Automotive trickle charger ???

[Zero Cool]

Its that time of year again here in MN. time to put the BMW into winter storage and this year i need to build myself a trickle charger to keep the dual batteries charged up.

I just cant bring myself to pay $70.00 for 1.5 amp trickle charger when i probably have all the parts needed right here in the shop. BUT. what i dont know is what is so special about a trickle charger? anything?

Some have said they have a IC inside to monitor voltage etc.

Anyone have any info on how to build one of these?

I have plenty of 2 amp 12VDC power supplys if thats all thats inside.

Zero

[Brian Donaldson]

Try some google searchs. If I remeber correctly, you need a regulated voltage, and the voltage varies with temp. If you know the temp, you could adjust the voltage as the weather changes. I've seen tricle chargers for a lot less here. Or use a automatic charger on a timer to let it run for 5 to 10 min a day or an hour per week or something.

[DigitalJunkie]

Maybe use a 12V 500ma "wall-wart" on a timer? run it maybe an hour every (other?) day or something. Simple,and should be really cheap. (especialy if you have a timer and wall-wart in your junk box.)
Or,if you have a smaller wall-wart,maybe 100ma,you could probably leave it connected continously without damaging the battery/ies.

[peranders]

You must look around more because battery charges are dirt cheap at least in Sweden.

If you have some parts in your junk drawers it's pretty easy to make a charger. Just fix current limitation and a fixed voltage of 2.35 volt per cell. LM317 will work fine.

[Brian Donaldson]

14.1 V is a little hot for a trickle charge. Take a long drive, then measure the voltage with the car running. Most modern cars use the computer to regulate the altenator and they have a pretty good charge algorithm that starts high untill the battery is recharged then tapers off to a trickle. Also it would be good to know how much of a load the car is putting on the batteries with its little BS accessories like keyless entry receiver, radio memory, etc. you'll need to overcome these losses. I would guess around 13.2v

[Stuart Easson]

I have a similar 3 headed setup for the cars I'm not driving...I just made a 250ma constant current source, attached a lighter plug to it and called it done. You'd need to make sure the current was enough to overcome parasitic losses, I made sure I had 150-200ma going into the battery.

Since you have the transformers, you'd only need a power device (mosfet), a small bipolar and couple of resistors...

Stuart