Accessory battery charger?

[Drunkskunk]

I know nothing about battery charger, but I would like to add a small 4.8v Nicad or NiMh battery to my bike to run a couple accesories, like led rear lights, and a servo to flip my motor switch.
I would need no more than a max draw of 2 amps, but that more load than I think the crystalyte controller can handle on it's 12 side, so what I'd like to do is tap off the 12 volt side, and have it charge a battery. the battery can give me all the power I need in short bursts, and be recharged by the controller when I'm not using it.

What would I need to do to make a passive charger like this?

 

[deardancer3]

one easy way-

subtract 4.8v from max voltage you are going to draw from. this is your charge voltage differential

divide your amphour rate by 25. this is your max charge current.

divide the charge voltage diff by the charge current- this will be your charger resistor,

mutiply your charge voltage diff by you charge current, multiply by 2.5 this is the wattage of the charger resistor.

example:

12v minus 4.8v is 7.2 v

if you have a 5 ah accesory battery, you charge current would be 200 milliamps.

dividing 7.2 volts by .2 amps is 36 ohms.

.2amps times 7.2v is about 1.5 watts; times 2.5 means use a 4 or 5 watt 36 ohm resistor.

Add a diode just in case.

 

[fechter]

What about a dc-dc converter? What is your system voltage?

 

[GGoodrum]

Another option would be to use an RC Battery Eliminator Circuit, or BEC, to drive your accessories, instead of the NiMH battery. Here is one that works to 60V and can supply either 5V or 6V at up to 2.5A: http://www.dimensionengineering.com/VHVBEC.htm. This will work great for any 36V or 48V setup. It looks like you are running a 72V setup, with two packs. Just hook this up to the first pack in series.

-- Gary