Page last updated: June 13, 2009
Batteries
More info on these Alpha Cells can be found here.
I'm starting with whatever I can get to get a functional vehicle. So that means mixing types. Since I'm building my own monitoring system. I think I can get away with mixing. But primarily, I'll be using these donated deep cycle SLA batteries.
I had to buy additional batteries, they are magnum flooded deep cycles, and I managed to get 4. I need 1 more to get full power, but I'm still experimenting anyways. The batteries were reconditioned and therefore cheaper.
To maximize my range, I was thinking of using 2 strings of 12V batteries up to the maximum weight capacity of the truck. This would also reduce the current draw on each battery by half of normal! The battery packs would be isolated by water cooled high current diodes. I think this is affordable because of what I can see at digikey. It may be best to get a few 50A diodes and put them in parallel. The system is fused for 400Amps, so if there were 2 strings of batteries, then I would only need diodes to max out at 200A.
Update: April 4, 2009
I found a very rare deal for some batteries! AGM batteries are known to be better than regular wet cell lead acid in every way except for costs. Well I scored 12 big ones for 60$ each (normally, they would be around $300) They are slightly used (not an exaggeration) hence the price. They came from a UPS/Generator system from a casino, so they are to bridge the gap from if/when power cuts out and the generator kicks in. Keeping the power on is so important to casinos, they'll use only half the life of the batteries and then replace them with new ones even though the warrent is not used up yet, (or so I'm told). Here is the datasheet for the battery showing that they are AGM type VRLA.
Since getting the truck weighed, there is no more room for additional batteries. So the whole parralel idea is scrapped! (I dont mind, it means less work)
Chargers (this has been redone)
(updated June 13 2009)
Ok, so I'll keep the old chargers listed here for reference. I decided that they were too heavy and upgrading them all to a switching power supply instead of transformers would have been too tedious/expensive. After much deliberation with myself, I decided to go with a single charger after all. I was originally against this because of the pack falling out of balance, however I also built my own balancers to go with it.
For starters, I went with the cheapest charger I could find, and there is only 1 that is below 1000$ (kinda smart considering my pack was only 720$) and that is the quick charge select-a-charge. 144V 10A version. This charger is very slow, but another word for slow is gentle! I bought the charger from charging chargers. And had it shipped to US address. I had to pay 72.90 in tax when I came back to Canada, however I havn't paid tax on my controler or most other things I bought in the states so I didn't mind. It is still cheaper than anything I could find here.
As for the balancers, I got the idea a long time ago when I was first researching DIY EVs. Metric Mind had something similar for his lithium ion batteries. I redesigned his to use high power mosfets. To explain what they do here:
No matter how fancy your single charger is, you may wreck your batteries because they never all finish their charge at the same time. Some charge faster than others, so to bring the pack to its nominal voltage, some batteries get over charged to make up for other batteries who my be lagging. This module "clamps" the voltage at the rated maximum for the batteries. In this case; 14.7V. In fancy-speak, it is a shunt that automatically bypasses the current going through the batteries to bring other batteries up to their proper voltage.
Building my own was pretty cheap, but I only recently found out that someone else did exactly the same thing as me and is selling the parts. He can't me making much money from the kits though, because mine worked out to be similar in price. Though I think mine are better because I can shunt more current. (I had a resistor too at first, but then took them out).
On the far right is the prototype balancers and their schematic. The other pictures are my nice tidy battery boxes and new charger! (with balancers hanging there... they are screwed down now)
Balancers
The whole reason I started with multiple chargers is because of the concept of keeping the batteries balanced. But after switching to a single charger, this because am issue, especially with sealed batteries which are sensitive to over charging.
From my research prior to starting this project, I came across the web site for metric mind, and one of his conversions. Specifically this page.
Since I've adapted these balancers to work with 12V lead acid batteries, a commercial version has popped up. (coincidence, I assure you). Since I'm constantly having issues with my balancers, (they are over heating because of my crappy heat sinks and under rated power resistors), I'd definitely buy actually balancers next time.
------------------------------the chargers mentioned below are for sale!---------------------------------
After, and thinking of what it would take to build my own charger, and searching for availability of cheap chargers, I decided to go with this charger:
From the beginning, I wanted to get a series of chargers to charge each battery separately. I know most people get one big charger, but I weight out the advantages over the disadvantages.
- A large charger is quicker, more convenient, easy to install, and CAN be cheaper, but usually isn't.
On the other hand, it cannot charge unbalanced batteries, or mixed batteries. One bad cell can bring down the whole pack. - Separate chargers are a pain in the but to wire together, generally charge slower, but are cheaper as a result of their slow charge. They also allow each battery to charge at its own pace. So over charging or under charging is never an issue.
It is important to note that you need a good battery monitoring system while driving/discharging the batteries when you use a non-balanced pack. See below for what I'll be using.
I had to modify these cheapo constant current chargers so that they would be more friendly to the SLA batteries that I got for free. It took me about a month of reverse engineering and trial&error trying to get these things to output a constant voltage. It may still be possible, but I have limited resources, so I decided to settle on pulsing the battery at 2Amps and shutting off when the voltage hits 13.8V (turns back on when it drops to about 13.2V)
Gel cell batteries are sensitive to over voltage, and these chargers will go up to 14-15 volts before they shut off.
The modification that limits the voltage is actually quite simple, it is only a PNP transistor, a variable resistor, and an input resistor.
I wanted to be fancy and have it charge at maximum current for the first leg of charging, and then automatically switch to 2A when it first hits around 13.8V. I added a relay to "flip the switch" on the charger.
Here are the rough schematics I made for my own notes while working on these chargers.
The third one here is the 2 circuits for my modifications. The upper one limits the chargers voltage, and the lower one switches the current. There is a regulator there because I accidentally bought the wrong relays and couldn't return them. (I also bought regulators... long story, I won't get into it).
Below, I have 5 mod boards like this first one, and 2 more like the second one (correct relay and no regulator) adding up to 7 modified chargers. (for 7 SLA batteries)
For measuring current draw, most people use a shunt. I found a way that is generally the same price or cheaper because I can use just about any volt meter. A solid state HAL effect transducer. datasheet
I found one that will work from 12-15V (perfect for automotive voltage range). It is also completely isolated from the high voltage line. I also chose this one because I am pretty sure that 2/0AWG cable will fit through the hold there.
I got mine from digikey, they only had the dual-power supply version available. I though since I'm measuring current in only one direction, it might work with only 1 power supply. WRONG. That's why LEM makes a single supply version. (which I will try and get next time). As a work-around, I got an additional chip to convert +12V to -12V (free sample from TI via a friend). I've tested this on the old starter-motor and a 12V battery. After the current spike, I measure 50Amps! I wasn't sure if that was correct, but the solenoid is an old design, so I guess it is feasible.
This is being hooked up to a 3V analog meter I got from Sayal for a few dollars. ( added a resistor and will re-print the label when I am comfortable that I have a suitable range, right now it measures 300Amps (represented by 6Volts)
I chose an analog meter because its cheap, and I can see current spikes that wouldn't show up on a digital.
Pictured here is my test setup. The meter will go to the dash board, and the blue thingy will go somewhere along the welding cable.
If I have any problems with the analog meter, I can swap out for an actual multimeter, (I have a digital and analogue).
Here is what I've installed in the engine compartment. After I got the circuit working, I sprayed it with rubberized undercoating and enclosed it in some plastic.
See video below (with temperature sensor) for more info
Temperature sensor This video covers the temp sensor, but also covers lots of other stuff.
As I mentioned in the Cost/Time section, I got this idea from other projects. I thought I would be able to get the display to be even smaller than forkenswift's, but I ended up soldering 100 LEDs for 10 rows to a proto-pcb. (did this while on a road trip for my job) Next time, I'll bite the bullet and do the more expensive option of actual led bars.
A work in progress, but I will be able to monitor up to 10 Batteries on a single board, I have 12 LM3914 chips from Digikey.
I have a simplified schematic that I found and modified. It uses less components than other user's versions.
R3 was a potentiometer that controlled the lower limit of the LEDs, but I found that by coincidence, 47K gave me the bottom level of 10.5-10.6V. Yes this is very low for Lead Acid batteries, but if you look at the image at the top of this page, the batteries are rated to go down to 1.75VPC (that's Volts Per Cell). Regardless, this is just the measuring range, I don't want to get down that low anyways (about 80% discharge is good).
The upper range is controlled by R2, I picked 12.7 because it was a nice average of where my existing batteries are at with a full charge. Some of them go as high as 13V, but with my testing, anything higher than 12.7 does not harm the circuit. (the top LED simply stays on).
I found it too difficult to use the LED board shown above, so I had to start over. That sucked, but now it is done.
I really wish I spent the extra money and used actual bar graph LED displays.
Video comming soon.
Each monitor will also be attached in series; meaning that the ground of one will be +12V for the next. This means that only 1 wire will be needed in-between each battery. (10 batteries would need 11 wires) each wire will have a 0.6A fast-blow fuse with an in-line fuse holder. The red box on the board you see in the photo above is a DIP switch, this will disable each meter individually.