Alternator size

[Sam_Steele]

I am looking to upgrade my domestic batteries to 320AH ( total) from 220AH. They will be AGM - and these are not used for engine starting, there is a seperate battery for that (also AGM). I have a 70AH lucas alternator and an Adverc and there is a blocking diode. When reading Nigel calder he recommended that the ratio between 3 to 1. But at the end of his chapter he said in reality it could be up to 5:1 ie 20%. Mine would be 4.6 to 1 - so I was a bit concerned, so i phoned up barden who said that alternator size really didn't matter - but shore power charger is the important part. So I am now stuck with conflicting opinions between a respected author and a respected suppler of batteries.

Can any one help. I am concerned that I will be under charging my batteries and could reduce their battery life.

many thanks
sam

 

[Vulcan]

I think it is just a question of charging time rather than absolute output. My shore power charger has isolated outputs for each battery and goes from bulk to float charge when the batteries are fully charged.

 

[VicMallows]

As VULCAN says, there is no right or wrong answer to this. It all depends on how much power you take out of the batteries, and how much you put back in. I'm afraid you will simply have to do the maths for your paricular situation. With an installation of that size, the first thing I'd be doing would be to install a device measuring total amp/hours in / amp/hours out. Quite a few on the market, 'Link' being one of the best known.

Vic

 

[pvb]

Urban myths...

There's a popular urban myth which suggests that a 70A alternator will always push 70A into the batteries. It won't necessarily do that. The current an alternator can generate depends on the battery bank it's connected to. Mainly, it depends on the Ah capacity of the bank, but battery type affects it too. With an Adverc to maximise charging efficiency, I think you'll be fine with your 70A alternator. You certainly won't be "under charging" your batteries, and there'll be no adverse effect on battery life. I have a 660Ah domestic bank, and when I had a 130A alternator I rarely saw much more than 90A going in to it. Now I only have a 90A alternator, and there are still no problems with it.

As regards the advice on the shore power charger, I'd be tempted to ignore it. If you have a smaller capacity charger, it'll just take longer to recharge the batteries. That usually isn't a problem when you're connected to shorepower.

And as regards Nigel Calder's advice, all I'd say is that - like all journalists - his pronouncements aren't always properly researched!

 

[jrt]

Just because it is in print doesn't mean that it is gospel. It is the author's opinion.

We have a Mobitronic 45 Amp shore power charger. The hand book says that it is alright for 500AHr of battery capacity. You shouldn't have any problems with yours.

Your alternator shouldn't have any probs either. It will only deliver it's max output for a very short time and only then if the battery is very flat. As soon as charge starts to go into the battery the the voltage of the battery starts to rise and consequently the voltage difference between the battery and charger output gets less and so the current starts to fall. As the battery gets to about 80% full (I think) the Adverc switches in and switches the charging voltage either side of the gassing voltage. This quickly brings the battery up to as near as makes no difference 100% charged.

We also fitted a battery monitor, the Mastervolt one. Amongst other things this monitors the AHrs flowing out of the battery and back in again when they are on charge. You have a running total of the battery state of charge. We call it the battery fuel gauge. If you are doing the sort of sailing that is not going from marina to marina then I believe this to be a real must have. /forums/images/graemlins/laugh.gif

 

[roger]

Those who sell smart alternator regulators/chargers say that without very prolonged charging you never really get your batteries fully chargec with a conventional alternator. The car shystems usually rely on having a fully chrged battery that loses a bit of charge only at start up which is pretty quickly made up on a car. However the well used and well dicharged leisure battery will suffer.
With a lot of faith rather than hard evidence I tend to go along with this. The smart reg and charger I now have have worked well for a season or two.
Smart regulators will to some extent make up for limited alternator capacity. The alternator will work harder though and you are likely to get more belt wear etc.

 

[mikemc]

I think I have the opposite problem - possibly too much charging capacity from shore power charger.

I've acquired a boat with an 85Ah starter battery and a 110Ah house battery. It's fitted with a Sterling charger with 3 outputs - 30Amp 12v. The Sterling literature says a charger rated at 10% of the Ah capacity is "ample". But my concern is can my 30A charger be too powerful? I've tried it a couple of times with just one battery connected (linking the unused outputs as per the instructions) and it pushes the volts up very quickly on "bulk" charge and goes immediately to "absorption". Is this because it is too powerful? Also, I'm not clear on how the three separate outputs work - is the "bulk", "absorption" and "float" controlled separately for each output? In other words, when more than one battery bank is connected, might one bank be on "bulk" whilst the other might be on say "float"? Presumably this must be the case otherwise a full battery bank could be overcharged whilst the lower battery bank is catching up.

Finally, I'm not sure what cable should be used at these amps. The motor is a 50Hp outboard so the starter doesn't draw much current, The alternator is 10A. All the existing cabling from batteries to Voltage Sensitive Relay and isolation switch also to motor and Sterling charger, are all 8AWG. Is this OK?

Any comments would be very much appreciated.

 

[Anonymous]

[ QUOTE ]
I think I have the opposite problem - possibly too much charging capacity from shore power charger.....

[/ QUOTE ]The Sterling can be set for three types of battery - Gel, Sealed or Flooded (open) so check that you have made the right selection. The outputs of my 20A Sterling were split by an internal pair of diodes with one remote voltage sense. All batteries should be of the same type and you should put the voltage sense on the battery that is likely to be discharged the LEAST if you wish to avoid overcharging. Actually overcharging isn't a big deal with open cells, it just costs you water. Assuming you have wired it per instructions and set the switches correctly it should be pretty foolproof - mine worked around the year around the clock for nearly five years with no problems. I was very happy. You can check your battery volts with a digital voltmeter. The maximum volts should be around 14.5 at which the current will have dropped to almost zero - this is the finishing stage of the charge and is essential to avoid sulphation. After that, it should drop back to around 13.7V and stay there even when current is being drawn by the domestic load.

If you have problems call Sterling. The owner, Charles(?) Sterling does know what he is talking about and is actively involved in the design. Or was back in early 2000, when I last spoke to him.

David

 

[boatmike]

Endorse all that and would add that the control of the alternator charging current is what is important. With 60 amps available and a 320 Amp hour battery bank, theoretically you could charge from 50% discharged to full charge in 2.7 hours running.
In practice, with 3 batteries this would mean each battery getting 20 amps charge which would in practice boil the water out of the batteries very quickly if done regularly and even deep cycle batteries don't last long at this rate so it's plenty big enough. Without a Stirling charger or similar the 60 amp alternator will reduce its output to a maximum of about 20 amps when the battery is "normally" discharged to about 20-25% capacity and full charge will seldom be reached. The automotive type of control on most alternators assumes the engine is running all the time current is being drawn and the only really high load on the battery is cranking to start the engine.
Also without stating the obvious the shore charger contributes nothing unless you are tied up in a marina.....

 

[Sam_Steele]

Thanks for all the advice
Regards
sam