imax b6 questions

[mr.chubbles]

I'm new to this forum and may or may not be posting this in the correct place. I have a new imax b6 charger from skyrc that i believe to be genuine. I'm attempting to charge 18650 cells that i have reclaimed from old laptop batteries. I am charging using the lipo selection at 1.0a(which drops from 1.0a to less as it nears 4.2v). The charger is reading that it is charged to 4.20v but continues charging (but lowers the amps). When I disconnect the battery and test its voltage it reads only 3.98 volts. My question is, is this normal? I have calibrated the charger the best that it will allow. If i try to reconnect the battery to charge it further it initially reads the correct voltage but then rises in a matter of seconds to the 4.20 reading. Is this due to the fact that these are used/old batteries that have been sitting for who knows how long? Is this a flaw in my charger? Or is this the normal way that this charger functions? So far I have only attempted to charge two batteries. One had a starting voltage (tested after i removed it from the laptop pack) of 3.51v, was charged to 4.2v(but read 3.98v on my volt meter), and was sitting at a consistent 3.90v after the first charging attempt. The other had a starting voltage of 3.58v, was brought up to 4.2v(also read 3.98v on my volt meter, and was sitting at 3.88v after the first charging attempt. The two batteries were removed from the same laptop pack. I have only attempted to charge each battery twice and have removed them when they sit at 4.20v for and extended amount of time(15 mins or so). It seems that the charger is dwindling the 1.0a down to 0a as it nears completion. I am hesitant to leave them connected at the 4.2v for too long due the safety concerns I have read about these batteries.

I have a couple of hundred 18650 cells that are all at different voltages that i have harvested from numerous laptop packs that I plan on charging, discharging to obtain the current mah potential, sorting out the good and the bad, and then reassembling into a 12s14p pack to build an electric bike. Realistically the electric bike is of little concern to me. Im mostly doing this because I am trying to learn about electric vehicles and their components. Any information would be extremely helpful. Everything i understand about this up to now I have researched on my own but I am still very new to this field.

 

[Gregory]

It is normal. The charger will probably say FULL when charge is complete. Completion is normally something like when the amps going into the battery reach C/10 i.e 100mA with a 1000mA starting current.

It is called the CCCV charge profile and looks something like this. It starts off at full current till the cell gets near 4.2V, then see how the voltage tops out at 4.2V and from there the current trails down to approach zero. Then it is full.



PS welcome to the forum.

 

[dnmun]

your battery cells show a resistance to the charge entering the cans so you are seeing the voltage needed to overcome the resistance of the can to further charging.

you will see the same voltage sag when you discharge them.

 

[dogman dan]

Exactly. Poor, or worn out cells will sit there at that finishing stage of the charge for a long time. Then in use, not a lot of capacity. So on discharge, they my drop to 4v very fast, then discharge more or less normally, but sagging a lot, from that voltage.

4v might be about all they are holding at this time.

 

[mr.chubbles]

Awesome. Thanks guys. That was very helpful.

 

[dnmun]

you will find that the used and discarded laptop cells are impossible to balance and have no capacity and the sag is severe.

new cans are so cheap it is hardly worth the trouble to determine which are useful and which are not. then you will end up scrapping 50% after you do the capacity and Ri tests.

you will have a hard time if an imaxB6 is what you have to use.

 

[mr.chubbles]

Thats disappointing to hear. But as I said I am mostly doing this to familiarize myself with electric vehicle components. So far I have invested only $30 for the Imax charger. If I were to buy any sort of new batteries I'm looking at between $3-$9 per battery, and seeing how I'm planning on using roughly 168 batteries I have no intention of investing $500+ into this battery. Hopefully it will work to start and if I want to upgrade in the future I will. But that is good to know. Thanks for the heads up.

 

[mr.chubbles]

Since I am going to give the used cells a shot is there anything that you can recommend to help to maximize my situation. I had planned on evaluating each batteries maH and then grouping them together in parallel with batteries that are similar. So hopefully when I try to balance, each parallel set will roughly work at the same rate.

 

[dnmun]

finding the actual capacity is the problem. lithium cells need to be stabilized by a few charge discharge cycles in order to create the maximum storage capacity. so you have to go through a few cycles on each can before you actually attempt to measure the capacity. but if you buy a single cell charger like the kingpan 40W 4V10A charger then you can connect a lot of the cans in parallel with some jumper wires to each set of cans. usually they are spot welded in parallel so you can charge them up and then connect them in series and discharge and measure the voltage as they discharge through a wattmeter to find the capacity where they hit the 2.8V level. then you have to remove the ones from the series as they hit the LVC, and start the discharge again to find the next lowest capacity when the next group hits the LVC.. and so on until you have a handle on capacity for all of them.

then you mix and match so your sums of measured capacity are the same or very close on each channel. then you charge them all up again and build the battery as these groups and then do another discharge to measure the voltage sag on each channel so you have a relative number for the internal resistance of that group on that channel. if one is way off you have to find the ones that forced it out of the average Ri and then replace them with similar capacity cans, and then do the entire Ri test again until the pack has balance on each channel and the same relative internal resistance on each channel.

 

[mr.chubbles]

welp.... this is gonna be fun....ha

 

[dnmun]

you will learn alot. my 24S 87Ah ping pack has about 450 pouches. i tested all of them for capacity at least once and for about 25% i tested them twice. and i discarded another 150-200 that i tested and decided they did not have enuff capacity or had unusually high internal resistance. took 4-5 months. and i have it down to a really simple technique too. which i just covered.

 

[mr.chubbles]

to speed up the initial charge/discharge you were talking about to stabilize the batteries I was hoping I could build a battery case that would allow me to use the balance charge option on my imax b6 to charge multiple batteries at once. I have tried to research the best way to go about this and I'm having trouble finding the right information. I think I can do 5 cells at once. Would I just link 5 battery holders together in series with the balance wires going to each negative lead?

 

[Gregory]

Your charger is capable of up to 6S charging/balancing.

Yes, e.g. six cell holders in series with wires going to each negative (starting at most negative point and working up from there) AND then the most positive makes up the 7 wires required for the plug on a 6S battery balancing connector.

 

[Hillhater]

to speed up the initial charge/discharge you were talking about to stabilize the batteries I was hoping I could build a battery case that would allow me to use the balance charge option on my imax b6 to charge multiple batteries at once. I have tried to research the best way to go about this and I'm having trouble finding the right information. I think I can do 5 cells at once. Would I just link 5 battery holders together in series with the balance wires going to each negative lead?

Separate the cells into voltage ranges. 2.5-3.0 volts, 3.0 -3.5 volts etc, ...
Get a flat piece of cardboard, and lay a 6" square of Aliminium foil on it.
Stand 10-20 cells of similar voltage, base down, in a group ( use a large rubber band to keep them together) and then place another piece of flat foil across the top, with a thin piece of foam (Yoga mat ?) on that, and something flat and rigid on that to hold it down such that the foil contacts all the cell tops.
Now connect your charger, set for single cell , max (6.0 ?) Amps, to the foils
...hey presto...mass charger !
You can think of many refinements/improvements on this, but it's quick , cheap and effective !
Watch carefully for any shorting between the top foil and cell casings.....but your charger will soon tell you.