how to undercharge

let’s say tryin to undercharge a 10s only up to 40V (asuming it’s a perfectly balanced pack)

so which of these undercharging method is the best?

a) Set the voltage to 42V then yeet the power supply immediately after it hits 40V.

b) Set the voltage to 40V then yeet the power supply after the current drops down to almost zero.

c) “Undercharging will extend the life-span of your pack.” is a myth.

d) Others. Please specify: ____________________

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Not sure which is best, but on my next build I plan on:
e) using a smart bms to set over voltage and balance voltage via an app (not sure if metr can do this, so I may have to write the app myself, since I don’t like the manufacturer’s).

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None are best, just different.

This will result in a charge that is lower than your other option as the pack never has a chance to top off. The voltage will settle at lower than 40V. This is fine.

How much lower depends on how fast you were charging the pack. The faster you charged the lower it will drop and settle after you disconnect the charger.

This will top off the pack at 40V and result in a true 40V charge. There will be very little settling of the voltage once the charger is disconnected. It’s not better than stopping as soon as it hits 40V, it’s just a slightly higher charge level.

IMO there won’t be much of a practical difference between these two. Pick the most convenient one or stop charging whenever you see the current tapering off.

You can try an experiment. Charge both ways and see if you notice a difference in range.

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Great idea! Will do some tests. Will take time tho.
Won’t do it with an actual board cuz there’s lots of factors affecting range. Instead, for a more controlled method, I’mma use load tester.

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You can also get a different charger, an 11s LiFePO4 charger is 39.6V so you can leave it on and it will stabilize at 39.6V

I regularly use a 10s Li-ion charger on my 12s Li-ion battery as it is a convenient way to get from dead to storage voltage


Would be super easy if you were using lipos.
Most chargers allows you to set the voltage. I had mine at 4.1 for the longest time.

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If you have an ESC with a METR module and a motor with temperature sensors you can bleed power off until you hit a lower voltage, still a waste of time and energy though

IMO this is a pretty hefty assumption. Could be fine for a while, but unless you’re using a smart BMS you can reprogram to turn the balancing on, no bms will be balancing without getting to or near full voltage.

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Search on google only yield “undercgarhing extends the life-span of lithium cells”. No papers talkin bout how to “partially” charge em.
So with the spirit of DIY, I did an experiment. Here’s the result.

You get slightly more juice from undervolting.
I wouldn’t accept the results in face value partly because all the stuff I used are shitty.
Needs more experiments… but I’m done with this!

Let’s have definition of terms used so that this experiment would sound kindda legit.
Undercharge - the process of partially charging the batteries
Full Charge - the usual CC/CV method of charging 3s up to 12.6V
Rested Voltage - the voltage of the pack after charging and then setting aside to cool down for at least 30min
Truncated Charge - charging with 12.6V source but will terminate when the packs’ rested voltage is exactly 11.5V
Undervolt Charge - charging with CC but the CV is below normal so that the rested voltage after charging is exactly 11.5v

here’s how it went
Got nine identical liitokala cells to make sets of three 3s1p using 18650 holders.
To distinguish from each other I taped one set red, the other set blue and the last got no tape.
Used my cheap load tester to measure how much Ah on the packs. 4A CC discharge and 8.4V cutoff.
I used my shitty CC/CV power supply set to 1.5A for all charging process.
Took many trials and errors to figure out when to terminate in truncated charging so that the rested voltage would be exactly 11.5V
Didn’t use bms or any charging/discharging protection modules. Trust me I know what I’m doin.
The cells’ individual voltages never really drift apart from one another but I bottom-balanced each set before every charge just in case.
By bottom balance I mean disassembling the 3s after a full discharge and then connecting all the three cells in parallel.

Since there are three sets of cells I decided to vary the test sequence for each set
just in case the preceding test may have unforeseen effects

here’s some pics of the setup

Didn’t burn me bed even tho safety is questionable with all the exposed stuff. Call me Mr. Reckless Lucky!

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You will lose out on a solid chunk of range if you don’t charge it all the way… I think under discharge would be a better middle ground. As you get most of your power from the higher voltage and it dwindles