Understanding discharge current for 18650

[Jordi]

Greetings Solar adventurers,

I was wondering If you could help me understand battery life in relation with the specifications with a type of 18650 batteries I have.

In the specifications sheet, this is found:


Standard Discharge Current
My understanding is that for an optimal battery life (amount of cycles) and nominal capacity perception a current no higher than 0,5 A can be drained. Whenever a higher current is drained, then the battery capacity perceived is lower and the battery can be slightly damaged over time. The batteries can however provide higher currents than that.

Max. Discharge Current
This is the maximal current a battery can provide without overheating. Over this current the battery overheats which leads to quick damage and a significantly lower capacity compared to nominal capacity. However, peak current might still be possible (1-2s) when switching on appliances.

Given the fact that I do not have many 18650 cells and therefore I need to drain more than 0,5 A from each cell to reach a decent power output. Which current would you recommend me to use as limit for continuous usage of home appliances; where (1) no significant damage will be caused to the cells and (2) no significant decrease on the batteries capacity will occur.

Do you have any experiencing on damaging 18650 cells due to draining more than the standard discharge current?

Can you please share your experiences?

Thank you!

 

[GXMnow]

I built up a few E-Bike 18650 packs. And before adding a BMS, my son and his friends managed to destroy 2 of them.

Based on the spec sheet for your cells, if you want a long life, I would not run them much harder than 3.75 amps for more than a few seconds at a time. That is the maximum current rating up to 60 degree C. Most of these cells will take high current surges quite well as long as they don't get hot. Certainly use a BMS that monitors cell temp. The more important part is not running the voltage too low. If you pull them below 3 volts, you are shortening their life. The spec sheet says 2.75 volts... That is too low unless you are happy with < 300 cycles. If you need 10 amps, parallel at least 3 of them, 4 is even better 1C rate. Then startup surges should not hit the 5 amp per cell limit.

 

[Jordi]

Thanks for the reply.

I have no BMS and did not plan to add one for the moment. Basically because I am still learning about them and because the system I am building will not be operational 24 hours; it is more of a back-up for now.

I hope and that is what I am willing to understand from your answer, that as long as I control the discharge current I am pulling (like keeping it low) and make some checks of the voltages from different cells or group of cells during usage, I will be able to use the battery pack with lower risk of damaging it.

Your son and friends, broke them because they pulled too much current from the pack?

 

[GXMnow]

Use a BMS. Running without one will end in a failure. I learned my lesson.

In a pack with cells in series, there is nothing keeping the cells balanced. All it takes is a few % difference in cell capacity and even though the charger thinks all is fine, you have one cell going too high and destroying itself. I used well matched cells, and used a balancing charger. But when the kids ran the e-bike, they ran it until the speed control hit low voltage cutoff. After pedaling home a bit, they noticed it would pull again, ran it to shutoff again. Well, one of the cells was showing NEGATIVE 2 volts. The charger refused to charge with the balance lead connected, and it threw a reverse polarity error. While I was trying to measure the cell voltages, it started hissing out of a hot cell. Yup, the reverse one. I had to yank the hot pack apart so it would not go into runaway and fry the cells next to it. Yes, this was an extreme case, but it is just not worth the risk. Even a cheap import BMS will save you a huge pain when it keep a cell from going nuclear. Those were 25 amp rated Samsung Cells, the motor is a 750 watt, that should top out at about 15 amps, so max current should not be a problem. That plan was for 2P, and have 50 amps available, but the pack of bulk cells only had 20 that measured full capacity, so I could only make a 1P 14S pack. I warned them to ride it light and only pedal once it hit cutoff. Well, that didn't work. Even 1P, it had near 20 mile range when I rode it.

Now all of my packs have a BMS and always will. My 2 ebike packs are now using a Daly 30 amp ones from Battery Hookup for just $22. Way cheaper than the batteries it is protecting.

 

[Jordi]

Ok, I see. I have made a small 4S1P 18650 12V battery and had no issues because I guess I just drain power for just 5-10 min and at a low wattage.

For the next pack I want to make a 4S12P 18650 12V. For display purposes I want to use 18650 battery holders; which translates into 12 battery holders with 4 cells each.



Could you please recommend me a BMS for this setup?
I ask it because I basically do not even understand how many BMS's I need for this set-up (1 per holder? or better to buy a bigger BMS for a group of 4 holders?), nor even If they will be compatible with this type of holder. I think I can attach the BMS cables to the springs though.

As we mention earlier each 4S cells will not drain more than 5A (unless for a milliseconds higher peak).

Given the fact that for now I only have a small solar panel, I want to have the option to take cells away to charge them with a regular AC/DC charger. Reason for which I want to use holders.

This will I will learn about BMS

 

[tim0shel]

I have something similar that I used to add on battery pack on a Bluetooth speaker but I'm using the board in the system to control the charge etc. Chemistry is close to the same as what was in it originally but I just triple the Ah.

 

[Jordi]

Guys, I still have a question and kindly appreaciate your say. When attaching a BMS (like the one in the link) to a group of 4 18650 cells, can I use the same BMS on more groups of 4 cells simultaneously? (Taking into account that all cells have same voltage and equally new/used)

I ask that because If I have 12 holders with 4 cells each, that is 12 BMS and wonder If I can cut the number down to save costs.

Or do I strictly need one BMS per group of 4 cells?

 

[GXMnow]

You can run groups of cells in parallel without a problem. Any cells that are in parallel will have to stay in balance and can be treated as a single cell. The only problem you run into is a mater of current. Each cell has a current rating. As you parallel cells, you multiply the maximum current. If you do not need all of the current, you can use a BMS rated for less than all of the cells, but it will limit how much the system will handle. So if you parallel12 cells, they may be able to deliver 120 amps, but if you only have a 60 amp BMS, that becomes the limit. Or you could split it into 2 groups of 6 cells in parallel and then the whole system can take 120 amps with 2 BMS units taking half each. Also keep in mind the cell balancing. IF your cells are well matched, then balancing will not require much current. But if the cells differ, they can drift out of balance. Most BMS unit use a small resistor to pull some current on the cell with the highest voltage so that the others can catch up while charging. The more cells you have in parallel, or even the higher capacity of the cells, you may find a BMS may not have enough balance current to keep the cells balanced. For this reason, you may also choose to use 2 or 3 BMS units to have the extra balance current available. My main pack is using a single BMS for 14S6P battery bank, but my BMS is rated at 200 amps and has 2 amps of balance current. My small E-Bike packs use a single Daly 30 amp BMS with just 0.03 amps of balance current for 14S2P of 2500 ma 18650 cells. But I can run 2 of them on the bike at a time. That way I technically have 14S4P with 2 BMS units.

 

[Jordi]

Thanks a lot GXM. You saved me time and money!