BMS that can taper current?

[keepsake]

Maybe someone on this thread can comment on a brand that will taper off the charge as cells are deemed 100%. I got 16 cells for 51.2 volts.

 

[Steve_S]

A small clarification on BMS' {they do not charge cells} | DIY Solar Power Forum (diysolarforum.com)

 

[keepsake]

If you had 280ah cells would you expect a longer life out of them is you only charged them up to 100 amps ?
Maybe I'm better off with a 100a bms, rather than a 200a or 250a.

 

[ArthurEld]

People say Chinese BMS advertised limits are overly optimistic so it's better to buy a BMS that can handle more than you need.
You probably should find someplace else to ask questions. The first post asks to avoid discussion

 

[GXMnow]

Maybe someone on this thread can comment on a brand that will taper off the charge as cells are deemed 100%. I got 16 cells for 51.2 volts.

That is not really the job of a BMS. Two of my RC car chargers that can charge up to 6S will back off the total charge power as any cell hits the target voltage, but I have not seen this in an external controller. There are a few reasons why this would be difficult to do. The first problem is that the BMS would need to be able to control the charger directly. Let's say the BMS had a switching equivalent of a variable resistor. Ideally, you want it at zero ohms most of the time. But when a cell get's too high, it would increase the effective resistance to drop the current. The problem is that the charger is still in the constant current mode. So the voltage would increase until it got back to the programmed current, or it hit the bulk charge voltage limit. The more out of balance, the more voltage would need to be dropped. If it is an efficient switching type converter, you are doing the opposite, it would drop the voltage to the battery, but the current would go up, or it has to throw away the extra power as heat. The switch mode energy balancing systems do this in a smarter way, but are limited in the rate they can move around. So the only way I see it working is having a cell voltage monitor that can actively dial down the set charger current. But to be honest, this is a fix for a pack with poor balance. A good top balance should eliminate the need for this and active balancing that is limited to near full charge should keep the cells top balanced good enough to keep this from being an issue. And if anything does go out of range, it still has the cut off switch to keep cells from popping.

 

[cinergi]

That is not really the job of a BMS. Two of my RC car chargers that can charge up to 6S will back off the total charge power as any cell hits the target voltage, but I have not seen this in an external controller. There are a few reasons why this would be difficult to do. The first problem is that the BMS would need to be able to control the charger directly. Let's say the BMS had a switching equivalent of a variable resistor. Ideally, you want it at zero ohms most of the time. But when a cell get's too high, it would increase the effective resistance to drop the current. The problem is that the charger is still in the constant current mode. So the voltage would increase until it got back to the programmed current, or it hit the bulk charge voltage limit. The more out of balance, the more voltage would need to be dropped. If it is an efficient switching type converter, you are doing the opposite, it would drop the voltage to the battery, but the current would go up, or it has to throw away the extra power as heat. The switch mode energy balancing systems do this in a smarter way, but are limited in the rate they can move around. So the only way I see it working is having a cell voltage monitor that can actively dial down the set charger current. But to be honest, this is a fix for a pack with poor balance. A good top balance should eliminate the need for this and active balancing that is limited to near full charge should keep the cells top balanced good enough to keep this from being an issue. And if anything does go out of range, it still has the cut off switch to keep cells from popping.

Not strictly true. My Orion BMS is controlling the charging parameters of my Victron equipment via CAN Bus control. So when one of my cells approaches the limit, charging is reduced and even stopped if it gets high enough. Then the BMS balancing algorithm can get the cells in line and eventually everything gets balanced out and the entire pack reaches target voltage.

 

[toms]

I have used REC, Batrium, and have just got a ZEVA BMS. All of these are able to throttle current at a different voltage setpoint to the end of charge voltage.

I put this at the top of my list for a BMS function.

 

[Dzl]

I think the important distinction (that these last three posts implicitly understand but is probably worth underscoring) is that the BMS itself does not do this and is not capable of this (current limiting / throttling / tapering) BUT advanced BMSes are able to command/control/communicate with a compatible charger to limit or taper current.

I think this ^ bridges the gap between what @toms and @cinergi are saying (which I agree with) and what @GXMnow is saying (which I think is also mostly true).

Does this ring true to y'all?

 

[Ampster]

Not strictly true. My Orion BMS is controlling the charging parameters of my Victron equipment via CAN Bus control. So when one of my cells approaches the limit, charging is reduced and even stopped if it gets high enough. Then the BMS balancing algorithm can get the cells in line and eventually everything gets balanced out and the entire pack reaches target voltage.

Yes, I have an EV charger that My Orion BMS can communicate with. I could actually set different profiles so most of the time I could charge my pack to 80% but another setting would go to 100% but cutback current earlier so that more time could be spent balancing. Since I cannot get that functionality out of my Outback Skybox I am thinking of setting it up for situations like the above.
But yes, technically as others have said it is not the fundamental role of a BMS to do that. It is a nice feature to have that kind of interoperability. Most of that time it would not be necessary but on days before a power outage I would like to push my pack a little harder to get the most kWhrs, especially if smoke or clouds might reduce my ability to recharge from solar.

 

[Ampster]

I think this ^ bridges the gap.........

Yes, I agree.

 

[toms]

I think the important distinction (that these last three posts implicitly understand but is probably worth underscoring) is that the BMS itself does not do this and is not capable of this (current limiting / throttling / tapering) BUT advanced BMSes are able to command/control/communicate with a compatible charger to limit or taper current.

That is correct. Main advantage being that you can use low current passive balancing for a longer period