Can Daly or LLT BMS’ turn on balancing for channels that are next to each other or do they alternate odd/even channel balancing to reduce the heating of the BMS?
If channels 3, 4, and 5 needed balancing could all three channels be turned on or would it balance 3 and 5 for a bit (1/2 second?) and then turn those off and balance 4 for a bit. I’ve seen video of a BMS doing this but can’t find the video.
The Daly’s balancing current spec is 35mA and the LLT’s is at about 55mA. If these BMS’ alternate channels, and don’t allow two (or more) adjacent channels to turn on at the same time, then the average balancing current is lower than those two numbers.
I’m trying to get a better idea of what the different BMS’ have for their balancing current levels but this channel alternating, if used, can affect the specs I looked up.
(Update) At least one LLT alternates when balancing adjacent channels. This results in about an average of 28mA of balancing current if it has the 55mA balancing current spec I saw.
Why do they do this? Is it just too much heat in a localized area on the PCB/heatspreader? Most or all BMSes have a separate circuit for each channel, don’t they?
Yup, to both questions.
There are separate balancing channels, often under control of a single monitor/balancer/protector chip or chips. There could be completely independent balancing channels but that is less common. But even completely independent channels are directly next to each other and can heat each other up.
To offer faster balancing the manufacturers increase the balancing current. This creates enough heat that two adjacent channels, much less several, cannot be on for very long before certain components get too hot or the balancer gets hot enough to heat the pack.
To avoid this they often alternate between the odd and even number channels so neither is on for more than half the time. This allows them to cool a bit but halves the actual (average) balancing current level.
This odd/even thing can be avoided by lowering the balancing current level or increasing the size of the balancer circuit board, spreading out the components so they don’t heat each other up. But neither of those two scenarios are desired by most of their customers and, except when a pack is first charged, you rarely need much balancing current anyway.