Battery Sizing - What's everybody's take?

My hand up up as a lifelong technologist (audio/broadcasting/live sound/electronics) but eBike newbie. I'll admit, before my heart attack I was a "pure bike" snob - boy have I learned my lesson. Not one of those anymore. Never should have been, as I've also always claimed to believe in "ride what you bring, ride without judgement." As I say, lesson learned in a way I hope nobody else has to learn it.

So, my next bit of learning. Battery sizes.

Having converted my trusty tourer/gravel to electric with Australian street legal, 36v, 250 watt, front wheel kit from TDR Moto in Melbourne's west, I opted for a 15Ah battery and with barely 20 rides on the beast yet, I'm regretting not getting a pair of 8Ah units. The kind of riding I want to do, multi day tours of my adopted state of Victoria, maybe even a ride to Adelaide, maybe the Nullabor... might be a stretch with my ticker, but ambitions, you know.

It seems to me, with 20 rides on the rig, that a single, long-range battery takes up a LOT of space in the frame and is barely being sipped from on most of my daily riding. Typically 16km (12 mile) distances, mostly never needing more than level 2 boost. I've been wondering about Ryobi pattern storage, as they do a nice range of 36V, 4, 6 and 8Ah packs that can run things like hammer drills, plunge routers, circular saws and lawn mowers. (Actually have a Ryobi One 36v lawnmower - it's stunning!) I don't work for Ryobi, I have no brand preferences, I cite them solely as an example and am considering them as one option. They look up to the job.

So, first question, What size pile (in watt hours if not running 36v) do you run in conjunction with type of riding do you do? Add your wattage as well, because I'm trying to find a solution for a 250w 36v system because that is the highest street legal power where I live.

Second question, based on using this article GitHub - pittxprojects/ebike and this blog Technical Stuff as my starting points, what links can you share related to solar charging Li-ion batteries (already onto a couple of leads for boost MPPT regulators, but always open to suggestions for "the best" to "the best value" range.

Final question, specifically for cycle tourists and longer haul riders, which is better? Should I go for a pair of "just enough range" batteries, solar charging one while powering off the other, or is solar still a "fool's errand."

I'm happy to hear diverse opinions - this is a research - but, if you disagree with another's opinion, please express your opinion, stand-alone, without commentary on others' opinions.

Thanks in advance for any forthcoming learning. Thanks for being here, this community certainly looks like a great find. Answers via DM also welcome.

Solar panel energy density is very poor.. they are heavy.. and have negative aerodynamic effects... you will immediately need a trailer to get enough power to propel the bike.

The right size battery:
A) Has enough power to feed the motor, with headroom.
B) Has the right amount of amp hours beyond that to satisfy your range needs.

Two 8ah units would only give you an additional 1 amp hour.. if you are really stressed for range and want to do more riding, there's 36v 25-30AH batteries out there!

I use a 48V 22Ah battery. I limit my bike's speed so the battery runs out exactly when I get to work 20 miles in, riding at full throttle the whole way. Could get to work half an hour quicker if I had a battery with more Ah capacity. Or if I was willing to stop and swap batteries out. I'm not, though.

I do have an older 48V 14Ah battery. It's annoying to use, though. The BMS shuts off at a max amp limit below what I normally set my bike to. So I have to reprogram to use it and it's slower. If I plug it into a battery blender with the other one, sometimes the blender is using the weaker battery, and again, the low max amps on the BMS can trigger the battery to turn off.

I don't really want to connect the two batteries in parallel since they are different capacities and I've seen people start fires that way, even when they thought they had the diodes all setup correctly. If I ever upgrade, I'd probably buy a duplicate 22Ah battery and connect them in parallel, always at the same voltage, or just upgrade to a better single battery and retire the old ones.

I enjoy running my kid's Powerwheels off Milwaukee M12 tool batteries. I hate the fact that the p-group contacts have resistors on them, though, so the tool batteries are tough to balance. Really chose the wrong brand for power tool battery hacking there.

If you only have one battery, then it might as well be a big one.

I've used the Ryobi batteries on my ebikes, seven or eight years ago, when ebike packs were like $300. I looked up my numbers, Running on my 500W hubmotor, at 10-12 mph, I maxed out at 11 miles on the 94Wh battery and 20 miles on the 180WH battery, but that wasn't normal ebiking. I was probably coasting the last half of the rides and not touching the throttle as it would shut off the bike, Let's call it 6 miles and 12 miles of no-stress riding,

Manufacturers multiply the battery voltage by the AH rating to get watt-hours (WH), Your 36V15Ah battery is noninally 540WH, You have to derate those numbers, Ebikes are run under different conditions than how the cells in the battery were tested to get their AH ratings, Should be at least 90%. I back that off to 80% to be conservative, That make it a 430 - 490 WH battery, I know I burn about 10WH/mile at 12 mph. So I would estimate about 45 miles for your range, maybe as high as 50 miles. What are you seeing?

I've mentioned this before, but I don't like range anxiety and I'm a country cruiser that likes to do all-day rides. I run a pair of batteries in parallel for a total of 37Ah (5p on the downtube and 6p in the trunk bag - 52v). Both packs use the same Panasonic 3400mah cells and same 2-wire BMS. Charging one pack charges them both. I charge them together but usually only to 80-90% for pack longevity unless I know I'm going to ride 75+ miles then I charge to 100%. When I used to run different cells with different BMSs I would never charge to 100% when they were together.

When I ran just a single 12.5Ah pack it always failed/ran out when I needed it most. Manually pumping the pedals uphill after 45 miles of riding that day left me near cardiac alert and leg cramps- heh! (I'm an old guy......)

If I get caught in the rain or if I get hurt, I like the knowledge I can lean on the throttle (BBSHD) and go home.

Like I say, I'm running on average a max of 40% (assuming my cheap TDR-sourced-from-China controller is linear - AKA 16Nm and 100w) but that would be my weekly climb to a friend's place, a consistent 1% over 11km. My 40m de-elevation into the city is across an undulant 12km and accumulates to about 200m and has a 200m hill that's close to topping hy heart out at the end of that few minutes. This hardly drinks more than 15 to 20% of my 540Wh, although again, I realise the discharge rate is a curve, not a line, and that probably translates to 3x that over a day, assuming no long hills. I've always planned rides based on total EG, but now I look for flatter rather than, er, "challenging"

Am I correct in assuming you guys are running a throttle as well as or instead of a pedal assist? Australia is closer to Euro ebike laws and way more strictly enforced than I believe Europe is in that regard. (Frankly, police here blame cyclists for everything, even the re-emergence of polio and bubonic plague... well, almost.)

I'm entering the design phase of solar assist and from an engineering perspective, charging one while riding the other, then swapping seems to offer potential for less need for power outlet charging at the end of the day, which can be much further apart than even my pre heart attack, non-electric range.

The rhythm of pedalling is my meditation, so I'm not a heavy pedal assist user, preferring to mostly use it to stop my heartrate topping out. Mainly using 60% assist and up for climbs, which tend to shorten my daily targets anyway. I've always aimed for 2500 to 3000 calorie/day energy burn and 100km a day. Anymore is too much like cycle _sport_ not transport especially these days.

Anyway, useful data all the same, gives me context of real world use and your motor power helps me correct Watt hours back to my motor power. I really appreciate your time and replies. Thanks.

Most of us are the in USA, and then there's some eurozone/aus people who don't care about laws.
I'm all throttle all day on 1000w-3000w bikes because nobody cares out here & also, i ioften have to take the car lane to get to where i want - no way around it.

Range estimation can be made with the ebikes.ca motor simulator. It should be very accurate.

Just remembered, I'm looking at a 300W (200w effective) panel that's 580mm x 970mm. Reliable Australian seller. My rear panniers, fully stuffed, make a bulge to bulge of 540mm across the rear, so width is fine. The rear overhang would be no more than 400mm (I estimate 350) behind the back wheel's trailing apex. This would provide 2 to 2 and a half hours safe flat to full on an 8Ah, which is about as much time as I spend in the saddle, in 2 sessions a day. (used to be 3, but hey, the "ticker" grumble grumble.) So, it "feels" like running one on the motor while charging one on the panel would work OK in the SE Australian spring, maybe needing a long lunch stop in the Aussie summer due to heating inefficiencies. (And nobody touring in Australian summers minds a long lunch break, trust me )

I do appreciate, greater inefficiencies come into it if mounted flat, but that also reduces headways and sideways drag and I figure is safer, if less electrically efficient. That said, indirect sunlight does less heating, and airflow while moving if air-gapped above the load, so they may be some "swings and roundabouts" benefit here, right? Cooling gains at least partially offsetting angled incidence light losses?

Keen to hear if anybody is using a rig similar to what I'm riding and the parts I'm proposing to add to my ride. 250w 36v motor, 200w (effective) panel, use one battery while charging the other, Aussie levels of daily sun.

neptronix said:

Most of us are the in USA, and then there's some eurozone/aus people who don't care about laws.
I'm all throttle all day on 1000w-3000w bikes because nobody cares out here & also, i ioften have to take the car lane to get to where i want - no way around it.

Range estimation can be made with the ebikes.ca motor simulator. It should be very accurate.

Click to expand...

Thank you so much for the link. That's a super useful tool! Ohh, training vids - real engineering!

And boy, I wish nobody cared about power here, or cared more about car horsepower than bike horsepower, at least. I've been stopped twice in the few months since I fitted the kit and "requested" to show proof the bike is legal. First the motor sticker, then the web page for the kit. Both times. Then I had to justify that there is no legal requirement for servicing of any vehicle to be done by a qualified mechanic. I was beginning to wonder if I'd made a mistake. ACAB, hey.

Yep, between that motor simulator and solare.bike's 3.5 multiplied by panel watts (200 in my case) Wh approximation, I think I might be able to 60km/day out of a pair of 8 to 10 Ah piles, "drinking" from one while "feeding" solar to the other, then "feeding" them both at lunch stop and end of day, most days. Provided I avoid serious elevation gain days. No more Great Ocean Road for me Who am I kidding, that horse bolted 4 years ago while I was lying in a hospital bed.

Batteries are very expensive, and having to replace an $800 battery every say 500 cycles would increase the cost of my ride by over $1.50/day, which on top of other bike maintenance expenses such as tires, cassettes and brake pads, is approaching the cost of the city bus. You can get a whole lot more life out of a battery if you avoid drawing too many amps, discharging below 30% and charging above 90%. My solution is a 52V 17Ah (would be about 900Wh at 90% charge) battery with smart bms that balances when charged to 90% capacity. My max draw at ~52V full charge is 640W but I like to pedal so it usually sits at around 300W. My cruising speed varies depending on the grade, but on level ground, it is around 42 kph on a full charge. My battery has enough Ah capacity that the current draw is always under 1C and capacity to complete my 30 km commute with > 30% remaining. I'm hoping this battery will last in excess of 2000 cycles - time will tell but the battery is already around 1500 charge cycles over 6 years and still seems to have similar range as it did new so I imagine it will make it. My climate is too cold and rainy to make good use of solar power.

We get our (un?)fair share of crappy weather in SE Australia, too, but probably nowhere near as bad as higher lattitudes of North America or Europe. Solar's a feasible option here, probably. The battery insights about more cycles shared over batteries being more expensive are helpful. Thanks

You have a solar panel you can hang on the back of a bike that puts out an effective 200W? I did not think that was possible, I see videos of solar riders pulling trailers or having roofs over recumbent trikes. How many watts will you get at the charger output to the battery?

Changing topic to public power outlets, most chargers are around 3 amps. and a 36V unit will put out about 100W to the battery. Most batteries have cheap barrel connectors that cannot handle more than that without getting hot. So an hour of charging gets you 100WH, enough for 8-10 miles.

What if you upgrade the connectors and use a higher current charger? The next limit is the allowable charge current. You have to look inside the battery and get the cell specs. A Sanyo GA cell has a max charge of 1.675A. A 40 cell 14AH battery with these cells could take 6.7A, but they will last longer if you give it 3A, or half the max rating,

Going back to solar, I could be wrong but think 100W to the battery is doing pretty good.

molybdenum said:

You can get a whole lot more life out of a battery if you avoid drawing too many amps, discharging below 30% and charging above 90%

Click to expand...

If you use only half of the battery capacity, then why not buy half size battery for half the price?

crunchysteve said:

And boy, I wish nobody cared about power here, or cared more about car horsepower than bike horsepower, at least. I've been stopped twice in the few months since I fitted the kit and "requested" to show proof the bike is legal. First the motor sticker, then the web page for the kit. Both times. Then I had to justify that there is no legal requirement for servicing of any vehicle to be done by a qualified mechanic.

Click to expand...

Wow. That is pretty crazy for a freaking ebike.

Justin (Grin Technologies) did a video on "Primer on Solar Ebike Systems" about three years ago.

Two more videos you might want to look at as well:

Being on the Southern border of Texas .... I try to avoid the solar energy ... especially this time of year.

It hangs a LOOOONG way out the back, nearly a metre long (970mm or 3' 2 1/2") It's Pmax is 300w, Pe 200, and if I get half of that, based on Pe x 3.5 for Wh/day, that'll charge one of the 2 batteries I'm considering while I ride on the other. Most days, anyway. As I average about 100Wh while riding level on undulant terrain (I pedal as well, remember, no throttle, level 2/5 PAS with 250 watt motor) That should have me covered. Been running a lot of numbers based on SOLAR ebike Trip Planner 2.1 and Technical Stuff

How far such a rig gets me before I have to plug in? Well, I'll suck it and see. If I make 60km/day these days in good weather, I'll be happy.

docw009 said:

Wow. That is pretty crazy for a freaking ebike.

Click to expand...

IKR!

LewTwo said:

Justin (Grin Technologies) did a video on "Primer on Solar Ebike Systems" about three years ago.

Click to expand...

Yes, found that but found it a bit light on for data at a more electronic engineering level. Grin have some great stuff online. Nice to find businesses that are genuinely in it to help.

Thanks all, the data today has been super helpful, If we were in the same town, I'd throw barbecue in your honour, free snags and free [insert drink of choice] for all. It being midnight on my side of the planet, I'ma turn in for the night. Cheers!

crunchysteve said:

a single, long-range battery takes up a LOT of space in the frame

Click to expand...

Just to mention a few things that come to mind.... You state a larger battery takes up space (which they do) ,then there is a solar panel discussion? Ummm - Talk about taking up space!

crunchysteve said:

Final question, specifically for cycle tourists and longer haul riders, which is better? Should I go for a pair of "just enough range" batteries, solar charging one while powering off the other, or is solar still a "fool's errand."

Click to expand...

Why charge one and run the other? Run them together and charge them together at the same time. No reason a charge circuit can't work concurrently.

You mention overnight touring (not camping) which I like to do but only for a few nights here and there. I ride all day on big batteries (as mentioned above) and roll it into the motel room for charging (3 amps to 90%) with me as I sleep. Never had a problem with having it in the room. Even at high rise hotels (Casino) with elevators - heh. (I do have a 10 amp charger to speed things along at home but I don't travel with it.)

lnanek said:

I don't really want to connect the two batteries in parallel since they are different capacities and I've seen people start fires that way, even when they thought they had the diodes all setup correctly.

Click to expand...

You don't need diodes; you definitely don't need a battery food processor. You only need to know that the batteries are at the same voltage when you connect them in parallel. Their voltages will not diverge from each other as long as they stay connected, and with no voltage gradient between them there will never be any current flow from one to the other.

Diggs said:

Just to mention a few things that come to mind.... You state a larger battery takes up space (which they do) ,then there is a solar panel discussion? Ummm - Talk about taking up space!

Click to expand...

Yeah, I'll wear that, fair comment.

However, my openSCAD 2.5D sketch of layouts of panels of varying sizes (2x 75w front and rear, 1x 200w & 1x 280w effective max) all are practicable, the second is probably prefered... (metric graticules)

Also, plugin at the end of the day is not impossible where I like to explore, just frequently car travel spaced, not old guy on a bike spaced. And, if I have to lie in my tent for a day, reading a book, waiting for a full charge? Meh, Not in any hurry.

And dude, 100TW ranking? I truly respect your experience! You've been at this longer than me.