Top balance? 3.65 volts or intended maximum charge voltage?

[ericfx1984]

In the past when I put batteries together I would top balance them at 3.65 volts.. But my question is should they be balanced at 3.65 volts if I intend to run at 3.45 volts? Or if I'm intending to operate at 3.45 volt should I do my top balance at 3.45 volts... My guess is 3.65


Just curious what everyone's opinion is on this... Maybe I will change the way I do things I don't know

 

[FilterGuy]

I top balance to 3.65V even though I charge them to a lower voltage. The theory is that I don't want them to get to the top balance voltage during normal use. However, if I am honest with myself I have to admit that if the BMS balancing system can't keep them balanced this won't help for long and if it can keep them balanced this won't matter.

 

[Warpspeed]

What I do now, is to first charge up to only 3.45v average, and see if any of the cells might be a "runner".
That just means that the "runner" started out at a slightly higher state of charge than all the other cells.

The way to fix that, is to charge up to 3.45v average for all cells, until charging current has fallen right back to zero.
Then very slowly and gently discharge the highest voltage cell back down to 3.45v with a one ohm resistor across that cell.
Only discharge the one cell, then put the battery bank back into normal operation and let it go through a normal full natural charge/discharge cycle over the next 24 hours (or longer).

Don't try to hurry up the discharge, or over discharge the highest cell trying to speed things up.
Also, only do this when charging current has fallen completely to zero.
Its also important to let the whole battery readjust itself over one or more normal full daily charge/discharge cycles before repeating the process. And only do this one cell at a time.

With a bit of patience, its possible to achieve excellent balance, and it will stay like that for a very long time.
The secret to this is gradually creep up on it, and be very patient.

For the longest battery life its best to always stay close to the flat part of the voltage curve, avoid over charging, and over discharging.
Higher than necessary charging voltages will definitely reduce battery life according to the literature.

The belief that you can always charge safely up to 3.65v, but 3.66v will permanently damage the battery is just not realistic.

 

[Grend]

Hi,
After you get cells to 3.45v, do you put them all in parallel for your charge / discharge cycles or in series with the BMS connected and managing it all?

 

[Warpspeed]

My thirty cells all remain connected in one long series string, nothing is changed or disconnected.

The whole battery charges and discharges normally each day, with the BMS, balancer, and everything else still connected up.
The voltage across each cell is measured when the full normal charging cycle has completed, and charging current has ramped right back to zero. If its a horrible grey cloudy day, and this fully charged zero current condition cannot be reached, just leave it alone and hope for more sun the next day.

The particular cell with the highest measured voltage is then very slowly brought back down to the exact 3.45v average, with a one ohm discharge load connected across that particular cell.
All the other cells, some will be slightly below 3.45v, some slightly above 3.45v.
Just bring back the worst highest voltage cell to the 3.45v average each time, one cell per day, one cell at a time.

If one cell is very much lower than all the others, bring the voltage up with gentle charging, at no more than about three amps.
Very important to only correct one cell at a time doing this each day, then give the whole battery at least 24 hours to go through another full normal days charge/discharge activity to let every cell readjust and the whole string settle down to the very slightly changed conditions.

The secret to success with this is patience.
Trying to hurry the whole thing up just leads to chaos, and the voltages will be all over the place and measure different each day.
If it does go crazy inconsistent on you, just leave it alone for a week or longer and when it has all settled back down, sneak up on it.