Lithium ion 2S safety circuit woes - Page 1
Forum section: electronics / projects

author: depot
author_rank: Contributor
timestamp: May 13, 2016, 10:20:51 pm
content: Some commercial (selling products) people I know are in trouble and I'd like to be able to help them out (with compensation). They're trying to use lots of technology and in this case I don't get exactly what's going on. I've asked a couple distributor types of people about it and so far nothing.They have battery packs of lithium ion cells of various numbers but always 2 in series for 8.4v to ~6v. These use R5460 for the safety circuit supplied by battery people,http://www.e-devices.ricoh.co.jp/en/products/product_power/bmu/r5460/. The Chinese supplier has tried to be helpful but just doesn't seem to know this detail. This is normally a great chip for safety but the way the product is right now any short circuit will keep the battery pack shut off. They have to connect it to a charger most of the time to get it working again, and the products tend to get used outdoors.Some electronics with the product that this goes into will draw at least constant 200uA. Even when just sitting there, it is waiting to detect some events like charging. And part of the reason behind that is compatibility with years of old products that had power charging the battery going in on the same wire as power from the battery. It isn't an option for the user really, but I tried disconnecting these electronics from the battery and safety circuit and then it recovered normally.Am I just missing something? I don't see a parameter for this and the datasheet doesn't appear to talk about what I'm seeing. Reducing this current to almost nothing would be kinda tough, although it would be nice for shelf life.I had thought I was pretty smart in putting together a zener diode circuit to detect when a 12v wall wart was plugged in for charging power (vs the usual 8.4v to 6v) but those parts accounts for about 80uA. Another bunch is the low power mode for some chip that detects various USB devices, particularly Apple, and puts them into charging modes from a 5v switching supply.Maybe I could quickly detect when the short is happening or when the safety circuit is opening and turn off everything for some time? I don't want to have to build another safety circuit external to the first one. Or another thought is would a slow start-up on some of these things that take power help?Really cooks my noodles.

author: akis
author_rank: Frequent Contributor
timestamp: May 14, 2016, 10:03:32 am
content: There is not much info to go on.Ideally, in my opinion, a "battery pack" should contain all the electronics to deal with charging and discharging, and it should only require a single DC entry / exit as a connection to the outside world. But it should be able to handle CC/CV charging, temperature monitoring, over current situations while discharging and under voltage on each cell separately of course.If you require an external charger then you have too many cooks. You also need to expose the individual cells to the external charger let alone temp. monitoring etc.Laptop batteries work in that way precisely. They accept a DC voltage and they get on with the charging / discharging / cell monitoring; the laptop does not have a separate battery charger inside.So the question is how much electronics can you fit inside those battery packs. My laptop battery's "module" is really tiny and it handles everything, charging, discharging, temperature, and provides an interface to the laptop as well. Ideally the battery pack should model the laptops' battery packs - only one connection to the outside world and through that connection current can flow bidirectionally, depending if the battery is charging or discharging.

author: Kilrah
author_rank: Unknown
timestamp: May 14, 2016, 10:38:06 am
content: Quote from: akis on May 14, 2016, 10:03:32 amIncorrect, for laptops/phones/other mobile devices the charger is almost always in the device. The battery houses the protection/cutoff and balancing circuitry, but no charge regulation.

author: PChi
author_rank: Frequent Contributor
timestamp: May 14, 2016, 02:16:25 pm
content: I haven't experience of that particular part but older devices did exactly as you say. They protect the cells from a short circuit by switching off which is latched. Connecting a charger resets the latch.The only way the Ricoh circuit could know if the short has deen removed is to pass current through the circuit which means power dissipation in the protection circuit and unwanted discharging of the cells.

author: depot
author_rank: Contributor
timestamp: May 14, 2016, 03:36:59 pm
content: I'll provide more information.The battery component basically looks like this:http://www.dx.com/p/aob-2-x-18650-li-ion-2200mah-7-4v-protected-battery-pack-279350?tc=USDThey call it a "battery pack" and it looks 2S by the voltage. It says "rechargeable" in the specs, but I don't think this includes an actual charging chip. I know from looking at schematics and boards for the one we're using that it doesn't have any charging capabilities on its own. Actually the safety circuit for this example doesn't handle short circuit or over discharge so I probably wouldn't use it.I guess I'll experiment with the other ideas like an power-on delay. I thought that was a chip-level thing. If anyone has super low current ideas, that'd be a help.

author: Marco
author_rank: Super Contributor
timestamp: May 14, 2016, 04:36:36 pm
content: The datasheet seems to be the victim of poor translation/formatting, but it's pretty clear what happens. After there's a short the external switches are disabled and an internal 45k resistor is switched in to the negative connection of the battery pack, the voltage across that resistor has to drop below 0.8V to reset the short circuit condition so. So the current has to drop below 17 uA ... bit low.

author: depot
author_rank: Contributor
timestamp: May 14, 2016, 05:22:41 pm
content: That's the sort of explanation I was looking for, thanks much! Consistent with what I was seeing in the lab.Is that resistance the Rshort parameter? If the resistance is actually 75k (upper tolerance) then the target is 10uA. Yikes. There are some microcontrollers in sleep modes and low quiescent current linear regulators that can do that though.Is there a simple solution with replacing R5460? One part from SII appears to have the same kind of issue with different numbers (0.5v, 20kOhm).http://datasheet.sii-ic.com/en/battery_protection/S8252_E.pdf

author: Marco
author_rank: Super Contributor
timestamp: May 15, 2016, 01:01:55 pm
content: The only option I see is to find something which only does overcharge/overdischarge protection (S-8209A/B for instance, although you will need two and they don't seem to be available much) and add your own overcurrent protection.PS. or add an extra MOSFET which disconnects the load for X seconds when DOUT goes low ... so it cycles automatically, but it will be cycling a potential short. Even for 250 us at a time that's a bit hacky.