Problems with 12V going into 4S A123 Pack?

I have a question for the smarter people here with electronics knowledge, I have an alarm on my bike, and it's powered with a 12V (true voltage closer to 13V) DC to DC power from my main pack, however, the main power switch is easily turned off thus rendering my alarm useless.

I have cobbled together a 4S A123 M1 cell pack from an old Dewalt 28V pack that had a bad cell, and wondered if there is any risk in having the A123 pack hooked up in parallel?

I'm only nervous because the A123 pack will have slightly higher voltage than the DC to DC when it's fully charged, and it's really only acting as a back-up or auxiliary power source that won't see any real major drain, but I'm wondering if I need to have a one-way diode or something to keep the higher voltage from the pack back-feeding into the DC to DC?

Ok, so I went ahead and wired up the 12V auxiliary pack, and the only thing is now when the power is off, the power from the 12V pack back-feeds through the DC to DC and powers my magic shine headlight.

Is this a bad thing to do to the DC to DC?

Not bad, as it is not actually going through the DC-DC converter, simply powering the light which I assume is also connected to the same output. Your power supply should be equipped with a diode to handle reverse current, so that really isn't an issue. Cheers,

Stew

stewrobb said:

Not bad, as it is not actually going through the DC-DC converter, simply powering the light which I assume is also connected to the same output. Your power supply should be equipped with a diode to handle reverse current, so that really isn't an issue. Cheers,

Stew

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Oh you're right!

LOL now I feel dumb, thanks for pointing that out. :lol:

I wonder if you could add a diode so the head lights do not turn on. I have a gorilla alarm I would like to wire too. I also have a decent quality small package dc- dc. I think a back up bat would be ideal.

You could add a diode but the voltage drop in the diode would mean the 4s battery would not get as much charging voltage. A Schottky diode would minimize the losses. I think it would work.

Thanks all for the feedback, I think I will leave it as is for now, and see what happens over-night. I have the main pack attached and on, so there shouldn't be a significant drain, only the green light on the back of my magic shine is on, not the actual main headlight, and this shouldn't be enough to worry about, but if I see a significant drop in voltage on the main pack, then I will see about it.

Another note however, is that my CA is wired in at the controller, and it's showing voltage, so I guess there must be some back-feeding through the DC to DC after all, not sure how else power would reach the CA unless the wiring is back feeding, but so far doesn't seem to be presenting a problem, and gives me a way to quickly check the voltage of the back-up pack.

That's curious, hard to tell without a wiring diagram or at least a decent description. What voltage is it showing on the CA? Where are the power leads? How is your DC-DC converter wired to the pack, and how is your controller wired, Do you use and ignition wire for one or both?

Btw, I don't like the diode idea, I admit it crossed my mind to add to my previous post. If you think about it though, the power consumed by the LED is probably much less than the power wasted via the voltage drop on the bulb, not to mention the slightly dimmer light. Obviously it depends on use of the light, but I figure you want those as bright as possible, and if you use your bike at night a decent amount, the power lost will be less, even with a resister network step down for the LED.

stewrobb said:

That's curious, hard to tell without a wiring diagram or at least a decent description. What voltage is it showing on the CA? Where are the power leads? How is your DC-DC converter wired to the pack, and how is your controller wired, Do you use and ignition wire for one or both?

Btw, I don't like the diode idea, I admit it crossed my mind to add to my previous post. If you think about it though, the power consumed by the LED is probably much less than the power wasted via the voltage drop on the bulb, not to mention the slightly dimmer light. Obviously it depends on use of the light, but I figure you want those as bright as possible, and if you use your bike at night a decent amount, the power lost will be less, even with a resister network step down for the LED.

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Well, I have a power wire for enabling the controller, so I guess when I turn on the CA it might be causing the drain by running the 12V back through the DC to DC backwards (I cannot see how else this would happen, as there is no other physical connection between the battery and the CA other than the leads of the DC to DC converter) and running the controller when I turn on the switch, so maybe what I need to do is test the voltage at the plug of my light, I wasn't thinking when I turned on the CA and realizing that meant power had to be back feeding through the controller too.

I will do some more testing, as now the drain is immense drawing down the pack to nothing in less than an hour, however that might be more the condition of these cells, I have a small SLA battery I can use and see if it does ok.

Without understanding how the wiring is done, it is impossible to help you diagnose the problem. Good luck in your testing, if you still can't figure it out, draw a simple diagram and post it.

For testing, with the main pack removed or unplugged, throw a multimeter between the battery and DC-DC, see what kind of current is happening. Then unplug the DC-DC from the input lines and test the voltage on the *power input lines*, Ideally, there shouldn't be any.