Capacitive balancer efficiency.

[paul.simon.eu]

Hello to all,
so an active balancer of the flying capacitor style (like the Heltec/Hankzor 5A modell) takes charge from the highest voltage cells and transfers it to those cells with lower voltage. I was wondering if anyone here has a rough estimate of the efficiency of this process? How much of the energy taken from one cell is actually ending up in another cell and how much is just dissipated as heat? Is it about 80% or even better? Or much worse? I really have no idea.
Background: My balancer is controlled by the programmable load output of my Victron MPPT solar charge controller. Absorption voltage is 14.2V for two hours and after that the controller switches to float at 13.5V (Victron recommended settings). At the moment the balancer gets activated as soon as the pack voltage exceeds 13.8V (3.45V per cell average) and is deactivated as soon as voltage falls below 13.75V (3.4375V average). After that my cells are all within 5 millivolts and I was wondering if I should enable the balancer a bit later and disable it sooner - even if I have to tolerate a few more millivolts of delta.
If balancing efficiency is really good, though, I would keep everything as it is (start balance @ 13.8, stop @ 13.75). Actually I am really happy with how the whole system in my van works right now - without the active balancer cells would drift just a bit too much over time.

Cheers
Paul

 

[time2roll]

I don't know the efficiency but there is no conversion losses that I know of. afaik the power is switched on to fill the caps, caps are then isolated from the battery and averaged out, then reconnected to each cell causing each cell to come closer to the average. Highest cell keeps putting net power in, low cell keeps getting power out from the capacitors. My money would be on higher than 80% efficient. Does not move a ton of power to waste anyway. Need a large differential to get close to the rated transfer amps.

 

[Luxpower_Gilbert]

I recommend NEEY Active balance, i have two with my 2*16s 277ah and 300ah battery pack, easy to set it up and works great…

 

[octal_ip]

I don't know the efficiency but there is no conversion losses that I know of. afaik the power is switched on to fill the caps, caps are then isolated from the battery and averaged out, then reconnected to each cell causing each cell to come closer to the average. Highest cell keeps putting net power in, low cell keeps getting power out from the capacitors. My money would be on higher than 80% efficient. Does not move a ton of power to waste anyway. Need a large differential to get close to the rated transfer amps.

Agreed, mine gives off no noticeable heat, which would be the key indicator of an inefficiency if it existed.

Edit: Mine is always running, and always connected. I've not experienced the problems others have described of this approach unbalancing their cells, it always keeps everything in check for my daily cycles, and helps keep the cells balanced during a discharge if any cell drifts for whatever reason.

 

[Alkaline]

I found this video that explains it very nicely

and I have 3 of these (only 2 are being used) and I have had no issues, I leave them on the whole time.

So basically the flying capacitor balancer acts like a parallel bus bar, this is why the balancing speed all depends on how far out of balance they are. In my case the highest I ever say was 1.2 AMPS.

So in other words there is no way for this to BOOST balancing capacity which is why at the flat part of the charge/discharge curve its balancing will basically be useless; and that is fine btw, as I only care about the balancing near the top end.

 

[paul.simon.eu]

Great, thanks to all of you! Didn't notice any heating up of the caps either. So I will keep my current settings and be happy with it.
If anyone is interested how to switch the Heltec/Hankzor active balancer from a 12V or 5V signal: I am using an IRF5305 P-Channel MOSFET for the task, but I guess most parts with similar specs will work just as well.

 

[basreflex]

there is a simple rule of thumb for the efficiency of a switched capactior circuit.: 100%-(relative voltage difference*0.5)%

example take a 1V cap of 1F and a zero voltage cap of 1F and connect them together. relative voltage difference is 100% so efficiency is 50%.

check: before connecting them we have 0.5CV*V=0.5J in one and 0J in the other. after we have 0.5V in both (2F) so 0.5*2*0.5*0.5=0.25J QED..

the other losses are the capacitive switching losses of tehe fet switches, and monitoring circuitry. they are specd at around 25mA, a lot more than the few uA's of the inductive solution. the ETA3000 does it's job but has a 100mV threshold cell difference; At one end of the stack an off-cell can only proliferate to the other side of the stack with 100mV difference per cell. at >10S that becomes a problem

 

[basreflex]

the most efficient way of doing the cap balancer is to vary the frequency when there are cell differences and lower the frequency once balanced

 

[meetyg]

I don't know how efficient these active balancers are, but for sure better than passive balancing, where 100% of the balancing energy goes to waste (heat).

 

[basreflex]

most of the background losses are in the dcdc buck converter, the halfbridge drivers, and the gate gate drive energies. that is where the 25mA background current comes from.