On 28 Apr, 16:58, David D wrote:
Yes, you are right. I am going to a local electronic store - bringing
the monitor with me - to see what they say. It would be nice to use
the monitor for 3-4 hours at a time, longer would be a bonus. I will
also ask about SLA batteries, thanks for that.

On Apr 28, 11:15 am, "DaveM" wrote:

"David D" wrote in message

oups.com...
On Apr 27, 2:54 am, wrote:

Ï/Ç David D Ýãñáøå:

I don't want to fry my security monitor, but I want to hook up a 12V
4.5A rechargeable battery to it. Since the monitor on the back says
"12V 3amp", would the monitor fry or would it just take what it needs
and be ok with this amperage?

Thanks

I 'm from greece and i don't know so much english. Yes you can use
this battery the problem is in Volts. If you have a batterry with up
of 12V don't try to use it with this monitor because you burn this
monitor. The 4.5 Amperes of battery say how much current you can use
from battery and if you have a monitor with up of 4.5A and conected
with this battery you burn the battery and maybe this can make the
battery to BOOOOOOOOM. You Understand?

'thanks for the replies.
Right now, I am interested in getting 12V battery with a 4.5
amperage. The only problem is I am doing it DIY and trying to make it
on the cheap - therefore I am stuck with finding a way to RECHARGE the
battery. Of course without LED lights and levels, I won't know know
if it is charged or not. And yes, I don't want to blow a $100.00
monitor either. My main purpose for this is to create a portable
monitor system. I went to Canadian Tire to find the 12V cordless
drill batteries, but they only pump out 1.5 Amp, so it is getting
harder then I thought...

Well, now we're getting to the root of the problem. The answers that other
posters have given are correct, for the most part. The issue now is.. how long
do you need to operate the monitor on battery? The ampere-hour (AH) rating of a
battery is useful in determining how long the battery can supply power. The
load current is a major factor in determining this time. The chemistry of the
battery is also a major factor in this calculation.

A fairly good tutorial on batteries, AH ratings, Loads and recharging is athttp://homepages.which.net/~paul.hills/Batteries/BatteriesBody.html. I suggest
that you read that with great attention and you will get a better understanding
of your situation.

For your portable application, I suggest that you look primarily at sealed
lead-acid (SLA) batteries to start. Newer chemistries such as Lithium-Ion and
others typically used in laptop computers have a higher energy/weight ratio, but
are significantly more expensive.


There seem to be some misunderstandings here.

Firstly there is no such a thing as a 4.5A battery, what you have is a
4.5Ah battery. This tells nothing about what current it will supply,
but rather its capacity.

2nd, you wont get 1.5hrs from a 4.5Ah battery on 3A, for 2 reasons:
a) Ah capacities are only correct for 20 hour discharge. High
discharge rates will reduce capacity considerably.
b) If you discharge a lead acid battery until flat it wont last long.
You need to stay above 20% charge at least.

For 4 hours of run time, if we momentarily ignore the 2 above issues:
4x3 = 12Ah
Add 50% (guessed) for a short run time - 18Ah
Add 25% to permit discharge down to 20%: 22.5Ah

So really you want a 25Ah or more battery to do what you want.

There is one last issue. Is the monitor 12 real volts or 12 battery
volts? What are known as 12v batteries arent really 12v. This is fine
if the monitor is designed to run off car supplies, but if it needs a
12v regulated supply it might be unfine - or it might be ok. Would be
wisest to check first.


NT