Electric Bikes - Reality check for battery size reduction

Hi!
I have an electric bike, 36 volt, 680 watt hub motor, came with three 10 ah sealed lead acid batteries. My commute to work used to be 10 miles each way, and if I added errands I'd be doing 25 miles on the batteries and they would be at 60% full on the motor controller's battery gauge.

I decided to get 12 volt, 5 ah batteries to save weight and because I moved to a smaller area and I'd never need more than 5 miles of range. Now it goes a little more than a mile before it's dead. By dead I mean the battery gauge goes to 10% when I give it throttle and then the motor cuts off, but the gauge goes to 80% with no throttle. I have two different battery chargers and between those and some testing I can say for sure that they're getting charged fully.

I am curious if that's the correct outcome to my battery reduction or if it sounds like something else might be wrong.

Thanks!

Your bike came with 3 batteries because it needs all 36V to run.

those 3 are wired in series to increase voltage. 12V+12V+12V = 36V

They are NOT wired in parallel to increase capacity 10Ah + 10Ah + 10 Ah = WRONG

You don't have a 12V 30Ah system. You have a 36V 10Ah system.

Additionally, that "extra" reserve you have after your ride is important because batteries don't like to "cycle" deep (discharge down far). By running your batteries until their voltage is kaput, you are damaging the chemistry inside.

If saving weight is your concern, and that is entirely reasonable, particularly of you need to lift your bike over obstacles; then, instead of going for smaller batteries, you need to looking into other types of batteries.

Here is a Summary of several popular battery types (http://e-bikekit.com/blog/2011/05/05/how-significant-are-the-battery-types-of-electric-bikes/):

here’s a recap of some battery types and with it, you should be able to narrow down on your choice of electric bikes with ease! There are different battery types available in the market:
Sealed Lead Acid (SLA)
It is the most common battery type and is highly recommended for beginners and for people who are looking for cost-effectiveness. It is affordable in terms of cost, as 36 Volt 8 AH SLA costs approximately around $160. But its heavy weight and questionable durability pose disadvantageous aspects to its usage.
Nickel Metal Hydride (NiMh)
It has got an edge over the SLA type batteries, as NiMh batteries weigh only half the weight of the SLA and last longer in terms of charges, i.e. 500-600 charges in a span of 2-3 years. This is a highly recommended option for people who choose the conversion kits to enhance the power of batteries, as most of the pre-built electric bikes come with either Li-Ion, or SLA.
Lithium Ion (Li-Ion)
Apart from being an expensive option, it is one of the most durable and light weight batteries available in the market. Plenty of laptops models are manufactured with Li-Ion type batteries. Three types of Li-Ion are used in the manufacturing of electric bikes:
• Lithium-Manganese (LiMnO2): It is a battery lighter in weight, but heavier in Li-Ion chemistry, used most commonly in the electric bike manufacturing. It typically ranges up to 800 charges and below.
• Lithium-Polymer (LiPo): Quite expensive compared to the LiMnO2 type, but has got a lighter Li-Ion chemistry.
• Lithium-Phosphate (LiFePo4): This is the most costly battery used in the electric bikes and one of the longest lasting types too, ranging up to 2000 charges.


SLA batteries have an advantage of being cheap and reasonably forgiving of poor charging practices. That is about it. Other battery chemistry's are significantly lighter; however, they are not as cheap. These are some examples (http://www.pingbattery.com/servlet/the-36V-LiFePO4-Battery-Packs/Categories), Ping is definitely not the only choice; he is just popular among many hobbyists when they are looking for, ready made, upgrade batteries for their bicycles.

I am curious if that's the correct outcome to my battery reduction or if it sounds like something else might be wrong.

Something else is wrong. Replacing three 10 A-hr 12V batteries in series with three 5 A-hr 12V batteries also in series is quite reasonable and should give you half the range that you had before. I suspect that at least one of your new batteries is defective and isn't giving you its rated capacity. Can you try running a known load with each of the three batteries individually? E.g. one 5A-hr 12V battery should run a 10W, 12V light for about 6 hours with reasonable brightness. Try it with each of the three to see how well matched they are and how close they come to this time.

I agree that you can get much lighter batteries using NiMH or Li battery chemistries, but your smaller lead-acid batteries should give you sufficient range for the trips you describe (I assume there aren't any major hills involved).

Lead acid batteries only seem less expensive because of the price. If you figure in the lifetime of use, lithium batteries are less expensive because they last so much longer.

Your bike came w...

How did I give you the idea I misunderstood series wiring?



If saving weight is your concern, and that is entirely reasonable, particularly of you need to lift your bike over obstacles; then, instead of going for smaller batteries, you need to looking into other types of batteries.
...


Thanks for the info. I shortsightedly left out that my old batteries were weak and in need of replacement, and that I wasn't specifically buying smaller batteries because I can't deal with the weight. For my purposes and situation I can't justify getting LiFePO4 batteries. I don't have to lift my bike or deal with hills, but some day I would like a working kickstand again.



Something else is wrong. Replacing three 10 A-hr 12V batteries in series with three 5 A-hr 12V batteries also in series is quite reasonable and should give you half the range that you had before. I suspect that at least one of your new batteries is defective and isn't giving you its rated capacity. Can you try running a known load with each of the three batteries individually? E.g. one 5A-hr 12V battery should run a 10W, 12V light for about 6 hours with reasonable brightness. Try it with each of the three to see how well matched they are and how close they come to this time.

I agree that you can get much lighter batteries using NiMH or Li battery chemistries, but your smaller lead-acid batteries should give you sufficient range for the trips you describe (I assume there aren't any major hills involved).

Thank you very much, I hadn't thought of an individual drain test like that or that one of them might be bad. No hills involved. I'll try a drain test when I can and report back.

Sorry for misunderstanding your original query.

Something else is wrong.


Thanks again. I took your advice, and you were right. I got a DC motor from a car fan to test the load and I didn't have to test current drain or RPM because it was clear pretty fast that one has a pretty low capacity. I'll replace it and hopefully enjoy electric biking again!

my experiment with electric bikes was very disappointing. Oct 2010 i got a kit for my bike. had a Lithium (LiFEPO4) 48v battery (10 amp/hr)
and one of the selling points brought up in this thread was a selling point. namely the 2000 charge cycles. Pretty decent i thot as thats 5yrs of
daily charging. By June of 2011 it was dying. After a few km of riding the motor would just quit. The distance i could go got shorter and shorter b4 it would
stop. I thot there was something wrong with the motor or a short circuit and i brot the bike in and they couldnt tell me what was wrong. They put a new switch
in the battery incase that was the problem.

by oct 2011 (when my warranty was over) they then tell me it is a weak cell in the battery pack. a new pack would cost upwards of 600+$ ... umm no thanks. not going
to replace the battery every year. i basically now ride with out the battery and once i get some extra money will be removing the whole kit and restoring my bike to its former
non electric condition.

as an FYI this is the kit i got. http://www.raybenmotors.com/#!__conversion-kits

my experiment with electric bikes was very disappointing

Sorry to hear that. For that investment and effort I wish you could have had a much better experience with electric bikes.


So, problem solved thanks to prathmann. With three, *good* 12v 5ah SLA batteries in the place of my old 10AH ones, it gets around great. I did two test rides, one was 4.5 miles and the other 4, charging in between. I made it home without loss of power both times with the battery pack at 80%.