what to set lvc to?

[Tench]

Hi, i am nearing the completeion of my current build, this is my first venture into lipo, i am using 6x 6s 8000mah 30c zippy lipo's. The 6 packs will be arranged in 3 pairs of 2 permanently parralelled packs (main cables and balance leads) these 3 pairs will be connected in series (main cables only) to run the bike on 18s 2p. For charging the 3 permanently parralleled packs will all be connected in parralel including the 3 balance leads from each pair via a 25 pin D-sub connector on my charger balance lead and my series/parralel change over switch for the main leads so that my 306b icharger can charge the whole pack as 6s and balance all of the cells in their 6p groups. the change over switch and D-sub have been constructed so incorrect connection accidents are impossible (I will show it off soon)
My question is with the pack configured like this and balanced every charge what should i set the LVC in the CA to? i will be using the CA throttle over ride to cut the power and control the LVC which they do very well. i will monitor the individual cells very closly during the first few charge cycles by charging to 4.0v then disconecting the balance leads to see if all the cells are closely matched. and check them again after the first few discharges.
My thoughts where to set the LVC to 66.6v (3.7v p/cell) then if i get stranded i can drop it 1 volt to 65.6v (3.64v) to help with limping home. Or do some of the more experienced lipo users have reason to think i should do it differently? could i set it lower, is there a benefit to the total AH i will get, i know they go over the cliff around this voltage and dont want to risk damaging them.
Some more info, the max current draw will only be 40a into a mid mounted HS3540, to start with anyway

Thanks Simon.

 

[Ykick]

Well balanced bricks I think you're being very conservative. 4.1V/cell top charge is plenty easy on em and under full load 3.7V/cell is gonna come quicker than you might think. I'd probably start with 3.5V/cell LVC and when that trips make a mental note what they're resting at. Then drop CA limit to 3.3V/cell and limp home.

But I'd be checking cells individually at that point to see if any weaker cells are noticeable? Obviously, abort any further use if any are resting around 3.5V/cell but they can dip into 3.3V under load and survive alright IF WELL BALANCED. A cell log meter is very handy for field checking bricks although I dunno how you'd do that with your connector setup?

 

[dnmun]

you should not rely on pack level LVC. go look at methods ultimate solution

 

[dogman dan]

Set the CA to 3.5v per cell. Likely you won't take anything disasterously low with that. If you did, you have a bogus cell that needs to get weeded out of the pack anyway. But I don't mean you should ride till the CA cuts it off at all.

Stop long before you get to that. 3.7 v is good, or 3.65. At 3.75v, you may still be up on the plateau of the discharge, and have a lot left. But by 3.7 you are nearly there to the cliff in voltage drop, and have only 20% or so left. 10% left that anybody but a moron would discharge. Don't even figure on that bit below 3.5v being usable.

That's resting voltage, so under load you'll need to add a few volts to your reading. Or get off the throttle for 60 seconds, and see where you really are.

My typical ride these days is 8 miles, 6 of it on rough trail. It puts me back in the garage with about 3.8v per cell. That light of a discharge pretty much never unbalances the pack.

You definitely don't need to balance charge every ride. Do so during a few break in cycles, but after that you shouldn't need it often at all. I tend to balance at most every 10 cycles, and with experience, that interval is getting longer and longer. Balancing every cycle is just going to take a lot of time, over .01v. Screw that, balance if you are .1v off, not .02 or .03. That much balancing is a pointless waste of time IMO.

 

[neptronix]

See that cliff at about 3.65v per cell on this lipo? yeah; stop right around there. What happens shortly after is not pretty

I say 3.5v/cell is a nice safe voltage to stop at.

 

[kfong]

LVC is all I use these days, it keeps my wiring simple. 3.65v is a good starting point with good packs. If you have a significant imbalance, then you need to weed out the bad cell. I use a balance charger, and always balance charge. This maintains a healthy pack and tells me if something is wrong. My charger of choice is the Hyperion 1420i.