Bike Headlight Chagring Interface Overheating

[mhumm2]

Hello,
I was happy to find this forum as I'm always putting something together. I have a recumbent trike on which I installed a headlight and battery combo that works very well. To charge the battery, I have to disconnect the headlight cable that connects to the battery cable, then connect the battery cable to my charger. I also have two tail lights with their own internal batteries. To simply charging, I built a charging box. The box contains two DC to DC converters, connectors and switches. When i mount the box to the trike, I'll be able to keep the switch in OPERATE mode and when I want to charge the batteries, I switch the switches to CHARGE mode. So here's the issue:

The DC to DC board for the headlight overheats and the board starts "singing" (board components are oscillating).

The original power supply for the headlight battery charger is rated for 8.4VDC at 1000ma. The specs on the DC board is 2A with a max of 3A with an additional heatsink on the IC. I figured that since the Charger that came with headlight battery is rated for 1 Amp, the DC board I bought should be fine, but it obviously is not.

To handle the load of the headlight and tail light charging, I purchased another DC power supply that outputs 12VDC @ 5A. Then the DC Boards I purchased (1 for the headlight and 1 for the tail lights) are adjustable, 8.4VDC for the headlights and 5.0VDC for the tail lights.

How much current would you expect the headlight battery to pull? I assumed it would pull less than 1A since its "wall wart" was rated for 1A and any more would overheat and damage the wall wart, but it's apparently pulling significantly more -- I'd say >3A. I will of course put an ammeter between the DC board output and the headlight battery to know for sure. Assuming it is too high, do you think a 5 Watt resistor in series with the output be a good fix? I'm unfamiliar with the loading affects of batteries. Please advise.

 

[gophert]

Ratings on the wall wart or a battery DO NOT indicate how much current they CAN supply, but, instead are warnings about how much the circuit you connect ALLOw them to supply.

also, the design of the battery determines how much current should be used to charge them at each level along the charging process.

 

[mhumm2]

Thank you gophert. As I stated in the post, the battery charging wall wart is rated for 8.4VDC @ 1A so what would you suggest I limit the current from the DC board to? If I put a large-wattage resistor across the output, I should be able to limit current easily. Do you think 500ma is a good starting point?

 

[gophert]

Start with your idea of checking current with ammeter. Remember, an ammeter is connected in series with the circuit, don't just touch two points on your existing circuit. You may have to get creative or actually cut a supply wire to inset your ammeter into the circuit path.

It is not unusual for dc/dc converters to sing at high loads (approaching their max rating).
Once you measure, report back and we'll look at options.

Also, is your description of "overheating" an actual failure in the circuit or just your impression because it feels hot?

 

[mhumm2]

Too hot to touch.

 

[gophert]

Too hot to touch.

Most people think 140F or 60C is too hot to touch but nearly all electronics can handle 85C (170F) and many can go much higher without damage. Check the datasheet.

 

[dr pepper]

Imported power supplys and converters tend to be rated optimistically, overate your parts by 2 or even 3x.
You can get clamp on type amp meters for Dc nowadays fairly cheap, good idea if you build stuff, you dont need to break the wires with a clamp.
I had batteries on my push hog, I got fed up charging them so I built a hub dynamo into the front wheel, its very good and its always ready.