Dream GreenSaver Charger - The GreenBMS

The LM5118 is my leading choice now for a buck boost converter. It's limited to 75V output so no help with your Prius as far as tying into the pack voltage with that chip. Has the Plug In Prius community made a boost converter?

Used Prius packs go for around $800 on ebay, http://hybridinterfaces.ca/assemblies.html looks interesting.

If all the batteries are very low, or if told to do so, it switches to an extend range mode, where the auxiliary battery charges all the batteries at a rate in which it will be depleted at the same all the other batteries are.

Same "time as" rather. I really need to start proof reading. :D

Hi Dave,

You can plug any DC power source up to 75V into the GreenBMS. Your method of charging would work given the battery you're using to store the solar energy stores enough energy to fully charge the scooter. It's also possible plug the solar panels directly into the GreenBMS, and you'll get higher energy efficiency from the solar panels.

Since the scooter comes with a BMS, we'll just have to see how the charger works then and write a customized charging script. It's likely to be beneficial especially for solar charging of the batteries since you won't need to convert DC to AC back to DC to power the charger it came with. If you're just charging from AC, it's unlikely to benefit you much except for the individual cell monitoring.

Yes it's completely programmable. You'll get full control over the hardware.

As for display, what kind do you guys think I should go with? Listed from cheapest to most expensive:

A)Two line monochrome LCD, like the PakTrakr uses.
B)A two color OLED 0.85"x0.44"
C)Palm III.
D)Small color Nokia LCD like on a cell phone 1.2"x1.2",

I went with the color OLED. They are amazing. The epaper is damn cool, and low power, but no backlight and slow refresh. Are you into uC's David?

Guys, I have some crappy news to report, the charger I wanted to build won't be much cheaper than the vector chargers everyone is buying. Big 500W-1000W DC power supplies, the big mosfets, and the rest of the parts, as well as assembly, makes the vector chargers a price I won't be able to beat. By the time we change our batteries, I suspect there will be a lot of other batteries with new chemistries that will be more affordable.

I'm still trying to optimize the circuit. Looking at the way modern RC battery chargers function has inspired me.

More to come... I built the auxiliary battery for testing. It's made from 8 used lithium laptop battery packs I bought off eBay. I would need a minimum of 40 packs to replace all the greensavers in the scooter, and that's assuming all the cells in them are good, which they are not. It's the cheapest solution, but require discharge current limiting, they are the kind of batteries that explode if discharged too fast. A metal box is required also just in case :-)

Some uC's allow you to program them in Java. The Propeller I believe is one of them. If I was to make something easily hackable I'd write it in Java or Basic, but for various reasons (mostly time and money) I'm writing it in C++. I'll heavily comment it so you'll be able to easily reverse engineer it.

I drew out my first user interface on paper. I want it to take advantage of the color screen and use the color for some functional way. I'm also planning on incorporating 3D graphics in it, just to give the GreenBMS a high quality feel. Writing your own charging script is the hardest part to keep simple, too many variables and just five buttons.

It's my first time making a user interface. If it blows, people like you can change it. Don't worry it will have the PakTrakr monitoring and Cycle Analyst speedo by tapping into a hall sensor.

I was just working on the software for the GreenBMS speedometer. I can have it show the usual boring MPH, KPH, RPMs, but just for fun I think I'll do meters/sec, and football fields per hour. Am I being too silly?

I mean it will have the standard plus some non standard units of measure. Just for you I'll put in footlongs per fortnight. I'm also going to put in cubits per hour, I'm gonna target that Vatican City market.

Hi John,

My target is $200 assembled or $150 as a kit with display. It will be modular so if you want just the minimum "keep my batteries safe price" will be even lower. I think I'll offer 500w and a 1000w versions. I want people to be able to use their string charger as a supply to keep price even lower. I've not finalized the specs yet, the biggest cost with these is the step down transformer. The more powerful the more expensive. Wireless will be longer range and cheaper then bluetooth, also zigbee is longer range and easier to interface on my end. Sparkfun has great ideas, but awful markup. If you want to buy their serial bluetooth adapter for $60+shipping it's doable, but you just raised the price of your GreenBMS an extra $55. My display will be a GORGEOUS large color oLED. I placed a large bulk order from these to get them at more then half retail price. If my BMS doesn't sell, I can sell them on online with my custom interface and recover my loss.

I want this unit to be professional and full featured while balancing price. I won't sacrifice quality. Period. That's why it's modular, you can get what you need and pay for nothing you don't want. It will outshine the competition by combining all the features people want in just one system, while offering redundancy, open source hardware and firmware and design with an eye for detail. I'll prioritize the development so that you guys with the XM-3500 can protect your batteries ASAP, then I'll ship the accessories when they are made.

Since my first post a month ago, I've been spending all my free time catching up on all the latest charging methods, state of charge measuring methods, various battery chemistry's, as well as looking at what the other chargers are capable of. I want it expandable so it can be used on any size pack with minimal upgrade cost, or even on solar and EV markets.

I hope to have a prototype ready this week, I'll test for 3 more weeks and if it goes well I'll have the circuit boards made and shipped. One month from now is the earliest for the main system. Two months is the latest.

Hi John,

Yes, it's modular in the number of cells also, that's a major design requirement so that it doesn't become obsolete. I'm expecting it to cost about $2-3 dollars a cell. It has two levels of redundancy to protect your investment, without costing more than your investment. The other inexpensive shunt based BMS don't have that redundancy.

Tell me how you'd like it configured for the xm-3500. Monitor each cell individually, or each group of four?

Hi John. Your wishes are my command. Each cell will get tender care. I will also use an SD card instead of wireless, Anyone else want to give me a wishlist?

There will be a PIC on each cell, they'll communicate with each other over a single communications bus to keep the wire nest to a minimum. Stay tuned everyone, a prototype working on lithium and lead acid cells should be on display by the week's end.

-=Jeff=-

Is there a way to measure the resistance of batteries with a standard DMM?

There is, it's called the DC load test, where you measure the voltage sag which correlates to a known load. or AC Method (No, not for finding roots of a polynomial :D) where you watch a phase shift between voltage and current.. I realize this is too much work for everyone. I've only had one person give me voltage readings even. So the GreenBMS will have to be able to take these measurements and learn while in operation.

Could it be adjusted for use with the 102 cell NiMH battery in my Vectux?

Yes, it will work on NiMH, it has all the necessary hardware. The only negative is I am writing the script to this one last. From what I've read it's more complicated than lithium and lead acid because it gives false voltage peaks. Each BMS has an optional temp sensor input, you can use that with good results but there are even more complicated ways that pulse the charger and apply some algorithms have even better results. There may be even better ideas out there, I'll have to experiment if no one else will.

This is open source, for this very reason everyone can expand on the functionality. If you're able/willing to write in C++, this can be your nearly immediate solution.

Which battery does it use? I'll design the BMS so it can screw onto the top of your battery if possible.

Hi HCT,

Glad to have made your day and to have your contribution to the design. My plan was to use the DC method because it's more simple to monitor current draw. When you're driving or charging the scooter the current and/or voltage will in the process very and this will give points to use in calculating resistance. I put a scope on a battery and operated the scooter, lots of noise. It will wreak havoc on the ADC. Filters are inevitable for voltage and current measurements.

What do you think?