starter motor battery

KREW2

KREW2

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In my manual it says that 70 ah is the max, however I have been using a 110ah leisure battery for the last 3 years. The engine is a vp 2002 the starter is rated at 1.9 hp all this has left me totally confused as to what size started battery I need to install
 
Haven't a clue why the Volvo Penta workshop manual says that, although you're right in saying it does. You can safely disregard that bit of Volvo advice!
 
yes, there is a reason. If you go cruising, you may loose your alternator and battery in one minute, if the battery is to big.
Example:
If you stay for 14 days on the same place, you lift on your departure date the anchor with your anchorwindless and it gets stuck, you increase the rev on your engine, to get more juice on the anchorwindless and that will burn your alternator and your battery. Because you do not charge the battery fully on the day before you departure, you will start the day with a reduced battery. The combined current of the battery and the anchorwindless will destroy the alternator. This happens only, if the batttery is sufficent discharged and if you are using one of this special regulators. The inbuild regulator usually is not strong enough to kill the alternator, only external intelligent controllers are capable doing this. As an added bonus, the battery is damaged as well.

Happens quite often during extenden cruising. I had it 2 times in 3 years. If your battery bank is sufficent big, you may destroy the alternator even without the anchorwindlass due to the high current going into the batteries for an hour or so.

Peter
 
I don\'t think so...

Not wishing to contradict your unfortunate experiences, but I know that what you're saying is basically wrong. Any alternator has a maximum output, regardless of the size or state of the batteries, and it shouldn't get destroyed in one minute unless there's something else very wrong. Similarly, with proper installation, even extended running at high output current shouldn't damage an alternator (although some boats do have problems due to poorly ventilated alternators).

In the quoted case of the Volvo Penta recommendation of a max 70Ah battery, the standard-fit alternator is 50A. There's no doubt that a 50A alternator will cope easily with significantly bigger battery banks. The Volvo recommendation makes no sense, and may in fact just be a simple translation error.

Just as an example, I run a 90A alternator with Adverc smart regulator fitted, feeding a 660Ah domestic bank. There are no problems with this.
 
Re: I don\'t think so...

Is this related to the starter motor rather than the alternator?

Normally the volts will fall to 9 volts when cranking or to 7 volts if stalled or jammed so presumably the starter motor design reflects the current which flows under those conditions. If a much larger starter battery is fitted then it will maintain a higher voltages. Could that be to the detriment of the starter motor as a result of higher currents flowing than it is designed for.

Logically there must be some reason for Volvo the state a maximum battery capacity. Many people fit very much larger domestic batteries without causing an alternator problems.
 
Re: I don\'t think so...

Yes I would go with the theory that a larger battery can over power the stater. Consider that if the battery is 9 volts when delivering 300 amps(2700 watts) then if a very large battery held 12 volts then it would deliver 400 amps which means the starter is dissipating 4800 watts or getting towards twice the power (heat).
Now of course the situation is not nearly as simple as the calculations because the current drops as soon as the stater stats to turn and reduces with speed. As this happens quicker with more volts you would get less current sooner.
You can perhaps see how VP might want to cover themselves against possible premature stater failure.
I think I would happily risk it and woulld be happy with a bigger battery. Just be carefull if you have to crank for extended period.

As for the alternator. Theoretically an alternator should be self limiting of current because of the internal resistance of the windings. So with full power to the field coil (you can't get more than 12volts) the voltage of the output of the alternator will start to fall at the rated current so it will firstly reduce any battery charging current (assuming partially discharged batteries) and to a lesser extent reduce current to other loads as voltage falls.
Normally with a battery you get no charge current if the voltage falls below the inherent voltage of the battery about 12 volts. But if the battery is stoned at say 6 volts then the alternator will be able to push current in even beyond it's rated current. This especially so for a large battery bank. (yes the excitation voltage will only be 6 volts also which will possibly limit the current.) A reasonable sized battery will quickly increase in voltage to closer to 12 volts.
So with really flat batteries beware of alternator damage but generally the alternator should be able to cool itself enough to avoid self destructing. It should be OK for short period though.

Practice of course can be a different story so kandoma may well have experience of alternator failure from battery abnk too big. olewill
 
Another argument in favour of a dedicated battery for the windlass (with its attendant charging complications) rather than running the windlass off the engine cranking battery (as is often advocated on this forum)?
 
Re: I don\'t think so...

[ QUOTE ]
Yes I would go with the theory that a larger battery can over power the stater. Consider that if the battery is 9 volts when delivering 300 amps(2700 watts) then if a very large battery held 12 volts then it would deliver 400 amps which means the starter is dissipating 4800 watts or getting towards twice the power (heat).

[/ QUOTE ]
The Volvo 2002 starter motor in question is rated at 1400W, so would normally take say 120A. The only way you would get the 300A or 400A currents you mention would be if the motor was stalled. Let's take a theoretical situation and assume the engine is seized, and the starter button is pressed - nothing happens but a high current flows. Let's also assume that the theoretical skipper is a complete dumbo, and that it takes him 5 seconds before he works out that the starter motor isn't turning and he releases the starter button. Using your figures of 2700W and 4800W, in the first case of 2700W the starter motor (assume 10kg) would increase in temperature by about 3degC. In the second case of 4800W, the starter motor would increase in temperature by about 6degC. I really can't see that the more powerful battery poses any real problem unless our theoretical dumbo skipper just keeps on pressing the starter button until he sees smoke coming out! Maybe Volvo are trying to protect idiotic boat owners against their own stupidity. /forums/images/graemlins/laugh.gif
 
Re: I don\'t think so...

Many thanks for all your replies I understood most of it. I gather it will be ok to keep my 2X 110 amp hour leisure batteries in one bank, and install an 85ah battery with 500 cold cranking amps as a dedicated starting battery. This will all be charged by the standard 50amp alternator, and a stepped 20amp sterling charger from shore power.
 
Don't agonise about it. Think of the smallest diesel car you know - I decided on a Golf diesel - go into Halfords look up the correct battery for that make and model of car and buy one of those. Car batteries are stacked high and sold cheap, if one's good enough to start a small diesel car, it'll start you Volvo 2002 no problemo...
 
Re: I don\'t think so...

Hi PVB I was using imaginary figures and I was not trying to justify the VP caution about using a larger stating battery but rather trying to show that there is a difference between a small battery falling to 9volts and a large bank which does not fall so much at high current.
I think I would go fo a bigger battery or at least not be worried by using a large service battery bank for starting at least occasionally.

Re the other poster picking me up on "you can't get more than 12 volts excitation" that was a slip when it should have been battery voltage ie up to 14 volts. However my point was that with large current drain on an alternator the output voltage will fall because it can't do any better due to internal resistance which means at theis point the system voltage is probably at 12 or less volts. olewill
 
I asked the same question about another engine and the manufacturere
s told me that the maximum size is quoted for two reasons. First if the battery is to large, if the engine is diificult to start and the owner just keeps on turning it the battery will give out before the starter motor fails. The secopnd reason is that if you churn the engine over and over you will fill up the exhaust system and risk a water lock, the smaller capacity battery will die before that happens. On my 20 hp engine I use a Land Rover battery, but then I have a big heavy flywheel. A marine engine will need a larger battery per HP than an engine based on an industrial engine with a small flywhhel.
 
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