Batteries....what rating?

[JumbleDuck]

That's correct Jumbleduck. Its the hydraulic analogy. Ahrs = volume, voltage = depth(head). Seems logical to me.

I'm sorry, but I can't see any useful hydraulic analogy which would work that way.

 

[ghostlymoron]

Let me explain my analogy. Imagine you have two water tanks standing on a level surface. Tank 1 has a volume of 85 units and is 12/13 ths full. Tank 2 has a volume of 45 units and is 10/13 ths full.
The two tanks are joined at the bottom by a pipe with a valve in it. When you open the valve, the level in tank 1 will fall to 11.2/13 ths and tank 2 will rise to the same level. But the combined level (equivalent to voltage) will be LESS than the original level in tank 1.

I'm sorry, but I can't see any useful hydraulic analogy which would work that way.

 

[JumbleDuck]

Let me explain my analogy. Imagine you have two water tanks standing on a level surface. Tank 1 has a volume of 85 units and is 12/13 ths full. Tank 2 has a volume of 45 units and is 10/13 ths full.
The two tanks are joined at the bottom by a pipe with a valve in it. When you open the valve, the level in tank 1 will fall to 11.2/13 ths and tank 2 will rise to the same level. But the combined level (equivalent to voltage) will be LESS than the original level in tank 1.

Doesn't apply, in at least four different ways.

First of all, it would depend on the shape of the tanks. Depending on the base areas, the initial depth of water in the smaller tank could be less than, equal to or higher than the level in the bigger tank, so the flow could be in either direction.

Secondly, it would take some time for the levels in the two tanks to equalise - just as it takes some time for the charge in two batteries to stabilise - so the available head at the joint outlet pipe would not change instantly.

Thirdly, the maximum capacity of the tanks is irrelevant. You could clearly increase the capacity of either tank by extending it upwards without changing the behaviour of the water.

Fourthly, the relationship between depth of water in a tank and head at the outlet is linear with no flow. The relationship between voltage and stored charge in a lead-acid battery is extremely non linear.

Hydraulic analogies are fine for very simple electrical systems, but they have to be carefully thought out and they don't deal at all well with non-linear components.

Here's another analogy.

 

[GHA]

This might be of interest in the slight thread drift...
http://www.zetatalk4.com/docs/Batteries/FAQ/State_Of_Charge_Ver_Voltage_2004+.pdf

 

[ghostlymoron]

Jumbleduck, I agree that the water tank analogy is not perfect but the combined voltage is bound to be less than the original voltage of the 85ah battery in my scenario.
Don't understand the relevance of the balloons - are you saying that the smaller, less charged battery will discharge into the larger, higher charged battery?

 

[GHA]

Jumbleduck, I agree that the water tank analogy is not perfect but the combined voltage is bound to be less than the original voltage of the 85ah battery in my scenario.
Don't understand the relevance of the balloons - are you saying that the smaller, less charged battery will discharge into the larger, higher charged battery?

The balloons are not what you would intuitively expect, but if you assume combiner surface area is proportional to pressure(voltage) then the smaller balloon emptying into the bigger balloon results n the lowest combined pressure, less than both balloons the same size.

I think a problem with the watertank analogy is that a charging current will results in a larger change in battery voltage than a similar drain current.

 

[JumbleDuck]

Jumbleduck, I agree that the water tank analogy is not perfect but the combined voltage is bound to be less than the original voltage of the 85ah battery in my scenario.

It'll be less all right and it will also be changing with time. Predicting how much less and how much it changes with time is complicated, though, and simple water tank calculations just aren't relevant at all. Sorry. Nothing personal.

Don't understand the relevance of the balloons - are you saying that the smaller, less charged battery will discharge into the larger, higher charged battery?

It's a good example of a case where intuition is often misleading. The pressure in a balloon goes down as you inflate it, then starts to rise again. Shows how odd non-linear systems can be.