Battery "fuel gauge" query

[KeithH]

Yesterday was a good day for cycling.
Starting with a fully charged battery 9AH bottle battery after 15 flat miles on the Lee towpath it was showing only 1 green LED . After 18 miles still 1 green LED. I stopped for some liquid refreshment and half an hour later I had 3 greens. This became 2 after a couple of minutes and stayed 2 for another 3 miles (then I arrived home).
My query is how do these "gauges" work and more importantly how accurate are they? I don't really enjoy unassisted pedalling any more.
Thanks all
Keith
PS new conversion, trek 7300, battery was "conditioned" once a couple of weeks ago.

 

[trex]

on Chinese batteries, the fuel gauge indicates battery voltage, linear to the voltage.
The voltage corresponds to a logarithmic scale of the charge.
If you have 3 bars, then they correspond roughly to 100% to 50%, 50%-25%, 25%-0%.
If you have 4 bars then they correspond roughly to 100%-50%, 50%-25%, 25%-12%, 12%-0%
You get the idea.

 

[flecc]

The problem you experienced has two parts. First, when you are using power the battery gauge is recording the power drain so reads very low content. Second, lithium batteries chemically tire when in use with high current drain, but recover somewhat when the current is no longer being drained.

The result is that the battery content is only fairly accurate when power is not being used, and only at best accuracy after the bike has been idle for around 15 minutes or so.

Of course batteries are chemical devices so the content cannot be instantly precisely measured electrically. Electrical methods can only accurately measure the content being used, not static chemical potential.

There's more information on this link
.

 

[peerjay56]

As Flecc and Trex have indicated, the gauges aren't that useful, other than as a general indication of consumption. There is an added 'fly in the ointment', which is ambient temperature. The colder it is, the less efficent the chemical process is. So you can expect a lower range on days like yesterday (bright and cold) compared to a summers day (bright and warm).

 

[KeithH]

Thanks all
I understand more having read flecc's thread, very useful. Unfortunately my forum search managed to miss it!
What really confused me yesterday was the difference between the "LCD dashboard" which I understood as a simple voltmeter and the LEDs on the battery. When I was tootling (and pedalling) along using level 1 assist I had lots of bars on the dashboard and one green LED on the battery.
Definitely an inexact science!!

Keith

 

[flecc]

I do what a lot of e-bikers do Keith, having got used to the range on my bike in various conditions I use mileage as the measure and ignore the meter.. For users who still worry about what a handlebar meter says, I've often advised them to just stick a strip of black pvc tape over the LED panel.

Two of the oldest hands at producing e-bikes and kits, Heinzmann and Giant, have both produced models without any handlebar metering, obviously knowing how little use it can be.

 

[KeithH]

I think what I really hanker after is a "reserve" just like on my motor bike in the old days. No fuel gauge and I usually got to the filling station in time based on mileage alone but with the comfort of knowing that an extra 25 miles were available at the twist of a tap.
Keith

 

[flecc]

Indeed, those two-way taps were ideal, I'd have been sunk without them a few times!

 

[Yamdude]

The system with a tap and reserve was far from ideal..... many people would forget to turn the tap after they filled up. So when they started to run out of fuel at a later date, they would go to turn the tap to reserve, only to find out they'd left the tap on reserve. They then had a long walk pushing the bike.

Far better now with fuel gauges on motorcycles and they almost all have a flashing light to tell you you're on your last 4 ltrs or reserve. My bike will even automatically switch on a mileage trip meter starting when the fuel light comes on.
This is a far better system.

 

[flecc]

The system with a tap and reserve was far from ideal..... many people would forget to turn the tap after they filled up. So when they started to run out of fuel at a later date, they would go to turn the tap to reserve, only to find out they'd left the tap on reserve. They then had a long walk pushing the bike.

Far better now with fuel gauges on motorcycles and they almost all have a flashing light to tell you you're on your last 4 ltrs or reserve. My bike will even automatically switch on a mileage trip meter starting when the fuel light comes on.
This is a far better system.

The nanny state way!

Some of us like to do things for ourselves.

 

[Yamdude]

Ah yes, the nanny state....... I expect it was the nanny state that took away our kick starts and made us have that complicated button that you have to push.

Nurse !.... Bring me my rose tinted specs will you.

 

[flecc]

Ah yes, the nanny state....... I expect it was the nanny state that took away our kick starts and made us have that complicated button that you have to push.

Nurse !.... Bring me my rose tinted specs will you.

Not rose tinted specs, I like less complication as much as anyone, my car has a keyless system for example. But two way taps are fail safe, the reserve exists. If the flashing light fails on your m/c you can get caught out, and many motorbikes don't even have them, my last one didn't.

Ideally today's m/cs would have two way taps as well.

 

[Yamdude]

The point is they aren't fail safe because of the simple fact it was so easy to forget to turn the tap from 'res' back to 'on' when you filled up. They wouldn't dream of having a stupid system like that on a car.

Another thing about them was you would run out of fuel in your main tank at the most inopportune moments. You'd be overtaking a car and the engine would start stuttering and you'd lose power. This is a time you definitely don't want to be losing power. So you would have to faff about reaching down below the tank to turn the tap to reserve as quick as you could.
So it was not only a stupid system, it could be dangerous as well.

 

[flecc]

Either you had no experience of them or are forgetful, the symptoms of low fuel show up long before cut out occurs. And as for returning the tap at fill-up, some of us have memories, I never once forgot with any bike.

You may hate them, fine, but they are ideal to me.
.

 

[KeithH]

But battery meters aren't accurate hence my hankering after a reserve.
Isn't the whole idea of a "reserve" hinted at by the name, it isn't a "reminder" (to refuel)
Keith

 

[flecc]

But battery meters aren't accurate hence my hankering after a reserve.
Isn't the whole idea of a "reserve" hinted at by the name, it isn't a "reminder" (to refuel)
Keith

I agree Keith, but obviously there's two points of view on this and I can also understand Yamdude's view.

A reserve on an e-bike could be in the form of a switch on the battery controlling two voltage cut-off levels, the first leaving a usable margin of perhaps 5 miles or so, depending on terrain etc, the second level being the default final safety cut-off to protect the cells.

The switch could be made fail safe if the charger connector could only be inserted with the switch in the reserve-available position.
.

 

[Deleted member 4366]

Both LCD and LED meters show voltage, which changes all the time depending on how much power you use, but it trends downwards as you use up the battery charge.

A wattmeter is fairly accurate in showing how much battery you have left. It measures amp-hours or watt-hours used, so if you have a 10aH battery and you've used 8aH, you have 2aH left, or if you have 360wh and you've used 200wH, you have 160wH left. Simples!

In theory, there's external factors that can affect battery range, but I've hardly noticed any variation in practice.

New Digital 60V/100A Battery Balance LCD Voltage Power Analyzer Watt Meter | eBay

 

[CoachMark]

If batteries weren't so heavy or expensive you could carry a spare - would it be possible to carry a smaller 'reserve' like a 2.5aH that are fitted into saddle bags?

 

[Deleted member 4366]

Yes, I do it sometimes, but you need accessible common connectors. I use lipos. You can get a pair of these for about £35 for 2.7aH. I use a 5aH pack, which costs a bit more. You need a charger, power supply and monitoring system as well. You need to know what you're doing with lipos. The problem with other types of battery is that with 5aH capacity or less, you can't get enough current out of them.

 

[Sheffmax]

So, yesterday I had a 21 mile ride including getting to the top of Roper Hill. On the way back climbing up out of several more valleys I had one indicator out of the 4 lit. When coasting home this went back up to 2/3 indicators lit. When I got home and after washing the bike it remained at 3 lit.

From what I understand this means that there would be anywhere between 50% and 75% charge left, enough to go round again and still leave me with between 25% and 50%. Or am I being too optimistic?