Over the years I've used several types of batteries. My first bikes had huge LiFePO4 bricks with A123 type cells, which could take a lot of my beginner's abuse but were very heavy and actually needed a cargo bike to carry them. I liked them a lot because it's a safe chemistry- there were too many reports of people burning their houses down with the other main choice, which was hobbyist model airplane radio control (RC) packs. I eventually moved on to Lithium NMC 18650 cells as they became affordable and Battery Management Systems (BMS) became better, because they were 2/3's the size and weight. I've stayed with 48 Volts, because I think it gives the best performance for the smallest wire and component size, while staying below most safety standards for high voltage and not needing more expensive components.
I prefer to use as large a battery as possible for 3 reasons:
I figure about 15 Wh per mile, (depending on the rider it can range from 10 to 20 Wh/m), so on average 840 Wh is good for 56 miles which covers most situations. I've told everyone I've built bikes for that they still have to pedal.
2. surge capability for climbing hills
Larger batteries won't have as large a voltage sag under load, and my riding is one long hill after another.
Lithium cells don't like to be fully charged or fully discharged, and they last longer if kept between 20% to 80% full. The first two batteries I owned lasted for only a couple of years, because I used to put them on the charger right after I got home to find out how many Watt hours I'd used on the ride. But after installing a meter on the bikes (Grintech Cycle Analyst) I stopped the frequent charging and took more rides between charges, and the batteries lasted twice as long. Then I started trying to charge the batteries only right before a big ride, and the last battery was a few months into it's 7th year before it did not have power anymore. Interestingly Stanford University researchers recently found that by charging a cell and then immediately briefly discharging it they could move plated out lithium back towards the cell electrodes, in effect rejuvenating the cell. I think charging just before a big ride has a similar effect and may help explain the 7 year life span Clean Technica wrote a summary of the research here:
To take more rides between charges, you'll need a larger battery.
I've found the down tube packs to have the lightest cases and the best bike balance, I also like the smooth nonconductive plastic with rounded corners so that I don't hurt myself as much. After I'd installed half a dozen different styles of down tube cases I settled on the Reention DP-6 case as my standard choice. It will fit 65 cells in a 13 serial by 5 parallel pattern (13s5p), which if assembled with 3500 mAh cells adds up to 840 Watt hours (48V x 17.5Ah). This is as good as I've been able to fit in the front triangle of most bikes I've converted. (Reention does make a DP-9 case but it's usually a bit too big and if more Wh are truly needed, I end up using a 1 kWh (13s6p, 48v x 21Ah) rear rack battery.) I often have a DP-6 on the shelf because I'm converting a bike. Having a standard battery came in handy for the Upper Valley Ebike Lending Library last summer when they dropped a battery. I was able to substitute one of mine while I glued the broken case back together. (PVC pipe plumbing glue from the hardware store works well.) Because of this background, the Stratus got a Reention DP-6, 48 Volt, 17.5 Ah, 840 Wh battery without a second thought.
An empty Reention DP-6 case ready to be stuffed with cells. When purchasing an already built battery, I look for one with name brand cells and a BMS that has at least a few certifications. I prefer this case because the base has these features:
A. it has 4 retention tabs down each side instead of 2
B. a full length aluminum spine instead of 1/2 way
C. 5 large spade terminals that can be doubled up
D. no large cavity for a controller that I don't need
For comparison most of the Hailong cases available have only 2 retaining tabs per side plus the empty cavity for a controller with a shorter spine, and use bullet terminals that can fill with water and dirt when the bike is transported with the battery removed.
However the DP-6 case could still be improved by adding these 3 features:
1. an IP-65 rating (to match the BBS02) and a more waterproof switch. These cases are OK in the rain, (I've ridden in downpours without a problem), but they can't take a garden hose pointed directly at the switch and connectors. In many ways they remind me of the electrical switches on the motorcycles of my childhood, which had bare switch contacts screwed inside a plastic shell on the handlebars that were very exposed to the water coming in. There were millions of these switches made for decades and they worked, but we can do better now.
2. a Vee channel molded up the center under the base to fit on down tubes more securely. Numerous companies make water bottle and accessory mounting rails with this groove, and Grintech's Bottle Bobs also have this groove, here's an example:
3. an XT60 socket instead of wires. Here is an example that I made for a mountain ebike conversion:
The terminal base of the DP-6 fitted with an XT60 plug. Normally the power wires would exit here.
The outer 2 spade terminals are paralled together for each pole. The short wiring is almost stiff enough to hold the XT60 in place by itself, but a RC connector hold down strap was also added. Note: live power receptacles should have recessed female sockets, similar to household wall receptacles. Using exposed male terminals that could easily be shorted is not good practice.
BBS02 motors come with a 20" lead, the battery cases have a 12" lead, and usually there is a 10" long adapter cable for a total power cable length of 42 inches. This is a bit long since the distance from battery to BBS02 is around 9" on most bikes. I cut a lot of this out and leave only one pair of connectors. However the Stratus motor is a foot forward of the battery so I kept the wires and hid an XT60 connector under the battery base instead of fitting an XT60 terminal into the base.
Connectors I have loved or left
(First loves on left, current connections on right)
AC-The first battery connectors I used were AC power cord plugs such as on a computer printer- big and heavy, just like the LiFePO4 batteries they came on.
APP-I used to use Anderson 45 amp Power Pole connectors, but found that the jiggling wires twisted the contacts apart slightly during riding, and then the contacts started sparking and pitting. This would melt the housing, and then all hope of getting home was lost. (Although hard to see, in this picture the red housings are melted, and the contact is pitted.) After the second meltdown, I moved on.
EC5 (blue)- I liked these, they're tough and stayed together, but they are completely open to water
HST (red)- a little smaller than the EC5, but the spring contact collar squished I was while playing with them on the bench, so I never put them on a bike. These are the same size as banana plugs though, so I'd consider using them for a bench testing setup.
BF- These are Bafang bullet terminals, which are not the same diameter as common 1/8" bullet terminals. I liked these a lot because they have most excellent silicone rubber boots. But they have a very weak neck between the wire crimp and the socket, and after breaking 3 of them off I now only use them for the connection inside the BBS02 between the controller and the windings.
WP- Delphi Packard Weatherpack automotive connectors (similar to AMP/Tyco/Molex) did not rate a trial, they are either too low in amps for a suitable size (20A on left), or too big for a usable amp rating (46A on right) due to their stamped steel terminals.
XT90- The early motor controllers I used had large capacitors in the input circuit, and there would be a very bad spark when plugging the battery in. The XT90S version has a resistor that moderates this spark and was a step up for my earlier bikes. But they are big and heavy, and my batteries have switches now. Normally this connector would have a solid yellow exterior casing but I sat down one night with a hacksaw and an Xacto knife and cut a connector in half to see the resistor, shown in the photo. .
XT60- My current fave. There are several versions shown in this photo. Machined brass terminals give a 60A rating in the smallest package, (my rides consistently have 12 to 18 amp climbs, with some momentary spikes to 30A, so this is a good safety margin). Because the standard yellow version on the left has a hooded design it can be reasonably water resistant if you put heatshrink and silicon caulk around the outside wires, and then smear the terminals and inside hood with silicone grease before plugging them together. I'm using the gray version on the right which has snap on outside covers. I still put heat shrink tubing over the wire solder joints to provide some bending strain relief for the wires, then I fill the area with silicone caulk and snap the cover on. After drying it's a pretty robust setup. I've tried to improve this by putting a boot over the connector, however the available boots (in blue) are too short. A substitute is a piece of heat shrink that is an inch longer than the connector with some caulk sealing the stationary end. (I place a wood shim in the tubing while shrinking it so that afterwards when I remove the shim there will be some slack.) However I think the best protection so far is putting the XT60 inside the battery base. The black aluminum U shaped RC model hold down strap for the connector is shown near the top. Note: the yellow bulkhead connector at the bottom with exposed male terminals is OK for an appliance that draws power, but shouldn't be used on a battery that is a power source.
All of the large forces on a battery case act to press the case and the downtube together, and the 2 small bottle mount screws are sufficient for most riding. On standard diamond frame bikes I prefer to install a third screw for additional sideways strength when the bike is ridden with the frame tilted. (The Vee groove battery base improvement would help here.) I use an M5 popnut that matches existing bottle mount hardware.
A popnut gun with M5 inserts. A bolt with nuts and oiled washers (center) can also be used to compress the popnut with some effort because it will try to unscrew while compressing
An easy way to find the center of the downtube is to clamp a stick on each side, and then put a mark on a piece of masking tape halfway between the sticks.
The Stratus didn't have any bottle mounts on the tube for the battery, and I decided not to drill holes in the frame. I switched to tee nuts held on by hose clamps instead.
A metal hole punch is used on the hose clamps. The 3 sizes of tee nuts I use are at the bottom, along with a piece of heat shrink tubing for covering the clamp. The tee nut in the red circle has been bent to fit the frame tubing.
Furniture Tee nuts come in many styles, I use the type with 3 small holes. (Metric tee nuts are not available here and I've been using M5 Weld nuts instead.) The 8-32 size will slip into most battery base mounting slots but it's a bit light duty. I often use the 10-24 size but the base slots need to be filed slightly wider. The M5 size requires a lot of filing for use with a battery base, but once in a while this thread size is necessary for mounting an accessory.
I start by tightening the hose clamp on the frame in the desired position and marking the location for the hole in the clamp band for the tee nut with a Sharpie marker. A metal hole punch works better than trying to drill a hose clamp. Drilling is possible but start undersize and then use a chainsaw file to smooth out the hole and move it back towards where it was supposed to be.
Using a pipe, vise and punch to bend a tee nut to fit the tube.
Sometimes the tee nut has to be trimmed, but most often the shim washers and hose clamp take up any excess height. I bend the tee nut to a radius that matches the frame tube, slip it into the hose clamp, and then fit a piece of heat shrink tubing over the clamp band (with cutouts to fit the tee nut and screw housing) to protect the paint on the bike.
A hose clamp with a tee nut fitted, then I cover this assembly with heat shrink.
The Stratus hose clamp mounts installed. The power cable to the motor runs under the base.
For the Stratus I tried to hide the screw heads up under the base, but they could be pointed down for easier access. The heads were useful in this position for holding the wiring and XT60 connector in place. I also painted the screw housings black to blend in. Hose clamps and tee nuts provide a very secure mounting, it's just that they aren't as clean looking or light as a pop nut.
I'll write one more short post to finish up this series about The Breeze - the Stratus, with a couple of the other small details that make it a nice bike.