After saying I wasn't going to go through the frustration of figuring out how to set up a balance charging rig, I did it anyway. Funny how things look simple after you figure them out.
(used ribbon cable (here and here) to connect to the balance adapter)
And not wanting to be limited to only using it when there are 4 cells to charge, I made the bus bars with a tail so the + clip from the charger can bypass any empty slots and charge anything from 1 to 4 cells in the same box.
After using it, I gotta ask... am I doing something wrong, or is balance charging supposed to take dramatically longer than the same number of cells being charged in parallel? Especially if one cell in the batch is nearly full (say 1 cell at 4.10v, one at 3.65v), it seems the charger cuts the current and does some balancing whatever, then charges a little more. If all the cells are close to the same voltage at the start it goes quicker, though still not as fast as in parallel.
For 4 2900mAh 18650s, what charge rate should I use for series/balance charging? In parallel I've always used 1A per cell and they end up reasonably full and it doesn't take forever. Does balance charging allow higher charge rates without terminating before the cells are truly full?
For series/balance charging, the number of cells determines the voltage you input into the charger (3.6V/cell times the number of cells), and if you wish a charging rate of, let’s say, 1A, set it at such, and all the cells in series will be charged at that same 1A rate.
In parallel, it’s the opposite. If you wish to use the same 1A rate, multiply the 1A times the number of parallel cells to be input into charger. So 4 parallel cells = 4A. The voltage set is still at 3.6V, regardless of the number of cells.
So roughly the same charging time in both settings if the cells are charged at the same starting voltage.
If, say, I have 8 cells, all at 3.6v, and charge 4 in parallel at 4A, and 4 in series at 1A, they take around the same amount of time.
BUT, if there is one cell at 4.10v, and 3 at 3.65v, balance charging all 4 at once takes vastly longer than charging 3 at 3.65v in parallel, and then charging the one at 4.10v in a separate session. And I don't mean just a little longer, I mean like 5 hours vs. 1 hour.
If I charge a single cell at 1C, or 2.9A, it terminates way early and isn't anywhere near 4.20v. I was just wondering if balance charging at 1C, instead of 1 amp, would work any better.
Balance charging burns off excessive current in internal resistors inside the charger and there is a limit to the current it can burn off, in cheaper chargers usually it is about 200-500 miliamp, so until the cells voltage is equal the lower voltage cells will be charged at that 200-500 miliamp until they are at whatever highest voltage cell is. In your case the 3.65 volts cell charge at low current to 4.10 volts then the charger switches to current set by the user.
Charging at 1C will hasten charging time termination but it takes a while for a cell to convert electrical energy coming from a charger into chemical energy in a cell. That’s why personally I always charge my cell at only .7A, at most at 1A, for a more ‘full’ cell.
I have never tried to deliberately balance-charge cells with large starting voltage differences though, but what I know is that good cells, specially the new ones, almost always, if not always, end up at the same terminal voltage, regardless whether charged in series or parallel.
So pretty much justifies my reluctance to mess with this in the first place. If you have a sealed R/C pack and can't break out the cells from their original config, balance charging is a necessary evil... and whenever possible, charge cells individually or multiples in parallel if their starting voltages are close enough.
When charging a single cell, increasing the charge rate usually results in a faster total charge time - up to a point. At some point the CC phase of the charge stops getting shorter and the CV phase starts getting longer, with the total charge time being about the same. But since the termination point of the CV phase is usually set at about 10% of the initial charge rate, setting the initial charge rate too high can result in an incomplete charge. I don’t think balance charging will change this behaviour unless your charger has an adjustable termination current.
Nice work on the cradle, BTW. I like the tabs idea for charging fewer cells.
I have a 2S balance charging magnet setup and a hacked 2 cell WF-139 setup as for parallel charging.
Testing with the same 2 cells I have found that charging at 2 amp in parallel and 1 amp in balance (for the same power), that parallel charging time is consistent and balance charging times vary a lot, sometimes taking more than twice as long as parallel charging.
My conclusion is that parallel charging is the best for flashlight batteries, and that balance charging should be left to R/C Lipo packs.
As Bluecbvc mentioned earlier, if the two cells being balance charged have a different state of charge it can take longer to balance charge them both. Are the two cells you mention the same age, capacity and SOC? What are the cells and the charger being used? Do you use both cells together in one light, or separately in different lights?
within 30mAh of each other when discharged separately - I don't remember the exact figures as I did this test several months ago but at the 1 amp discharge rate each had just under 2100mAh.
I used the chargers discharge function to discharge them before doing each charging test - they were at around 3.2 volts and within .01 volts of each other when charging started.
The batteries came from a laptop pack that had been used for only 3 months - before the laptop tried drinking beer.
Sounds like the cells should be in good shape, and close enough to each other not to retard the charge by any balancing being done. I have lots of pack pulls in sets of 2, 3 and even 4 cells that are not as close together as yours. I balance charge them and they seem quite consistent with respect to charging time. Charge times only vary depending on how deeply they’ve been discharged.
If in doing your tests you are discharging the two cells to the same level but the charge times are varying significantly I would wonder if the issue might be with the charger. The Accucell 6 is inexpensive and the tolerance of components in them can vary from one unit to the next. I have read about balance chargers sometimes not even terminating because they are waiting for the cells to balance. This thread talks about such a situation.
I have a $35 X-Charger (B6 clone) that works well enough except for the balancing function. I had it for a while before I tried balancing, but when I did I quickly found out how poor it was. I started with a pair of cells that were 40mV apart and when the charge completed they were about 50mV apart — so much for the balancing! I eventually bought a better charger that I’m happy with.
If your priority is to get a fast charge then I think parallel charging may be a bit faster than balance charging, if only because you avoid any potential delays due to the balancing process itself. Excluding the balancing, and assuming that cell voltage differences are minimized and the charger involved has the capacity to deliver the current and voltage required, there is very little difference between parallel and serial charging.
