Choosing The Right Battery Management IC for Li-Ion 18650

[itsdonny]

I'm trying to find the right solution for charging a product I'm having designed. Here are the details: The unit consists of 3 18650 Li-Ion batteries. Depending on the unit the batteries will range from 1400mah for the entry level units to 3300mah each for the higher end units. I want to be able to charge the batteries either with USB or with a separate charger at 12V for faster charging. Also I'm wanting to give users the option to purchase an extra battery pack consisting of 4 18650 batteries at probably 3200mah each.

If I wire the batteries in parallel I can use less expensive battery management chips like Skyworks AAT3620 however this would require extra circuitry to allow charging by both USB and the 12v additional charger. Alternately I could use a chip like Maxim Integrated MAX8677 which is a dual chip and allows me to charge both USB and 12v.

The Skyworks chip is $.83 in 1000 increments. The Maxim chip is $2.28 in 1000 increments. I'm looking for suggestions on alternate ways to do this to keep the production costs down. Please let me know if you have any suggestions or ideas.

Thank you

 

[LiteYear]

To get the AAT3620 working you'll need an additional DC-DC converter and some changeover circuitry. I think that'll quickly gobble into the price difference. Having designed something similar (except changeover was on the output side - supply the load via batteries or external input), I think it's worth going the integration route where possible - the problem is that if you start to have multiple SWPS on the board and your battery charger is trying to monitor sensitive voltage levels from the battery to determine state of charge, it's easy to run into noise coupling issues that will make the charge process unreliable.

 

[itsdonny]

LiteYear, thank you for your reply. The MAX8677 has a maximum output of only 1.5-2A. This would be pretty slow to charge a battery pack at almost 10,000mah. Do you have any suggestions on higher current rate chips? Thank you.

 

[LiteYear]

You might need to be a bit careful there. Battery chargers are typically limited to about 1C charge rate for a few reasons - to prolong battery life and because charge time doesn't fall much beyond that are the two big reasons. When you put a few cells in parallel you'd want to ensure one battery doesn't take too much charge. One way would be to limit total charge to about 1C, so even in the worst case you wont over-supply one battery. The other way would be to have separate chargers for each cell. The first option doesn't improve your charge rate and the second requires almost 3x space and 3x cost. I don't know if there's another chip option, but suspect it might involve a bit more design risk.

 

[mikekilroy]

in case you are not aware, most critical life series and/or parallel li-ion cells today have a BMS consisting of individual FET/R loaders across each cell to 'equalize' (google bms equalizer) the charge