This is the second in a series of technical articles covering some of
e-bikes complexities in quite full detail so that all aspects of the
subject can be appreciated.
It's impossible to accurately measure what electric current there is
stored in a battery since they are chemical devices, the electricity
existing in a different form as a change of chemical state.
Therefore, what these battery meters on our bikes do is use the fact
that a battery voltage declines very slightly as it discharges. They
check the voltage coming from the battery and register it's level on
the LED lights. This isn't very accurate but gives a rough idea of the
state of charge when a bike is standing still.
The problem comes when riding though. If the throttle is opened and the
motor runs, it draws current and the voltage coming from the battery
drops due to the increased load drain, and the LED lights reflect this
drop in voltage. On a steep hill when the motor is working very hard,
the drain of current is greatest, the voltage drop is also greatest, so
the red or lowest light can come on.
The only way to measure the battery content on these meters when riding
is if you shut the throttle and stop pedalling for a moment and then
look at the meter.
Some owners use the battery meter as an economy meter while riding,
trying with careful throttle use to avoid the lights dropping and so
get better range, but it's fiddly doing this. If this type of usage is
intended, it's better to buy one of the rather expensive (£50 to £100)
consumption meters like the "DrainBrain" or "WattsUp". These measure
the current being used while riding and can assess what the battery had
in it once all the current is used. It's then possible to predict with
some accuracy in future the remaining content in a battery, and this
type of meter is also very easy to use to monitor consumption as you
ride, and thus keep it low to get the best range.
There's no economic case for using these though, since the minute
saving in charge current which could be achieved by careful riding with
one could mean it would take around 10 to 24 years to recover the cost
of the consumption meter, so they have the status of executive toy for
most users, good fun but of limited practical use. Bicycle testers and
reviewers find them invaluable of course.
Many thanks to Flecc for permission in reproducing this article.
As always, if you would like to discuss this topic further then please visit our Technical Forum
Article Series
This article is part 2 of a 2 part series. Other articles in this series are shown below:
Comment #1
(Posted by Jill Worth) Rating
I take your point that when the yellow light comes on I am putting the battery under a heavy load. However yesterday I found that even after I ceased to use my throttle the yellow battery light remained on and did not revert to green until I switched off and started again. My question is - is this just a false indication or is the battery leaking power whilst the yellow light is on. I ride a eZee Sprint.
Comment #2
(Posted by an unknown user) Rating
Nobody buys a proper amp-hour meter in order to save pennies of charge current, what a silly point to make. You ride with one so that you don't find yourself stranded unexpectedly with a flat battery pack, and having to heave and haw a heavy bicycle back home. If you put a dollar figure on that experience a few times, then the expense becomes very easy to justify.
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