A123 Quick-Change Battery System Build

[JeremyBP]

Hi all,
This is my first post here, so please pardon me if I'm putting this in the wrong place.

I build tiny electric vehicles (go-kart like things, pocket bikes, scooters, etc). One of the events I regularly participate in features an endurance race, during which I have to change my batteries several times. So I decided I wanted a quick-change battery system. This thread will chronicle my attempt to build one. Please, if you have any thoughts or feedback, jump right in. I'm not exactly an expert at this.



I'm starting with these. They're surplus A123 prototype modules that were once used for starting jet engines. They're 26.4v nominal, with a built-in BMS and all the associated niceties. Each one of my battery modules is going to have two of these in parallel.



I made some copper bus bars to attach them. They might be excessive, but if this whole thing works out, I'd like to keep using them for various projects in the future, and I want them to last. So big copper bus bars it is.



And then I put the whole thing in a box. This is my module. I want to cut a keyway in one side, and key the slot it will slide into so it's impossible to put it in backwards.

What I envision for the receiver is a box with terminals at one end that I can just slide a battery module into and latch it in place. Then I want to change batteries, I can just slide out the old one, and slide in the new one. Which brings me to my question. What do I want to do for terminals in the receiver? I was thinking more copper bars (with beefy wires coming off the ends) mounted on springs, but I'm not sure. I'd love to get some ideas and feedback from you guys.

Cheers,

Jeremy

 

[docnjoj]

Those are cool. What are the amp/hours of those batts and what type of A123 cells do they use?
otherDoc

 

[JeremyBP]

Each of the finished packs is 26.4v nominal, at 5Ah. They use some kind of 26650 cell that I haven't seen before. I don't have any modules to spare or I'd take one apart, but they aren't the usual M1 cells. I'll see if I can get a part number without completely dismantling the module.

 

[docnjoj]

Thanks JeremyBP. What is the rate of amp draw that you intend to use? 10C would be 100 amp draw.
otherdoc

 

[JeremyBP]

The car I have them in now has a peak amp draw (full throttle from a stop) of around 90A, but that really quickly drops off to ~6A to maintain speed.

 

[999zip999]

How much are these moduels and are they available. Nice setup and good cell base. So that's two 8s blocks and a 9 pin access form the bms ? Do you have the female plug for monitoring ? With a 8s cellog's or ect.. What charger do you have ?

 

[JeremyBP]

The modules are surplus, so I don't know about availability. I have no idea what the pinout for the data port is.

I just charge them off a CCCV power supply.

 

[999zip999]

You charge them connected as a 48v or 16s block and the BMS's are hooked how ?

 

[JeremyBP]

The modules are 8s, and I charge them as 8s. I just plug in a CCCV supply at 28.8v and let it rip. The supply I have now can only charge at 10A, but I'm in the process of building a higher capacity one. In other words, the batteries will drink 10A until they reach 28.8v, then stay at that voltage and the current will drop off. The BMS takes care of all the balancing. They're built in to the modules. I'd be more specific about how they work and what they do if I knew. Sorry.

 

[JeremyBP]

So I've been thinking about how to connect to the bus bars. I'd like to make some kind of holder much like what 9v batteries fit into. 9v batteries use springs to press against the contacts. I can't really do that, as springs aren't exactly renowned for their current carrying capacity. So I was thinking about having another set of copper bars (with wires running off the ends) mounted on springs that would press against the bus bars on my packs. That way there's good contact and good current capacity. What do you guys think? Is this a bad idea? What would you suggest?