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Lithium battery with PCB

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SteveC69

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I'm trying to connect 3 Tenergy lithium 18650 3.7 volt batteries together, for a 12 volt flashlight application. I bought the protected type for safer charging and discharging. The problem I have encountered is they only work when I flip the switch a few times. I seems to never light on the first time. I'm wondering if the PCB is not liking the instant power draw and is stopping it from powering the bulb. When I connect a volt meter it will always show voltage. What ideas do you guys have on this?

Thanks,
Steve
 

audioguru

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An incandescent light bulb draws up to 10 times its normal current when it is first turned in and is not warm. Maybe the protection circuit prevents it always starting at the very high current.
Maybe the cheap Chinese battery cells drop their voltage when loaded then the protection circuit disconnects the load until the bulb warms up a little.
 

SteveC69

New Member
I'm not actually using a switch, just touching the wires together. I found tonight that an MR16 led bulb seems to work fine, but the halogen has a lot of trouble. I also found one of my 3 batteries dead. At least it showed no voltage. I just charged it a couple of days ago. I'm starting to wonder if I should just use unprotected batteries. Which leads me to the question of do I need to balance charge the battery pack?
 

KeepItSimpleStupid

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The protection PCB is supposed to protect against discharge too, so I'm in agreement. You see what happens with unprotected batteries. I doubt that's a good idea.
Touching wires isn't good either. You could have damages the protection PCB.

Balancing should only be required if your charging the batteries in series with the proper charger. If your charging them separately, I don;t see a problem.

A reminder that switches are rated for both AC and DC breaking and making.
 

audioguru

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I also found one of my 3 batteries dead. At least it showed no voltage. I just charged it a couple of days ago. I'm starting to wonder if I should just use unprotected batteries. Which leads me to the question of do I need to balance charge the battery pack?
A Lithium rechargeable battery cell or pack that has been discharged lower than about 3.2V per cell is dangerous to charge. I read somewhere that then the lithium ions have plated pure lithium inside the cell that is very reactive and could short the cell or catch on fire when charged. Lithium metal burns something like magnesium (flares) or titanium (see the "ium?"). That is why most charger ICs measure the cell voltage first then if its voltage is low it begins charging at a low current and if the cell temperature does not rise and if the cell's voltage does rise it continues. Temperature rise or voltage not rising causes the charger to stop charging and issue a warning.

Of course a pack of lithium rechargeable cells should be balance-charged to avoid the weakest cell from being over-voltage and exploding or catching on fire.
 

dr pepper

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Most Helpful Member
Sounds like you've put the protection pcb in permanent protection!

Halogens are much lower resistance when cold, the protection probably 'saw' it as a short at turn on.

One possible simple way round it would be to use a ntc thermistor of a few ohms to limit the inrush, or maybe a 3 position switch, off, pre warm (power through a resistor), and On.
 

audioguru

Well-Known Member
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Many years ago we used an NTC thermistor in series with any incandescent light bulb to prevent it from suddenly burning out by the high inrush current when the bulb was cold.
 
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