Protecting of 16S LiFePo pack

[Lu.Sochr]

Hello to all,

I have 16 15ah lifepo cells I bought in order to assembly 48V15Ah pack and I have problem to decide between two options of protecting and balancing cells.

First option is to buy full BMS system for my pack: http://www.ev-power.eu/SBM-CBM-1-1/Simple-Battery-Management-Board-16-cells-48V-60A.html

But I am concerned about voltage limits (3,9V HVC and 2,0V LVC), which seems too over the edge, I believe that those tresholds should be between 2,5V - 3,65V right? And another concern is about balancing current (72mA), which is pretty low. Here is how I believe this BMS work. I have misbalanced pack and one or more cells reach voltage over 3,65V, so BMS will start bleeding excessive power in form of heat, but because balancing current is too low, voltage continue to rise till reach 3,9V when BMS will cut off the charger, till those high cells are bleeded back down, but again because balance current is too low, those cells will stay above 3,65V for too long time right? Maybe I do not understand this whole thing right and everything would be OK with this setup, that is why I wanted to ask you

Those concerns brought me to option number two:

I would be two of these buzzer alarm: http://quadcopters.co.uk/lipo-battery-warning-alarm-buzzer-1-8s-182-p.asp which would be set to 2,7V LVC alarm and then buy 16 of these ballancing modules: http://www.ev-power.eu/SBM-CBM-1-1/Cell-Balancing-Module-3-60V-1-7A.html. They have balancing current 1,7A which seems sufficient for me (I will be using only 2amp charger), so they will not allow those cells to go above 3,65V. Drawback of this option is more cables and more room for potentional mistake but I should handle it

So what do you think guys?

 

[dnmun]

nothing wrong with the limits. totally normal settings.

but that item is not available anyway but you can buy it from BMS battery and from bestekpower for half that price i think.

http://www.bestekpower.com/512v16spcmbmspcbforlifepo4batterypack/PCB-D131.html

 

[dogman dan]

More and more, I like the approach of using a bms, but not completely trusting it.

Get a decent bms, and use it. Then run your pack in such a way as to make it nearly redundant. Monitor whole pack voltage, use a wattmeter to monitor capacity used, trying to rarely
every actually let the pack get to bms lvc. I don't mean stop before you are home, but slow down and conserve in time if the wattmeter tells you this trip is cutting it fine.

And add two 8s jst balance taps. They splice into the wires leading to the bms. The balance taps allow single cell monitoring with a variety of devices, as well as manual charging or discharging at 2 amps when you may need that.

Typically, 16 good cells will only get very unbalanced after a deep deep discharge. So if you just did that, that is the recharge to monitor, perhaps switching to single cell charging or discharging as the charge finishes. This way 15 cells don't spend all night getting charged to 3.9v over and over waiting for one slow cell to catch up.

 

[Lu.Sochr]

Thanks for your advices. It seems that you would prefere option 1 and use regular BMS board right?

This BMS is available at store in our local Czech vendor, so It shouldn't be problem to get it. This link to ev-power.eu was only for you, because it is english speaking website

So if I am understand this right, I could normally ride with this BMS and have no fear about cell protection and balancing unless I would drain battery to 100%DOD and then I could monitor charging proces and take care about single cells and then ride normally again right?

 

[dnmun]

the BMS will stop that battery from being overdischarged so you could actually go to 100% discharged and it will stop.

if your charger will create 59V or so, up to 60V then that BMS will force the cells to balance when they become out of balance.

as with all BMS you have to disconnect it when you leave the battery unused for long periods of time.

you can also put a switch in the little wire that goes to the thermal switch screwed to the heat sink and that will turn off the BMS circuit current and it will not drain down the battery when left unused over the winter.

or you can use that little switch to turn off the battery so that when you connect and disconnect from the controller you do not get the big sparks, or when you just want to turn the battery off for safety. the switch has to be on for charging and for discharging.

if you are building a new battery it is critical that you balance each of the cells to the same voltage before you assemble them into a pack or it will take days and days for the pack to balance after it is assembled the first time. after that the cells will stay balanced. balancing first is critical.

 

[dogman dan]

Yes. Only if you have a known weak cell, or if the bms fails to function would you need to do anything. Even if you drain it 100%, your bms may be quite capable of handling balancing it.

But I suggest adding some 8s plugs so you can quickly and easily check up on if the bms is performing right. Then if it's not, you have other easy options for dealing with an imbalanced cell. For example, a turn signal light bulb plugged into the highest voltage cell bringing down the high cell faster than a bms can do it. Having the plug makes this easy compared to removing a pack from it's protective container.

In addition, at the very least ride with a voltmeter or wattmeter, so you don't make simple mistakes like ride off with a pack you think got charged, that did not.

If you get 16 decent cells, it could be years before you need to actually use those balance plugs to take action. But right away, it can be very reassuring to plug a cellog 8 or similar device into the pack, and quickly and easily see what is really going with your pack. Watching that display, you can actually watch the bms do it's work.

 

[Lu.Sochr]

Thanks again for your ideas.

This is how I will do it: I will buy that BMS system together with 2x 6s + 1x 4s JST-XH balance plugs and battery medic, which I will use it for occasionally checking single cells and BMS function and balance all cells if necessary.

Celllog is great device, but big disadvantage is that it cannot directly balance. If they would implement this function into celllog, it would be invincible device

 

[dogman dan]

Sounds like a plan. I just use the cellog to monitor. Then If I have to take a big swing at balancing a cell, I'll connect a light bulb to a high one, or an RC charger to a low one.

The main thing is that having any kind of plug allows an easy connection of something.

Again, it might not be needed for years, but as the end comes to and older pack, It's really nice to be able to single cell charge or discharge with ease. Meanwhile, the peace of mind when the pack is newer is priceless.

 

[dnmun]

you do not need the battery medic to balance if you have a BMS. the BMS will balance the pack. that is all you need.