Any advantages to going with two 6s VS three 4s VS four 3s?

[dogman dan]

Agreed. A properly functioning bms has a very low draw. But how do you know it's properly functioning if you are a moron like me?

When I had a bms kill my first ping, it did not shut off the pack when I left a discharged battery connected to the bike. I had a 103 fever at that moment, and frocked up. The bms lvc had died but I never knew it till my mistake let it kill the already very old pack. The lvc had worked for years, but when it failed there was only one way to find out.

.

Best bet, is to have a bms, but not to trust it blindly. (have a voltmeter too at least) That's why until the day I came home with the swine flu, I had been in the habit of unplugging the controller, and starting a recharge when I came home. It made sense not to rely on the bms, if the 3w draw of the turned on controller drained the pack.

I'm not saying no bms is best. But I'm saying, with a volatile cell like RC lipo, the same thing that happened to my ping could burn the house down. With lifepo4, all that happened is the battery doubled in size, but never got hot.

 

[dnmun]

Agreed. A properly functioning bms has a very low draw. But how do you know it's properly functioning if you are a moron like me?

When I had a bms kill my first ping, it did not shut off the pack when I left a discharged battery connected to the bike. I had a 103 fever at that moment, and frocked up. The bms lvc had died but I never knew it till my mistake let it kill the already very old pack. The lvc had worked for years, but when it failed there was only one way to find out.

.

Best bet, is to have a bms, but not to trust it blindly. (have a voltmeter too at least) That's why until the day I came home with the swine flu, I had been in the habit of unplugging the controller, and starting a recharge when I came home. It made sense not to rely on the bms, if the 3w draw of the turned on controller drained the pack.

I'm not saying no bms is best. But I'm saying, with a volatile cell like RC lipo, the same thing that happened to my ping could burn the house down. With lifepo4, all that happened is the battery doubled in size, but never got hot.

on a ping pack, failure to shut off at LVC is because of a shorted output mosfet. when a mosfet is shorted it conducts current even when the LVC gate drive is turned off.

when a mosfet is shorted and conducting current it will cause the mosfet to get extremely hot and usually shows up as melted solder on the BMS and if it is still wrapped up in plastic the plastic will melt around the mosfet.

if this is a condition your BMS displays while in use you should repair the BMS immediately because it is very damaging. usually you can determine which mosfet is shorted by observation and if you cut the source leg of the mosfet then the other mosfets in parallel will function to provide the LVC protection until you get the bike home to repair the BMS.

 

[friendly1uk]

I think your saying Radio control chargers offer no protection while riding so could cycle a cell down to 3v while the others were still at 4v, and in fact that 3v could drop to zero volts on that cell before controller lvc operated. Then your charger would likely just carry on charging the pack regardless, as overall it is a dumb charger.

No. What I was saying was that a BMS will do this (the part in bold), if the LVC is not adjustable. Setting a total pack LVC on the controller will not help. A RC charger will not charge begin a charge if a cell is under or over-discharged.

I imagine a rubbish or broken bms could be as bad as no bms at all. But overall not having a bms is the sure fire way of letting this happen. My bms wouldn't allow it.

 

[Punx0r]

Ignoring the copious homoerotica for a moment...

I think your saying Radio control chargers offer no protection while riding so could cycle a cell down to 3v while the others were still at 4v, and in fact that 3v could drop to zero volts on that cell before controller lvc operated. Then your charger would likely just carry on charging the pack regardless, as overall it is a dumb charger.

No. What I was saying was that a BMS will do this (the part in bold), if the LVC is not adjustable. Setting a total pack LVC on the controller will not help. A RC charger will not charge begin a charge if a cell is under or over-discharged.

I imagine a rubbish or broken bms could be as bad as no bms at all. But overall not having a bms is the sure fire way of letting this happen. My bms wouldn't allow it.

Your BMS (what one is it?) has adjustable cell LVC? I.e. you can set it to something like 3.6 vpc rather than 3.0 vpc?

 

[Ypedal]

holy crap... guys.. chill out.

I just hacked off a bunch of posts from this thread. fyi... live with it ( they were of no use to the OP's question anyway )

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To answer the original question.. buying 3S, 4S or 6S packs is a matter of economics, if you buy Turnigy 20C cells, all those various S number packs are made from the same cells, and QC is not much different.

What does factor into a pack build significantly is the cost of building a wire harness for the balance and power leads, it adds up fast, connectors, wire, solder, shrink....

So , if you intend to build a 12S pack.. buy 6s packs and make it easier on yourself....

Then, regarding charging, as has been said over and over and over, RC packs are no good for those who like to plug it in and go to bed... they REQUIRE monitoring.. even when the pack is in good condition, working well and you have put dozens of cycles on it, keep checking the cell voltages regularly.

Personally, i run all my Lipo packs without BMS, and i bulk charge with an pack voltage averaging cells 4.15v, i check EACH pack , with a cell log before i plug in the charger, and i check the cells again when the charge cycle is nearly done.. and after i disconnect the charger, i check them again.. ON EVERY CYCLE..

this may seem like a lot of work, but it's not, it's 10 seconds per charge cycle of monitoring.

Series for discharge and separating the packs to charge is a recipe for disaster, one wrong move and POUF, sparks... even with idiot proof wire harness, it only takes one bad connection when the pins start to wear out and voila.. problem.

Build the pack in the way you intend to use it, make the wire harness in such a way that you can monitor the cell voltages via the balance leads while charging, follow safety advice to the max and life should be good.

Even the test packs i have BMS in use, i've build the balance harness so that i can visually check what's going on while the BMS is being used... i've found more than one bad BMS, in fact i have a big box of funky one's i removed in time to save it's pack from.. and the packs are still in use.. BMS in a dusty box.

RC lipo is perfect for high powered machines, if your ebike only needs 20 amps.. buy a boxed pack pre-built from lower C rated cells, give it a good inspection when you get it, and ride easy.

If you intend to run your 48v pack on a 50+ amp controller, and want a 10ah pack, RC lipo is the only way to go... so go ahead but play safe.