How many batteries?

[anhar]

This bit about sawing a 100Ah battery in half, does it make a difference which direction you saw it. Ie length or breadth. (i mean about 50Ah 12.6v or 100Ah 6.3?

 

(running to hide in a bunker smilie)

Returning to the thread title and ignoring whether or not you should mix capacities, isn't it the case that, rather like sex, you can't have too much of them?

 

regards

Steve

[Gibbo]

Returning to the thread title and ignoring whether or not you should mix capacities, isn't it the case that, rather like sex, you can't have too much of them?

 

regards

Steve

 

Rather like sex it's no use having a huge one if you can't fill it properly. Or is that not quite what you meant?

 

Gibbo

[anhar]

Rather like sex it's no use having a huge one if you can't fill it properly. Or is that not quite what you meant?

 

Gibbo

I see your point Gibbo. Joking apart you mean presumably that too large a bank may be unable to be recharged fully by the alternator if that is the sole source when one does not have access to a shore line or generator.

 

regards

Steve

Edited July 2, 2007 by anhar

[Gibbo]

I see you point Gibbo. Joking apart you mean presumably that too large a bank may be unable to be recharged fully by the alternator if that is the sole source when one does not have access to a shore line or generator.

 

regards

Steve

 

That's right. Not fully recharging is the cause of most premature battery failures. It wrecks them. So if you have limited charge capability you're better off with a smaller bank and fully charging it rather than a huge bank and leaving it only half charged (there's room for a few more jokes - maybe something about an alternator controller being viagra for batteries).

 

Gibbo

[magnetman]

So having one battery makes more sense than anything else (assuming not cruising maybe winter mooring so limited engine hours) cos that way you save money on the batteries and get one 'proper' cycle daily.

[Gibbo]

So having one battery makes more sense than anything else (assuming not cruising maybe winter mooring so limited engine hours) cos that way you save money on the batteries and get one 'proper' cycle daily.

 

Well no. What use is that if one battery will only provide you with enough power for 3 hours?

 

Gibbo

[anhar]

So having one battery makes more sense than anything else (assuming not cruising maybe winter mooring so limited engine hours) cos that way you save money on the batteries and get one 'proper' cycle daily.

That can't be right MM. Seems to me there is no absolute answer, battery capacity required has to be a balance between your electricity needs supplied by the batteries when not being charged against the capacity of the charge source to replenish the consumption. This will vary between boats.

 

The excellent free Victron book on boat electrics - Energy Unlimited - explains it all to laymen quite well but broadly, the battery capacity needed is calculated from the adding up the likely consumption over a period of all the equipment on board and comparing that with the available output from the alternator.

 

After explaining all the complexities involved, the book gives a simple rule of thumb on calculating domestic battery capacity in practice:

 

The capacity should be at least three times the expected discharge during the period the batteries are not being charged.

 

Thus if consumption in say an overnight stop period of 12 hours is 120Ah, then the capacity should ideally be at least 360Ah. This is derived from the figure shown in the book that a battery is often really only good for about 32% of its nominal capacity. The reason this is not the often quoted figure of 50% is primarily to account for short recharge periods which may not recharge the batteries fully, a 20% loss of capacity as the batteries age plus a margin of error and other reasons. This assumes the boat has a suitable alternator able to supply the batteries' capacity effectively.

 

regards

Steve

Edited July 2, 2007 by anhar

[KenK]

You are missing something I'm afraid Ken. The load resistance is NOT the total resistance of all the batteries. The load resistance is the LOAD to which the batteries are supplying current eg: water pump, lights, fridge etc etc etc.... or to be more precise:

 

Load resistance = volts applied/current drawn

 

Chris

 

Thanks Chris, given the nature of the discussion I wasn't sure whether you were ignoring the external load and just considering the charging circuit.

 

Ken

[magnetman]

If your energy use was 40Ah a day (yes i'm a caveman and food shops have their own fridges) then one single 150Ah would be about right, better than my always a bit flat 440Ah which i can't (be arsed to) charge properly. Is that right?

[Gibbo]

If your energy use was 40Ah a day (yes i'm a caveman and food shops have their own fridges) then one single 150Ah would be about right, better than my always a bit flat 440Ah which i can't (be arsed to) charge properly. Is that right?

 

Top phrasing!

 

You're getting close to the 50% rule there. Have a look at http://www.smartgauge.co.uk/50percent2.html for a boring explanation.

 

However that is regarding most economical battery useage. You still have to put back in what you take out. Makes no difference how big or small the bank is.

 

Gibbo

[anhar]

If your energy use was 40Ah a day (yes i'm a caveman and food shops have their own fridges) then one single 150Ah would be about right, better than my always a bit flat 440Ah which i can't (be arsed to) charge properly. Is that right?

Yes that would be comfortably right on the Victron analysis. It assumes that after consuming that 40Ah, (ie. about 0.5 kilowatt hours on 12v) you then run the engine long enough and at sufficiently high revs in order to recharge.

 

regards

Steve

Edited July 2, 2007 by anhar

[smileypete]

Gibbo, can you have a look at post #49?

 

cheers,

Pete.

[Guest]

That's right. Not fully recharging is the cause of most premature battery failures. It wrecks them. So if you have limited charge capability you're better off with a smaller bank and fully charging it rather than a huge bank and leaving it only half charged (there's room for a few more jokes - maybe something about an alternator controller being viagra for batteries).

 

Gibbo

A mistake I made (batteries not viagra)

 

Thevenin and Norton Equivalent Circuits anyone?

[chris w]

Thus if consumption in say an overnight stop period of 12 hours is 120Ah, then the capacity should ideally be at least 360Ah. This is derived from the figure shown in the book that a battery is often really only good for about 32% of its nominal capacity. The reason this is not the often quoted figure of 50% is primarily to account for short recharge periods which may not recharge the batteries fully, a 20% loss of capacity as the batteries age plus a margin of error and other reasons. This assumes the boat has a suitable alternator able to supply the batteries' capacity effectively.

 

regards

Steve

 

One of the major reasons for this low figure of around 30% is that, without a proper multistage mains charger or a multistage alternator controller, your batteries will probably never get to 100% charge. A more realistic figure would be about 70-80% charge. Since it is inadvisable to discharge a battery to less than 50% charge, only the difference is available, viz: 80%-50% = 30% or worse 70%-50% = 20%.

 

Remember too, that because of the efficiency losses in charging (charging is only about 70% efficient), you need to put back close to 1.5 times what you took out. The rest is wasted as heat etc in the batteries.

 

Chris

[Gibbo]

Gibbo, can you have a look at post #49?

 

cheers,

Pete.

 

I did. I thought you were having a laugh.

 

Gibbo

[smileypete]

I did. I thought you were having a laugh.

 

Gibbo

 

Very well, how about quick summary of when it's OK or not to parallel batteries?

 

cheers,

Pete.

Edited July 3, 2007 by smileypete

[Gibbo]

Very well, how about quick summary of when it's OK or not to parallel batteries?

 

cheers,

Pete.

 

How quick?

 

When they are not the same type.

 

Will that do?

 

 

In reality you would get away with paralleling certain different types (eg normal gel and one particular type of AGM would be OK together) but the difficulty is identifying what can and can't work together from the scant information available on the battery label.

 

But........

 

Different capacities of the same type is NOT a problem. At all. Ever.

 

Gibbo

[Sir Nibble]

S'troo.

[smileypete]

S'troo.

 

I'd tend to agree, I reckon most people's problems are down to the existing batteries not being charged fully, let alone more new ones as well.

 

It would be pretty easy to prove with a DC clamp meter, to show how much is coming from each battery.

 

cheers,

Pete.

[Gibbo]

It would be pretty easy to prove with a DC clamp meter, to show how much is coming from each battery.

 

That's more likely to show that the batteries are badly installed.....

 

http://www.smartgauge.co.uk/batt_con.html

 

 

Gibbo