Dual batteries, in stealth mode! (updated with range test)

[Altema]

I was thinking of different ways to add a second battery when needed, and I thought of an old Popular Mechanics article about a man with two classic VW Beetles. One of them had the normal VW engine, and there were always suitcases in the back seat. The Second Beetle had a fiberglass replica of the same suitcases, which was actually an engine cover for a mid-engine Ford V8. I thought the idea was pretty slick, and since I always have a rack bag on my RadMini, I decided to install the second battery in the bag, and hide the cables.

The new battery is a 48 volt, 15 amp hour unit, and hooks in parallel to the existing Rad battery. Alternately, the bike will run fine with one battery or the other, but if you run both, you need to make sure the voltage level is similar before connecting them. The rule of thumb is run together, charge separately, and I think I've seen that mentioned in these forums as well. The neat thing is that, if you want, you can use one battery while the other is being charged, and just swap them as needed. You still have to remember to balance the voltage before putting both back in though

The basic overview is that I removed and disassembled the Rad battery bracket, soldered the new wires to the terminals inside the bracket, put it together, and ran the wire through the rack support tube itself, then into my rack bag after adding an XT90 connector for the new battery. This combo brings my battery capacity from 14 Ah to 29 Ah, and should extend my range to 70 or 80 miles. Photos are below and I'm open to questions, but keep in mind it snowed where I live, and range testing might be delayed for a bit!

[Ryan]

Nice work! Thanks for sharing.

[Altema]

I forgot to add photos of the battery installed IN the bag, so those are below. The shipping foam that the battery came in is the smooth but firm type, and was perfect to use as a protective insert. It's been several days since my last full charge, and my actual range is 60 miles using the bike like a moped without pedaling. I usually do pedal though, and it's looking like I'd be able to get up to 90 miles if I limit myself to PAS 3 and moderate throttle use.

[Altema]

I've been running the dual batteries for a bit, running tests on different terrains and with different riding styles, and thought I'd report back on the findings.

Riding wild like a dirt bike, with full throttle and top speed as much as I can get away with it, the range is 50 miles. I can technically go further by waiting for the motor to start working again and babying it, but the batteries hit 39 volts under heavy load and the motor cut out at 49.31 miles, so I'm gonna call it an even 50 and be done with it.

Riding normally with PAS level 2 or 3, but still using the throttle when I need a boost, I was able to get 75 miles.

I didn't do a full test of just being careful with low PAS and avoiding throttle completely, but the shorter runs I did like this resulted in 1.2258 miles for each 1% of battery used. On paper, that would be 122 miles, but nobody rides like that in the real world, and all it takes is one twist of the throttle when the battery is at 43% to make the voltage dip to 39 volts, and the system will cut the motor. So realistically, I'd put the "Being careful" range at 90 miles.

[Altema]

I thought I'd add some notes on handling, so here they are!

Out of the box, the RadMini 4 is surprisingly balanced. You would think that with the battery on the back of the seat post and a heavy hub motor in the back wheel, this bike would be very tail-heavy. However, the balance point is right around the crank, and you can pick the bike up by the frame directly above the crank.

Adding an aftermarket rack didn't change the balance enough for me to notice. When I started loading that rack with locks and tools and put rear foot pegs on the bike, the balance began to change a bit, but it didn't affect the handling enough for me to care.

Adding eight pounds of battery did make a significant different though, especially since my rack sits high, and the load change the weight distribution AND the center of gravity. Is it manageable? Absolutely, and I put over 200 miles with both batteries installed with no issues. It handles fine, but you have to remember that your body weight is now balancing out a heavier load, and you have to compensate for that. I was fine for bike paths and dirt trails, but when I came to the entrance of my favorite mountain bike trail with it's sharp turns and zig-zags through the trees, I said "Nah".

My rack placement is high but I like it, as it visually balances out the proportions of the bike and eliminates the "short bike with stalks" look because of my seat being all the way up. But if I was going to keep the rear battery on permanently, I would lower the rack all the way down for the sake of handling. Fortunately, I don't NEED 75 mile range all the time, so there's no reason to carry around the extra weight on shorter rides. Then when I feel like exploring, it's simple plug and play to ride all day

[Altema]

I tried it without the stock battery on the bike, and was pleasantly surprised by the handling. The bike feels light in front, but at the same time does not feel tail heavy, which was a surprise. I'd be totally comfortable going for rides with the stock battery at home, which would be a benefit for wintertime. The second battery is snug in a foam holder inside a closed rack bag. Thanks to the small amount of heat generated from use, and being insulated, the battery is usually around 70 to 80 degrees F after a ride in below freezing weather.

[Altema]

I went to a metropark yesterday that was linked to a state park (different from where I usually go). I went in dual battery mode because I planned on exploring, and about an hour in, I just got cold and bored. The metro park was ok, but not very many hills. The state park was another story though, and the patched-up path just went on as far as the eye could see. Below is a photo from my camera mounted on the bike. I think the most interesting aspect was a railway crossing, and I used cruise control most of the time with no pedal assist.

Anyways, I held it at full throttle for miles on the desolate path. It has cruise control, and it's easy to go full speed and turn the cruise on.  I got back to the car and checked the battery after an hour of high speed riding, and it was at 77%. Although my feet were freezing, the experience gives me high hopes for extended exploring this summer.

If I had to start over with a stock bike, would I do dual batteries again? Yes!
Would I do it exactly the same way? Nope!

The current setup has actually been very good, and the positives far outnumber the few negatives. After a couple hundred miles, I have a few pros and cons.

Pros:
1. Great range.
2. Nobody knows you have dual batteries.
3. You have a choice to run one battery or the other, or both at the same time.
4. You can leave your expensive Rad battery at home to save wear and tear.
5. Even when running both batteries, you extend the life of your Rad battery by draining and charging it less.
6. You still have space inside the rack bag for a lock and tools.
7. Motor has more torque because there is less voltage sag in dual battery mode.
8. Range tests of the stock battery are easier. When you run out, just use the other battery to get back home.
9. You can get range back quicker because you can use two chargers!

Cons:
1. Extra weight is notable if you carry the bike up stairs.
2. The extra weight, and it's placement, makes the bike less nimble on tight trails.
3. Uses most of the space in your rack bag.
4. Charging is more complicated. The batteries must be charged separately, and measured before reconnecting if used in parallel.

If I had to do it all over, I would buy a rack battery and use the rack that comes with it. This would simplify mounting, give me a lower center of gravity, and give me back all my rack bag space. I could still leave the second battery at home if I wanted. But, if I wanted to get crazy, I could run the stock Rad battery, and the rack battery, AND the stealth battery!

[Altema]

I noticed a heavy spark when connecting the second battery if the first battery is offline. So, I replace the battery connector with the anti-spark version. The connector is by the same company (Amass), and fits the other connectors perfectly. The negative terminal is straight through, but the positive terminal has a two stage connector. Power from the initial connection goes through a 5.6 ohm resistor, which starts the power flow without sparking, then when the plug is seated all the way, the connection is direct. There is no resistor between positive and negative sides, so there is no battery drain by the connector. The resistor is only on the positive side, and you use th eplug as you would normally. There is no downside, so I'll be using the anti-spark version from now on.

[radrover4fun]

I just wanted to say thanks for keeping us updated, especially the "if I were to do it over" stuff.