Rapid charge Rad External Battery Pack, using Luna Charger @3A?

[JohnE]

Hi,

I am making regular longish out and return trips on my Radrunner where I need to recharge before setting back on the return leg. Sometimes I'm a bit pressed for time before heading for home, and I have wondered whether it is safe and technically feasible to 'rapid charge' the battery before setting off.

I have been using the 48V Advanced 300W Luna charger for some time now, which provides switched options for charging to 80%, 90%, or 100%, and for charging at 1A, 2A, 3A, 4A, or 5A.

My usual charge will be to 80% unless I am doing my long out and back trip. I also always choose to charge at 2A to match (more or less) the current that would be delivered by my origional / supplied RAD charger.

So I'm wondering; would it be okay to charge to 100% at 3A?

I appreciate that this could have a detrimetal effect on overall battery life evventually. I would be interested to know how quickly that might happen. But I'm more concerend about overheating and fire risk, and / or the possibility that the battery BMS would struggle to handle cell balancing at that current.

Wondering what you good people think?

John

Luna charger: https://lunacycle.com/luna-charger-48v-advanced-300w-ebike-charger/
Rad External Battery Pack: https://radpowerbikes.pxf.io/y2097N

[DickB]

Samsung specifies a normal charge current of 1.7A per cell (6.8A battery) and 1.02A per cell (4.08A battery) for cycle life. Other cell manufacturers have similar specs. You can charge at up to 4A with no detrimental effect.

The Rad Standard Battery BMS balances at the end of a charge cycle when current draw is low, so there is no problem in that regard. There is no increased fire risk.

[handlebar]

Thanks, Dick.

[JohnE]

Thanks Dick,

I appreciate the guidance.

If I stop a charge before it reaches the target capacity (be that 80%, 90%, or 100%) because of the need to set out on my home leg, will that compromise battery health eventually because balancing wasn't completed during the later stages of the charge?

I suspect that the answer will be yes.

I wonder then if it might be better to select a charge target that is less than 100% rather than cut the session short if choosing 80% or 90% would allow time to complete cell balancing and still provide sufficient charge to get me home?

Your a star!

John

[DickB]

It is better to select a lower charge target than cutting a charge short, but batteries don't need to be balanced that often. As long as you run a complete charge to any level every month or so, no worries.

[John Rose]

I wonder then if it might be better to select a charge target that is less than 100% rather than cut the session short if choosing 80% or 90% would allow time to complete cell balancing and still provide sufficient charge to get me home?

I'd be very surprised if it wasn't the the same thing.
By that I mean if you've selected say, 80%, doesn't the charger simply stop charging before it goes higher?

I doubt that the charger itself is able to address individual cells to balance them.

[DickB]

Chargers are Constant Current / Constant Voltage, meaning that they limit current at the start of the charge (while voltage increases), then limit the voltage (while current decreases) to what is called the float voltage. If you set the Luna charger to 80% it simply lowers the final float voltage.  You're correct in that the charger does not balance cells, but the BMS balances if needed when at the float voltage and charge current is small. If you shut the charger off early rather than lower the float voltage, current remains high and balancing does not take place.

[John Rose]

...  You're correct in that the charger does not balance cells, but the BMS balances if needed when at the float voltage and charge current is small. If you shut the charger off early rather than lower the float voltage, current remains high and balancing does not take place.

That sounds very likely.

Did you find that graph somewhere, or is it from your own set of measurements?

[handlebar]

Did you find that graph somewhere, or is it from your own set of measurements?

I'll bet it's from his own measurements. I don't have equipment to record charts, but I can estimate current by monitoring watts. In an unbalanced battery, some cells are less charged than others. Imbalance will lengthen the tapering as the lower cells catch up. Today I charged my Radmission battery for the first time in 16 weeks, in which I've ridden it only 30 miles. The charged tapered pretty steeply to about 1 watt ( a few milliamps ), showing that it had stayed pretty balanced.

[DickB]

...  You're correct in that the charger does not balance cells, but the BMS balances if needed when at the float voltage and charge current is small. If you shut the charger off early rather than lower the float voltage, current remains high and balancing does not take place.

That sounds very likely.

Did you find that graph somewhere, or is it from your own set of measurements?

My measurement.