DIY 7s Li-ion Pack with BMS and Fuel Gauge

[lja246]

I'm planning on making a LED panel and powering it from a 7s Li-ion pack. As part of the pack I wanted to include a BMS and a fuel gauge. I'm planning on using the BQ76942 for the protection and the BQ34Z100 for the fuel gauge. I was wondering if anyone has had any experience with these parts and whether you think they would be suitable for this. I have attached a draft of the schematic which I made basically following the example schematics. I'm not sure if the schematic is right so if you have used these parts before I would appreciate it if someone could have a quick look over it to check for obvious mistakes. I haven't really made a complicated schematic before and didn't want to get a PCB ordered before getting it checked.

[jbb]

Secondhand, but someone over at https://www.eevblog.com/forum/renewable-energy/how-to-predict-battery-status/msg5700489/#new did mention having some trouble with the BQ34Z100. You might want to check that - or even message the user directly - before proceeding too far

[lja246]

Thanks for that,

I don't suppose you would know of any good alternatives? I've found some others but I guess they could all have their own issues that aren't documented.

[mtwieg]

I'll re-post my original response on the BQ34Z100-G1:

I had tried this one out (or rather the BQ34Z100-G1, an earlier revision I guess) a couple years ago for use with a SLA battery (it still seems to be the only fuel gauge chip which supports PbA chemistries, for some reason). Bought the dev hardware along with a specific battery model which I knew matched one of their existing chemids. Long story short, it was a huge waste of time because their development software BQstudio is a raging dumpster fire. The process for customizing the chip for your application is thirty steps long, and I couldn't get more then five steps in before some show-stopping bugs manifested. I'm convinced that the only chance of successfully using these chips is with direct support (as in from the factory via an FAE, likely under NDA. Support on their public E2E forums was unhelpful).

Perhaps most of the other "BQ" chips which are just AFEs and not gas gauges are a different story.

Maxim also makes a family of gas gauging ICs, look up "modelgauge". I had played with the dev tools for one of them, and while complicated it at least seemed to function. Unfortunately it didn't support my SLA chemistry.

The Maxim fuel gauge chip I had tinkered with a bit was the MAX17205. Ultimately that also wouldn't work for us because their firmware couldn't support PbA chemistries (no idea why), but at the very least their software tools seemed far more stable compared to BQstudio.

However, I would still assume you will need support from the manufacturer (either TI or Maxim/ADI) somewhere. And from what I've seen, their user support forums (e2e, engineerzone, etc) do not offer sufficient support for these chips. Just search e2e for BQstudio or BQ34Z100-G1 and see how many issues actually get resolved, vs how many are left unresolved or are taken "offline" (meaning the user had access to more direct support). If you can't get that direct support (usually only an option if your EAU is thousands or more), you face a very difficult path...

If you're working with an Li-ion battery and also want other BMS features (cell balancing, protection, etc), then that might make TI's parts more attractive, as they seem to have a more fleshed-out ecosystem. But AFAIK it all uses BQstudio, which is the main problem IMO.

[lja246]

Thanks,
I think for now i'll leave the BQ34Z100 in there but just not populate it and try to work something else out for SOC. I've tried to ask questions in the E2E forum at TI but I can't because I don't have a company so I can't really get any technical support from them that way.

It's a shame because from the datasheet it looks like a really nice IC. I was only planning on programming the IC from a MCU anyway because to use BQStudio I think you need the EV2400 programmer which is like $400 AUD and i'm guessing if its a pain to get working with their own software and programmer it would be even worse to try and use my own.

I think i'll try to stick with the BQ76942 for the protection because it looks promising but that may also have its own issues with programming.