Lithium Battery Project

[Scrap]

Pretty neat project Nuke. When we built our 2nd pack I worked for a project engineer that was totally anal about thermal shorts. I never did figure out his deal, but you may want to hit it with one of the thermal imagers while figuring out your new to you pack, charging, etc. I do think you should hook up the water loop. It can just be a simple thing but it makes a huge difference in temperature uniformity through your groups. Especially with a Volt pack where you've got those neat little plates between cells, the best TMS in the industry, and really it would be a shame not to use them. But then again, the glycol loop caused their one fire too... Still even if you don't use a chiller, just a small reservoir and one of those 12v coolant pumps will go a long ways to keeping it all uniform. I know you aren't going for any regen braking or a 8C discharge like it can do stock but you know that once you get it all up and going you are going to be eyeing those air conditioners. I'd set the TMS up pretty stout from the get go. The 70-72 F the Volt batts are used to is pretty darn hard to achieve. We haven't matched that on a truck yet. Anyways, I see Ebay has a Volt reservoir and even a real cheap Volt chiller. You need to get that chiller just to get it in front of you to get the gears turning. You'll come up with something to run it.

 

At first read I was going to say you are absolutely crazy to put it outside without using all of the (uber expensive) sealed HV connectors and a HVIL loop. But I keep forgetting that it isn't 390V anymore. So I dunno. I think my opinion would be to do my first pack indoors/basement to eliminate all of the dirt & water variables and work on the water loop to maintain your temps.

[Nuke-E]

Scrap--I had intentions of hooking up the water loop and still might. The heat exchangers I bought for that task are serving as a foot rest under my desk, so actually putting them to use creates a new problem.

 

As far as I know, 70-72F is what's considered ideal, but the cooling system for the battery only keeps it below 104F (40C). There's also an interview with the cooling system engineer floating around where they talk about the heat generation in the battery being very small, and that it's mostly the external environment that drives the heating/cooling system. I'm maybe a little less scared of thermal runaway than some--the chemistry is essentially what we have in our phones, laptops, etc. I've had the experience of using a laptop and phone on a very hot 8-hour bus ride this summer--running calculations on the laptop (100% CPU usage), while surfing the 'net tethered to my phone wirelessly (which makes the phone hot under normal circumstances) with less than ideal air circulation and direct sunlight (glass roof) beating down on everything, with both the phone and laptop charging. I didn't notice it until later, but both batteries had swelled--the laptop battery doubled in thickness in the middle, the phone a little less noticeably, but both continue to work. Based on what the normal battery temperature on the phone is while tethering (~110F in a 70F room), I'm guessing the batteries were well into the 150F range.

 

There's a group actively working to decode all of the communications between the on-board stuff--from the charger, HVDC-12VDC converter, cabin and battery heaters, etc.--with the hope of being able to use what's already there. But for the time being, I'm content to measure cell voltages at the connectors for the battery monitoring boards on top of the battery, and those connectors also let me get at the temp sensors.

 

The DIY EV guys are really putting these things to the test--there's a guy who has 1 4kWh section (i.e. 96V, 84lbs) that he's tested sitting on the passenger side floorboard of a smart. He's seeing good performance at 15C--which figures out to 10 more hp than the smart gets out of its engine, plus it'll do wheelies! And that's without water cooling yet. If they keep it up, I hope to have a "stealth" mode with a motor hooked up to spin the driveshaft. What I'd give to sneak up behind someone with a 45' motorhome and an air horn...

 

I was also able to confirm this evening that the Volt's on-board charger stops charging at 406.1V, which figures out to 50.76V for a 48V pack, or 4.23V per cell module.

[rbertalotto]

Did you see my post on using a WHOLE Chevy Volt as a toad AND a battery pack?

[Scrap]

What I'd give to sneak up behind someone with a 45' motorhome and an air horn...

 

 

I, kinda, mighta, um, well the sergeant of security guards on his Segway can't tell your inverter whine from his sled so he might be a good target......... Be sure to lock your doors when he's got his hand on his taser! (and sneak up from beside him and not behind him...)

 

Voltec class was 3 some years ago so I wouldn't doubt it if I got my numbers jumbled. Setting our spec to ideal and beating ourselves up for not meeting it is par for the course for us too!

[Nuke-E]

Setting our spec to ideal and beating ourselves up for not meeting it is par for the course for us too!

 

Perfect is often the enemy of good enough. In all of the discussions and sales literature about battery balancing, I've seen very little data showing that the balancing systems actually do much balancing. Especially in the context of other people doing the same conversion, it's worth finding out just how necessary it is. Consensus on the Volt packs is that it isn't necessary, and GM even says that variations in the self-powered monitoring system are the leading cause of imbalance. But they obviously kept it in there too.

[Imurphy907]

Just picked up the cells for my own battery project

 

 

25KW worth.

[Imurphy907]

 

Perfect is often the enemy of good enough. In all of the discussions and sales literature about battery balancing, I've seen very little data showing that the balancing systems actually do much balancing. Especially in the context of other people doing the same conversion, it's worth finding out just how necessary it is. Consensus on the Volt packs is that it isn't necessary, and GM even says that variations in the self-powered monitoring system are the leading cause of imbalance. But they obviously kept it in there too.

 

 

From my reading and playing with these things in an industrial setting. Bottom balancing is more important that top balancing like most BMS systems. You are more likely to nuke a cell by running it to dead, than nuking it with a slight overcharge. The reason you need to watch the bottom end of the cell is to prevent discharging past the voltage curves knee. all cells can be at the exact same floating voltage, but will have varied AH capacities. If you have a cell with 40ah less cap then the rest you will end up over discharging that cell to failure. The way around this is a BMS that will monitor and alarm on low cell voltage not low string voltage. use that alarm to drive a contractor to isolate each string if it goes into alarm.