Li-Ion protection circuits and charging

[gregallenwarner]

I'm wanting to design a power supply for a Raspberry Pi based security camera that I'm working on. Basically, I want it to run from the mains power, but include a lithium ion battery backup in case the mains go out. I'm familiar with the dangers of Li-Ion chemistry, but I am almost a complete noob when it comes to working with them.

My question is in regards to those little protection modules that you get that are sometimes included as part of a battery pack. They say they protect against over charge/over discharge and short circuit. Here's an example of what I'm talking about:

http://www.batteryjunction.com/pcbfor7li181.html

My question is this: do these things typically handle the charging of the batteries themselves, or do I need to couple them with a proper charging circuit? I'm going to be using 2 cells in series, so I will want a balancing charger to make sure the cells stay equally charged. If I want balanced charging, am I going to need more than that little $3 protection circuit?

[sleemanj]

You need a charging circuit.

The protection circuit is just that, a protection circuit, it cuts out when over-voltage/current or under voltage, it does not manage the CC/CV charging profile in any way.

[gregallenwarner]

Thanks. So what do these little protection modules do? Do they just cut the connection to the battery if it detects any improper condition? (Such as too high of a voltage when charging or too low when discharging?) Would I be better off not relying on these little guys to offer much protection and instead design my charger/power supply to take care of the batteries itself?

[amyk]

The protection circuit just limits the max/min voltage and current, sort of like a cicruit breaker. You need a current-limited, well-regulated source to charge them. For series cells you should use a balancing charger IC.

[Rasz]

in series = in addition to charger you will need a balancer

[ohmineer]

You will need both a charging and a protection circuit. Charging modules usually include secondary protection method in case of primary ones fail.

There are plenty of charging IC (Linear and Switching) that implement CC/CV profile and you just have to configure current output during CC stage. In my opinion, this is the safest approach you could take.

Protection modules usually consist of an IC that detect unsafe events and a couple of MOSFETS that act as circuit breakers. For example, Mitsumi and Seiko are reputable brands of detection ICs.

Finally, if you are using two cells in series "cell balancing" is recommended but not a must.

[gregallenwarner]

Thanks for the info guys. One thing: Since I'm new to all of this, can you explain what "CC/CV profile" stands for/refers to?

Anybody have a part number of a charging or protection IC that you've used in the past and that you would really recommend?

[Neilm]

CC = Constant current
CV = Constant Voltage

Li-Ion batteries are typically charged at a constant current until the maximum voltage at which point the charger sticks at that voltage and gradually decreases the charging current.

I have used the BQ24600 chip from TI for battery charging.

One point on the protection module - I believe that it is required to pass the UN standard for safe transportation. If the battery pack has not got this it has to be shipped as class 9 hazardous goods.

[ohmineer]

For a protection IC, look for example at http://www.mitsumi.co.jp/latest/Catalog/ic/battery/index_e.html.

Each series consists of several devices that differ in current and voltage levels of detection. A safe approach in terms of voltage is 4.25 V and 2.6 V per cell in over voltage and under voltage specifications respectively. For selecting overcurrent levels you have to determine your circuit power consumption first.

I have not try them personally but they appear in lots of application notes from manufacturers such as Texas Instruments.

Bear in mind that this modules only provide the detection and the activation (the truth protection) is done by a couple of MOSFETs in series with the cell stack. The IC controls the gate of this transistors.

Hope it helps.

[gregallenwarner]

Yes, all this helps a lot. Thanks, guys.

According to the documentation I could find, the Raspberry Pi model B requires 700 mA, but it's also got a WiFi dongle drawing power from the USB port, so let's say 1 A to be safe. I calculated my IR LED array will draw approx 500 mA, so for a safety margin let's round the whole thing up to 2 A total.

The plastic housing I'm intending to put all this in has a nice molded AA battery compartment with center tap, so I figured I'd use 14500 sized Li Ion cells to take advantage of that compartment. A lot of 14500 lithium cells are advertised as having protection PCB's integrated right into them. If this is the case, can I omit the protection IC from my design, as long as I mark it for use with protected batteries only? Or would you recommend a 2nd protection IC in the power supply/charger anyway?