Sunday, February 22, 2009

Maximizing battery performance

To get the absolute most performance out of an Optima lead-acid battery pack, the batteries should be balanced. In the original plan, batteries were arranged in series. They were charged with a single 4A 48V charger. The charger has a regulator that shuts off the charger when the overall pack has sufficient voltage across it. However, as batteries age is possible that some batteries may be lacking charge, while other batteries may be overcharged. If the imbalance becomes too great, it will lead to a shorter pack life or less than optimum performance.

Fortunately, someone else has thought of this problem and has made some electronic modules which dump (or take) excess charge from a neighboring battery. Ordering the parts from EVSource took a while since they apparently had a backlog of orders. I've since learned that Harvey Coachworks also carries these modules. I can't recommend these modules enough. I have operated my pack without these modules and it doesn't take too long before each battery voltage starts to vary by quite a bit. I then need to manually charge some batteries more than others and it's basically a big headache. With these modules installed, charge averaging is all taken care of. I'm convinced my pack would not have lasted as long as it has if I hadn't had these.

Anyway, the battery terminals were bought from Autozone, a local car parts chain in my area. Spades and terminals were mixed since I didn't have enough terminals for all the wires. The color of the wire is important. Each module MUST be connected with Yellow wire (+), then purple (-) on other battery, then white (com) on the common terminal between the batteries. Failing to do this will make the module think there is a massive imbalance in the battery arrangement and it may burn out. So, for wiring reliability, the connectors should be soldered, not just crimped on so there is less possibility of having a wire detach and cause a problem. Also, when connecting the (-) there will usually be a visible spark. Also, it's VERY important to connect and disconnect wires in order according to the instructions since it's possible to damage the modules.

The battery terminals also need some preparation. The nuts holding the bolts can fall out when there was no bolt present. Since nut dropping is not something that should be happening when attaching touchy expensive electronic modules, something needs to be done. One solution is to melt some solder to the underside of each terminal so the nuts stay on even when bolts are out. As you can see, attaching solder to the terminals doesn't look pretty, but it works. It also is something that cannot be seen by the casual observer since this side of the terminal connector faces down.

After connecting all the powercheq modules, they need to be mounted somewhere so they can't rattle around in the battery compartment. So, I made a small mounting tray made out of wood covered in duct tape so the modules can be zip tied to it. The wood could be painted instead. However, covering with duct tape is faster than waiting for paint to dry. To hold the tray, two flat head screws pass through and screw into a battery terminal. The clamp is then attached to an unused battery terminal. The little red terminal cap on the bottom of the tray is there so you can see the battery terminal better.

My first mounting attempt going across looked OK. The powercheq modules blinking on and off when equalizing gave a high-tech look to the battery pack. However, I could no longer stow the stick I use to prop up the fender during maintenance! So, I rotated the tray 90 degrees. To do this, I needed to extend the length of some powercheq wires so they would reach the battery terminals.

3 powercheq modules
6 battery terminals
2 flat head screws
3 zip ties
9 terminals that fit 1/4" battery terminal end screws (avoid using spade connectors since they pull out)
Wood for shelf
Duct tape (or paint) for shelf.
12Ga wire (to extend powercheq wires when tray is mounted sideways).

Jigsaw (to cut wood)
Drill with 1/4" for ziptie holes and countersink bit (for flathead screw holes)
Mini ratchet w/ 2 sockets for securing bolts on battery terminals.
Soldering station (solder pen, helping hands, solder, wire stripper, heat shrink tubing, ect...)

Around 3 hours, to think of and construct solution, not including blog time.


I have run my Optima batteries with and without battery management modules. I definitely think the BMS system was a good purchase. It saved me the trouble of manually rebalancing the cells in the pack. Plus, I later found it also had the benefit of allowing me to use a single inexpensive 12V charger rather than a more expensive 48V charger. To do this, I just attached a cigarette lighter port to one battery. Then, I charge the battery through that port! The BMS then balances out the charge equally with each battery. Charging this way isn't as fast as doing it in series. However, 12V chargers are rather inexpensive, widely available and light enough to just throw in the cargo area just in case field charging is needed.

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