Charging and Balancing Questions for LG HG2 pack (14S/20P)

Hello. I am using a 14S/20P pack of LG HG2 cells (“18650 lithium ion”) for my prototype electric go kart.

What is a safe charging rate for a 20P configuration? I typically charge at 15-20 amps (CCCV)

What is a normal “Minimum Voltage per cell”. I have an Orion JR. BMS that is set for 2.9V minimum but I am wondering if that is too conservative. The spec sheet for the LG HG2 shows 2.5V as the minimum but I do not want to go that low for a variety of reasons.

What is typical “safe difference” in cell voltages after discharge? I pull approximately 100 Amps (continuous) and 350 Amps (maximum “bursts”) on my electric go kart and usually have about .03V to .05V difference between the highest and lowest cells. Is this a good range?

Thanks for any responses or suggestions.

Gary

Hi Gary,

The standard rated charge current is 1500mA per cell, so 15-20 amps for 20P seems safe. I would leave it at that.

The minimum cell voltage of 2.9V isn't conservative enough for my taste. I would say around 3.0V - 3.1V per cell for the BMS and 3.2V - 3.3V per cell or 44.8V total pack voltage for the controller's low voltage cutoff. You will get more cycles this way and avoid over discharging under load.

You haven't mentioned it but if you also charge to a max of 4.05 to 4.1V per cell, that will also net you more cycles.

A cell string voltage difference of .03V to .05V seems high. I would shoot for 0.01V difference or less.

Good luck with the build.

pwd said:

You haven't mentioned it but if you also charge to a max of 4.05 to 4.1V per cell, that will also net you more cycles.

A cell string voltage difference of .03V to .05V seems high. I would shoot for 0.01V difference or less.

Good luck with the build.

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Thanks for the response. I typically charge to a maximum of about 4.07V Per cell so I it sounds like I’m OK in that regard. After balancing, the cell variance can reach about .01V. It can be higher after a long day of testing and tuning but it seems to balance out nicely. I really do appreciate your feedback. Thanks again. Gary

cell variance at .01V should be the trigger for balancing to start.

20-30mV should be where they end up.

LVC really should be higher if at all possible for longevity,

voltage sag depends on your peak loads

should end up no lower than 3.4Vpc after resting bounceback.

john61ct said:

cell variance at .01V should be the trigger for balancing to start.

20-30mV should be where they end up.

LVC really should be higher if at all possible for longevity,

voltage sag depends on your peak loads

should end up no lower than 3.4Vpc after resting bounceback.

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Thanks Sir. Really appreciate the response.

pwd said:

The minimum cell voltage of 2.9V isn't conservative enough for my taste. I would say around 3.0V - 3.1V per cell for the BMS and 3.2V - 3.3V per cell or 44.8V total pack voltage for the controller's low voltage cutoff.

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I know it is not a perfect analogy, but Tesla recommends keeping the State of Charge between 20% and 80% on their vehicles. Would the recommendation above be similar to these percentages? Thanks.

garolittle said:

pwd said:

The minimum cell voltage of 2.9V isn't conservative enough for my taste. I would say around 3.0V - 3.1V per cell for the BMS and 3.2V - 3.3V per cell or 44.8V total pack voltage for the controller's low voltage cutoff.

Click to expand...

I know it is not a perfect analogy, but Tesla recommends keeping the State of Charge between 20% and 80% on their vehicles. Would the recommendation above be similar to these percentages? Thanks.

Click to expand...

That was just the low end, not top.

But no, there is very little usable energy left below even 3.4Vpc at rest for those chemistries.

80% DoD as a normal stopping point does greatly extend life, but often just is not practical to pull over and sleep there.

For stop-charge, just 1.0-1.5V below the max spec is plenty conservative, and again sacrifices insignificant range in practice. Would be silly to stop any lower.

Charging at a lower current has a great impact on longevity, as would gentle driving patterns.

Thank you. I really do appreciate the response. Gary.

Thanks again for all of the great responses. Would higher numbers of cells in Parallel make lower voltage strings acceptable? For example, if I used 14S/30P instead of 14S/20P, would this allow for lower voltages per cell string? Just curious. Thanks. Gary

No, really separate parameters.

Ah capacity will help support amps discharge rate, lower stress on the cells, and of course range.

The loads - in this case motor + controller - will determine what voltage you want, taking desired top speed into account.

But going faster drastically lowers range.

So if you had an adequate but fixed number of cells, balancing top speed vs range would be your judgment call.

john61ct said:

No, really separate parameters.

Ah capacity will help support amps discharge rate, lower stress on the cells, and of course range.

The loads - in this case motor + controller - will determine what voltage you want, taking desired top speed into account.

But going faster drastically lowers range.

So if you had an adequate but fixed number of cells, balancing top speed vs range would be your judgment call.

Click to expand...

Excellent information. Thanks Sir!