Aug 2011
by larry

My DIY SLA Trip Batteries

Test drive to Sheldrake Point

Test drive to Sheldrake Point

Caution: shop talk blog post intended for do-it-yourselfers. For my recent 240-mile journey I created what I call my “trip batteries”—batteries that I can attach to my bike to augment my regular batteries, but that I don’t intend to carry around on a daily basis. As such, the main design criteria for these batteries is that they be inexpensive. I don’t want to pay the big bucks for a battery that I only use once in a while. The obvious choice is SLA (sealed lead acid) batteries. These are the same kind of batteries used in cars, and the technology is almost 100 years old. E-bikers out there may poo-poo this choice of battery. After all, compared to my lithium batteries, my SLA batteries are heavy (20lbs vs. the lithium’s 15lbs), not quite as powerful (600wh vs. the lithium’s 720wh), don’t last as long (300 charge cycles vs. the lithium’s 1,500) and they are dumb (that is, they don’t have a battery management circuit board in them to prevent human error from damaging them, although most controllers provide the necessary protections). But they are cheap. I can put together a 10ah 36v battery for about $120 versus a 10ah 36v battery for $600.

Furthermore, there are many reasons to have some SLA batteries around. One is that their native voltage is 12v. I’ve created custom connectors for my batteries so that they operate at 36v when they are on my bike, but I charge them at 12v (see the images below). I find that 12v chargers are much more reliable than chargers made to output other voltages. I’ve had several 36v and 48v chargers self-destruct.

I can also power 12v appliances. 12 is a magic number in the appliance world. The boating, camping, and RV industries produce all kinds of 12v appliances. I have some small solar panels and those too output 12v. And I purchased an 800w inverter ($80) to power 110v household appliances. I recently used it to power my electric weed-wacker when I was at too great a distance for a power cord to reach the weeds. I do have a 12v converter for my lithium batteries, but it can only output about 240w.

How did I make the batteries? My bike operates at 72v, so I made two 36v 10ah batteries that I connect in series when they are on my bike, one battery on each side. Each 36v battery is made up of three 12v SLA batteries in series. As I mentioned, I can quickly convert the 36v 10ah battery to a 12v 30ah battery by switching from a series connector to a parallel connector. I used scooter batteries since I figure they are designed for a similar application. I connect the batteries with 10awg wire. Thick wire is essential since these puppies will be outputting plenty of juice. The wire has spade connectors on the battery side and Anderson connectors on the output side—Anderson connectors are an awesome tool for the hobbyist. They are the Lego of the connector world. I should probably put a fuse in my battery pack. I then wrap up my pack with a layer of duct tape.

I considered making a special battery box but I decided that the batteries are waterproof enough, and they are so ugly they are probably theft-proof too. So I simply strap the batteries to the “footsies” on my Xtracycle bike. Footsies are wooden platforms that my daughter rests her feet on when she is riding with me. This spring I did a test drive out to Sheldrake Point on Lake Cayuga, some 25 miles from my house. The trip batteries performed admirably and took me almost the full 25 miles at 20mph, drawing 600wh in the process. (Note that I probably could have gone 50 miles at 12mph.) Then I switched to my lithium batteries for the ride home. Yes to switch batteries I have to actually stop, get off my bike, physically disconnect the spent batteries and connect the fresh ones. Someday maybe I’ll connect my SLA and lithium batteries in parallel, but I understand it’s important to put some electronics between batteries using different chemistries.

Let me know how it goes making your own SLA battery packs!

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