Li Ion batt. pack

[Zac Hall]

Hi, I'm attempting to make a battery pack for my bike light (the ones for sale online typically aren't waterproof, and I ride all-weather. )

I realize I need a protection circuit to make it safe, so I purchased a Tenergy PCB 7.4V pack (item# 32089 on Amazon battery superstore).

I have the basic idea how to hook it up, but have run into two issues;

1) there are 2 terminals labeled "BM". Which one hooks in the middle of the battery pack? (between batt1 + and batt2 - )

2) can I use my balance charger to charge this pack (2S X 2P) even with the safety IC installed? It seems the charger can charge through the P+ and P- terminals on the safety IC, but I don't see where the middle terminal from the charger would go.

Any input would be helpful. Thanks!

 

[(*steve*)]

but I don't see where the middle terminal from the charger would go

Between the batteries, probably

 

[Audioguru]

A 2-cells Lithium battery is balance charged so the battery and charger have 3 wires. One wire connects between the two cells.
Ternigy is an extremely cheap and poor quality battery from HobbyKing in China. Much better ones are available buy they cost more.

 

[ag273n]

there's a another forum - Endless Sphere - everyone there does DIY electric vehicles from Electric skateboards to electric bicycles.

about that protection circuit - you won't need if you're already using a balanced charger. what i recommend is better to put is a low voltage alarm that you can set to beep. some of these items include a voltage checker for cells in series. so even if the pack gets to 6S, it should be ok.
the protection circuit might even interfere with balance charging

the danger in Li-Ion isn't really with overdischarging - i've seen ones that reached below 1V - nothing happens to them, they just stop working, the real danger is with overcharging - this part is covered by your balance charger.

 

[Audioguru]

I have heard that when a lithium battery is discharged too low then the lithium ions form plated Lithium metal. Then if the cell is normally charged it is shorted and explodes or catches on fire. A Lithium battery charger IC detects that a cell has been discharged too low then begins a test charge with very low current. If the voltage does not normally rise then the IC stops charging and raises an alarm.

 

[ag273n]

Li-Ion cells aren't really as dangerous as Li-Polymer batteries. Li-Polymer batteries can easily catch fire upon overcharging or any small puncture.

 

[Zac Hall]

Hey, thanks for all the input guys! Sorry for late reply, life's been a li'l crazy lately lol.
About that protection circuit, it's good to know I won't need it for charging, thanks for telling me.

For the protection against possible short circuit overdraw, I'm still inclined to use protection circuit while batt. pack is in use. So, I'm still wondering which of those 2 "BM" terminals is connected between batteries 1 and 2.