Battery pack layout / DIY

[paul2129]

Hi guys, a quick question if I may. I'm trying to work on a little project of modifying a Dyson cordless vacuum battery pack, the current one is worn out, and I'd like to change the batteries to a larger size in a different orientation. The quantity of batteries will be the same, as will the voltage, just the capacity will be more.
The current pack is 4 cells in series, with a small circuit board. I'm unsure of the exact purpose of this, I guess it handles charging, will there be a function of monitoring the cells also ?
So, in order to explain this pack layout, imagining the batteries laid out end to end, in line pos to beg. There's positive and negative connection on the top and bottom ends, (the heavier gauge wires) and a thin wire back to the circuit board go to the same places. Then one cell in, i.e. Between pos and neg, the closest positive to negative battery connection to the outside of the pack, there's another connection to the circuit board. What do these actually do ?

Any explanation, or link to an explanation would be very useful to me.

Many Thanks

Pos Y+(0000)-X+(0000)-+(0000)-X+(0000)- Y Neg

Y is thin wire to circuit board
X is also connection to other part of circuit board
(0000) represents each cell

 

[paul2129]

I'm just reading that the circuit board is a PCB, handles protection for maximum charging voltage, maximum current drawn, guards against short circuit and cross polarity ? Do they generally also monitor cell temperature and shut off the device if the cells are getting too hot ? ...... and it shuts off the charger when the cells are full ?

 

[kdog]

Without knowing what you have actually got I'd say you are describing a small lithium battery with a bms ( the circuit board). The bms ( battery management system) regulates the voltages of the individual cells ( P group cells are treated as one) in the battery. Lithium ion is very susceptible to too high or low a voltage. So the bms makes sure during charge and discharge the stay within limits generally 2.5-4.2 volts. If a cell goes too high or too low-it shuts the battery down.
You can put more cells in a p group to make a bigger battery but without extensive reworking you can't add more in series. I'd say you can get a better run time by getting good new cells rather than the crappy ones that come in most appliances. Don't buy any old bs cells of eBay like ultrafire 6000mah. The best on the market for 18650s is about 3500mah. If you've got space make up a 4s2p battery. Do a bit of reading before you launch in cause they are a little dangerous if mishandled.
K

 

[paul2129]

Thanks very much for your help and advice. I guess that if I retain the same quantity of cells (i.e. Same voltage) then it's ok to have the second set in parallel ? It's technically still the same number of cells as far as the BMS is concerned ? (It's only really bothered about voltage ?) so I can reuse the BMS which came with the original pack ?

 

[paul2129]

(Bump

 

[kdog]

Yes should be ok if you keep same series number. You can reuse the bms ( assuming it is working). Remember you need to parallel first then series's up the P groups. One sense wire will go to each P group and the bms will just see it as a bigger single cell .With extra capacity the bms may have yo work a bit harder balancing so it might get a bit warmer, keep an eye on it ( most have a thermal cutout). The bms might need a quick charge to turn it on after the rebuild. Good luck. Don't burn the bms by putting the sense wires in wrongly!
K

 

[kdog]

And... Some Bms's that are more integrated with the control circuitry on an appliance like to lock if they sense a zero or very low voltage on a cell. There is a chance it wil never work again...but it's slim.

 

[paul2129]

Ok, I'm trying to take the next steps with my mini project. The previous experimentation was with another Dyson battery pack, it was a 4 cell, similar, but this BMS circuit is different. I now have the correct layout, a faulty 6 cell, correct type pack. Disassembled, it has a knackered cell, looks like rust around it, I guess it's leaked, the others have good voltage, this one has next to nothing. The wiring for the circuit board is completely different. I have a diagram and will copy it for the replacement pack. All of the wires to the cells on this pack are metal strip rather than wire. It seems that there is a connection to the negative of the (effectively) last cell, and a thicker connection to the first cells positive. Then there's a sensing wire between each pos/neg connection of each cell.

So, if I needed a replacement BMS, how is this worked out, you specify the number of cells (or the voltage) and the required maximum draw of current ?

How about charging a pack if it's completely home made ? Would you use a smart charger which has selectable modes ?

Forgive all the questions, I'm keen to learn... and I have learned a lot from reading online, but for some questions I need to ask


If there are any good websites or links that you could suggest, so I can learn some more of this theory, would be much appreciated

Paul