adding extra batteries

[boatystuff]

hello all

 

I am currently resiting the batteries on my boat and am thinking of putting an extra battery ( currently have 3 x 110 amphour with a 75 amp alternator). I realise that I should uprate the alternator, and intend to put a 100A alternator on eventually ( when time, money, and the rest of life permit) .

 

My question is will it do any harm in the short term to charge 4x110Ahr batteries off a 75A alternator

 

My second question is am I likely to be able to easily uprate to a 100 A alternator. Will I be able to run it off the same pulleys, and a single belt. I presume I will need to uprate the wiring from the alternator

 

Any advice suggestions etc would be welcome

 

 

Tim

[bottle]

hello all

 

I am currently resiting the batteries on my boat and am thinking of putting an extra battery ( currently have 3 x 110 amphour with a 75 amp alternator). I realise that I should uprate the alternator, and intend to put a 100A alternator on eventually ( when time, money, and the rest of life permit) .

 

My question is will it do any harm in the short term to charge 4x110Ahr batteries off a 75A alternator

No, it will just take the same amount of time to re-charge them for the equivalent dis-charge. If you take 50amps out of 3 batteries it is the same as taking 50 amps out of 4 but each battery will have dis-charged a little less, you still have to put the same amount back, approximately 75amps. tThe advantage of the extra battery is the batteries do not 'work' so hard and you can last a little longer between charging. Having said that, you will have to charge longer

 

My second question is am I likely to be able to easily uprate to a 100 A alternator. Will I be able to run it off the same pulleys, and a single belt. I presume I will need to uprate the wiring from the alternator.

The answer to all these is probably, it will depend on the engine and the space you have (physically I doubt there is much, if any, size difference in the alternators).

 

Any advice suggestions etc would be welcome

 

 

Tim

 

Edit: the difference between a 75 and a 100 alternator is very little, try looking at fitting a bigger one, in the region of 120/150 but then but then belts and pulleys will need to be looked into.

Edited April 4, 2010 by bottle

[Keeping Up]

It would do no harm at all to charge from a 75 amp alternator, as long as you run the engine long enough to charge them up fully.

 

If you take the same number of amp-hours from your 4 batteries as you used to take from the 3, before recharging, then the recharge time from your existing alternator will be (broadly) similar to what it it has been in the past. There is a temptation however to make use of the increased battery capacity, and take more energy from the batteries before recharging than you could have taken when you had fewer batteries; if you do this, remember to allow more time for recharging.

 

Personally I'd not bother upgrading the alternator unless you need to (eg because your existing one breaks down), it seems a lot of bother and expense for not a lot of benefit - that's why I'm still charging my 4 batteries from a 70 amp alternator just as I did when I had only 2 batteries - but it's your choice.

[Gibbo]

Other points to consider...

 

Most narrowboats have badly pulley'd alternators. They simply don't spin them fast enough. The consequence being that you get seriously limited charge from the alternator at low engine revs.

 

If you do lots of charging whilst moored with the engine running at low revs you might get a better increase in charge by changing the pulleys rather than increasing the alternator size. If this is the case with your boat then increasing the alternator size is actually likely to make things worse because bigger alternators usually have to be spun even faster.

 

If, on the other hand, you do most of your charging with the engine running sufficiently fast to spin the alternator properly then a bigger alternator will help. But nowhere near as much as you might think. Going from a 75 amp alt to a 150 amp alt makes many people think they will cut the charge time in half. This isn't the case because most of the charge is limited by the batteries not by the alternator. Roughly speaking, moving from 75 to 150 amps on a 400ahr bank might reduce the charge time by about 20%. Going from 75amp to 100amp is unlikely to make much difference at all, all other things being equal.

 

Also, check the charge voltage you are getting. If it is below 14.0 volts then sorting that out will have a much greater impact n recharge times than putting a bigger alternator on. It's also easier.

[boatystuff]

Thanks for the information. The impression I am getting is that its probably best to stay with the existing alternator, and take the boat out more often.

 

It generally has plenty of time to charge, but what I really need is a bit more capacity so that the managment can watch the TV whilst the beautiful english country side is slipping past gracefully and still use the hair dryer at night.

 

I presume the best way to check the output voltage is stick a voltmeter across the battery with engine off and on. What are the likely reasons/ remedies for sub 14V output

 

thenks Tim

[Gibbo]

Thanks for the information. The impression I am getting is that its probably best to stay with the existing alternator, and take the boat out more often.

 

It generally has plenty of time to charge, but what I really need is a bit more capacity so that the managment can watch the TV whilst the beautiful english country side is slipping past gracefully and still use the hair dryer at night.

 

I presume the best way to check the output voltage is stick a voltmeter across the battery with engine off and on.

 

Yes, but you have to wait until the voltage stops rising during charge (ie batteries nearly charged) then take the reading.

 

What are the likely reasons/ remedies for sub 14V output

 

Insufficient cable size between alternator and batteries causing voltage drop.

 

Older alternator which were regulated at a lower voltage than the more modern ones.

 

There are various cures but let's see what you've got first before we explore that avenue.

[bottle]

Thanks for the information. The impression I am getting is that its probably best to stay with the existing alternator, and take the boat out more often.

 

It generally has plenty of time to charge, but what I really need is a bit more capacity so that the managment can watch the TV whilst the beautiful english country side is slipping past gracefully and still use the hair dryer at night.

 

I presume the best way to check the output voltage is stick a voltmeter across the battery with engine off and on. What are the likely reasons/ remedies for sub 14V output

 

thenks Tim

 

Hi Tim

 

How long is 'plenty of time to charge'?

 

Extra capacity is fine but whatever you take out of three/four or five batteries has to be put back +++.

 

More batteries does mean extra capacity but it also means more charging, if you use that capacity.

 

If the management are watching TV whilst you are on the move then the batteries are being charged less than if everything is turned off.

[boatystuff]

Thanks all for the input

 

I can't check voltage at the moment as I've disconnected the batteries and dismantled the engine room. As soon as its all back together I will investigate.

 

Just to clarify ,Gibbo, are you saying that voltage should be above 14V with batteriees virtually fully charged.

 

My other major plan is to buy the managment a book!!

 

Tim

[Gibbo]

Just to clarify ,Gibbo, are you saying that voltage should be above 14V with batteriees virtually fully charged.

 

Yes, but only while you are actually charging.