Protected 18650 cells?

I'm building a project where two 18650 cells in series are the "desired" power supply. I'm thinking two 185650 cells with built in protection (not of ebay) in series. Can I just charge with a 12V supply with a series current limiting resistor? The ones I'm looking at are 2.4Ah so at 0.5C they need about 1200mA. discharged ~7V so around a 5 ohm resistor?

BTW, anyone know the current medical equipment power supply requirements? Are USB chargeable devices considered to be connected to the mains?

Mike.
Edit, as I can get these around the corner and I know the company won't sell junk, these are the cells I'm thinking of purchasing. https://www.jaycar.com.au/18650-2600mah-li-ion-protected-battery/p/SB2299

Pommie said:

I'm building a project where two 18650 cells in series are the "desired" power supply. I'm thinking two 185650 cells with built in protection (not of ebay) in series. Can I just charge with a 12V supply with a series current limiting resistor? The ones I'm looking at are 2.4Ah so at 0.5C they need about 1200mA. discharged ~7V so around a 5 ohm resistor?

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I would say not, and SERIOUSLY not - as far as I'm aware the protection circuits are 'mainly' to stop the battery been over discharged, and 'may' also have a degree of over-voltage protection - I don't think they provide charging circuits?.

But Li-Ion are easy enough to charge - for your two in series you want a regulated constant voltage of 8.4V - this should also be current limited to your required charging current - and personally I'd aim for 0.2C rather than 0.5C, as it will charge the battery to a greater level, and also considerably increase the batteries life.

So basically you charge with constant current until the battery voltage equals the maximum (4.2V per cell) and then you switch to constant voltage and keep charging until the current falls to about 0.05C.

If you're got a 12V supply, then there are plenty of cheap and easy charger boards from China, you set the constant current to what you want, and the constant voltage to what you want, there's usually also a third pot, this sets the fully charged point, and simply lights an LED when charged.

I use just such a board in my current project, and monitor it via a PIC and a IN219 I2C current/voltage sensor as well.

So, how can they claim a charging cutoff voltage of 4.2V (added in the edit). I've used none protected cells before and used cell protection circuits but I thought these had them built in.

Mike.

Pommie said:

So, how can they claim a charging cutoff voltage of 4.2V (added in the edit). I've used none protected cells before and used cell protection circuits but I thought these had them built in.

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Do you want to risk it? - it's essentially a desparate emergency measure, not a replacement for a proper charging system.

Nigel Goodwin said:

Do you want to risk it? - it's essentially a desparate emergency measure, not a replacement for a proper charging system.

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If I was buying from china then No, I wouldn't risk it. However, I figured paying a local trusted company three times more I should be getting a quallity product that I can trust.

Do you have a link to a trusted 2s balanced charger protection module?

Thanks,

Mike.

Pommie said:

If I was buying from china then No, I wouldn't risk it. However, I figured paying a local trusted company three times more I should be getting a quallity product that I can trust.

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To be fair I imagine they simply buying them in from China, and making a nice profit on the deal

Do you have a link to a trusted 2s balanced charger protection module?

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I don't think I've ever seen any combined charger/protection boards?.

A charger circuit for a lithium battery detects a full charge then shuts off. A protection circuit does not because it is not a charger circuit.

So, are you saying that if I buy protected cells from a company like Panasonic, I can't trust them not to set on fire?

Mike.
Edit, think I'm just going to use the guts from a power bank I have handy.

Look for Big Clive's YouTube videos on the subject. You'll get good information.

Panasonic is a good Japanese company, not a fake Chinese one.

audioguru said:

Panasonic is a good Japanese company, not a fake Chinese one.

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I know that. What about their protected 18650 cells?

Mike.

Fake Chinese sellers on ebay and on the other Chinese auction websites sell cheap bad copies of anythng like batteries. Buy good real ones from a good electronics parts distributor.

audioguru said:

Fake Chinese sellers on ebay and on the other Chinese auction websites sell cheap bad copies of anythng like batteries. Buy good real ones from a good electronics parts distributor.

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I know that too but it still doesn't answer my question.

Mike.

"Protected" cells have a circuit which switches off the external terminal if the voltage gets too high or too low.
If you are trying to charge series cells, that's not what you need, as switching off any one kills the charge current through both/all.

The weakest or nearest full cell will reach full voltage, cut out, and prevent any other cell from charging.

Cells with internal protection are only really suitable for individual use in a stand-alone charger, where cells do not affect each other.


For what you want to do, you need a "balance charge" circuit that monitors and regulates the voltage on each individual cell.

You could use a board such as this that incorporates a high-current balance circuit (70mA) and cell voltage protection.
You do not then need "protected" cells.

You still also need a current limited and voltage regulated (8.2 - 8.4V) supply for charging.


Note that there are three types of board sold as "BMS" units on ebay.
Voltage protection only (like the internal circuits in protected cells);
charge balance control with with various balance currents - often very low, and
combined voltage protection and balance.

One of the third type with a relatively high balance current rating, 50mA or higher, is the best and simplest option.

You can otherwise use a voltage protection board and a separate balance board or external balance charger.

rjenkinsgb said:

The weakest or nearest full cell will reach full voltage, cut out, and prevent any other cell from charging.

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Thanks, I hadn't thought of that even though it's kinda obvious. As it's just a prototype I'm going to use the guts out of a power bank as that seems the quickest and easiest option.

Mike.