Lipo damage voltage for 18s = sign of the devil

[whereswally606]

I think its probably generally excepted that lipo's lower voltage limit per cell is 3.7v. I decided to work out what my new 18s pack would read at 18x3.7 to see what voltage to stay above. To my surprise. 66.6 spooky eh?

In other news today finished my battery box for 18s 2p of zippy flightmax 40c 3s packs. The B&Q had some 100mm rectangular pvc ducting ala hyena builds. Its ridiculously snug but it fits like a glove. only cost 5 pounds woop! and i have a section left to house the other 4 spare packs. At the moment Im not looking to push my hua tong to 24s2p, think i need to upgrade brakes first. pictures tomorrow when its light again.

regards Andy

 

[ZOMGVTEK]

3.7V is generally quite the shallow discharge for RC LiPo. Generally, 3.65V is a great lower limit, 3.5V if you really want to eek capacity out of a well balanced pack.

It's OK to discharge below 3.5V, it's just the capacity gains are going to be extremely minimal, and it will degrade the cells more rapidly than keeping the voltage closer to midband.

 

[neptronix]

^--- fully agree

However, things do get quite dicey below 3.6v.
I only dip into 3.4-3.5v/cell average because i know the calibration of my iCharger is spot on, and all my packs are fully tested so there aren't any underperformers on the team that could dip to 3.0v by then..

3.7 is real shallow. You have a good bit of energy 'till your RC Lipo packs start fallin' off the cliff.

 

[liveforphysics]

3.7 is shallow, but stopping there generally means a very long happy pack life.

If you have the money and weight and space budget to simply over-size the battery a bit so you don't need to charge past 4.1v or discharge below 3.7v, you sure end up with a long lasting happy battery. (unless you had bad cells from the get-go)

 

[whereswally606]

Yeah trying to make this last since it cost enough to buy and replacements will take time to be delivered. Thanks for the education, I'm new to lipo but I'm getting the hang of it, first kff the other day hopefully the last.

 

[ZOMGVTEK]

Definitely shoot to keep the discharges above 3.65V. I'm just saying don't be worried if you need to take them a little below that, and don't go setting the LVC on your controller to 3.7V/cell average unless its way out of balance.

Generally, I assume people are going to have the pack fully charged (to 4.20V/cell) and then discharge it to something like 3.85V and think its better for the battery. This stuff tends to prefer sitting near mid-band. Ideally, have them sit in a cool room, never below freezing, store at 3.8-3.85V/cell, charge to 4.0V or lower, discharge to 3.7V or higher. This only really works if you can afford and rationalize the larger and more expensive battery to still maintain the range needed with such severe over provisioning. Charge the pack up just enough, so that when you arrive back home, the cells average voltage is 3.85V. Charge only when you need the power in the pack, avoid having the pack sit at a high SOC.

If you REALLY want RC LiPo to last a long time, you need to be an over-protective BMS. Or, you could do whatever and hope for the best. It's a whole lot easier.

I'm fairly 'nice' on my battery, and just cycled all my packs. Most cells gave 4.7-4.8 of the rated 5AH, on a 4.15-3.5V discharge. They rate the capacity at something like 4.2V-3.0V, so my packs still have nearly the rated capacity. However, the internal resistance did go up roughly 1mohm per cell, after the 50 or so cycles/1 year of operation they have. My brother probably has <5 cycles on his pack, and his cells averaged 4.5AH, but 1mohm lower internal resistance than mine. According to his cells resistance, they are like new. The capacity tests show otherwise, which is most likely due to his 'store at whatever SOC' mentality. The pack sat for months at 4.1V+, in below freezing temps. Otherwise, he almost never uses the thing. Never balanced, never monitored, barely used, lower capacity than my pack thats somewhat used.

 

[auraslip]

Can someone settle the debate about how much damage storing at 4.2v or 4.15v will do to the battery.

I'm thinking that maybe after a year of the batteries sitting pretty much at 4.15v 24/7 that it may be more damaging then the ~1 discharge I average a day.

 

[Xanda2260]

I remember Docbass posting some data from RC folks that said a 4.2v charge gave around 500cycles, while 4.1v gives around 1000. So I assume 4.15 will give around 750...?

 

[sn0wchyld]

personally I've got my lvc at just under 3.7 volts, though I think I'll turn this down soon... in no small part due to the normal sag of the battery causing my CA to limit power when there's still a couple of amp hours left before resting hits 3.7. As others have said, set it to about 3.6/3.5, and just keep an eye on your packs net voltage to get a good idea of what's left, and try and not let it get below 3.7 if you can help it.

Can someone settle the debate about how much damage storing at 4.2v or 4.15v will do to the battery.

I'm thinking that maybe after a year of the batteries sitting pretty much at 4.15v 24/7 that it may be more damaging then the ~1 discharge I average a day.

I dont think anyone has enough hard data on the differences (atleast as far as I've seen), but if every company that sells these batteries stores 'em at 3.8 ish volts i'm taking that as a big hint that they're better stored at about 50% soc rather than around 90%. charging them up before your commute in the morning probably isnt a big deal, storing them for a few weeks at 4.1 might be though. I've also noticed a higher passive drain when left at 4.1 vs 3.8. my packs will sit for months on end at 3.8 with no notable change, but at 4.1 they're starting to drift apart and self discharge after a few weeks/months, drooping to 4.05 - 4.08 ish volts.

I remember Docbass posting some data from RC folks that said a 4.2v charge gave around 500cycles, while 4.1v gives around 1000. So I assume 4.15 will give around 750...?

I'd be amazed if its a liniar scale. it might be you'll still get 900, or maybe cycle life will drop to 600 from that extra .05v. Also, check how much power you actually get out of the extra .05V... I used to charge to 4.15, but now I only go to 4.1 once I discovered the difference was less than 5% of my capacity. it also makes balancing with battery medics easier. if you really miss the extra 5% then you probably better off getting a bigger pack. Alternatively charge to 4.1 most of the time, and only add the extra topup when you know you'll be going a long way.