Selecting a battery managment IC for my project - Page 1
Forum section: electronics / projects

author: roli
author_rank: Regular Contributor
timestamp: November 10, 2021, 07:20:18 pm
content: I am in the middle of a slightly unusual project and I need some help. My project is creating a replacement battery pack for old Sony Aibos (from the early 2000s - for those unaware - they are robot dogs). They use what is essentially a "smart battery pack" that you would find in a laptop. Problem is that the ones out there are almost all dead or nearly dead. There are some folks recelling them but there are also a lot of dogs being sold without battery packs. And there are pretty much no battery packs being sold. And this is the problem that I am trying to solve - by creating a replacement battery pack so I can run all of my dogs.The second generation (that I am currently focusing on) uses 4 18500 lithium cells (2 in series, 2 in parallel) to give 2300mAh. I am looking at simplifying this and using 2 18650 cells instead. There is just enough space to fit two inside a custom 3D printed plastic case. And this should give a roughly similar capacity. Problem is battery management. Because these are smart battery packs they need to talk to the dog in order for the dog to actually boot. The original battery packs are based on the TI BQ2040 battery gauge (which uses SMBus to communicate) so the replacement must be compatible with that. For testing I used an ATtiny 85 micro to actually spoof the SMBus communication and the dog booted up and worked normally with my custom battery (I just used a RC lipo for testing). Now I need to condense this into a more usable form.What I was thinking of was using an Attiny 85 to handle the communication part and some of the shelf battery management/battery protection IC to do the cell protection. And if possible I would like to deal as little as possible with battery management on the Attiny side. In order to spoof the battery I really just need the battery voltage and current. Now if those can be provided to me by the battery management chip in a digital form it would be really nice, otherwise I guess I will need to do it myself. Cheap Chinese BMS modules all seem to be based on HY2120 chip and they are very simple designs. And that is what I am looking for but with a bit more features. Mainly what I am missing is cell balancing. Now if anyone knows of an integrated IC that speaks SMBus that will be compatible with the original BQ2040 but won't require me to build a spaceship of a schematic - I am all ears. But if not then I am going with the original idea. The features that I require are pretty much what is needed to keep the batteries safe - short circuit/over current protection, undervoltage protection, cell balancing, ...For reference... this is a reverse engineered schematic of the original battery:https://raw.githubusercontent.com/lpollier/battmon/master/example/ERA-201B1_reverse_engineering_schematic.pdfI am really hoping for simpler (less components) design than this, which should probably be doable - after all this is late 90s/early 2000s tech.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 10, 2021, 09:23:22 pm
content: Question, and sorry if I missed the obvious, but why wouldn't you use a BQ2040 here? It's probably flexible enough for your new setup.

author: roli
author_rank: Regular Contributor
timestamp: November 10, 2021, 10:04:03 pm
content: Mostly because I am looking for a more integrated solution. If you look at the reverse engineering schematic of the original battery pack... not exactly simple. The BQ2040 doesn't even have an internal eeprom and requires an external one. Also... it's not a protection device... it's just a battery gauge.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 11, 2021, 01:06:21 am
content: I see, but while you can emulate the battery gauge with a MCU, the protection circuitry should still IMHO be pure hardware. If you thought on implementing protection also on the MCU (+ maybe a couple MOSFETS), that's not a good idea.Do not rely on software for battery protection pretty please. So in any case, you'll probably need an additional IC just for this.The original circuit used a S8232AAFT, which was a Seiko battery protection circuit. It's obsolete, but there are many recent ones you could use here. Just for TI parts, you can have a look there:https://www.ti.com/power-management/battery-management/protectors/overview.html

author: roli
author_rank: Regular Contributor
timestamp: November 11, 2021, 06:46:45 am
content: Exactly my thoughts, yes. My intention was to use a off the shelf protection IC and then spoof the TI battery gauge with a micro. If I don't have to spoof the gauge even better. The point of this thread is finding a suitable IC for protection... what is simple enough to implement and fits my needs.

author: roli
author_rank: Regular Contributor
timestamp: November 12, 2021, 08:15:23 pm
content: Well after some researching I think I am going to use MAX17320 (https://www.maximintegrated.com/en/products/power/battery-management/MAX17320.html?intcid=para). By the looks of it, it's a single chip solution that can serve as both protection and gas gauge. And it is SMBus compatible which means that I don't need to fake anything. If anyone thinks that this won't work - please let me know.