Interesting blog making the case for Li-ion

[MtB]

Just now, WotEver said:

No idea, I’ve not done enough research. 

 

Nor have I. I've investigated casually several times and never found a credible answer. I emailed asking that company in Hayes that offers multi-cell arrays to boaters made up from used cells and just got a load of techno-waffle-babble back. 

 

[MtB]

 

"Our market-leading super BMS system handles all that. You don't have to worry about it." Or something along those lines. 

[George and Dragon]

8 minutes ago, Mike the Boilerman said:

 

"Our market-leading super BMS system handles all that. You don't have to worry about it." Or something along those lines. 

Of course. So anyone who wants to know how the magic happens has to buy their BMS

[MtB]

6 minutes ago, George and Dragon said:

Of course. So anyone who wants to know how the magic happens has to buy their BMS

 

Except there isn't any magic. The truth is the bod answering my email doesn't understand the question.

[Dr Bob]

2 hours ago, doratheexplorer said:

*thRush

 

I know having spelling corrected can be hard to swallow.

I'm puffin keeping up with you.

[nicknorman]

Individual cell balancing is done by having a “bypass” across each cell in series, so that current that would go to overcharging the cell is sent on to the next one. Model aircraft/boats/cars have such systems pretty much as standard. Although the cells may be balanced initially, slight variations in efficiency and capacity mean they drift of of balance unless there is a balancing strategy.

 

eg https://www.rc-airplane-world.com/balancing-lipo-battery-packs.html

Edited July 11, 2018 by nicknorman

[PeterF]

The BMS manages this as the battery reaches close to full and the charge current starts backing off, the BMS will feed small currents into individual cells so that at the end of charge all cells are at the prescribed voltage. My RC plane lithium charger will deliver up to 40A through the main charging leads to the battery +ve and -ve and can divert up to 2A through the balance wires. The BMS connects to each cell +ve and monitors each cell, so even if one cell tries to go undervoltage during use, it isolates the load rather than waiting for the whole battery to go undervoltage.

 

What is really required are slightly more intelligent chargers for lipo that sense the BMS going to end of charge and shutting down in a safe manner.

[MtB]

1 hour ago, PeterF said:

The BMS manages this as the battery reaches close to full and the charge current starts backing off, the BMS will feed small currents into individual cells so that at the end of charge all cells are at the prescribed voltage. My RC plane lithium charger will deliver up to 40A through the main charging leads to the battery +ve and -ve and can divert up to 2A through the balance wires. The BMS connects to each cell +ve and monitors each cell, so even if one cell tries to go undervoltage during use, it isolates the load rather than waiting for the whole battery to go undervoltage.

 

What is really required are slightly more intelligent chargers for lipo that sense the BMS going to end of charge and shutting down in a safe manner.

 

I can see how that works very well on a stack of three or four cells in series but how is the BMS arranged on one of these 'grid' format batteries composed of hundreds or even thousands of tiny cells in series/parallel grids?

[PeterF]

The cells in parallel are all interconnected at each positive voltage level, you do not have say 4 parallel strings of 4 cells in series, so if you have this battery with 16 cells of 3.2V in a 4 x 4 grid, all the first 4 cells in parallel are tied together at 0.0V and 3.2V, the next 4 are all tied at 3.2V and 6.4V etc. The cells in parallel all then keep the same voltage during charge and discharge as long as the stack is built properly with good joints between the individual cell terminals / tags. 

Edited July 11, 2018 by PeterF
Terminology

[mrsmelly]

Nowt like keeping it simple eh? And Li ion is nowt like keeping it simple.