Tonight

[Silly Sausage]

Back to the batteries....

They are described as "3Ah" and we know they are 1.5V. So the total energy stored in each is 1.5 x 3 Wh = 4.5Wh (= 4.5 x 3600J = 16.2kJ). However you connect them up, the total amount of energy contained in 4 batteries is 4 x 4.5Wh = 18Wh. (4 batteries x 3Ah x 1.5V).

4 batteries in parallel, you have 1.5V. 18Wh / 1.5V = 12Ah.
4 batteries in series, you have 6V. 18Wh / 6 = 3Ah.

Hope this is helpful

Yeah, but you're driving 4 times the current through a load with the batteries in series than parallel.
i.e. 4 x Power

The Ahr is the same for both configuration, just that the batteries will last 4 times longer when in parallel.

 

[happysteve]

Yeah, but you're driving 4 times the current through a load with the batteries in series than parallel.
i.e. 4 x Power

Oh aye, agreed (assuming the same resistive load).

The Ahr is the same for both configuration, just that the batteries will last 4 times longer when in parallel.

No, for the parallel configuration you've got 12Ah at 1.5V (=18Wh) and for the series configuration you've got 3Ah at 6V (=18Wh).

This confusion is just because the term "amp hour" isn't complete, as it assumes a certain voltage.

(Similar confusion arises when you say, "This is a 1kW [resistive] heater." It's only 1kW at the intended voltage (eg 230V). If you run it at 115V, it's only a 0.5kW heater.)

 

[davesparks]

And then of course the Ah value of a battery is only ever achieved at the specific test conditions that the battery was tested at!

If a battery is delivers 1A for 10 hours under test conditions then it is labelled as a 10Ah battery. But if you tried to get 10A out of that battery for an hour you would be sadly disappointed!

 

[happysteve]

Agreed, definitely. When I lived on a boat and relied on a bank of 12V batteries (3x110Ah 12V in parallel, plus a starter) I reckoned on roughly one third of that energy actually being useful (before the voltage dropped too much).

 

[Silly Sausage]

Oh aye, agreed (assuming the same resistive load).



No, for the parallel configuration you've got 12Ah at 1.5V (=18Wh) and for the series configuration you've got 3Ah at 6V (=18Wh).

This confusion is just because the term "amp hour" isn't complete, as it assumes a certain voltage.

(Similar confusion arises when you say, "This is a 1kW [resistive] heater." It's only 1kW at the intended voltage (eg 230V). If you run it at 115V, it's only a 0.5kW heater.)

'n' batteries have the same 'AmpHour' capacity whether they are connected in series or parallel.

I'll bung my proof up later!

It's a brave man that challenges Tony!!!

 

[happysteve]

'n' batteries have the same 'AmpHour' capacity whether they are connected in series or parallel.

Well of course each "cell" remains a 3Ah 1.5V battery. Unconnected you have four 3Ah 1.5V batteries. Connected in parallel, you have a 12Ah 1.5V battery. Connected in series, you have a 3Ah 6V battery.

I'll bung my proof up later!

I'm sure I'll agree with it. I think in your case (and Tony's) it's more about semantics than fundamental understanding.

It's a brave man that challenges Tony!!!

True dat.

Had to grow a pair some day, though...

 

[Silly Sausage]

Well of course each "cell" remains a 3Ah 1.5V battery. Unconnected you have four 3Ah 1.5V batteries. Connected in parallel, you have a 12Ah 1.5V battery. Connected in series, you have a 3Ah 6V battery.


I'm sure I'll agree with it. I think in your case (and Tony's) it's more about semantics than fundamental understanding.

True dat.

Had to grow a pair some day, though...

No, you'll have 12Ah 6V battery, it'll just drain in a 1/16th of the time of that of a 12Ah 1.5V battery (Power proportional to V^2), sameb load.
I was wrong in my 1st post saying 4 x the Power!

I think you're not taking account of time, possibly

It's good to stand up against his Lordship once in a while, keep him in check...slightly!!! :smilielol5:

 

[HandySparks]

(Similar confusion arises when you say, "This is a 1kW [resistive] heater." It's only 1kW at the intended voltage (eg 230V). If you run it at 115V, it's only a 0.5kW heater.)

Hi happysteve. I agree entirely with your reply to Tony (I thought about it and then decided to let it lie).

You might, however, care to correct your passing statement above.

 

[davesparks]

What's wrong with his statement about the 1kw heater?

 

[HandySparks]

What's wrong with his statement about the 1kw heater?

If you halve the voltage on a resistive load, the power reduces to a quarter. 1kW at 230V > 250W at 115V.

 

[davesparks]

Theoretically yes, but only if the temperature and resistance remains constant, however the temperature and therefore resistance will change and the change in power won't be quite so simple to calculate

 

[happysteve]

Brain mong!

 

[happysteve]

Yes, Handysparks (and also Davesparks), I have had a total brain mong here.

P = IV. P = V[SUP]2[/SUP]/R P = I[SUP]2[/SUP]R

If you halve the voltage, the current will halve (for fixed resistance). But the power will go down to a quarter of what it was.

Gosh, you have no idea how silly I feel right now. :redface:

 

[davesparks]

I have a very good idea how silly you feel, we all make these silly mistakes sometimes.

 

[HandySparks]

What's wrong with his statement about the 1kw heater?

Ask happysteve.

Brain mong!

No probs, it's a easy one to miss. :biggrin: