Guru
Joined: 15/07/2006
Location: AustraliaPosts: 676
| Posted: 08:26am 08 Feb 2012 |
I believe this balancing issue is really related to a few operational characteristics.
They don’t stray too much in voltage if…..
1) there is a low cell count in the series string (I have 45 cells, less is less cell variation, although the LYP seem to be more consistent in quality anyway. I have LFP)
2) used between 3.0v and 3.6v (I charged repeatedly to 4.2v when they were new, I don’t anymore. I have drained a few of them below 2.5v too, not good. Charged to 4.6v is very bad)
3) all cells kept at an even temperature (the middle of my pack is always much hotter than the edges. This is where I first noticed cell problems)
4) low charge & discharge currents (I am frequently discharging with 200 to 400amps, puts much more strain on them)
I have 4 cells x 90ah LFP in the Hilux EV as the 12v pack and has not been balanced since I tested the first Relativities Active Balancing circuits from Gordon. I have no problem with them.
I have a 12v block of 200ah LYP I use on my camper trailer. They have never been balanced. Currently I have this 200ah block on my house wind/solar in series with 3 x 12v 200ah lead acid batteries. Initially, every day I check them and they are within reason. Now I check them every week or 2 and the voltage difference is still reasonable.
The cells do not have to be exactly the same to operate. The key is not to have any cell go above 3.6v and no cell below 3.0v. I have said it before, but worth repeating, in a worst case senario, if one cell was at 3.6v and one cell at 3.0v then you can't charge with out damage and you can't discharge without damage. There is no problem if a cell is at 3.6v and a cell is at 3.5v.
For peace of mind, I believe Relativities Active Balancing is the answer to keeping the voltages all balanced all the time. Gordon has tested the Relativities on his Wind and Solar setup for the past, must be near 3 years, and have never missed a beat. They have been cycled to 80-90% DOD every day. His recent capacity test showed no loss of capacity in the LYP cells whatsoever.
We are still working on Relativities to become a commercial product.
The issue we had seen originally with Relativitities is that it takes a long time to transfer power from a high volt cell in one end of a long string to a low cell in the other end.
I bought some 14.8v chargers with the idea that I could use them to assist in transfering power quicker in a long series string (not tested yet). Gordon is currently working on a set of Relativities circuits to test on the Hilux EV based on his latest design.
Trev @ drivebynature.com