps at absorption voltage (14.4 or whatever) only then are you approaching fully charged. 

When the current (at absorption voltage) doesn't drop over a 45 minute period you've finally achieved 99.9% charged. 

 

Thanks everyone for taking the time to help me.  This is all so new to me but I'm learning, fast.  The meter now indicates 13.91v 17.5A and dropping.  97% SOC - this is via the pretend, shhhh genny.

[WotEver]

3 minutes ago, jono2.0 said:

1.6m with a fuse in the line too.

Thanks everyone for taking the time to help me.  This is all so new to me but I'm learning, fast.  The meter now indicates 13.91v 17.5A and dropping.  97% SOC - this is via the pretend, shhhh genny.

Forget the 97% SoC. like I said, the BMV tells lies. 

If the charger has dropped to <14.4V now is the time to turn it off and on again as I said above. 

[jono2.0]

14 minutes ago, WotEver said:

Do not fall into the trap of thinking "It's gone into float, good, the batteries are charged."  They almost certainly won't be. Turn the charger off and back on and you'll probably see it go into absorption again.

1

No probability about it - it's done exactly what you said it would :-)

Now, 14.51v and 23.60A

How many years did it take you lot to get used to generating your own power! 

[Robbo]

10 minutes ago, jono2.0 said:

1.6m with a fuse in the line too.

Thanks everyone for taking the time to help me.  This is all so new to me but I'm learning, fast.  The meter now indicates 13.91v 17.5A and dropping.  97% SOC - this is via the pretend, shhhh genny.

 

I would up the cable to 50mm2 if possible as that will reduce voltage drop, and dont use a fuse that's even close to the alternators output, as these cables can take 300+ amps, just stick a big fuse in it like a 300.  You don't want really want the fuse to blow unless it's a dead short as it will take out your alternator if on.   So route the cable and tie the cable so it's away from heat sources as this is properly the main way to short the cable.

[jono2.0]

4 minutes ago, Robbo said:

 

I would up the cable to 50mm2 if possible as that will reduce voltage drop, and dont use a fuse that's even close to the alternators output, as these cables can take 300+ amps, just stick a big fuse in it like a 300.  You don't want really want the fuse to blow unless it's a dead short as it will take out your alternator if on.   So route the cable and tie the cable so it's away from heat sources as this is properly the main way to short the cable.

 

Thanks for this.  I've got a length of 95mm that I can use instead.  Bought too much previously.
I have a feeling the fuse is a 400A but not sure of that until I take a look.
All the cables in the engine bay are within that black plastic flexible corrugated split conduit.  I have the 35mm in one from alternator to fuse and then again from fuse to battery - will this still be ok for 95mm?

[Robbo]

6 minutes ago, jono2.0 said:

Thanks for this.  I've got a length of 95mm that I can use instead.  Bought too much previously.
I have a feeling the fuse is a 400A but not sure of that until I take a look.
All the cables in the engine bay are within that black plastic flexible corrugated split conduit.  I have the 35mm in one from alternator to fuse and then again from fuse to battery - will this still be ok for 95mm?

If you have spare 95mm that's more than enough, but remember you'll need to do the negative as well.  plastic melts as well so it's no protection against a hot exhaust!  Fuses are there to protect the cable from shorts, but it's better to protect the cable from getting shorts in the first place!

 

what size cable have you going to the Victron, 95mm2 is good for that.

[jono2.0]

2 minutes ago, Robbo said:

what size cable have you going to the Victron, 95mm2 is good for that.

 

I currently have 95mm but I will be changing this as it should be 2x 50mm as per manual and terminals on the Multiplus.

I have run out of space on battery terminal to have all these big cables so have ordered 2x 400A bus bars.  I'm planning (with changes to alternator as suggested) to have:

One for + with 95mm from battery bank to it and then, 2x 50mm cables to MultiPlus, 1x 95mm to alternator via 400A (I think) fuse, 1x small monitor cable to shunt, 1x 35mm to 12v supply in cabin.

One for - with 95mm from battery bank to Victron shunt, 95mm from shunt to bus bar and then, 2x 50mm cables to MultiPlus, 1x 95mm to earthing stud fixed beneath engine, 1x 35mm to 12 supply in cabin.

[WotEver]

Don't bother changing the Victron cables. The 5mm difference won't make enough difference to make it worthwhile plus, and it's a big PLUS, you can fuse 1x 95mm cable far more simply and effectively than 2 x 50mm. 

[ditchcrawler]

11 minutes ago, WotEver said:

Don't bother changing the Victron cables. The 5mm difference won't make enough difference to make it worthwhile plus, and it's a big PLUS, you can fuse 1x 95mm cable far more simply and effectively than 2 x 50mm. 

That makes good sense