How much battery capacity do I need?

[Tom]

Hi guys. Just got a BMC geared rear hub motor w/ 25A controller for my mountain bike and now am looking for a battery and debating what capacity to get. My primary usage will be for biking mountain logging roads, maybe 10 times a year. I don't need to go fast and I plan to pedal some (at least that's what I tell myself). Typical logging road will be 7 miles uphill (won't need any juice on the way down). Typical elevation gain might be 500' in the first 3 miles (average 3 degree slope) and 1500' in the last 4 miles (7.5 degree average slope). The road will be bumpy in spots and will involve some starting and stopping. My bike is currently about 30 pounds (will be around 40 pounds after adding the motor and controller sans battery) and I'll be carrying a 20 pound backpack. I'm 180 pounds. I'm thinking a 36V 15ah battery from Ping might be enough. Thoughts?

Another option I am considering is two 36V 10ah batteries and bringing one or both depending on trip. It would be nice to shave 8 pounds of battery on shorter trips (vs 20ah pack), but I'm reading that a higher capacity battery (with the same c rating) will allow me to draw more continuous amps in steep sections without damaging the battery, so perhaps going with a larger battery might be better despite the extra weight?

 

[pwbset]

I climb 1,600ft during a 7.5mi commute and I used between 4.5-5ah and I peddle a lot and I draw between 15-40a sustained. I don't want to knock Ping packs as they are clearly awesome for what they are, but I doubt it'll handle your needs happily. If these are dirt/singletrack/off-road style roads expect your amperage to increase a lot because of rolling resistance... you'd be amazed how much power you save on pavement vs. gravel for example. You may want to look into LiPo or high rate Nicad or a toolpack setup etc. I have a feeling your controller is going to be a limiting factor at 36v25a anyway so you probably could get away with a 15ah Ping now that I think about it. Sounds like a fun ride!

 

[Tom]

Thanks for the feedback. Yes, this will be dirt/gravel roads so sounds like I'll probably need at least 10ah. It's a geared hub so I'm not too concerned about the 25 amp controller being a limiting factor (presumably it will gear down if it doesn't get the amps it needs). Based on your comments I'm a little tempted to go with the two 36v / 10ah option I was considering as the seller claims they are rated 3c / 50a max so should be able to feed my 25A controller and two batteries would give me 20ah total.

 

[dogman dan]

If you go duct tape battery you will need 20 ah to keep the battery happy. Climbing hills with 10 ah will likely cause cut outs when the amperage spikes. You will be better off with a 20 than two 10's because with two you will have to deal with protecting the bms with diodes etc. Much easier to just get the 20 ah. If funds are really that tight, at least the 15 ah.

With a123 or PSI cells, you can go with a 10 ah pack since they can discharge a lot higher c rate.

 

[Tom]

Thanks, funds are not necessarily tight in terms of 15 vs 20ah. More hoping to keep weight down on shorter trips, but it sounds like the higher ah packs are worth carrying the extra weight for the higher continiuous discharge capability. FWIW, the 10ah I was considering were the shockley batteries. In addition to being rated 3c I liked that they came in something other than duct tape and had an on/off key.

 

[GGoodrum]

I think I'd go with the 20Ah packs as well, because even the 15Ah version would be pushing it, in my opinion, and you'd probably end up with a lot less "life" in the pack. I think most people that have these less expensive packs have been getting good performance and so far they seem to be holding up, but I think it is mainly because the packs are being stressed too much.

-- Gary

 

[dogman dan]

While the cells in a given pack may be rated 2c, 3c, 4c, etc. Untill they show me a test results , I consider the battery to be a 1 c battery if that is the only c rate I see a test on. In any case, for range purposes only, it is not so inconvenient to carry enough battery to keep discharge rates in the 1 to 1.5 c area. Lifepo4 is light man, you won't mind carrying 20 ah of it.
I wouldn't do 10 ah to climb hills even with a gearmotor. The shockleys come in a nice can, but it's not that hard to find a nice toolbox or rack trunk for a 48v 20 ah. Some report that the golden cases, similar to the shockley, had too much sharp stuff on the inside.