Li Ion BMS modules..What alternatives for low power circuits ?

[MalcolmZA]

Folks,

I have a couple of questions regarding charging Li Ion batteries.

I’m building a small security device. It operates from 12vdc and must be able to run for at least 48hrs from its rechargeable batteries. Whilst a SLA battery is the simplest solution, I want to use three Li ion cells in series to keep the size down. The unit draws only 50mA, and occasionally 1amp whilst the siren runs.

The smallest 3S BMS module I can find is rated at 10amp, its way oversized for my application.
The battery will be charged from an off the shelf 12v 1A psu adjusted to supply a little less than the batteries full charge voltage. So when they’re almost fully charged, the psu will only supply the alarm pcb.

For short circuit protection, I’ve thought of putting a PTC resettable fuse inline with the battery. This would limit the current out of the battery and prevent a catastrophic failure.

What is everyone’s opinion... would this arrangement cover safety whilst keeping it simple ?

Regards
MalcolmT

 

[Nigel Goodwin]

Folks,

I have a couple of questions regarding charging Li Ion batteries.

I’m building a small security device. It operates from 12vdc and must be able to run for at least 48hrs from its rechargeable batteries. Whilst a SLA battery is the simplest solution, I want to use three Li ion cells in series to keep the size down. The unit draws only 50mA, and occasionally 1amp whilst the siren runs.

The smallest 3S BMS module I can find is rated at 10amp, its way oversized for my application.
The battery will be charged from an off the shelf 12v 1A psu adjusted to supply a little less than the batteries full charge voltage. So when they’re almost fully charged, the psu will only supply the alarm pcb.

For short circuit protection, I’ve thought of putting a PTC resettable fuse inline with the battery. This would limit the current out of the battery and prevent a catastrophic failure.

What is everyone’s opinion... would this arrangement cover safety whilst keeping it simple ?

Regards
MalcolmT

No, you're not using any charging circuit, you can't just use a plain PSU. One of the main points of a BMS is to balance the charge across the cells, another is to prevent excessive discharge which damages Li-Ion cells - the most minor point would be too avoid too high a charging voltage, and the charging circuit should take care of that anyway.

You can buy specific Li-Ion chargers, they look like a normal 'brick' PSU, but have a two colour LED on them - and they charge the battery in the correct way, and stop when fully charged (and show it on the LED's). They are available in various voltages, to match how many series cells you have - you can get them from Farnell, and numerous other places.

It's VERY, VERY important to charge Li-Ion cells correctly.

 

[rjenkinsgb]

You MUST have a balance & protection module connected with any series-cell lithium battery setup, as any imbalance can cause one cell to overvoltage before the pack reaches full voltage, resulting in failure, bursting or fire.

 

[rjenkinsgb]

A 3S 2 - 3A example:

Battery Protection Boards 1S 2S 3S 4S Cells 18650 Ion Lithium Polymer BMS PCB | eBay

Find many great new & used options and get the best deals for Battery Protection Boards 1S 2S 3S 4S Cells 18650 Ion Lithium Polymer BMS PCB at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[MalcolmT]

Nigel, The only Li Ion chargers I've seen here in SA are the type you'd pop your cells in to charge, or small PCB's which charge one cell from a USB cable. I've not found anything on RS Components, and Farnell isn't available here in SA. Is there a chance you can post a link to something suitable to help me track it down here ?

Rob, I was about to order a few BMS boards and noticed they don't have cell balancing.
So I assume they must be avoided ?

 

[Nigel Goodwin]

Nigel, The only Li Ion chargers I've seen here in SA are the type you'd pop your cells in to charge, or small PCB's which charge one cell from a USB cable. I've not found anything on RS Components, and Farnell isn't available here in SA. Is there a chance you can post a link to something suitable to help me track it down here ?

Here are a couple on CPC-Farnell, to give you an idea:

https://cpc.farnell.com/mascot/2241000059/charger-li-ion-3-stp-3-cell/dp/BT03480

https://cpc.farnell.com/ansmann/2000-0001-05/universal-charger-li-ion-0-7a/dp/BT06584

Or RS:

RS PRO Battery Pack Charger For Lithium-Ion Battery Pack 3 Cell 2A with AUS, EU, UK, USA plug | RS

uk.rs-online.com

RS PRO Battery Pack Charger For Lithium-Ion Battery Pack 3 Cell 0.7A with AUS, EU, UK, USA plug | RS

uk.rs-online.com

2000-3013-520 | IPC12 Battery Charger For Lithium-Ion Battery Pack 3 Cell with AUS, EU, UK, USA plug | RS

uk.rs-online.com

Rob, I was about to order a few BMS boards and noticed they don't have cell balancing.
So I assume they must be avoided ?

Perhaps?.

Personally I've been removing them from one of our products, as we've had a few come back with duff batteries, and the BMS's have been killing them - so I've ended up removing the BMS's. However, I charge them with a micro-controller circuit, and my algorithm does various different tests during charging, including the batteries being too flat, or taking too long to reach an acceptable level. If the batteries are too far out of balance, then it will detect it and stop charging - then be returned for repair, and we can charge for fitting new batteries

So far, the only ones returned with duff batteries are ones that originally had BMS's fitted.

 

[MalcolmT]

Gent's,
Thanks a mill for your info. I think its much safer staying with SLA batteries rather than risking burning someone's house down.
Quite a few 'Robotics' suppliers have sprung up in SA. Many seem to be one man shows operating from home and catering for Arduino experimenters. Quite a few sell BMS boards and their website implies connect it to three cells, stick 12.5 - 13v on it , and bingo, you've got a charger. It seems like its more of a potential incendiary device.

I'm going to do more research, and testing in early November ;-).
Many Thanks
MalcolmT

 

[rjenkinsgb]

I was about to order a few BMS boards and noticed they don't have cell balancing.

Oops... Well spotted, I normally only use high current ones - it makes no difference with the battery function, just use a series fuse or polyswitch etc. for lower current applications.

This is the type I normally use for three cell packs:

3S 40A 11.1V 12.6V 18650 Lithium Lipo Battery Protection Board PCB BMS Module 2 | eBay

Find many great new & used options and get the best deals for 3S 40A 11.1V 12.6V 18650 Lithium Lipo Battery Protection Board PCB BMS Module 2 at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[Externet]

MY opinion; explore the possibility of doing/modifying it with a circuit working on 3.7V lithiums. Charge the cells (paralleled if current requieres several) to only 4.00V fixed and no BMS.
I have circuits recharging their lithium cell daily for 3 years now with zero failures, zero overcharge, zero overdischarge events.

 

[ChrisP58]

Folks,

I have a couple of questions regarding charging Li Ion batteries.

I’m building a small security device. It operates from 12vdc and must be able to run for at least 48hrs from its rechargeable batteries. Whilst a SLA battery is the simplest solution, I want to use three Li ion cells in series to keep the size down. The unit draws only 50mA, and occasionally 1amp whilst the siren runs.

The smallest 3S BMS module I can find is rated at 10amp, its way oversized for my application.
The battery will be charged from an off the shelf 12v 1A psu adjusted to supply a little less than the batteries full charge voltage. So when they’re almost fully charged, the psu will only supply the alarm pcb.

For short circuit protection, I’ve thought of putting a PTC resettable fuse inline with the battery. This would limit the current out of the battery and prevent a catastrophic failure.

What is everyone’s opinion... would this arrangement cover safety whilst keeping it simple ?

Regards
MalcolmT

Other than the siren, what parts of your system actually need 12 Volts? I'm guessing that much of the circuit would run on 5 Volts or less.

I'd use a single Li Ion cell feeding a boost converter to make 5 Volts. Then switch in a 12 Volt boost converter only when needed to feed the siren. Charging is from a single cell charger

If you need more runtime than a single cell would give, you can add more cells in parallel. But, since they're not in series, you don't need a balancing bms, since there's nothing to balance.

 

[MalcolmZA]

Hi Chris,
The circuit uses a couple of PIR sensors which need 12v.
I've been down the road of a single cell with a 12v step-up module.
And yes, it would simplify everything. How ever the step-up modules struggled to provide sufficient power to run a decent siren reliably.
These little alarms have been running well on 12v SLA batteries, hence a 12v lithium alternative is the goal.

 

[Externet]

There is plenty of PIR sensors that work on single lithium cells used in solar patio motion lamps.

And are daily recharged with no BMS.
There is very loud sirens that work on single 4V lithium, and several can be paralleled, instead of driving the leds as above.

----> https://cdn-learn.adafruit.com/downloads/pdf/pir-passive-infrared-proximity-motion-sensor.pdf

----> https://randomnerdtutorials.com/modifying-cheap-pir-motion-sensor-to-work-at-3-3v/

Perhaps you should explore other alarms and equipment in the market that can perform as you wish or even better.