Custom 7.4V Battery Pack with Panasonic 18650 Cells

[ta1234]

Hi Everyone,

I'm planning to build a 7.4V battery pack to power my bike light. My current battery has died as it was left for a long time. All cell voltages are now well below 2.0V so I've disposed of it.

My pack will contain 4 x 18650 CGR18650CG cells in 2S2P configuration, as shown here. I realise you can buy these packs all over the place on ebay or amazon; however, my reason for building my own pack is I have some good quality 18650 Panasonic cells (CGR18650CG) from a laptop battery and having had experience with the cheapo ones you get on ebay I'd rather use the Panasonic ones.

As I'm building these cells into a 2S2P battery pack I'll need a protection circuit for charging, discharging and ensuring that the cell voltages stay balanced. My questions is regarding this battery protection circuit. The datasheet for the cells I have can be found here. The datasheet recommends that I don't discharge the cells below 3V. All the battery PCM boards that I have found so far have over-discharge detect voltages way down at 2.4V or so, with discharge release up around 3.0V. This is too low for the cells I have, so does anyone know of any good PCM boards with over-discharge detect voltages at 3.0V and discharge release at say 3.3V or so?

What do you think? do you think I'm being too cautious? I could just put a 2.4V detect voltage PCM on but according to the datasheet it looks like the cells I have will go of the cliff edge at less than 3.0V. My old pack used much cheaper cells but I'm wondering if they died because the PCM on it allowed them to go down to as low as 2.4V per cell.

My other option is to wire up the cells with balance leads as shown here. I could then use my balance charger to charge the cells safely; however, this configuration does not provide any over-discharge protection so I'd have to be very careful not to flatten the cells.

Any help would be much appreciated.

Thanks,

Tom

 

[racookpe1978]

Don't know enough to criticize the EE side, but from the mechanical side, how will you shield the battery cae against spray and water damage?

If the bike is left locked up (outside a store for example) how will you prevent theft or damage of the case from passers-by?

I disliked the nuisance and drag of a generator, and so like your battery idea.

 

[ta1234]

Hi racookpe,

I was planning to use the small case that came with the light and battery pack that I bought. It is just a fabric thing with a velcro strap. It's in no way waterproof or IP67 or anything like that, but it's adequate for protecting from a bit of rain. It goes in my saddle bag so it doesn't get wet when I ride. I always take the saddle bag off too if I leave the bike anywhere for any length of time so security isn't an issue.

I'm only talking about a small battery pack here, just 4 x 18650 cells wired to give 7.4V. I essentially want to re-make the cheap Chinese pack that came with the light but using better quality, higher capacity cells from my laptop battery. I plan to heat shrink all the cells individually to prevent the easy-shorting problem that these cells have straight from the factory. I will then heat shrink the whole assembly of 4 cells, wires and protection circuit to hold everything together and provide a bit more protection.

My problem is finding a suitable protection circuit that will stop the battery discharging below 3.0V per cell. All the circuits I have found so far let the batteries reach 2.4V which will damage the better quality cells I have.