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charging SLA battery off vehicle auxiliary power outlet

Oct 18th 2013, 02:04

K0HLB

Joined: Apr 28th 2009, 11:25
Total Topics: 0
Total Posts: 0
I usually operate a 2 meter radio in my Honda Pilot by installing a SLA battery in the back for 12 volt powering. I then take the battery to ghe garage on occasion for charging. I have an auxiliary 12 volt power outlet in the back that is energized when the switch is on. How can I charge the battery from this power supply? R.O. K0HLB
Oct 18th 2013, 14:06

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

It should be possible, but I can see two potential issues.

The first is that the power outlet is fused. You should know where the fuse is hidden and have a replacement. The initial charge current can be quite high if you just connect the battery to the outlet without some current limiting circuitry.

The second is that the alternator/engine needs to be running for the battery to charge.

Zack Lau W1VT
ARRL Senior Lab Engineer
Oct 19th 2013, 04:58

K0HLB

Joined: Apr 28th 2009, 11:25
Total Topics: 0
Total Posts: 0
Thanks - I will make sure I know where the fuse is located and have a spare. I was thinking more about the issue as follows:
1. You are correct - I need to have some circuitry to limit the current when the vehicle first starts up - perhaps with just a current delay before the charger is first energized. I have made couple of 12 volt chargers that only apply about 500 ma max to the battery and when the battery is fully charged the current is reduced to just a trickle. What if I used a similar circuit only left off the 120 volt to 16 volt ac transformer and rectifier to dc replacing with the 12 volt from the power outlet. Seems to me that would work? K0HLB - Ralph
Oct 19th 2013, 09:48

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

You won't get enough voltage to power the "similar circuitry."
The 16 volts is the average voltage--the effective voltage for charging the capacitors is the peak voltage minus the diode drops minus resistive losses--roughly 20 volts, not 12 volts.

Designing and building a good switching supply with all the attributes a ham needs is a very difficult challenge.

Zack Lau
ARRL Senior Lab Engineer
Oct 19th 2013, 13:37

K0HLB

Joined: Apr 28th 2009, 11:25
Total Topics: 0
Total Posts: 0
Thanks Zack - I have many priorities right now but something to work on. The auxiliary source note says 12 volts at 120 watts. The voltage is 14.6 on the volt meter when the car is running. I will put the scope on it sometime to see what it looks like. I have looked for more info on this but find non as yet. If you have any ideas what I can do please let me know. Ralph Ohm - K0HLB

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