Charging batteries by running engine

[Miker]

I have two 110 AH batteries for services and a third for starting the engine. Last Saturday I was in a hurry and forgot to switch off the electrics. On visiting the boat today, the batteries were showing as flat but I was able to start the engine and let it run for about ten minutes. The marina seems to be having problems with the electricity supply so I am unable to leave the battteries on charge. Assuming that I can start the engine the next time I go for a sail, for how long will I need to motor to charge all three batteries fully?

 

[daveyw]

depends on your alternator and how flat your batteries are. A flat 110A Batt needs about 55 Ahrs to top it up so a 100 A alt will take just over 30 mins. 3 batts, about 100 mins

 

[mikehibb]

That will put some power back into them, but not properly recharge them.

They really need a long slow charge over night or longer on mains power.

I would forget about the sails next trip, just motor around all day.

 

[pvb]

depends on your alternator and how flat your batteries are. A flat 110A Batt needs about 55 Ahrs to top it up so a 100 A alt will take just over 30 mins. 3 batts, about 100 mins

Sorry, that's wrong and very misleading! A 100A alternator won't automatically give 100A charging current - it depends on the battery's ability to absorb the current. A typically discharged 110Ah battery might take 25-30A charging current initially, but that will rapidly drop off as the charge is replaced. If the OP has a smart regulator on his alternator, it will help, but it can't overcome the basic physics involved.

We don't know what alternator the OP has, or how it's controlled, or what type of batteries are involved, so it's impossible to guess how long it would take. The one sure thing is that it will be much longer than 100 minutes.

 

[Miker]

Thanks everyone. The engine is a Volvo 2010 with a 60A alternator. It sounds as though a 50 mile sail with just an engine blast out of the marina would not be wise.
I will have a go at the marina. I managed to get electricity from the marina to heat a soldering iron and kettle but the switch on the battery charger when connected just lit up briefly then went dead. After that, the kettle would not work either. I've tested out the shore power lead at home and all seems well. I will get my crew who knows about these things, to test the battery charger with his circuit gizmo, then complain to the marina.

The boat has an Adverc battery management system.

 

[planteater]

I'd take the batteries home and charge them overnight. That way would know that they are basically good for a day sail.

 

[planteater]

To discharge 220AH in 4 days means that something was potentially drawing over 2 amps despite not being used. Even a 50% discharge means a continuous 1 amp draw.

 

[Miker]

Yes. GPS on, VHF blasting away, depth and speed on, also compass light and Wind 315. Got the marina to fix the electrics so batteries now on charge, unless the marina electrics trip out again.
With the weather in the Irish Sea, it looks as though I will just about get in a day sail if I'm lucky, so should be OK with the charge in the engine battery anyway.

 

[Pete7]

The one sure thing is that it will be much longer than 100 minutes.

Quite.

Miker, we left Alderney last week heading home but with no wind had to motor sail, so volvo 2003 with 50 amp alternator running at 2000 rpms. The gauge said we had a shortfall of -29 amps when we departed. 5 hours later we were still short of a couple of amps and it took another 5 hours to replace these. So whilst your 60 amp alternator will give a good charge to flat batteries initially, the charge will drop off quite quickly. One of the reasons why the after market alternator regulators are popular. This is worth a read:

http://www.sterling-power.com/support-faq-1.htm

Pete

 

[mitiempo]

While an aftermarket regulator will help in the initial stage of charge it does not get around the fact that any battery, flooded, gel or agm, will accept a lot less current when it reaches 75 or 80% state of charge. The last 20% can take 8 or 10 hours as the current drops more and more. That is why if cruising continually most will draw the batteries down to 50% SOC and charge to 80% SOC if using the engine for charging. The last 20% takes many hours of run time for a full 100% charge. Of course if solar or wind gens are present the batteries can be fully charged quietly but the time factor is still there.

 

[[2574]]

Calder's 80% rule

In Calder's manual he talks of a 30% capacity engine charging window to make best use of an engine driven recharge. A battery should never be depleted beyond 50%. It is hugely energy inneficient to charge a battery via engine beyond 80% (because of the charge drop-off rate is described above). Therefore one should only ever engine charge between 50% and 80% giving the 30% charging "window". Effectively, if you have a three stage alternator controller, this means that you only ever bulk phase charge via alternator. Absorption (and eventually float) phase should be via shorepower driven charger.

On my boat this means that with my 550ah domestic bank I have a 30% window of 165ah. I have a 110A alternator with a three stage regulator. Generally in bulk phase the alternator will have an input average of about 70A meaning the 30% recharge is achieved in about 2 1/2 hours.

 

[charles_reed]

While an aftermarket regulator will help in the initial stage of charge it does not get around the fact that any battery, flooded, gel or agm, will accept a lot less current when it reaches 75 or 80% state of charge. The last 20% can take 8 or 10 hours as the current drops more and more. That is why if cruising continually most will draw the batteries down to 50% SOC and charge to 80% SOC if using the engine for charging. The last 20% takes many hours of run time for a full 100% charge. Of course if solar or wind gens are present the batteries can be fully charged quietly but the time factor is still there.

In fact the smart chargers are designed to get round this problem. I personally, prefer the Adverc regulator to the Sterling (makes far less claims and does deliver them).

An Adverc would replace 95% of full charge, from 50% discharge, in about 4 hours of running (alternator size relatively immaterial) - a switch mode would put in 85% in about the same period and the best a modern alternator regulator would get to (however long it worked) would be about 80% of full charge.
With the older in-alternator regulators fitted to most (archaic) marine-engine alternators I doubt battery banks ever get above 75% of full capacity.

The above refer to flooded lead acid batteries, gel and maintenance-free are far less efficient.

I don't think - whatever their claims - anyone manages to fully charge flooded lead acid batteries.

All IMHO of course!

 

[ghostlymoron]

"Last Saturday I was in a hurry and forgot to switch off the electrics"
There are often posts like this, sooner or later everyone (wait to be challenged) will forget to do it or their crew will. Read 12 volt bible p115 'install a battery isolator".
A 55amp alternator will only supply 55amps when the battery is flat, after an hour it will drop to 10amps according to Pat Manley. You may be talking 10 hours to charge to 70%. See p81 of Simple Boat Maintenance.

 

[mitiempo]

It is not what the alternator will supply but what the battery will accept. Any battery has a much lower acceptance rate after it is 80% charged and no regulator can get around this.

 

[mcframe]

"Last Saturday I was in a hurry and forgot to switch off the electrics"
There are often posts like this, sooner or later everyone (wait to be challenged) will forget to do it or their crew will. Read 12 volt bible p115 'install a battery isolator".

Or: "Leave the galley light switched on (at the fitting) *and* at the switchboard, and *only* switch it off via the isolator.

'course, then you've got to remember to switch it off when you get to the boat, but that might be less of a problem....

 

[alan_d]

I have a four-line mantra I mutter to myself before leaving the boat:

Gas Off.
Electrics Off.
Seacocks Off.
Feck Off.

Works for me.

 

[DavyMac]

An Adverc would replace 95% of full charge, from 50% discharge, in about 4 hours of running (alternator size relatively immaterial) - a switch mode would put in 85% in about the same period and the best a modern alternator regulator would get to (however long it worked) would be about 80% of full charge.
With the older in-alternator regulators fitted to most (archaic) marine-engine alternators I doubt battery banks ever get above 75% of full capacity.

Can a wise person explain whether an Adverc (or similar) does anything significant other than boost voltage thus allowing faster charging during the bulk phase? The various papers produced by the sellers of these things always look a little misleading to me, e.g. the Stirling one linked to a few posts up compares a battery charged at 13.2 volts with one with the bulk charge boosted to 14.8 by the Stirling regulator. This is no doubt correct, but how many alternators actually regulate at 13.2? Ours is over 20 years old (Paris Rhone) and regulates at 14.2 as measured with two different digital meters and my understanding is that newer marine alternator often regulate at 14.6 anyway, so the differences are much more marginal.

Is it not the case that once the battery gets to around 80% charge the charge rate drops whether you've got a booster or not because the boosters change to float mode and drop the voltage anyway?

 

[Heckler]

In fact the smart chargers are designed to get round this problem. I personally, prefer the Adverc regulator to the Sterling (makes far less claims and does deliver them).

An Adverc would replace 95% of full charge, from 50% discharge, in about 4 hours of running (alternator size relatively immaterial) - a switch mode would put in 85% in about the same period and the best a modern alternator regulator would get to (however long it worked) would be about 80% of full charge.
With the older in-alternator regulators fitted to most (archaic) marine-engine alternators I doubt battery banks ever get above 75% of full capacity.

The above refer to flooded lead acid batteries, gel and maintenance-free are far less efficient.

I don't think - whatever their claims - anyone manages to fully charge flooded lead acid batteries.

All IMHO of course!

Charles
The most common sense thing Ive seen written here in a long time! All the toothsuckers go on on and on about charging and voltage etc but yours explains it all!
Stu

 

[Heckler]

It is not what the alternator will supply but what the battery will accept. Any battery has a much lower acceptance rate after it is 80% charged and no regulator can get around this.

Sir
basically wrong, raise the voltage and the charge rate goes up, thats why the Sterlings etc do what they do, wack 14.6 volts in and then regulate to avoid damage to the battery. They tread a fine line BUT a good one will get the bank up to 95% pretty quickly, take note of Charles Reed, one of the more practical sages on here
Stu

 

[pvb]

Can a wise person explain whether an Adverc (or similar) does anything significant other than boost voltage thus allowing faster charging during the bulk phase?

All "smart" regulators work on the basis of increasing the charge voltage so as to push more current into the batteries. Different models work in different ways, with varying degrees of sophistication. But, at the end of the day, if you increase the voltage, the charging current will increase.

Newer alternators tend to have internal regulators set at a higher voltage than used to be the case, so "smart" regulators have less of a benefit.

Is it not the case that once the battery gets to around 80% charge the charge rate drops whether you've got a booster or not because the boosters change to float mode and drop the voltage anyway?

No, it isn't necessarily. Boosters such as the Adverc and Sterling can only increase the alternator's voltage - they can't reduce it to a "float" voltage. This is because they work in tandem with the alternator's own internal regulator - whatever that's set to is the minimum charging voltage.