John in CR
100 TW
I've been using these LiMN cells in Makita and Bosch tool packs for 18 months and I can definitely confirm that these cells do have the tendency toward self balance during subsequent cycles. 100% discharges and overcharging tend to throw them out of whack, but without doing anything other than using them and charging conservatively them gets my packs back toward balance. In a year and a half of daily use of 30-60% typical DOD cycles, and during dry season I was riding so much that I was often doing 2 and 3 of these cycles, so we're talking about way over 500 cycles with only bulk charging. Though I'm sure capacity has decreased, it's nothing very significant, because I get so close to the same range before the pack gets to nominal voltage, except in the period after the 2 full discharges I accidentally did.
In that 18 month period I have never balanced the pack, and haven't had to replace a bad cell or string. My main pack is made up of 2p20s strings that are tied together only at the ends. with the used cells from Makita packs, I just did a very crude capacity matching by charging the 2p 4s or 5s strings to identical voltage, and then discharging them equally in one long series string with lightbulbs as my load. Afterward I let them rest and match the sub-strings by voltage to make up my series 2p strings.
Out of the hundreds of used toolpacks I've processed, other than the dead pairs of cells, only a few toolpacks had out of balance strings, and most of those were obviously cooked with heat damage and discoloring of the green plastic sleeves on the cells. The good cells are typically in a +/-.03v range after being used in a high power tool with no BMS or balancing of the cells at all. I've been running them the same, just with the sub-strings soldered together into the longer series strings I add together in parallel to make a pack.
During the first 5-6 months I used to check the pack for balance. I kept the end of the cells exposed and checked them with a multimeter, which was a pain, because I had to check each 2p pair since there is no parallel tie at the cell level. Only after running out of juice on a longer than planned ride was the pack out of balance. Out of laziness, not wanting to charge and discharge 100 2p groups to get the pack in balance, I just cycled them very shallow and used a timer on the charger to cut it off early before reaching the full charge voltage. I didn't even check again for balance in the subsequent 2 weeks, because I didn't want the bad news. Finally I readied myself for the big manual balancing job I had been dreading, and when I started measuring the cells, the darn things were back in balance, all within +/-.03v .
I stopped even bothering to check for cell balance, and that was just over a year ago. I pull in, plug the charge wire to the bike using a polarity correct plug coming from my 2 Bosch chargers wired in series, push the ON button for the timer that I don't even set anymore, and go and turn it off and unplug sometime later after the chargers turn off automatically at a pack voltage of 80.8v . I don't have my cycle analyst on the bike, so I do check pack voltage before charging. I used to do that before every charge, but I've become so familiar with my pack and added some more capacity a few months ago that I only check voltage a couple of times a week. My only other pack monitoring is to very occasionally feel the duct tape exterior of my pack after heavy hill climbs and after charging to check for hot spots in the pack that would tell me there's a problem, but I've never had one.
What is it that makes these cells migrate toward balance? I don't think it's higher self discharge of cells charged to higher voltage, because the pack voltage drifts very slightly downward while resting after charging, maybe only from 80.8v to 80.7 . The only thing I can figure is that during charging the cells must become less efficient and give off a tiny bit more heat as the cells reach higher voltage. It must be pretty small though, because unless it's been in the sun, the pack has never felt at all warm.
As long as you don't run these cells near their limits, they have to make the most care free packs that exist. It's lead to real complacency about putting into service the 2kwh+ of LiFePo4 that I own. I've been waiting for BMS's to come out that are cheap and reliable enough that I'm confident of similar worry free use.
John
In that 18 month period I have never balanced the pack, and haven't had to replace a bad cell or string. My main pack is made up of 2p20s strings that are tied together only at the ends. with the used cells from Makita packs, I just did a very crude capacity matching by charging the 2p 4s or 5s strings to identical voltage, and then discharging them equally in one long series string with lightbulbs as my load. Afterward I let them rest and match the sub-strings by voltage to make up my series 2p strings.
Out of the hundreds of used toolpacks I've processed, other than the dead pairs of cells, only a few toolpacks had out of balance strings, and most of those were obviously cooked with heat damage and discoloring of the green plastic sleeves on the cells. The good cells are typically in a +/-.03v range after being used in a high power tool with no BMS or balancing of the cells at all. I've been running them the same, just with the sub-strings soldered together into the longer series strings I add together in parallel to make a pack.
During the first 5-6 months I used to check the pack for balance. I kept the end of the cells exposed and checked them with a multimeter, which was a pain, because I had to check each 2p pair since there is no parallel tie at the cell level. Only after running out of juice on a longer than planned ride was the pack out of balance. Out of laziness, not wanting to charge and discharge 100 2p groups to get the pack in balance, I just cycled them very shallow and used a timer on the charger to cut it off early before reaching the full charge voltage. I didn't even check again for balance in the subsequent 2 weeks, because I didn't want the bad news. Finally I readied myself for the big manual balancing job I had been dreading, and when I started measuring the cells, the darn things were back in balance, all within +/-.03v .
I stopped even bothering to check for cell balance, and that was just over a year ago. I pull in, plug the charge wire to the bike using a polarity correct plug coming from my 2 Bosch chargers wired in series, push the ON button for the timer that I don't even set anymore, and go and turn it off and unplug sometime later after the chargers turn off automatically at a pack voltage of 80.8v . I don't have my cycle analyst on the bike, so I do check pack voltage before charging. I used to do that before every charge, but I've become so familiar with my pack and added some more capacity a few months ago that I only check voltage a couple of times a week. My only other pack monitoring is to very occasionally feel the duct tape exterior of my pack after heavy hill climbs and after charging to check for hot spots in the pack that would tell me there's a problem, but I've never had one.
What is it that makes these cells migrate toward balance? I don't think it's higher self discharge of cells charged to higher voltage, because the pack voltage drifts very slightly downward while resting after charging, maybe only from 80.8v to 80.7 . The only thing I can figure is that during charging the cells must become less efficient and give off a tiny bit more heat as the cells reach higher voltage. It must be pretty small though, because unless it's been in the sun, the pack has never felt at all warm.
As long as you don't run these cells near their limits, they have to make the most care free packs that exist. It's lead to real complacency about putting into service the 2kwh+ of LiFePo4 that I own. I've been waiting for BMS's to come out that are cheap and reliable enough that I'm confident of similar worry free use.
John
