Question for you electronics gurus

[soundbeltfarm]

ive got a battery charger and it says 5watts R.M.S
and i charge up my tractor battery with it , how do i work out how long it will take to charge any battery?
when it charges a battery it seems to start out at about 3 or 4 ( i guess thats watts) then it goes down to about 1.
is this because as the battery gets charged it decreases the charging input?
can the battery be overcharged? I just leave it on overnight with anything i charge.

also i was thinking of getting a solar charger to hook up to our small electric fence unit down at thew run off , it runs off a car battery and lasts about 2 weeks.
can i hook the solar unit onto the battery at the same time its running the fence unit?
will it even be worth trying it ?

cheers
didnt know how to make my questions more compressed so apoligize for all the reading.

[kro]

If it's a fairly recent model charger, you could leave the charger on for days, and not harm anything. I have left my Arlec brand charger on for a week, with no ill effect, I am still using the battery it was hooked up to, and that happened 2 years ago, so I don't think there's any issue with it.


The watts RMS is kind of irrelevant in some ways, it's a trickle charger, so it's designed to go slow, at low output.

[Biff]

ive got a battery charger and it says 5watts R.M.S
and i charge up my tractor battery with it , how do i work out how long it will take to charge any battery?

it depends on how much charging your battery needs. Here's a tool I sometimes use for geeky things:

http://www.csgnetwork.com/batterychgcalc.html

when it charges a battery it seems to start out at about 3 or 4 ( i guess thats watts) then it goes down to about 1.
is this because as the battery gets charged it decreases the charging input?

Yup - The battery draws less current. Then the charger probably reverts to trickle charge mode, if it's a half decent unit.

can the battery be overcharged? I just leave it on overnight with anything i charge.

Yes - if you've got a crap charger then you can do permenant damage to a vehicle battery by overcharging them. But most modern chargers have protection circuits built in. Basically - when the battery is fully charged, using a cheap/crap charger, the battery will get hotter and hotter. This effects the plates in lead acid batteries. A similar thing happens to Lithium Iron batteries, but they generally have a conditioning chip in them in order to avoid this.

also i was thinking of getting a solar charger to hook up to our small electric fence unit down at thew run off , it runs off a car battery and lasts about 2 weeks.
can i hook the solar unit onto the battery at the same time its running the fence unit?
will it even be worth trying it ?

Yup - no problem. But again, this depends on what kind of power the solar panel gives out. A shitty little one won't be of much use. But there are plenty of solar powered car battery chargers out there. Here's a Dickie Smith unit:
http://www.dse.co.nz/cgi-bin/dse.sto...uct/View/O3307

Good luck.

[jonbuoy]

5 Watts RMS is a very random thing to out on a charger - RMS usually refers to an AC output or Audio/Radio output signal. Its a way of averaging out a power measurement. But some older chargers barely output decent DC, they rely on the battery doing a lot of the smoothing work so maybe thats what they mean.

Some older chargers are just current limited so they will keep trying to charge your battery even if its full, you'll see the voltage creep up when the battery is fully charged - eventually it will start to boil. If its a more modern one it will go from a constant current mode to a constant voltage mode - ie it won't over charge your battery. If you measure the charge voltage after its been on charge for 12 hours and its settled down to say 13.8-14v and the current or power output on the needle is at zero ish' its probably a constant current/voltage charger and is safe to leave on all the time.

The solar cell will work to delay having to charge on you battery for the fence, it might give you a few more hours between charges, depends on how much the electric fence pulls. Youll only be charging up during the day but the fence will be going all night I guess.

Cheers