Question re: Li-Ion pack building

[herulach]

Firstly let me start by saying that I've built NiMH packs before, but never Li-Ion ones.

My current nimh battery (for my bike lights) is on its last legs (about 3 years old) and I'm looking to replace it with a Li-Po/Ion one. Looking around the best bang/buck/size appears to be packs built from 18650s. Its to power a maxflex boost driver, although occasional use on my old 6V halogens might be handy, so I'm looking to go for a 7.4V pack.

Now I know this is 2 18650s in series, but I hear a lot of mention of peope wiring li-ion cells in parrallel. I know this is ok for discharge, but is it safe to charge in this configuration?

The order Im placing wil be for 4 tabbed 18650 cells, with 2 2 cell protection circuits. Will a pack wired in this manner be suitable for charging off a standard 7.4v charger? (although rated at 8Ah I appreciate it will take a while). If it makes a difference everything will be coming from battery junction, which appears to be best balance of stuff to international (uk) shipping cost.

If theres a simpler ( or cheaper) way to get where I'm going please letme know.

Cheers

 

[steve6690]

I asked the same question re parallel charging and was advised here that it's fine. I have just built my pack (4s2p) with balance taps and it works great. FWIW I find Battery junction charges ridiculous shipping to the UK so I bought some tagged Li-ion 18650's from US Ebay.

http://myworld.ebay.co.uk/victorstudio

steve

 

[yellow]

Now I know this is 2 18650s in series, but I hear a lot of mention of peope wiring li-ion cells in parrallel. I know this is ok for discharge, but is it safe to charge in this configuration?

7.2 V Video-cam packs (6600 mAh) are 6 18650s. 2 strands in serise of 3 pc. connected in parallel.
Only problem is the 1st connection to solder them together (when they are at different Voltage, this is basically a short).
Best way is to connect on side, and get some resitor in between on the other and have the cells resting so for a few days. With low current the cells slowly get the same condition.


PS: if You plan 7.2 V packs, the cheapest way were to get some for Your video-cam. They have protection built in, can be charged by the Cams chager, are wired and cheap
(search for Canon BP945, f.e.)

PPS: just read Your post again,
You plan 2 pieces of 2-cells-in-a-row-packs?
8 A is brutally too much. Even 1C charging rate might be not too good (thats 2 A)
(or do You want a 2 parallel + 2 in series pack (4 cells)? Then You need only one 2 cell protection circuit (because thats simply a "2 in series pack" with "larger" cells) and 8 A will still be too much.

 

[herulach]

7.2 V Video-cam packs (6600 mAh) are 6 18650s. 2 strands in serise of 3 pc. connected in parallel.
Only problem is the 1st connection to solder them together (when they are at different Voltage, this is basically a short).
Best way is to connect on side, and get some resitor in between on the other and have the cells resting so for a few days. With low current the cells slowly get the same condition.


PS: if You plan 7.2 V packs, the cheapest way were to get some for Your video-cam. They have protection built in, can be charged by the Cams chager, are wired and cheap
(search for Canon BP945, f.e.)

Thanks for the suggestion, will have a look around, although at first glance interfacing with them appears to be the problem, although I suppose cracking the casing and inserting a flylead would be relatively straightforward.

PPS: just read Your post again,
You plan 2 pieces of 2-cells-in-a-row-packs?
8 A is brutally too much. Even 1C charging rate might be not too good (thats 2 A)
(or do You want a 2 parallel + 2 in series pack (4 cells)? Then You need only one 2 cell protection circuit (because thats simply a "2 in series pack" with "larger" cells) and 8 A will still be too much.

Its the second one, however, I said 8 Ah, not 8A, meaning that a pack that size would take ages to charge.

 

[droo1210]

(or do You want a 2 parallel + 2 in series pack (4 cells)? Then You need only one 2 cell protection circuit (because thats simply a "2 in series pack" with "larger" cells) and 8 A will still be too much.

Is this because the parallel cells will balance themselves? if so, how would I go about wiring the pack up?

I have bought Batteryspace packs before, but really don't feel like pulling them apart to see how it's done, and buying more is not an option cos they charge more than what the pack costs to ship to the arse end of Africa....

 

[LuxLuthor]

Unlike NiMH, you can charge (& discharge) Li-Ion cells in parallel....so 2s2p or 2s3p or 2s6p are all fine. They balance voltages in parallel configuration when connected....but one set of parallel cells then still needs to be balanced against the other set of parallel cells.

Just balance charge your setup as 2s up to 1C rate, and double check your voltages periodically...but the balance charging should handle that.

Wiring the pack is the same as any LiPo/Li-Ion 2s2p setup, with heavy wire on ends for charging current (although 2-4 Amps is not much current to worry about wire size/length resistance), and thin wire for low voltage balancing leads.

 

[droo1210]

Thanks dude