Need some math help

[tlawler]

I recently sold my Sartek HID to a gentleman here on the board and it turned out one of the batteries wouldn't hold a charge. The light used a 8.8 Ah battery pack, two 4.4 Li-ion packs in parallel.

On another website I found what I thought was an exact replacement. This battery has the same physical dimensions, configuration, and voltage as the OEM battery, but it is listed as 2200 MAh or 2.2AH. Given that the cells should have the same power density and the number of cells is the same, what gives?

I don't have the original pack in front of me and only have the link to the other pack, so I may be missing something obvious. How does Sarteks' OEM pack double the output of the aftermarket pack? Their packs may be proprietary, but I'm pretty sure they are assembled with off the shelf parts and just re-branded with the Sartek label.

Any insight you guys can give me would be appreciated, even if it is to point out something obvious I missed.

 

[Gombessa]

Is it the really same configuration? Powerizer says it's 3 cells in series (11.1v), three packs in parallel (2250mah). That's not the same as two 4.4ah packs in parallel.

To get 4.4AH with the same battery pack, it seems like you'd need 18 cells--three cells in series (11.1v), 6 packs in parallel (4500mah)?

My guess would be that the cells in the Sartek pack are higher density.

I'm not an expert here though, take the above with a grain of salt.

 

[Gombessa]

Followup: My guess is that the Sartek is actually made with the "18650" 2200mah cells here: Li-Ion Single Cell - Li-Ion Cylindrical Cell Series - 18650 series Li-Ion Cells

That gets you: 3.7v x 3 = 11.1v (I don't know if these 12v ballasts are better driven off 11.1v or 14.8v for Li-ion) at 2.2AH per pack, two packs in parallel gets you 11.1v at 4.4AH, 4 packs in parallel gets you 11.1v at 8.8AH. That's 12 cells at 11.1v, or 16 cells at 14.8v.

 

[tlawler]

After reading your posts a couple times, it's beginning to make a little more sense.

If memory serves me, the OEM batteries were in 3 x 3 config, total of nine per 4.4 AH pack, not in a 3 x 4 setup (the length of the cannister, which accomodated two packs was just a bit over 12 inches inside depth). If my math is correct, they must have been 1500 MAh individual cells: 3 x 3.7v = 11.1v, with 3 sets in series and each set producing 11.1v @1500 MAh. That would give each pack 11.1v@4500MAh(or 4.5AH), which is pretty close to what Sartek quotes. I just looked at the aftermarket battery and did realize that each cell is only 750 MAh. Anyway, I think that answers the question. I guess I didn't realize that there was such a difference in the individual cells.(they range from 750 MAh to 2800MAh each, maybe more)

Interesting what you mentioned about the difference between 11.1v and 14.8v being used to drive the 12v ballasts. I would like to hear what others think about that too. Does it lead to shortened ballast life, or is it actually better? If so, it would seem to make sense that when replacing the OEM packs (assuming you needed to replace both of them, or just wanted to upgrade), you would opt for single cells with a much higher power density like 2800MAh each, then configure them for an output of 14.8v in a single pack. I'm not sure what the advantage to having two packs instead of one is, perhaps redundancy since one pack will still fire the ballast; maybe quicker charge times using two chargers?