Alternator And Charging System

[mikeyboy1966]

Hi all,

im just home from 10 days on my boat and I’m scratching my head a bit!.

last year when running a prestolite 70 amp alternator with a diode type isolator and an analog ameter it only ever showed 20 amps charging when 1st fired up.

fast forward to this year after winter upgrades,

same 70 amp alternator,

victron argofet battery isolator,

sterling power external regulator,

digital blue sea battery monitor 

all new very heavy duty wiring.

3x 120 amp domestic batteries (new this year)

it still only charges at a max of 20 amps dropping to @15 amps after a while.

I’ve only noticed this week as the boat is hooked up to mains power in the marina and the batteries maintained on a victron 30 amp charger and we’ve only had weekends out leaving with batteries at 100%.

on our extended trip with a dometic 80 litre fridge and a dometic freezer running the alternator was only just keeping the batteries topped up to @65% despite 6hr cruising days.

I’ll take a good look at things next weekend but my immediate thoughts are the alternator is not very good?

[FairTmiddlin]

Only thing you haven't changed is the ammeter.

Would remove it and test the resistance across it should be in point something Ohms. under .2 would be good. alternatively, a pp9 battery and a pair of croc leads should make it max out. Swap the leads round if it's low

[Meantime]

Look at the output curve for your 70ah alternator. It will only give 70 amps once it reaches a certain amount of revs. You'll struggle to reach maximum output at Broads speeds. 

If that is the issue you might be able to change the pulley for a smaller one to spin it faster, within the alternator limits off course.

However with a smaller pulley you might need to go for a notched belt as these handle tighter radius better.

[mikeyboy1966]

I replaced ammeter with a digital battery monitor.

the boat is on the great Ouse,so can run at decent speeds ,even if I rev the motor to 3k revs it doesn’t go over 20 amps

[mikeyboy1966]

The only thing unchanged over winter is the alternator,

 

[mikeyboy1966]

The engine is a Perkins 4108 lowline 

[Bikertov]

Silly question - where is the ammeter measuring the current draw ? ie, where along the positive is the shunt located ?

Could it be that you are just not measuring what you think you are ?

[mikeyboy1966]

Ammeter shunt is on the main earth IIRC

[mikeyboy1966]

Top left on the battery box yep,definitely main earth

[Bikertov]

OK, thought I would ask the obvious !

Have you tried measuring the current with a DC clamp meter, to verify the ammeter reading, including various different cable branches ?

Getting technical, you can use "Kirchhoff's first law" of electricity that applies to currents at a junction in a circuit. It states that at a junction in an electrical circuit, the sum of currents flowing into the junction is equal to the sum of currents flowing out of the junction. 

So you can track your current draw at various points in a circuit to work out where it is all going and to make sure you are not missing or underestimating the draw of an item or a wiring leg.

[Rover]

Look at the output voltage of the alternator. Usually about 14.4V. If you look at the charge characteristic curve for a lead acid battery at that voltage, it rises steeply to about 80% and then flattens and the last 20% can take many hours to achieve. In practice, just relying on the alternator you are unlikely to do much better than 80% charged.

In addition to charging the batteries, the alternator will also be supplying current to power the connected loads; fridge, freezer etc.

The optimum solution with present technology is a battery to battery charger from Victron or Sterling or similar. These have a multi stage charging profile and will charge the domestic battery many times quicker than relying on the alternator.

[ZimbiIV]

Back to basics, if it is the same as before the mods, check the belt is not glazed  and slipping under load, change the belt anyway.

paul

[LondonRascal]

Is it down to the fact you've just not discharged the batteries enough? On my boat (90Amp alternator fitted 2022 replacing the old 55Amp one) - all new wiring, batteries, charge controller etc but it wont do much if my batteries are only 20% discharged, but if I discharge them more - it will (as expected) begin with a higher amperage charge. You don't want to force too many Amps into old fashion batteries, same with my shore based supply - while it will charge at 14.4.v from the get-go (unlike the alternator which starts at a lower voltage and rises as the amperage deceases) the charger will not put out full 50Amp if the batteries are not discharged too much.

If you really want to go for rapid recharging ditch the batteries and go Lithium-Iron and have charger a multi-stage alternator to battery charger to take all the Amps and manage them to the battery bank. Never charge lithium off an Alternator without current limiting, because it will overheat an Alternator sucking the full 70Amps.

[mikeyboy1966]

Thanks for the replies and advice,

Ive ordered a new alternator,

I can’t see it being an issue with the wiring or other equipment,

It’s  a 10 minute job to swap it and it will confirm things one way or the other.

[grendel]

27 minutes ago, mikeyboy1966 said:

It’s  a 10 minute job to swap it and it will confirm things one way or the other

and then at least you will have a spare onboard

[Karizma]

3 hours ago, mikeyboy1966 said:

Ive ordered a new alternator,

Hi mikeyboy1966, did you order a 'like for like' replacement or upgraded to a larger output alternator?

[annv]

Hi As it's not fully charging the battery's it's likely the alternator suggest you remove it unscrew the brush box and check for obverse faults (worn brushes) and/or take it to a Auto electrician to get it checked out, it might only need a new brush box. John

[mikeyboy1966]

48 minutes ago, Karizma said:

Hi mikeyboy1966, did you order a 'like for like' replacement or upgraded to a larger output alternator?

I’ve gone like for like,the 70 amp is ,I believe already an upgrade over standard.

Added to that,without fitting serpentine belts and pulleys 100 amp is the limit for a single vee belt .

It  starts to get expensive!

[mikeyboy1966]

23 minutes ago, annv said:

Hi As it's not fully charging the battery's it's likely the alternator suggest you remove it unscrew the brush box and check for obverse faults (worn brushes) and/or take it to a Auto electrician to get it checked out, it might only need a new brush box. John

The brushes looked fine when I removed them over winter to fit additional wiring for the sterling power regulator,

I will get it tested /repaired though if it proves to be at fault as it’s nice to have a spare.

[mikeyboy1966]

On 03/09/2023 at 23:21, LondonRascal said:

Is it down to the fact you've just not discharged the batteries enough? On my boat (90Amp alternator fitted 2022 replacing the old 55Amp one) - all new wiring, batteries, charge controller etc but it wont do much if my batteries are only 20% discharged, but if I discharge them more - it will (as expected) begin with a higher amperage charge. You don't want to force too many Amps into old fashion batteries, same with my shore based supply - while it will charge at 14.4.v from the get-go (unlike the alternator which starts at a lower voltage and rises as the amperage deceases) the charger will not put out full 50Amp if the batteries are not discharged too much.

If you really want to go for rapid recharging ditch the batteries and go Lithium-Iron and have charger a multi-stage alternator to battery charger to take all the Amps and manage them to the battery bank. Never charge lithium off an Alternator without current limiting, because it will overheat an Alternator sucking the full 70Amps.

The batteries were discharged to 50%.

on Saturday 8hrs of cruising only brought them up to 65% charging at 15/17 amps.

regarding lithium batteries there is no way I’d be fitting them onboard!
regardless of the cost of them I’m not completely convinced they are safe?

[Bikertov]

Lithium batteries are not quite a like-for-like drop in are they anyway ?

I thought you needed to spec your electrical system to take them, including different chargers etc

[Jonathan]

mikeyboy1966,

Something is not telling you the truth, either the Ammeter or the battery state monitor.  Ammeters are usually quite simple devices whereas battery monitors need careful installation and “setting up” to give correct results.  Although you say that you are now using the digital state monitor as an Ammeter so it should be self-consistent!

You have three 120 Amp hour batteries (total 360 Ah), so if you have discharged to 50% you have used up 180 Ah of their capacity.

You say you were charging at between 15 and 17 Amps.  Charging at 15 Amps for 8 hours (allowing a few Amps to be used elsewhere by the fridge etc.) will put back 120 Ah of charge.  180 + 120 = 300 Ah or 83% of 360Ah.  This assumes that the 15 Amps is being divided between all three batteries equally.

If your meter shows 65% charge (which is 234 Ah) it suggests that only 54 Ah of charge have been added, which is a rate of less than 7 Amps for 8 hours, or a third of that into one battery.

I appreciate that this doesn’t help you solve your problem at all, but it does suggest an oddity in the battery configuration or the monitor.

One worry is that if you discharge lead-acid batteries down to 50% or less it can affect their long-term health (there are lots of references to this on the internet)!  So you do not want to let this continue.

[mikeyboy1966]

1 hour ago, Jonathan said:

mikeyboy1966,

Something is not telling you the truth, either the Ammeter or the battery state monitor.  Ammeters are usually quite simple devices whereas battery monitors need careful installation and “setting up” to give correct results.  Although you say that you are now using the digital state monitor as an Ammeter so it should be self-consistent!

You have three 120 Amp hour batteries (total 360 Ah), so if you have discharged to 50% you have used up 180 Ah of their capacity.

You say you were charging at between 15 and 17 Amps.  Charging at 15 Amps for 8 hours (allowing a few Amps to be used elsewhere by the fridge etc.) will put back 120 Ah of charge.  180 + 120 = 300 Ah or 83% of 360Ah.  This assumes that the 15 Amps is being divided between all three batteries equally.

If your meter shows 65% charge (which is 234 Ah) it suggests that only 54 Ah of charge have been added, which is a rate of less than 7 Amps for 8 hours, or a third of that into one battery.

I appreciate that this doesn’t help you solve your problem at all, but it does suggest an oddity in the battery configuration or the monitor.

One worry is that if you discharge lead-acid batteries down to 50% or less it can affect their long-term health (there are lots of references to this on the internet)!  So you do not want to let this continue.

That all makes sense,

out of that cruise time we had to go through 4x locks all of which were set against us( engine off whilst resetting lock and passage through)

we had an enforced stop due to a blocked water intake and we had a lunch stop

[mikeyboy1966]

Shiney,I like shiney

[annv]

Hi Mike Just had a thought you have attached the field wire to the correct terminal ! not the rev counter terminal. Just a thought. And have you checked the output voltage at the alternator terminal. John