Battery question

[CaptainCrash71]

Just a couple of quick battery questions, I’m sure they’re common sense to some people, but I’m just a bit stumped.
Home-made battery, 36v 11.6ah using Panasonic 2900mah cells. Battery was 35.5v when started charging, charger turned itself of 42.1v. I’ve only done about 1.5 miles, and voltmeter on lcd3 is now reading 40.7v.
First of all, am I right in thinking that the first couple of charges will deplete quicker than normal?
Secondly, at which point would a battery be classed as depleted, i.e. the lowest you should allow it to go before charging?
Thanks I’m advance for any help.

 

[Deleted member 4366]

The controller will switch off at 31v, which is about as low as you want to go. The BMS will normally switch off somewhere between 31v and 25v. 2.5v is the absolute limit for any single cell, but you shouldn't plan to go that low.

Depletion isn't linear voltagewise. It drops fast at first, is more or less linear between 40v and 34v, then starts to accelerate downwards.

 

[CaptainCrash71]

Cheers D8veh, I knew I’d be able to rely on you. It was kind of the answer I was expecting, didn’t know about the non-linear discharging though, that’s very interesting. I plan on doing a couple of longer rides over the next couple of days, then off work for 2 weeks so will have chance to test further. I only need the battery to do about 12 miles, but would like to avoid recharging every day, so ideally would like to get 24 miles between charges. Time will tell.

 

[Danidl]

Just to amplify d8veh comments. .. whether the battery will store more energy after a few cycles is uncertain. In some cases the charging circuits can include intelligence in the form of a microcontroller and memory and may nudge a few more columbs of charge.. But and this bit is real, the charging discharge cycles of the cell does mechanically stress it internally.. there is a physical change in internal dimensions between a charged and discharged cell , which causes internal fractures and reduction in capacity. This is not obvious from looking at the metal casing as it is occuring at a microcrystalline level. The effect is worse when the battery is brought from very full charge to depletion. It is better therefore to limit the high voltage level to say 4.1 v per cell and the low voltage to a higher level not the 2.5 v. .. charging the battery after each ride is a better strategy than allowing it to fully deplete.

As d8veh has said, the amount of additional energy stored in the 4.1to 4.2v range is very little, so there is not much lost in keeping it a little lower and a lot to be gained in cell lifetime.

As the cell depletes, the actual voltage inside each element of the cell does not deplete much, but what happens is that the amount of non conducting material increases, as the ions are used up. This means that the internal resistance within the cell increases, almost linearly with the amount of state of charge. If the current drawn from the cell remains constant, then the voltage as seen outside the cell, will drop at an almost constant level. If you were to turn off the load current , and then measure the voltage it might read closer to full. Simple meters based on measuring the voltage , give a reasonable measure of the state of charge, whereas more elaborate meters, using microcontroller which measure the current and then calculate the charge give a better estimate.

 

[Deleted member 4366]

Just to amplify d8veh comments. .. whether the battery will store more energy after a few cycles is uncertain. In some cases the charging circuits can include intelligence in the form of a microcontroller and memory and may nudge a few more columbs of charge.. But and this bit is real, the charging discharge cycles of the cell does mechanically stress it internally.. there is a physical change in internal dimensions between a charged and discharged cell , which causes internal fractures and reduction in capacity. This is not obvious from looking at the metal casing as it is occuring at a microcrystalline level. The effect is worse when the battery is brought from very full charge to depletion. It is better therefore to limit the high voltage level to say 4.1 v per cell and the low voltage to a higher level not the 2.5 v. .. charging the battery after each ride is a better strategy than allowing it to fully deplete.

As d8veh has said, the amount of additional energy stored in the 4.1to 4.2v range is very little, so there is not much lost in keeping it a little lower and a lot to be gained in cell lifetime.

As the cell depletes, the actual voltage inside each element of the cell does not deplete much, but what happens is that the amount of non conducting material increases, as the ions are used up. This means that the internal resistance within the cell increases, almost linearly with the amount of state of charge. If the current drawn from the cell remains constant, then the voltage as seen outside the cell, will drop at an almost constant level. If you were to turn off the load current , and then measure the voltage it might read closer to full. Simple meters based on measuring the voltage , give a reasonable measure of the state of charge, whereas more elaborate meters, using microcontroller which measure the current and then calculate the charge give a better estimate.

That's theoretically correct, but ebike batteries have to be charged to 42v or 4.2v per cell, to balance them. If you only charged to 41v, the battery would go out of balance and reduce in capacity. You need a special BMS if you want to charge to anything less than 4.2v per cell. For most people, that means stick your charger in until the light goes green.