Building a battery pack and deciding on an appropriate BMS

Thread Starter

HenryKittensworth

Joined Sep 3, 2019
1
Hi,
I'm currently working on a personal electronics project for which I'll need to build a battery pack from scratch. I already have a few new-ish 18650s lying around, so I'd like to use those.
I'm pretty much dead set on a ~12V pack in a 3S3P configuration. However, I'm having trouble choosing the appropriate external hardware here, as this is my first battery pack (previous projects used wall power).
I'm aware that there are quite a few cheap BMSs on sites like ebay which support the basic cell configuration I want, however I'd also want to avoid a fire and can't tell if they necessarily support the features i want (seeing as they're often poorly documented).

On top of the basic over-current, overcharge protection and temperature protection, I'd ideally want cell balancing and a state-of-charge indicator (though this can be built separately as far as I'm aware). I'd also want a way to use the battery as it's charging, though I don't think that's specifically a feature of the BMS itself (most of them seem to have a single charge-discharge connector). Typical power consumption should be around 30W, so about 3A continuous for margin's sake.

Again, I am fairly inexperienced here, so feel free to point out if anything I've mentioned isn't possible or is relegated to separate circuitry.
What type of hardware should i be getting here? Any recommendations?
 

oz93666

Joined Sep 7, 2010
667
For a start stop worrying about fires!!! I blame all this nonsense on Youtube .... You cannot have a fire unless you hammer a nail through one of these cells ... so , resist the urge to do that and you'll be fine , such cells are in billions of products world wide with no problems..

BMS units on eBay are fine , I have many different types in use .....

Be aware your 3s3p pack will only deliver about 25% of it's capacity before voltage drops below 12V ... make sure the things you plan to drive are happy at 11V and hopefully lower ... at 11V your pack will still only be half empty.
 

pmd34

Joined Feb 22, 2014
368
Hi Henry, I've been trying to get to grips with BMS myself, and after some time, I think I have figured it out a little!
- You need some form of undervoltage lockout - that prevents the battery from being discharged below a certain level
- You need an over voltage lockout - that prevents the battery from being over charged.
- You should have some form of current limitation, for charging and discharging.

These functions are normally given to ICs that are connected directly to each cell - especially when you are joining some in parallel (and cant rely on simple observing the voltages / currents of the combine cells.)

For balancing it seems the consensus is that you balance the batteries at the top of the charge cycle - you bypass / discharge cells as they reach the maximum voltage. Its a big plus if this is done by the individual cell BMS ICs as they are designed to work off very low currents so you don't have to worry too much about how rapidly you are discharging the batteries, potential leakages and keeping monitoring ICs active all the time.

Once you have some dedicated cell ICs you can then be more relaxed in the management and do the actual charge and discharge monitoring with something like a microcontroller and current sense amplifiers. This can then be the brains to add an additional lay or protection - battery pack voltage, charge current.. discharge current.. constant current charge... constant voltage charge, charge level indication etc. This would be the part that lets you charge and discharge at the same time - ie. charge current in > discharge current out.

You can find ICs that try and do everything, but usually you find that the currents are not what you want, or the voltages are a bit off, or its missing balancing.. etc..etc.. So the individual cell ICs seem to be the best bet (especially for cells in parallel), and give you a more modular, expandable approach.
If you search the net you can find people who have hacked some of the chinese battery management boards, so you can get an idea of what they have used for the protection ICs - usually 2 sot-23-6 ICs one for protection one for balencing, and 2+ mosfets that isolate the individual cells.
You get a phenomenal array of tables from most of the suppliers, as to voltage and current cut offs but once you have ruled out what is not suitable, there are actually very few variants that are still available!
As you mentioned you can just buy protection boards for the cells, or even get cells with them built in. Then you only need to do the microcontroller management but it depends a bit on what you want to achieve, and how much you want to do yourself.

Apart from physical damage to the batteries, you just have to be aware of thermal runaway. This is most likely to occur with over charging, or taking too much current. The individual cell protection should prevent this as a last resort but it never hurts to design things so you have an additional level of protection with your microcontroller, fuses.. etc. that prevents it ever happening. You can also add a temperature sensor to ensure you never get close to such a situation.

Hope that helps a little!
 

oz93666

Joined Sep 7, 2010
667
Hi Henry, I've been trying to get to grips with BMS myself, and after some time, I think I have figured it out a little!
- You need some form of undervoltage lockout - that prevents the battery from being discharged below a certain level
- You need an over voltage lockout - that prevents the battery from being over charged.
- You should have some form of current limitation, for charging and discharging.

These functions are normally given to ICs that are connected directly to each cell - especially when you are joining some in parallel (and cant rely on simple observing the voltages / currents of the combine cells.).....
I think you started off very well , pmd .... but then descended it a great deal of confusing complexity ... I also don't understand the highlighted part above ... connecting cells in parallel causes no issues ... of course the aim is always in series packs to have each 3.6V unit with close to same capacity .....

Bottom line TS just needs one of these , connect as shown and forget ...

There are dozens of different designs of these , but not one allows you to set the top voltage , a great pity. This has a great effect on the number of cycles you can get from a cell ( before capacity drops to 80%) .. charging to the usual 4.2 gives 400 cycles , charging to 3,93 gives 3,000 cycles (but only 65% of the capacity you get at 4.2)
 

pmd34

Joined Feb 22, 2014
368
@oz93666 haha fair comment... the point of the "especially when you are joining some in parallel" was to highlight that you cannot simply wire 2 cells in parallel and then treat them as a single cell, they need to have individual cell voltage and current protection. Whereas with cells only in series, you can potentially just monitor the current in / out of the battery pack as a whole.

Regarding your comment about "not one allows you to set the top voltage" the top voltage is defined by which variation of charger protection and balancing IC is used, for example the S8209 protection IC:

BMS.PNG
 

oz93666

Joined Sep 7, 2010
667
@oz93666 haha fair comment... the point of the "especially when you are joining some in parallel" was to highlight that you cannot simply wire 2 cells in parallel and then treat them as a single cell, they need to have individual cell voltage and current protection.
No ... that's definitely wrong ... Taking the BMS I pictured as an example .. it shows a 2p 3s arrangement ... but you could use the same BMS and put 100p 3s ... 100 18650s in each pack (300 total) and nothing else ... it will work perfectly fine , everything is protected , just take a long time to charge up ... as always try to keep the capacity of each pack the same ...

As for your chart ... well I'm not going to build the BMS myself ... I want a ready made product on eBay
 
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