Truma Motor Mover help!

[WoodlandsCamper]

There isn't the room (length) for both the van AND the car on our drive, hence why we use the mover to put the van in forwards, off side to a wall and we still then have access to the door to get into the van.
I'll take a photo next time it is at home.

 

[ProfJohnL]

Hello Bertie and Dusty.

Manufacturers of movers will suggest having a large capacity leisure battery to run a mover for two reasons.
Firstly the construction of leisure batteries and their intended function is to provide a modest current over a long period, and to be able to withstand being left in a significant state of discharge for extended periods. This usually means their peak current capability is quite limited and that could compromise the operation of a mover. However it is generally the case that bigger leisure batteries also have bigger peak current capabilities
.
The relationship between terminal voltage and the level of charge in a battery is not not linear It tends to flatten in the middle discharge section. Consequently the terminal voltage of a leisure battery that is 75% full, is not too dissimilar to the same type of battery that is only 25% full.

Mover manufacturers will be aware of these issues and will only be concerned if the terminal voltage falls much lower than this. To be fair if a battery voltage does fall that much then it points to a battery in a very low state of charge or even permanently damaged.

The second reason is that in most UK caravans, the battery also supplies power to lights and other appliances You therefore need a bigger battery as its serving these other services.

Dusty, I have re-read my posting, and I can't see how you have drawn those conclusions from it,. A linear relationship is one where the two related criteria are plotted on a graph the plotted points fall on a straight line. an inverse relationship can still be linear, it just the slope of the line means one criteria get small as the other get bigger.

But let me try to cover your points anyway; a battery is like barrel of water , You can deposit water , and you can take it out again. Consider if you put in 110 litres (Like a fully charged 110AH battery) You could withdraw money at the rate of 1L every hour and its fairly obvious the barrel would be empty after 110 hours.

You could withdraw could withdraw it at a rate of 110 litres per hour and the barrel would be empty after 1 hour. Batteries are in principle the same - but there may be practical limits to the maximum rate you can withdraw the current.

But with the barrel you could draw waster at a rate of 60 litres an hour but only do it for 5 minuets. that would be 1/12 of an our in which case you would only draw 1/12 of 60 litres which is only 5 litres, so the barrels capacity has only dropped by 5 litres.

I hope this helps.

 

[bertieboy1]

Prof,
At the moment of starting a mover the battery terminal voltage will reduce or collapse (depending on the state of charge).The bigger the Ah's available the less voltage reduction will be experienced.So for a 35Ah battery (flat line or not)the voltage reduction will be far greater than say a 75Ah capacity battery with the same percentage charge and the voltage may reduce to such a value as to cause the control circuits to stop operating.

 

[ProfJohnL]

Prof,
At the moment of starting a mover the battery terminal voltage will reduce or collapse (depending on the state of charge).The bigger the Ah's available the less voltage reduction will be experienced.So for a 35Ah battery (flat line or not)the voltage reduction will be far greater than say a 75Ah capacity battery with the same percentage charge and the voltage may reduce to such a value as to cause the control circuits to stop operating.

Hello again Bertie,

What you say happens to a battery voltage under load is perfectly true, and it is also true that a bigger battery will have a greater capacity, But let me modify that thought just a bit; What you have suggested is true only where the batteries are of the same construction. Batteries of a different construction will perform differently and its a question of choosing one for the job in hand..

Leisure (Caravan) Batteries have a different construction to cranking batteries, and generally leisure batteries cannot sustain the high current discharges that a cranking battery can. That is why I specified a car battery not a leisure battery becasue a car battery can maintain a better terminal voltage under the likely load. Also with the OP's limited usage of the mover, it will not exceed the car batteries AH capabilities.

In reality any battery in poor condition is capable of dropping its terminal voltage below the movers working thresholds. So its question of battery maintenance and keeping them properly charged.

 

[Dustydog]

Hi Prof
Thanks for your explanation and illustration.
If I follow your logic then the battery will yield it's full number of amp hours.
I 've found the thing from school which is why I thought a 110ah will never actually give the full wack. Maybe I got the wrong end of the stick. It was Peukert's Law.
I shall welcome your feedback. Thanks

 

[ProfJohnL]

Hello Dusty,

Good call,I wasn't aware of who defined it ( Peukert's Law). but I did allude to this when I wrote

"... there may be practical limits to the maximum rate you can withdraw the current..."

Perhaps I have underestimated the influence of Peukert's Law, but having reconsidered it, I am still satisfied that the use of a 35AH cranking battery will more than meet the needs of the OP.