I ordered a 60ah lfp EVD from rmartin last week (7/9/09) and am awaiting delivery. I have a few questions for those that have one. I want to verify that the batts are balanced but given the multitude of ways/opinions on the subject, I am not sure which way to go. Until the warranty is over I have to use the thundersky charger so my automatic options are limited. So, if someone has any advice on how to do it while keeping the warranty intact. I am assuming a few things here.
1. Batteries that start off balanced will slowly drift apart.
2. The further the drift, the faster they will spread.
3. It is possible to manually balance a battery pack given one has access to all the batts.
Here is my understanding of the problem. My battery pack has 21 batteries. Due to the varying AH capacities/internal battery quality they will not all charge/discharge at exactly the same rate. For instance I may have a battery in the pack that is 62AH and one that is 58AH or one that seems more stubborn to take a charge and is always undercharged. Ok let’s look at the stubborn one as an example. Let’s say for argument it’s just this one battery that is a problem in the whole pack. It is always charged less than the others because it won’t charge as fast. So while the others start out at 3.6V the "bad" one is at 3.3V. While that is in the "safe zone" for these batts (2.5V-4.2V) it is possible that it would drop below 2.5 while the rest of the pack are safe at 2.7 because it started out lower. This problem would never "fix" itself since the next charge would bring it up to 3.3 again and not 3.6. The reverse may also be true. I may have a bat that always is high compared to the others so one bat may go higher than 4.2 while the others are at 3.9 at the end of charge (to settle down around 3.6.) Again this wouldn’t be fixed with a new charge since its higher than the others when one started charging it.
Ok now I have a few questions.
1. Wouldn’t over time the high battery "charge" the rest of the batts in the pack and they would self balance (or vice versa the low batt gets a charge from the rest in the pack.) Sort of like water seeking the lowest point. Now, this doesn’t appear to happen given what I have read. I am not an electrical engineer so I am missing some of the basics on the subject. Just for my edification can someone explain why they don’t self balance (a Google search doesn’t really help all that turns up is balancing charges/bms systems.)
2. It seems to me this isn’t really any sort of magical problem that is difficult to solve manually. If there is a batt with too much charge on it, drain the charge off the battery until it is close enough to the other batts in the cell (say all have a .1V spread.) Then check it every month (or more often if one finds monthly isn’t frequent enough and rinse and repeat.) Or if there’s a stubborn cell get a 4.2V charger for lfp batts and charge it on its own until it’s close to the others. (I realize there may be low and high batts in the pack so both methods may need to be practiced on certain cells.) Am I being overly optimistic in thinking I can manually balance the batteries with these methods and they will stay close for the month? I.e. is it too much of a balancing act (pun) for a person to attempt oneself or too time consuming to be worthwhile?
I don’t have the scoot yet so I can’t tell how (un)lucky I am with the pack yet perhaps it won’t be much of a problem. I would also if someone has a bit of time like some guidance in what range I should be shooting for. I’ve read the specs on the batts and they say 2.5-4.2 but I would like to know what people are comfortable with in terms of swing between the low and high batt in the pack. I am assuming take readings fully charged and then depleted after my longest ride so I can determine if they are all in the safe zone and how far they drifted to then get safe max distance traveled.
Also, I would like some guidance with a single 4.2V lfp charger so I can address the stubborn ones if needed (or a bank charger if it turns out I have several low ones.) And also what would be a good safe method for draining off excess charge from the high ones (i.e. hook a light bulb up to it for a while etc.)
Once the warranty is up, I am thinking of going the bank charger way so each cell is charged on its own. Can anyone suggest a solution that can handle 21 cells. My thought is to get the batts close to full with the thundersky charger and then finish them off with something else that charges each cell on its own. But there may be better ways.
Thanks to everyone in advance for any help you could provide.