The T.I. bq78pl116 and bq78pl116EVM

[sparc5]

Thanks John,

That was very informative. What would happen if the board resets when I'm riding and using the BMS as TI intended?

 

[CamLight]

IIRC, the MOSFETs are turned off. But it's been a while since I forced the chip into reset so I don't remember for sure.

They would turn back on after the cell voltages rose back up though. The chip would reset, check voltages against the firmware settings, and then turn the MOSFETs back on. IIRC, some lifetime voltage/current/power measurements were reset to zero when this happens though. But, this was on the '114 and I don't know if it happens on the '116 yet.

 

[CamLight]

For anyone considering using the '114 or '116, I very, very strongly recommend reading ALL of the documents listed on the '114 and '116 product pages. There are a lot of things to consider and do before you can use either chip.

For example, you need to run a 12+ hour test on any pack you're considering using in order to create a special data file that tells the chip how your pack behaves when charging and when under load. Without this file, the chip cannot even begin to track the state-of-charge of your pack (or perform any calculations based on the SOC).

All the safety settings still work (with disabling some alarms to prevent pack turn-off), but that's bypassing a lot of what makes this chip so incredible.

 

[sparc5]

What about using a SCR instead of mosfets for discharging? They are easier to cool and will stay on even after the gate power turns off as long as the current is drain is sufficient. That might help out if the chip ever does reset. A separate board with MOSFETs means your current loop will be large and you'll get ringing with the MOSFETs.

 

[CamLight]

What about using a SCR instead of mosfets for discharging? They are easier to cool and will stay on even after the gate power turns off as long as the current is drain is sufficient. That might help out if the chip ever does reset. A separate board with MOSFETs means your current loop will be large and you'll get ringing with the MOSFETs.

I think, IIRC, the voltage drop across a SCR is a lot higher than a MOSFET.

And once the SCR turns on, there's nothing that can turn it off except waiting for the current flow to stop some other way (0% throttle?). If that's the case, I say skip the SCR since it's not really being used for anything. Also, back-to-back FETs allow for bidirectional current flow so you can have independent charge and discharge flow control in one wire. If using SCR's, you'd need to use separate charge and discharge wires (or use them in just the charge wire, but then it never turns off). SCR's are really best for use with AC current where you have an opportunity (typically) 50-60 times a second to turn them off.

IMHO, MOSFETS are the best choice here. With careful planning, you can keep the inductance of the gate wiring to the FET to a minimum. I have a 3" gate lead run without problems. The FETs can turn off in microseconds, even milliseconds, and still protect the MOSFETs from a short circuit so the speed of the driver can be fairly slow (compared to motor FET drivers). And even if the gate does ring a bit, that's OK as long as max Vgs isn't exceeded (and you can protect against that) or the FETs bounces on/off.

See the Design Guidelines for a description of the FET drive circuit. You can balance the drive's turn-off speed against its quiescent current draw as needed.

If you're set on using the EVM, it's best to get one, fire up the scope, and just start playing! It will be easy to see when the number of FETs you're using, or their wiring/arrangement, start to be a problem.

 

[perbear]

I have made a 12S BMS based on the bq78PL114 chip. It is especially made for ebikes and works fine with standard RC LiPos using standard 2.5 mm c/c balancing connectors. A description of the BMS can be found on http://groups.google.com/group/nohassel. The balancing current is less than 100mA but it balances continously so this is no problem as long as the cells are identical in the beginning.

I am currently making a revised board using bq78PL116. It will support up to 16S. The initial design target is 50A continously and up to 100A peak with the enclosed heatsink for both charge and discharge currents. The 16S board will be ready this fall.

If enough people want it I will offer the 12S as bare PCB. I would like to order 50-100 pcs, that would bring the cost down to less than $10 for each board, that should be acceptable for a high quality four layer board with dual sided soldermask and silkscreen, plus electric test. I paid $300 for the two first PCBs

Two friends are currently developing a BMS user interface/front end for both BMS boards. One version uses the USB dongle that TI sells and this SW runs on a PC and is used for setting up the BMS. The other piece of SW uses a small microcontroller board (Atmel USBKEY will work) that fits on the bike. It has a bluetooth module so that a mobile phone can be used for BMS communication

 

[auraslip]

I'll take two of the 16s versions... maybe one of the 12s

edit: btw - we can't view your site without membership

 

[sparc5]

Awsome perbear. That 16S sounds like what I am trying to make now. I am playing around with the eval board, doesn't play nicely with my Headway 10AH cells since they often drop below 2.5V, but it will be useful once I figure out how to get this thing to charge. Currently I connect the charger (5A common CC/CV Chinese model) and the fuse light on the board goes on- unless I connect the charger with the switch turned on, but then the board disconnects from the computer. There is probably some setting I'm not seeing, this is strange.

 

[CamLight]

Currently I connect the charger (5A common CC/CV Chinese model) and the fuse light on the board goes on- unless I connect the charger with the switch turned on, but then the board disconnects from the computer. There is probably some setting I'm not seeing, this is strange.

I was wondering if there's a big inrush of current when the charger is turned on and you're hitting a Secondary Overcurrent limit but those limits start at 2 seconds (and go longer).

There aren't a lot of things that can trigger the SPROT output. Perhaps there's a big burst of EMI when the charger is turned on and the chip freaks out, thinking that there's a LAN failure, temp sensor failure, etc.?

 

[perbear]

I'll take two of the 16s versions... maybe one of the 12s

edit: btw - we can't view your site without membership

I am unhappy with Google groups so I will make my own homepages. The web hotel is up and running, only need to transfer contents from Google groups to the new site. Will try to do this real soon now. The new site is http://www.nohassel.org.

Cheers,
Per

 

[auraslip]

I am unhappy with Google groups so I will make my own homepages. The web hotel is up and running, only need to transfer contents from Google groups to the new site. Will try to do this real soon now. The new site is http://www.nohassel.org.

Cheers,
Per

Google sites is free, incredibly easy to use, and allows custom domain names. I made the site in my sig with it in an a few hours. It probably has tools to allow you to import some of your google groups stuff as well!