# Extending the range of an electric bike...

Discussion in 'Electronic Design' started by [email protected], Apr 7, 2007.

1. ### Guest

Hi,

In the last few days,
I've bought a 22 inch electric bike 2nd hand for £120.

It's been pretty impressive so far imho.

Problem is,
as people probably already know, is the battery...

At the moment,
it does 12 miles at 17 mph quite comfortably with no input from the
rider.
(36v 12 amp lead acid battery)

Just curious to what you think of this idea of extending the range.

A leisure battery from a scrap yard 12v 110 amp - £30
A fast charger 22amp from Argos - £40
A 12v to 36 dc to dc convertor - £70
A current limiting diode.

ie 12v at 110amps,
probably equals 36 volt at 30amp = 2 times as far = 25 miles,
(taking into account the extra weight and the loss of electric
convertion)

but I reckon it should be good for an approx range of 30 miles of
effortless riding.

Thanks,
Dave

2. ### Rich GriseGuest

To me, it sounds like a complete non-starter. Don't EVER get a scrap
battery and expect to do anything but pay somebody to haul it away.

You'd be better off to just get another 36V battery just like the
one you have, slap it in parallel with the existing one, and either
beef up the charger or charge it twice as long.

Or, get THREE of the big 12V honkers, and wire them for 36V, and
just be done with it.

Although, you still have the weight issue. Other people might have
other ideas about battery technology/power density, etc, but I'm
at my limit here.

Good Luck!
Rich

3. ### Paul Hovnanian P.E.Guest

Do you mean 12 Amp-hours?
Are you planning on replacing the existing battery with the above? Or
paralleling the converter output with it?

What exactly do you mean by 'current limiting diode'? A blocking diode
for parallel operation?
You could replace the 110 Ah, 12V battery with 3 40 Ah batteries in
series (giving you 36V) and do away with the dc-dc converter, its losses
and weight.

One thing you might want to consider before investing \$\$ (or ££) in a
lot of range enhancing hardware: Test the existing battery capacity
first. You said you bought this thing second hand and its possible that
its battery capacity has diminished.

Hook up a voltmeter, ammeter and a suitable load (a couple of 12V
headlights) and plot the battery current and voltage over time. No sense
in re-engineering the whole contraption until you have established a
good baseline for performance.

4. ### kellGuest

If you want extra watt hours, get a bigger 36 volt battery or just add
another 36 volt battery in parallel with the one you already have.

5. ### Jan PanteltjeGuest

Is not putting Lead Acid in parallel creating the possibility for giga-amps
egalization currents?

6. ### MassiveProngGuest

"egalization"???

Also, if "wired for 36 volts"... that would be series, not parallel.

This is very unlike you Pan Janteltge! ;-] Have you been drinking?

7. ### kellGuest

Yeah, they say you should only parallel identical batteries of the
same amp hours/brand/batch/age (new, in other words).

I owned a diesel Suburban with two batteries hard-wired in parallel.
Must have been many thousands of those vehicles on the road, and I
imagine a lot people ended up putting dissimilar batteries together,
so I guess it's not a huge danger.

I know I would want to make sure at least that both batteries are in
good condition, similar amp hour ratings and well charged before
making the connection.

8. ### MooseFETGuest

12 miles at 17mph takes: 0.7 hours

36V * 12AH = 432WH

432WA / 746 = 0.579 HpHr

0.579/0.7 = 0.8Hp

You have huge losses somewhere in the system. Making a bike go at
17MPH takes less than 1/4HP.

9. ### skenn_ieGuest

Does it use regeneratice braking ? If not, employ it, if it does, it
may not be very effective. Either way, MooseFET has a good point.
Does it use resistive speed control, or PWM ?

10. ### martin griffithGuest

Pump up the tyres

martin

11. ### MooseFETGuest

It may not have any control at all other than on/off.

A system for running on 12V, 24V or 36V is better than simple PWM but
requires more smarts from the driver.

12. ### ehsjrGuest

You must know some things no one else knows.
Like: the weight of the rider & the bike, rolling resistance,
the slopes encountered in the 17 miles, headwinds and tail winds,
the state of charge at the end of 17 miles, how much energy
was extracted from the battery etc.

In other words, we have no idea of the work done
nor the energy expended.

Ed

13. ### MooseFETGuest

Yes I know lots and lots of things.
humans. The width across the shoulders matters more. I am on the
large end of the scale and assumed that since the person had not said
anything about the subject they were not abnormally wide.
All bikes weight 50 pounds.
A 10 pound bike needs a 40 pound lock and chain
A 20 pound bike needs a 30 pound lock and chain
A 30 pound bike needs a 20 pound lock and chain
A 40 pound bike needs a 10 pound lock and chain
A 50 pound bike doesn't need a lock and chain.
I assumed that the bike was a normal bike and that the tires were
pumped up and that the various bits hadn't rusted together.
There may be light poles and phonepoles but there were no math poles
enclosed during the ride.
I assumed the wind did not change and that it was low enough not to be
remarked on.
The OP told us the "range" was 17 miles. The battery was discharged
at the end.
Almost all the energy that could be.

What's this "we" paleface. I made a fairly good estimate based on
what I know from experience.

Well, that rating is for a longer discharge time. Maybe 5Hr but
proabably 10Hr or 20Hr. The useable capacity with a 3/4 Hr discharge
will be lower, probably substantially lower.

Robert

15. ### Jeff LGuest

Hi,

In the last few days,
I've bought a 22 inch electric bike 2nd hand for £120.

It's been pretty impressive so far imho.

Problem is,
as people probably already know, is the battery...

At the moment,
it does 12 miles at 17 mph quite comfortably with no input from the
rider.
(36v 12 amp lead acid battery)

Just curious to what you think of this idea of extending the range.

A leisure battery from a scrap yard 12v 110 amp - £30
A fast charger 22amp from Argos - £40
A 12v to 36 dc to dc convertor - £70
A current limiting diode.

ie 12v at 110amps,
probably equals 36 volt at 30amp = 2 times as far = 25 miles,
(taking into account the extra weight and the loss of electric
convertion)

but I reckon it should be good for an approx range of 30 miles of
effortless riding.

Thanks,
Dave

Check the capacity of your batteries, they may be aging - other similar
bikes running on 36V get a lot more range, see the link below (12V, 12Ah
batteries weigh a little over 4 kg, this one has 14 kg of battery, so about
a 36V 12 Ah battery). Do a google search for many more hits

http://www.bicycle-mountain-bike-cycle.co.uk/index.php/product/LB01-dd.html

New batteries are fairly cheap (likely around £50 - 60) - don't waste your
time with used batteries (especially don't waste your time with car / truck
starting batteries for storage), mixing batteries, or voltage converters.
Doing so will not give the expected results and could be dangerous. Ever see
what happens when a car battery shorts out? They are capable of sustaining
several thousand amps of current into a direct short. This is enough power
to turn a section of the frame of your bike into a red hot chunk of steel,
or vaporize a wire shorting it out. High power DC to DC converters are not
likely a good answer. There are several very good reasons why a 36V system
was initially used, and there is no reason to change it to a lower voltage.

16. ### Tim WilliamsGuest

Nope, it all factors out. It's just a comparison.

Do you honestly know of any bicyclist who can put out 0.8HP for 42 minutes?
The typical human can do about 1/4HP as I recall, making that figure a good
basis for the power consumed at 17MPH. Depending on bearing quality/tire
pressure, wind resistance, "uphill both ways", etc., of course.
Presumably, the energy expended was 432Wh, since the OP implied that 12
miles is the maximum range on the given batteries.

It's all just math and makes sense when looked at. Of course, facts don't
which reflects badly on his attitude...

Tim

17. ### martin griffithGuest

FWIW the average human can produce about 3w/Kg body weight, so about

martin

18. ### MooseFETGuest

A bike racer can put out more, but they go a lot faster than 17MPH.

[....]
That was an attempt at humor.

19. ### MooseFETGuest

I think that is a bit optimistic. The problem is with "average". You
have to include a lot of the very unfit, the sick and the elderly if
you say "average".

20. ### MooseFETGuest

This would be part of the losses of the total system, as would the
losses in the motor. All together it is a rather poor efficiency.