Battery adapter

[DaveC1337]

I have a small screen for the car, it has no built in battery for operation, I take it camping alot and annoys me that I have to be around the car when whatching. It takes a 12v 1200Ma adapter, I'm not to bad at wiring, just unsure of power flow, I was thinking about making a battery pack of my own with just normal batteries. Can any1 help me in working out how many batteries I would need, and if it would work ?

 

[Doomed]

In theory yes, your average battery provides 1.5v, so put 8 in series and you will have your 12v dc.

However, how long it will run for is a diferent matter, using the formula:

where
Q is the battery capacity (typically given in mAh or Ah).
I is the current drawn from battery (mA or A).
t is the amount of time (in hours) that a battery can sustain.

your I is 1200mA, you will need to look at a battery for Q and that will tell you if its worth your time and effort.

 

[boatnik1]

Building from ni-cads or similar, not worth the hassle as you would need 10 cells for 12v and a multiple of packs paralelled to last any time. 10 1200ma cells will only last around half an hour before voltage drop makes it useless.
12v gel battery is what you want. If you're only running 1200ma then a 6a one should last about 4 hours before recharging. You need a larger one if you want to run longer. Maplins supply batteries/chargers and connectors.
If you're not near a branch then google them and order from their catalogue on line. Couple of days for delivery.

 

[DaveC1337]

Ok .. this type of battery

2x D 10000mAh Rechargable Batteries Ni-MH NEW a1-uk9b on eBay (end time 22-Jul-09 17:54:11 BST)

... is 1.2v and 10000mah, so 10 of these make 12v, the screen needs 1200ma, so this calculates out at 8 hours and 30 mins using 10000mah(Q) divided by 1200ma(i) giving you the time(t) in hours.

does having more batteries not increase life time tho ? like one is 10000mah, I know this is the flow, could you explain a little more on how this works out.

 

[Mr Brady]

Or perhaps something like this:http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/469725.xml
More expensive I know, but also more versatile.

 

[boatnik1]

10 10000 batteries in series = 12v @ 10000mah
10 10000 batteries in series = 1.2v @ 100000mah.
still think the project is not realistic, those batteries from China would be costing you over £6.00 each and if one out of the ten dies you have a useless pack. i.e. £60.00+ for the pack. if the customs don't catch them and charge you the duty also..
Mr Bradys suggestion is much more practical. These can be bought for around £30.00 in Halfords and similar outlets and come complete with charger from car and mains. The 17ah battery in these packs would give you about 13hours use before Voltage drop made the screen stop working.
The mathmatical theory dosn't work in practice as the voltage drops off through in use and stops the screen working before the battery is completely flat.
The Maplins version is:- http://www.maplin.co.uk/module.aspx?moduleno=19363

 

[DaveC1337]

Wow, thanks for all this .. I'm confused about another thing tho. The adapter that you plug in says OUTPUT 12v 1200mah, so all of these batteries I'm looking at go over 1200mah does this matter ? or would the screen regulate the power coming in ? also does the 1200mah only matter because its going into the socket in the wall ?

Thanks for all your help guys !

 

[Mr Brady]

The 1200mah is a capacity rating, and it's saying that the battery can deliver 1200 milliamps for one hour or 600 milliamps for two hours or 300milliamps for four hours and so on.

 

[boatnik1]

The screen will only use the ma it is rated at regardless of the battery capacity. The important bit is the voltage which should be 12vDC up to a max of 13.8V (Car battery voltage on full charge) So the car powerpack is probably your best and cheapest option. It also has the added advantage of being a standby battery to start the car, or to blow up airbeds dingys etc, as long as it's kept charged.