Couldn’t find a thread on this after a quick search.
What’s the recommended BMS charge settings for good pack life?
Specifically:
- Overvoltage
- Overvoltage release
(LLT smart BMS)
I believe @BenjaminF made this
**just realized that didn’t answer the question lol. It depends on what pack youre using. As a general rule, charging within 1c is considered ‘safe’ or standard. I believe there was a similar conversion a few weeks ago discussing something like this, lemme see if I can find it
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Can you push the balance wires directly into the lipos JST plug then secure with hot glue?
That is the eventual end state, yes.
The problem is, that the charger can easily outrun the balancer - The charger has no way to know the voltage of any individual cell, so if you have 5 cells at 4.2 and 5 cells at 3.8, the charger will see 40 volts (average of 4.0v/cell) and keep right on charging, because it hasn’t reached 4.2v/cell.
If you have a 4-amp charger, you would need a 4-amp balancer to keep draining the 4.2v cells fast enough to keep them from rising above 4.2v.
That’s why you’ll sometimes see a BMS terminate charge, wait a few seconds or minutes, then begin charging again - It’s stopping the charger from overvolting the high cells, while it drains them back down low enough that it can begin charging again.
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You can do that, but it’s a really terrible idea.
Don’t do this.
Seriously. Don’t.
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Appreciate it. Although that’s not what I’m after either. I vaguely recall @Battery_Mooch talking about this a few times, but i couldn’t find specifics. If you’d weigh in sir, I’d appreciate it -
I was asking about the max cell voltage to charge to for best pack life. As in, you could charge to 4.2V for a lo-ion cell, but i remember reading somewhere that 4.1 or even 4.0 is safer.
Additionally, in the xiaoxang LLT BMS app, the overvoltage (as in @BenjaminF 's screenshot) is what the charger will charge up to, but at least on my pack the cells voltage all settle/balance down to the overvoltage release limit.
So, if your cells go over 4.2V (if your overvoltage is set to 4.21V) is that potentially damaging? And/or even if they almost immediately balance down to something lower?
What’s a healthy li-ion cell voltage full charge target?
Let’s get one thing straight charging, useing and age of battery’s causes damage that why you have a limited cycle life. If no damage happens then thay would last for ever and you would never need to a new battry. Unfortunately technology is not there yet and all battery’s have a limited lift cycle.
Li-ions cells are happiest around 3.7v the feather away from this the more thay push there limits (cause damage).
Reducing there max charge. Cutting the discharge early, limiting the amp draw, keeping cells at storage voltage, temp all help to reduce the aging of cells and will make them last for more cycles.
Manufactures spend a lot of time and money finding the best balance of all these and I would recommend following there advice on the data sheet.
4.2v hard cut off at 4.25v is common but there is others parts of charging li-ion cells other than voltage. One part of charging that is hugely over looked is the charge current cut off that changes depending on the max charge ampage. Dose your charge have a cut off and is it correct for the battry your charging?
This is a example of the Samsung q30 data sheet
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How should I go about connecting them?
I would suggest any of the following:
A: Buy an adapter to convert your BMS’s connector to the balance connector, or
B: Make such an adapter by soldering together some pre-crimped connectors, or
C: Get some empty connector housings, a crimper and some terminals, and crimp your own, or
D: Cut the balance connectors off your lipos and solder the wires directly to the balance connector (probably not the best option.)
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“Best” pack life will be subjective and will depend on your priorities. Charging to a very low voltage, perhaps 3.6V or 3.7V, will probably give you a life of thousands of cycles. But your capacity will be very low.
Charging to 4.2V can give you the rated cycle life for the cell, assuming no other abuse. Charging to 4.1V helps to extend the life of the cell but you get less capacity. Charging to 4.0V extends the life of the cell even more but results in even lower capacity….and so on.
Choose which is more important, cycle life or capacity, and start experimenting to see what different charging voltages do to your range. As a starting point perhaps compare 4.2V and 4.0V? If you don’t lose too much range then you could stay there or even try 3.9V. Experiment and see what works for you based on your priorities and preferences.
I think that’s just a coincidence. The resting voltage after charging stops will change depending on the charge rate. Higher charge rates can result in a bigger voltage drop once charging stops.
As @Darkie02 mentioned, cells age when sitting around, when charging, and when discharging. This aging is actually damage but is just a normal part of how these cells work.
Most of the cells we use are rated for a 4.20V charge with a 4.25V max. If the BMS is accurate there is essentially no difference between 4.20V and 4.21V in terms of cycle life in my opinion.
The voltage the cell drops back down to, after a couple of hours, is the true voltage of the cell.
4.20V is what they’re rated for.
A bit lower, perhaps 4.15V, could help extend cell life a bit but won’t cost you much capacity. Experiment and see what happens for you and how much range loss you are willing to tolerate. The lower the charge voltage the greater your cycle life will be.
How far you discharge the cells can also greatly affect cycle life. The less you discharge them the longer the cell will last. But I think the increase in cycle life isn’t worth the loss in range once you’re over 3.0V, perhaps 3.2V (depends on the cell and how hard they’re used).
This might need a low voltage cutoff setting of 2.8V, or perhaps even lower, to result in the cells bouncing back up to 3.0V (or whatever you choose). The harder the cells are used the bigger the voltage bounce back up to their true resting voltage after the discharge stops. As always, you’ll need to experiment to see what your setup does.
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The other thing a lower charge voltage can buy you — is if you live or work on top of a hill, you can start your ride with hard braking without overcharging. Depending on things.
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two of these soldered to the bms balance wires would work?
Yes, that’d be fine.
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You’re an absolute treasure @Battery_Mooch. i really appreciate the detailed and thorough response, it answered every question.
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What happens if sensor wire connectors get water in them? Does it just depend on the water? I’m thinking like rain water. What happens if rain water gets into sensor wire connectors? Does it cause problems?
The sensors stop working and the motor doesn’t work well or at all unless above about 3mph (4.5km/h)
If you kick push above that speed, it will work fine.
Unless you’ve shorted +5V to Gnd with brine…
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Is this all I need to build a charger for two LiPos outside of the XT90s? I’m doing it the @xsynatic route with externally connected BMS, just wanted to make sure I’m not missing anything before waiting a month or two.
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It’s already in the mail, although it took 3 weeks and an email for him to ship it… I didn’t become aware of the disliking of him until after I bought it
That’s nice to hear actually! My initial thought was that it’s one of those ocd things that just drives your crazy, but I’m starting to come to terms with it, so hearing that it’s not so bad is uplifting. If it ends up being too annoying I’ll probably get a Boardnamics hanger. He on the other hand is very fast with shipping, he literally shipped my item within 40 minutes of ordering and that being outside business hours, very impressing!
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Don’t celebrate until you verify it’s the correct thing, built correctly, and it doesn’t break.
eskating•eu in a nutshell
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Hahah good point!