Battery DIY.

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[D8ve]

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Are you admitting to something SW?

 

[Wicky]

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Clacton-in-Sea color gas explosion just round the corner from my Mum's gaff - thankfully my mum died peacefully in hospital at that time with her lead battery powered bike not plugged in - though I was worried for a while for her roof (my inheritance) from the resulting debris...

Amazingly they did pull out one shell shocked survivor but they haven't been quite the same since.

 

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[Danidl]

You can't use a lab power supplies to charge a lipo pack. You need a balance charger to balance the cells. Without balancing, the lab power supply can make the pack explode.

Yes I was working on the assumption that the battery pack would have a BMS battery management system fitted.

If Renax is unfamiliar with the concept ... Basically it is an electric connection from each tier in a multiple cell battery and it ensures that no cell experiences a voltage greater than 4.2v . If the voltage difference across any cell exceeds this threshold, the charger prevents any current flowing.
Why might this occur .... even if all the cells were identical at the manufacturing stage, those in the middle will have a different thermal environment than those at the edges and will degrade differently. A cell with less capacity will have a slightly higher resistance and basic electrical theory says that in a series circuit of a number of resistance s the larger voltage drop occurs across the components with larger resistance.

 

[Danidl]

ofc, don't know what I was thinking.
A lab power supply can then easily be used to charge a lipo battery, but there will be no cutoff switch, so if I forget about it it will still be charging (altho at constant voltage and very low amp) a long time after a dedicated charger have shut off?

At the constant voltage charging regime , say you had set the voltage to 4.18v , the charging to 4.14v will be rapid, but the closer it gets to 4.18 the longer it takes to charge the lastl ittle bit ... It actually takes forever and the charging current becomes vanishingly smal.l

Yea, it looks good for most uses, but can't be used for my 48V Li-ion pac for example.. not that I need it to, but I might in the future want to buy a similar battery, and if i now buy a charger that can charge such a battery I can save my future self both money and shelf space. that being said who knows what kind of battery my future self will actually buy, so at this point in time the B6AC seems like a reasonable compromise...

 

[Renax]

In the interest of learning: I get how BMS work, but in a 10S5P setup you only have balancing leads from each group of 5 paralell cells? whats stopping them from having different voltages across? Or is the problem only that one in series can have 4.2V and another 4 so the charger assumes both at 4.1 and keeps pushing current?
I was thinking the scenario Danidl writes about with temp differences in the pack can easily be affecting those in paralell too?

 

[anotherkiwi]

Yea, but how safe is it to charge lipo's with just limited volt and current, isn't there some sorcery involved in charging lipos? (again, I don't know but assumed there was because lipo chargers are so much more advanced than a standard li-ion charger?)

Being there for the whole charge, just in case, is the sorcery. As well as doing it in a fire resistant place. If you respect your batteries they are safe which is why Lipos from R/C cars and planes are not, they have been crashed and often asked for way too many Amps in a very short period of time...

You need a charger that will charge your batteries in an hour at 1C, during that hour you can do other useful things while keeping an eye on the charge. Your charger will charge any kind of battery up to 6 cells including your car battery (careful how many Amps you set the charger too, they like trickle charge). It can't charge a Li-ion battery because the BMS in the battery wants 42 V @ 2 to 4 Amps depending on manufacturers specs.

 

[IR772]

I have to laugh out loud when I read some of the "information" on here.

Below is a link to a BMS circuit, you will see it connects individually to each battery in the pack.

I have not read through this all but it would be a labour of love to split a spot welded battery pack and then fit a BMS.

It is not easy or straightforward. That is why battery packs are expensive.



http://www.ebay.co.uk/itm/48V-54-6V-13S-30A-Lithium-ion-Li-ion-Li-Po-LiPo-Polymer-Battery-BMS-PCB-System/332032296692?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=40794&meid=cf7d4035bbf64ba0b7d7e658e742679a&pid=100005&rk=5&rkt=6&sd=222314566051

 

[anotherkiwi]

I have to laugh out loud when I read some of the "information" on here.

Below is a link to a BMS circuit, you will see it connects individually to each battery in the pack.

I have not read through this all but it would be a labour of love to split a spot welded battery pack and then fit a BMS.

It is not easy or straightforward. That is why battery packs are expensive.



http://www.ebay.co.uk/itm/48V-54-6V-13S-30A-Lithium-ion-Li-ion-Li-Po-LiPo-Polymer-Battery-BMS-PCB-System/332032296692?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=40794&meid=cf7d4035bbf64ba0b7d7e658e742679a&pid=100005&rk=5&rkt=6&sd=222314566051

Count the wires, they are soldered to each of the 12S paralleled groups of cells and they control the state of charge of however many cells are in parallel in that series group. Once the cells are paralleled they all hold the same charge. If one is weak it drags the whole group down.

 

[Danidl]

In the interest of learning: I get how BMS work, but in a 10S5P setup you only have balancing leads from each group of 5 paralell cells? whats stopping them from having different voltages across? Or is the problem only that one in series can have 4.2V and another 4 so the charger assumes both at 4.1 and keeps pushing current?
I was thinking the scenario Danidl writes about with temp differences in the pack can easily be affecting those in paralell too?

All the cells in a single tier are electrically connected in parallel, so they will all be at the same voltage the . There are multiple connections between these cells so the intercellular resistance is very low, so they are all at the same voltage level. The lead samples the voltage present at all the cells and brings it to the BMS....
The only problem which might arise is when the cells are brought together for the first time. All cells in the same tier have to be at the exact same voltage state of charge or else massive currents can flow from the cell at the higher voltage to the other cell, while the voltages initially stabilise. This can be worked around by joining the cells via a resistance to restrict the current while the voltages stabilise.
Effectively a group of cells in parallel behaves like a single cell of larger capacity.
If one cell loses capacity or goes open circuit, the voltage on ,the tier remains as before.
If one cell fails short circuit then all the energy from the remaining. Cells in the same tier gets dumped into it....

 

[Danidl]

All the cells in a single tier are electrically connected in parallel, so they will all be at the same voltage the . There are multiple connections between these cells so the intercellular resistance is very low, so they are all at the same voltage level. The lead samples the voltage present at all the cells and brings it to the BMS....
The only problem which might arise is when the cells are brought together for the first time. All cells in the same tier have to be at the exact same voltage state of charge or else massive currents can flow from the cell at the higher voltage to the other cell, while the voltages initially stabilise. This can be worked around by joining the cells via a resistance to restrict the current while the voltages stabilise.
Effectively a group of cells in parallel behaves like a single cell of larger capacity.
If one cell loses capacity or goes open circuit, the voltage on ,the tier remains as before.
If one cell fails short circuit then all the energy from the remaining. Cells in the same tier gets dumped into it....

In a 10s5p arrangement you should have 5 cells in each tier sharing a BMS lead and 9 BMS voltage sampling leads excluding the 0v ground connection. The peak charging voltage would be 42v

 

[Danidl]

I have to laugh out loud when I read some of the "information" on here.

Below is a link to a BMS circuit, you will see it connects individually to each battery in the pack.

I have not read through this all but it would be a labour of love to split a spot welded battery pack and then fit a BMS.

It is not easy or straightforward. That is why battery packs are expensive.



http://www.ebay.co.uk/itm/48V-54-6V-13S-30A-Lithium-ion-Li-ion-Li-Po-LiPo-Polymer-Battery-BMS-PCB-System/332032296692?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=40794&meid=cf7d4035bbf64ba0b7d7e658e742679a&pid=100005&rk=5&rkt=6&sd=222314566051

The post is useful back up information. One needs to be careful about terminology. .. a cell is a single device. A battery Is the collective term for the entire set of cells. I call the group of cells connected in parallel a tier, others may use different terms. The number of tiers in series determines the battery voltage.

B

 

[Danidl]

The post is useful back up information. One needs to be careful about terminology. .. a cell is a single device. A battery Is the collective term for the entire set of cells. I call the group of cells connected in parallel a tier, others may use different terms. The number of tiers in series determines the battery voltage.


It would indeed be a Labour of love to redo a spot-welded battery pack. But using a voltmeter, it would be easier to identify the various tiers and solder a BMS lead as required

 

[Deleted member 4366]

Hmmm! I got confused by all those answers. here's a picture that should explain it. You need one sense lead for each cell-group (in series), so 10S5P has 10 sense leads:

 

[Danidl]

Hmmm! I got confused by all those answers. here's a picture that should explain it. You need one sense lead for each cell-group (in series), so 10S5P has 10 sense leads:

Dave , you confused? I think not ... But a very useful graphic