Battery Meters

[flecc]

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" (now known as the CycleAnalyst) 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.
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[JillWorth]

Originally Posted by flecc

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.
.

I appreciate that the yellow light comes on when under pressure on a steep hill but yesterday it remained on after I have ceased to use the throttle and did not revert to green until I had switched off and the on again? Am I still draining the current? I ride a eZee Sprint.

[flecc]

That's correct Jill, the yellow indicating the amount left in the battery when you had the throttle off, the proper meter function, not influenced by the motor. In other words, you didn't have much charge left in it, the exact distance left depending on territory etc.

Li-ion batteries do have some degree of chemical recovery though, and in switching off and on, you'd helped that to happen. If you'd been in a situation where you could have parked at that moment, some while later it would probably have returned to green for longer due to the very slight recovery of charge.

This happens because this type find it difficult to get the charge out quickly enough when the demand is high, and a chemical blockage occurs and builds up. When the battery is rested, the blockage disperses and allows a better flow of current, returning the meter to green for a little while longer.
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[mr-motorvator]

I am working on a new meter that measures voltage (a simple static guide), Amp/hours consumed since recharge (aka coulomb-counting) and has an audible feedback on the actual current being consumed.

Coulomb-counting is a good measure, but you need to calbrate it as the battery tails off over charge cycles. That can be done dynamically by storing how many Amp/hours came out between maximum voltage (when freshly charged) and when the controller cuts out (usually 30V or 20V depending on battery nominal voltage)

The 'proper' way to measure charge state is to measure its impedance - how its terminal voltage changes with load, and so the intuitive way is to ride trying to keep out of the Amber!

The audible feedback (akin to the ticking of a freewheel) is intended to help the rider manage the power without looking away from the road! The voltage measurement gives some idea of charge state without actually sucking power (SLA, NiMh and LiIon all exhibit falling voltage with charge.

I remember my Father had a vacuum gauge on the inlet manifold of his Hillman car and drove to achieve least depression - I had a Jag. with an 'econometer' that did just the same, but why would you want that on a 3.2S!!!

I am a reformed character now....

[jlanger]

I have an old Panasonic battery that I have not used for over a year.It is almost 5 years old.I stopped using it for my lafree because it was not lasting very many miles.

I have 2 more recent batteries which I have been using frequently for my 22 miles round commute.

For interest sake I recharged the old battery from scratch,to see if there was any life left in it.To my amazement it now travels over 50% more than it used to do.What is the explanation?

Julian Langer

[flecc]

That's very unusual Julian. It's more usual for an NiMh battery to refuse to accept a charge if it's been left months without a charge and the cell voltages have dropped to very low levels. However, that's not an absolute.

I think what might have happened in your case is that the hydrate has recovered and normalised without losing all it's hydrogen content, that enabling a recharge.

NiMh are actually a form of fuel cell, but a closed circuit one. The fundamental principle of their working is that charging hydrolyses water, turning it into free oxygen and hydrogen which combines with rare earths, forming the hydrate.

When connected to a load, the hydrogen is released from the hydrate and recombines with the free oxygen, releasing the stored electrical energy. Your rested and revitalised hydrate has given you more range, but it might be a bit early to celebrate it since the gain might be very short lived, returning to the short range you had before, or even less.
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[oldosc]

Originally Posted by mr-motorvator

I am working on a new meter that measures voltage (a simple static guide), Amp/hours consumed since recharge (aka coulomb-counting) and has an audible feedback on the actual current being consumed.

Coulomb-counting is a good measure, but you need to calbrate it as the battery tails off over charge cycles. That can be done dynamically by storing how many Amp/hours came out between maximum voltage (when freshly charged) and when the controller cuts out (usually 30V or 20V depending on battery nominal voltage)

The 'proper' way to measure charge state is to measure its impedance - how its terminal voltage changes with load, and so the intuitive way is to ride trying to keep out of the Amber!

The audible feedback (akin to the ticking of a freewheel) is intended to help the rider manage the power without looking away from the road! The voltage measurement gives some idea of charge state without actually sucking power (SLA, NiMh and LiIon all exhibit falling voltage with charge.

I remember my Father had a vacuum gauge on the inlet manifold of his Hillman car and drove to achieve least depression - I had a Jag. with an 'econometer' that did just the same, but why would you want that on a 3.2S!!!

I am a reformed character now....

Hi
how did the meter pan out..I would love one for my Aguattu in ageeky way,
BUT would like one for my electric outboard moter on a folding mirror dingy. I have done some really stupid things with boats and bikes, but now Lady J often accompanies me in the boat, I don't take any risks..the motor draws from 10 to 25 amps from 2 55ah gels, depending on speed (controlable) but wind conditions, uncontrollable over maybe a day out on the lake, sea..see the problem
Thanks in advance Oldosc