I have a 12v 2.5Ah 20HR battery to run some LED lighting for my camp site. When its low I want to plug it into a fuse box I have fitted in the cab of my Lux. Question is, does the cable between this battery, and the main battery, need to be the same size as my standard battery cables, or can I draw through 4mm cables and not melt them ??
click Here For Short Term Satellite Phone Rental
For a battery that small it shouldn't matter to much. Just use the cheap shit wire that you buy from supercheap or anything you have lying around. Dont leave it connected to long or you will fry it. Might pay to check if jaycar have a charger kit for small SLA batteries.
.........................................
click Here For Short Term Satellite Phone Rental
I'd be bunging in a small regulator in the circuit to the plugged in bubba battery to restrict the current going into the little bugger.
The wire from the added fuse box to the ute battery can be 4mm but be sure to have it fuse protected just in case of bigger currents.
I intend on living for-ever....so far so good
A regulater ??
click Here For Short Term Satellite Phone Rental
Something like that http://www.altronics.com.au/index.as...=item&id=N2075 will do and if you want later on you can have a solar panel as well.
You just don't want to pump in huge currents into the battery.
The current needs to be controlled in someway.
I intend on living for-ever....so far so good
As finga said, a regulator is the go if you want to play it safe. If you look at the cheapie rechargable laterns that you buy they will use an unregulated charger, which is fine until you leave them on charge to long. The problem with no regulation on you car means that a lot of current = quick charge, so battery fries if left connected to long. A quick trick is to charge it with a small fues inline, say 3-5amps and if that pops you should use a regulator.
I am not sure if such a beast is available, but I have been thinging about designing (i am/was electronics engineer) a 12V multistage charger for selaed lead acid and AGM batteries. Ie, similar to the CTEK chargers but one that runs of the car. Perhaps its time to start looking into it a bit more. Not sure how many kit builders are in here but if there is enough interest I might start looking into it a bit further.
Just wack a resistor in series to the battery you want to charge. That way you will limit the peak currents that can go into and out of the battery. 4mm cables will be fine.
I would recommend something like a 10W 3.3ohm ceramic resistor for this sort of application. (Make sure the output you are using is fused - 5A should be fine). Just be careful as the resistor will run a little hot when charging.
Don't run the leds at the same time you are charging the battery.
(I'm assuming its an SLA battery - if not disregard this advice)
oddbudman
Ihave remover my post here because the information contained was either irrelivent or wrong.
refer to my later post for explanation.
cheers
Last edited by oldboot; 18-01-2008 at 04:31 PM.
Actually a resistors prime purpose in life is to limit current. The only problem with this is that they are a fairly crude current limiting device, and one of the side affects is the the develop a voltage drop across them. So if the resistor mention before (3.3 ohm) is limiting the current to 1amp, then there is actually 3.3V drop across the resistor. The biggest problem with this is that as the battery becomes charged, there is less current drawn and the voltage dropped across the resistor decreases, which means you battery has a higher charge voltage. This is how the crude chargers work in the suprecheap et al rechargeable (they use SLA) laterns and lights.Originally Posted by oldboot
The diode is good to prevent current flowing back into the battery.
I will draw up a little circuit later than cam be built for under $10 bucks that will solve the problem. Stay tuned.
Heh,
Lets keep this thread interesting....
Interested to see your circuit.... I'll throw my hat in the ring.
Much depends on the terminal voltage of the battery in the car when the car is running. Unless you want to get ultra tech and boost / regulate the voltage.
oddbudman
Hi Bungie, for charging small batteries like yours, why not just use the same simple method used by most domestic home security systems use to charge their backup battery.
Simply fit a 5 or 10 watt 24v festoon globe in series in the positive wire and it will control the charge going into the battery.
It’s a simple system and the globe automatically self regulates the current based on the state of charge of battery.
The lower the state of charge of the battery, the higher the current going into the battery because of the larger voltage difference across the globe.
As the battery charge rises, the voltage difference across the globe reduces, in turn reducing the amount of current going into the battery.
The only thing you have to take into account is that this is a slower ( but safer ) way to charge a small battery.
Just a suggestion.
I normally use a program called protel, but I have not used it for 3-4 years, so I cant draw up a circuit. If I find the CD i will install it, its around somewhere.
Anyhow, the circuit i was going to suggest was based around an LM317 voltage regulator configured as a constant current regulator as in here:
http://diyparadise.com/yhlmccs.html
if you want to get real tricky you could even add another LM317 after the first configured as constant voltage regulator.
TheRealAndy,
The problem you will find with the lm317 is it has a high dropout voltage (ie the voltage it needs to drop to accurately regulate). So when the available charging voltage is so low (as is the case with a car trying to charge a 12V battery), its always going to be an uphill battle. In the circuit you posted you are dropping 1.25V across the resistor and also voltage across the regulator (at least 1.5v) itself, so this sort of circuit would really only work well if the charging voltage was up around 16V.
This is why i originally suggested a simple resistor to do the charge, as the charging voltage is likely to be at voltages a SLA battery can handle to begin with. The resistor was just a simple way to stop too much current going into the battery in the early stages of charge (many SLA batteries have this max charging current and voltage printed on them). The resistor could still cause problems as oldboot suggested such as overcharge, but this would depend on the terminal voltage in the car, how long the battery was left on charge and the SLA battery charge state at the time.
Anyway, I guess we have now bamboozled bungie, without giving him a clear answer.
Bungie -
were you planning on running the car when charging the battery?
do you know the voltage at the car fusebox?
oddbudman
YEah goid point oddbudman, didn't even think of that, all I was thinking was simplest way to to make a constant current regulator!
Its been in the back of my mind for a while to do this properly. I was asking about on an electronics forum cheap microcontrollers with free c compilers the other day for this exact purpose. There is a few semi manufacturers that make some shit hot parts for battery charging. My biggest problem would be the design of the magnetics for step up/step down as I have never don it before. You seem pretty clued up oddbudman, whats the story? You in the field? Its been about 4-5 years now since I have done any design, maybe longer. <-- this still does not help bungie!!
As for bungie, I would still be hooking the battery directly, and only using a resistor if the battery draws to much current.
Posting Permissions
Reply With Quote