What Voltage is best to charge batterys to?

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Hi. so i'm building a simple EEB type bike with 120 lg hg2 cells wired to 20s 18ah for the battery pack. and now i'm looking at buying my charger. i've been looking at these two options here:

https://goo.gl/zVOe0E

https://goo.gl/Js5SDP

Now for my question. i understand that only charging your pack to 90% is much better for the overall life-span of the cells. So is it ok that these chargers charge the battery all the way to 84v?, or should i perhaps contact the sellers and ask if they would be able to program a charger to a custom voltage for me, Say 82v?. or i could just find a seller that advertises they will change the charger voltage for me such as this seller:
https://goo.gl/oaB9h5 (although i'm not sure i like the look of the black plastic on this charger that will most likely get very hot)
So if i could get a seller to change the voltage what would be a good voltage to go for to give me the most charge in my battery without hindering the life span of my pack. thank-you and i'm sorry for all the stupid questions

 

[d8veh]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

 

[kappa7]

The best that you can do is to find a charger that allow to choose the charging voltage with a switch (80%~= 80V, 90%~= 82V, 100%~=84V) and then you can fully charge the battery only once a while (every 10-20cycles) to let the BMS balance. When you fully charge the battery you should leave connected the charger for several hours (10-20hours) after the battery is fully charged to allow the BMS to balance the battery. Then you should use quickly the battery at least until it reaches about 80% of the charge: it's when the battery sit at 100% that degrades faster (even if not used).

If you can't find a charger with a charge selection switch you can put some diodes in series with the charger output and short circuit them when you want do a fully charge. Every diode will lower the voltage about 0.6-0.7V, but you have to pay attention to the heat that each diode will generate (0.7 * Icharger).

 

[\ (•◡•) /]

The best that you can do is to find a charger that allow to choose the charging voltage with a switch (80%~= 80V, 90%~= 82V, 100%~=84V) and then you can fully charge the battery only once a while (every 10-20cycles) to let the BMS balance. When you fully charge the battery you should leave connected the charger for several hours (10-20hours) after the battery is fully charged to allow the BMS to balance the battery. Then you should use quickly the battery at least until it reaches about 80% of the charge: it's when the battery sit at 100% that degrades faster (even if not used).

If you can't find a charger with a charge selection switch you can put some diodes in series with the charger output and short circuit them when you want do a fully charge. Every diode will lower the voltage about 0.6-0.7V, but you have to pay attention to the heat that each diode will generate (0.7 * Icharger).

thanks for the help. The advice with the diodes defiantly seems like a viable option. So heres my follow up questions (hope this doesn't sound to stupid, i have a small understanding of electronics): how do i wire the diodes. say i wanted to use 4 in series do i have to make a chain of 4 diodes in series and wire one chain to the negative and one chain to the positive?. could i then soldier the ends of the diodes to the charge connector while also having the charge output wires soldiered into the same connector and then have a 2 position on-on switch such as this one https://goo.gl/eb6tqE between the normal positive and negative wires going straight to the charge connector and the positive and negative wires with the diode chains in them also going to the same charge connector, heres a crummy diagram trying to explain what i'm attempting to say
http://imgur.com/a/QlXnJ
and then i could possibly incase it in a hobby box (although as you said it will get hot). also what diodes should i use. as i said i really dont know any electronics that isn't to do with rc. sorry for all the stupid questions . thankyou

 

[\ (•◡•) /]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

Hmm, that kind of sucks. this is the BMS i ordered https://goo.gl/QsXd4B. but i guess that makes sense. thanks for the information. much apreciated!

 

[d8veh]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

Hmm, that kind of sucks. this is the BMS i ordered https://goo.gl/QsXd4B. but i guess that makes sense. thanks for the information. much apreciated!

As you can see, balancing happens above 4.19v an then only with 50-60mA., so you only get a small amount of balancing with each charge. I'd say that you need to charge fully to 4.2v every charge to avoid problems..

 

[kappa7]

thanks for the help. The advice with the diodes defiantly seems like a viable option. So heres my follow up questions (hope this doesn't sound to stupid, i have a small understanding of electronics): how do i wire the diodes. say i wanted to use 4 in series do i have to make a chain of 4 diodes in series and wire one chain to the negative and one chain to the positive?. could i then soldier the ends of the diodes to the charge connector while also having the charge output wires soldiered into the same connector and then have a 2 position on-on switch such as this one https://goo.gl/eb6tqE between the normal positive and negative wires going straight to the charge connector and the positive and negative wires with the diode chains in them also going to the same charge connector, heres a crummy diagram trying to explain what i'm attempting to say
http://imgur.com/a/QlXnJ
and then i could possibly incase it in a hobby box (although as you said it will get hot). also what diodes should i use. as i said i really dont know any electronics that isn't to do with rc. sorry for all the stupid questions . thankyou

Yes your drawing seem correct but you can connect all the diodes and the swithc on the positive wire, the negative wire can go direct to the battery. In any case I've attached a simple scheme that will drop the charger voltage between about 4V (with 10mA of charging current) and 4.75V (with 5A of charging current). It's simpler to build and the voltage is more stable than 4-5 series diode.
You have to mount the transistor T1 on an heatsink (the BDW93CFP case is already isolated so you won't have to do anything to isolate the heatsink).
The darlington transistor can be found here and the zener diode here

 

[kappa7]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

Hmm, that kind of sucks. this is the BMS i ordered https://goo.gl/QsXd4B. but i guess that makes sense. thanks for the information. much apreciated!

As you can see, balancing happens above 4.19v an then only with 50-60mA., so you only get a small amount of balancing with each charge. I'd say that you need to charge fully to 4.2v every charge to avoid problems..

I see the problem. I think that there is a safe solution to gently balance the battery once on a while and the rest of the time limit the charging to 80%:
When we charge the remaining 80% to 100% we could limit the charging current at about the same value of the balance current, when any cells rise over 4.19V it will simply stop to charging (the whole current will pass in the balancing resistor) and so no cells will be overcharged.
I see however two problems with this method:
1) the charging from 80% to 100% will be very slow (for example 40hours for a 10Ah battery with a balance current of 50mA).
2) You have to find how to regulate the current at 50mA.

Personally I have a 10S battery pack and I use programmable voltage/current regulator (sadly limited to 50V output) and I normally charge the battery at 90% and once in a while I slowly charge it to 100% with limited current (about 200mA). At the moment the battery is well balanced but it has only about 50 cycles.

 

[\ (•◡•) /]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

Hmm, that kind of sucks. this is the BMS i ordered https://goo.gl/QsXd4B. but i guess that makes sense. thanks for the information. much apreciated!

As you can see, balancing happens above 4.19v an then only with 50-60mA., so you only get a small amount of balancing with each charge. I'd say that you need to charge fully to 4.2v every charge to avoid problems..

Yea it seems i may just have to charge the battery to 100%, I mean how bad can it really be?. it just sucks because the cells are so expensive :/

thanks for the help. The advice with the diodes defiantly seems like a viable option. So heres my follow up questions (hope this doesn't sound to stupid, i have a small understanding of electronics): how do i wire the diodes. say i wanted to use 4 in series do i have to make a chain of 4 diodes in series and wire one chain to the negative and one chain to the positive?. could i then soldier the ends of the diodes to the charge connector while also having the charge output wires soldiered into the same connector and then have a 2 position on-on switch such as this one https://goo.gl/eb6tqE between the normal positive and negative wires going straight to the charge connector and the positive and negative wires with the diode chains in them also going to the same charge connector, heres a crummy diagram trying to explain what i'm attempting to say
http://imgur.com/a/QlXnJ
and then i could possibly incase it in a hobby box (although as you said it will get hot). also what diodes should i use. as i said i really dont know any electronics that isn't to do with rc. sorry for all the stupid questions . thankyou

Yes your drawing seem correct but you can connect all the diodes and the swithc on the positive wire, the negative wire can go direct to the battery. In any case I've attached a simple scheme that will drop the charger voltage between about 4V (with 10mA of charging current) and 4.75V (with 5A of charging current). It's simpler to build and the voltage is more stable than 4-5 series diode.
You have to mount the transistor T1 on an heatsink (the BDW93CFP case is already isolated so you won't have to do anything to isolate the heatsink).
The darlington transistor can be found here and the zener diode here

Thanks for the diagram and the advice. as i said i really am a noob with electronics, but that makes sense.

 

[\ (•◡•) /]

It depends on how your BMS works. Most of them only dothe balancing above something like 4.15v per cell, so if you don't fully charge, the battery will never get balanced. Balancing is an importantant function of the BMS.

Hmm, that kind of sucks. this is the BMS i ordered https://goo.gl/QsXd4B. but i guess that makes sense. thanks for the information. much apreciated!

As you can see, balancing happens above 4.19v an then only with 50-60mA., so you only get a small amount of balancing with each charge. I'd say that you need to charge fully to 4.2v every charge to avoid problems..

I see the problem. I think that there is a safe solution to gently balance the battery once on a while and the rest of the time limit the charging to 80%:
When we charge the remaining 80% to 100% we could limit the charging current at about the same value of the balance current, when any cells rise over 4.19V it will simply stop to charging (the whole current will pass in the balancing resistor) and so no cells will be overcharged.
I see however two problems with this method:
1) the charging from 80% to 100% will be very slow (for example 40hours for a 10Ah battery with a balance current of 50mA).
2) You have to find how to regulate the current at 50mA.

Personally I have a 10S battery pack and I use programmable voltage/current regulator (sadly limited to 50V output) and I normally charge the battery at 90% and once in a while I slowly charge it to 100% with limited current (about 200mA). At the moment the battery is well balanced but it has only about 50 cycles.

Wow. the programmable voltage step down module defiantly seems like my best options. although i also cant find any step down regulators with a input voltage of higher than 65v. although what i did find was a voltage step up regulator that could possibly step up its input voltage of 60v and input amp of 5a from the charger and convert it to 80v 0.05a. although then i still have the problem of how do i supply this board with 60v?. possibly i could charge the battery to 80% with the normal charger with the circuit that kappa7 sugested and then switch the this board with a 60v power supply powering it. Although it seems like a lot of effort over kappa7's diagram. It's going to take me 72 hours to charge from the remaning 20% of the 18ah battery (3,600mah) at 50ma

 

[kappa7]

I use this programmable step down module with an external mean well power supply (48V/220W).
The only programable high voltage charger that I know is the cycle satiator (the 72V version can go to 103V output) and you can go down with the current to 0.2A. It's quite expensive (not too much if you consider what you get) but as you say batteries are even more expensive. The gain in cycle life if you can limit the battery between 80-20% should be about 4x.
There is also to consider that good quality cells like you want to use are less likely to go out of balance fast, so the trick to charge at 100% (even with a standard charger) only one time every 10 charging cycle may be safe. The worst thing that can be happen is that the weakest cell go over the BMS overvoltage trip voltage (4.25V?), then the charge will be stopped by the BMS and the balancing circuit will slowly "eat" the excess of voltage (from 4.25V to 4.19V) of that cell group. After resetting the overcharge trip and rest your battery pack for a while you can continue to charge the battery until all cells are balanced.