Can someone help me out with Lithium BMS please!

[the evil stepson]

Hi,
The BMS is one part I don't understand, why do the battery makers list Max continuous Discharge current is 100amps, but the Max continuous charging current is just 50amps? Is there two seperate circuits? If so how would that work with loads & charge with just two exposed terminals on the outter battery?

thanks

 

[RvDreams]

Hi the evil stepson. Welcome to the CRVL forums! To help you learn the ins and outs of these forums, this "Tips, Tricks and Rules" post lists some helpful information to get you started.

Most of our rules boil down to two simple over-riding principles: 1) What you post should provide good information (like your introductory post), and 2) Any response to someone else's post should make them feel glad they are part of this forum community.

We look forward to hearing more from you.

Why don't you go over to the newcomers corner and introduce yourself?

 

[B and C]

This sounds like a question for the manufacturer.

Got a link?

 

[Trebor English]

For the 100 amp hour batteries Renogy seems to be 50/100 while Battle Born is 100/200 so it 's not just one vendor.

The plot thickens.

 

[B and C]

That still goes back to the manufacturers even if they are different. How are we supposed to know what their engineers were thinking when they designed the system unless we have one of their engineers on board here? Doesn't sound like it has anything to do with battery chemistry. If it makes a difference to you and you can charge above 50 amps, don't buy the renogy ones.

 

[B and C]

After thinking about it some more, it probably has to do with what components that they use in the BMS. Pay more and get better components that are rated higher?!?

 

[Weight]

Some BMS have separate charging cable connection. Most just use the one connection for loads and charging. The other side of all go to the battery negative. The amount of charging to discharge amperes is how the battery manufacture has built the battery. It is listed often as C rate. A 100ah battery with a .5 C will have a charge limit of 50 amps. The BMS will be set for the battery specifications. What brands are you working with?

 

[jonyjoe303]

with lithium lower amp charging is better, yes you can probably charge at 100 amp since its the same terminals but as soon as the bms see's max voltage it will shutdown. The only reason they say to charge at 50 amps is to get a fuller charge on your battery. The batteries have balancers that bleed off excess voltage from the high cells, but they won't keep up if charging at higher amps. Even at 50 amps is probably too high, it might get you to 80 percent and then the bms will shutdown the charge as soon as the surface voltage reaches the limit. Some bms will actually shutdown if it sees more than a 50 amp charge rate, that will be built into the bms.

There are some bms that do have seperate charge/discharge ports, those are the cheaper bms. They are kept seperate because you won't be able to charge from the discharge ports.

Thats why charging through a cc/cv charger is the best way, since the closer the battery gets to full, the charger will reduce the amps. If I charge my 65ah lithium without a cc/cv charger at 12 amps, it will never get more then 90 percent before the bms shutsdown, with the cc/cv charger I get to charge to 100 percent everytime.

 

[the evil stepson]

The amount of charging to discharge amperes is how the battery manufacture has built the battery. It is listed often as C rate. A 100ah battery with a .5 C will have a charge limit of 50 amps.  The BMS will be set for the battery specifications. What brands are you working with?

But these cells can do 1C normally right? So why does a 125ah battery of 4 large format cells say 50amps max charge current? My guess is to avoid trouble with overcharging with high voltages with the average folk. My brand is some ebay cheapy.

 

[the evil stepson]

with lithium lower amp charging is better, yes you can probably charge at 100 amp since its the same terminals but as soon as the bms see's max voltage it will shutdown. The only reason they say to charge at 50 amps is to get a fuller charge on your battery. The batteries have balancers that bleed off excess voltage from the high cells, but they won't keep up if charging at higher amps. Even at 50 amps is probably too high, it might get you to 80 percent and then the bms will shutdown the charge as soon as the surface voltage reaches the limit. Some bms will actually shutdown if it sees more than a 50 amp charge rate, that will be built into the bms.

There are some bms that do have seperate charge/discharge ports, those are the cheaper bms. They are kept seperate because you won't be able to charge from the discharge ports.

Thats why charging through a cc/cv charger is the best way, since the closer the battery gets to full, the charger will reduce the amps. If I charge my 65ah lithium without a cc/cv charger at 12 amps, it will never get more then 90 percent before the bms shutsdown, with the cc/cv charger I get to charge to 100 percent everytime.

Thanks, that makes sense to me I think. I thought some BMS had a bit of overhead for the cell V sutdown like 3.7-3.8? So you could charge at 12amps & hit 3.65 without a shutdown?

 

[Weight]

I have been reading many 'white papers' from various engineers and Li manufactures. They all recommend a charge rate of 0.3 C as a maximum charge rate. Li can sustain very higher charge rates, but it is not recommended. 125 amp hour would be 37.5 ampere charge rate to 3.65 volts and then stop charging. I haven't seen any LiFePo4 battery recommend maximum over 0.5 C charge.

 

[bullfrog]

Boy am I sorry I read this post, as I get confused easily. So are you saying with these batteries different manufacturers batteries have charge rates and times that may take twice as long to fully charge?  How does this compare to flooded cell and AGM batteries. Could you actually have more solar than you could use with the new batteries?

 

[Weight]

No I'm not. Li are not lead acid. Stop thinking in terms of lead acid. Li actually can be charged very fast if they are under constant control. Many banks die because they are over charged and that comes fast. Read up first. http://nordkyndesign.com/lithium-battery-banks-fundamentals/

 

[B and C]

From my understanding, LiFepo4 batteries charge at the rated charge rate until full unlike lead acid batteries that require an absorption phase which would make them charge far faster than lead acid. Could you have excess solar? Sure, just like LA batteries, when you get to absorption, you have excess solar. Nice to have the excess solar to use while the batteries stay topped off so they can be used after dark.

I still haven't convinced myself to change even though my AGM's are getting on their last leg.

 

[Weight]

Leaving LiFePo to any charge source after they are full will degrade and kill them. Best to keep them partial charge voltage and occasional boost to reset cell memory.

 

[bullfrog]

Thanks for the information. AGMs have worked well for me although maybe not the cheapest it has been simple after initial solar set up. I guess I’m just not ready for a battery I have to keep in a climate controlled space and have a BMS to control.

 

[B and C]

Leaving LiFePo to any charge source after they are full will degrade and kill them. Best to keep them partial charge voltage and occasional boost to reset cell memory.

I thought the BMS took care of this assuming charging parameters are set according to the manufacturers recommendations.

 

[Weight]

Please read the link I posted above at a minimum. Best to read their following papers. http://nordkyndesign.com/lithium-battery-banks-fundamentals/

 

[B and C]

Please read the link I posted above at a minimum. Best to read their following papers.  http://nordkyndesign.com/lithium-battery-banks-fundamentals/

They are the same link.  They didn't address a battery management system either.  Yes, over charging can be hazardous to your health, same with FLA batteries.  I will restate my earlier position, a properly set charge source according to the battery manufacturers recommendations and a BMS will mitigate the risk, just like with FLA.  Over charging FLA will give off explosive hydrogen gas and yet people are still using them.

There are no guarantees in life, some risks have to be taken.