Discharge is OK, and easy to implement, just set protection so it prevents discharge below 25.8 volts.
Forget all this stuff about charging to 70%, its not practical. Consider instead a policy of charging at a current less than 0.5C, with a charger target voltage as low as practical, whilst being adequate for passive balancing to occur if needed.
Any voltage over 3.4 volts per cell will given time charge to almost full, (27.2 volts).
To charge within a practical time frame use charge target volts of ideally 27.6 to 28 volts and stop charging when the target is reached. Any float set should be related to the desired rest voltage of the battery, 26.8 to rest near full, and 26.6 for somewhere around 70%. The charger used may need a 're boost' setting that restarts the charge to target once the battery has been discharged.
With a pre built battery or a diy battery with cells that tend to go 'out of balance' its probable that its required to spend some time with the battery on charge, at a voltage above that where balancing takes place, this will be typically above 27.2 volts. This may need the target volts higher at 28.4 ( or slightly higher), together with a absorption duration or boost duration where the voltage is held constant.
There is a second requirement to charge to 'full' to avoid memory effect that will occur if the battery is short cycled continuously, ( leads to an effective reduction in capacity). A full charge cycle removes this effect.
A good policy for many applications is to charge the battery and use it, recharging when it gets 'low'.
Having the battery at a very high state of charge, or a low state of charge, for long periods, is considered to reduce battery life.
Note that there are considerable changes in all areas of performance with lithium batteries with small changes in voltages. Thus the calibration accuracy on equipment should be taken into account when evaluating the system.
Mike
