Running out of battery power, and power remaining display.

Page may contain affiliate links. Please see terms for details.
I have never completely discharged an e-bike battery and had to return on pedal power, not seen the display show less than 40%, but how much can we trust the display? I would assume it does not measure the power used and take it off from total, but it is some sort of volt meter? So what warning do you get?
 
I have experience of 4 ebikes

My first one - I got it second hand in 2011 - was variable. When it got very low on power remaining it would often cut out then come backa bit later.
It would do this several times - the recovery time becoming longer each time and the time with power getting shorter.
It seemed as though the battery would recover a bit when the BMS refused to take any power for a while.
The battery display was pretty much useless as it was on the battery - which was under and behind the seat, so couldn't be seen without stopping. But as it only had about 3 leds it was not very detailed.
Also the power assists had only 2 levels - all or nothing!!!

My current bike - an 18 month old Bosch system - show a fairly accurate indication based on a decent display. The power indicator has 5 blocks. When it gets down to one bar left the next stage is when the last bar starts flashing - at that point there is very little left but at least you get a warning. When it cuts out it never comes back until you recharge it!

In between is my wife's folding ebike.
The system has 3 power assist setting (plus off) and 5 leds on the controller showing remaining charge.
If you are travelling along a nice flat road the charge drops evenly.
However, if you have to go up a hill the charge indicator can drop very quickly - but then recover when you reach the top (or stop!!)
If it is already low and you go up a hill this can lead to the BMS thinking you are very low and cutting the assist - but then restoring it a few minutes later!
It is no where near as bad as my first ebike - butit can be annoying when you are low on charge and have just one more hill to get home - and it cuts the power half way up the hill - then restores it when you have gone over the top!!!


The problem is due to the way the BMS measures remaining charge. Something to do with balance and charge. Hopefully someone who knows it in more detail will come along and explain so I won't confuse things my giving a have understood explanation!
 

CXRAndy

Guru
Location
Lincs
If you're fortunate to have a display that shows battery voltage, you can get an accurate gauge of when you're going to run out of power.

Lithium cells have a nominal charge of 3.7 Volts. Fully charged 4.2V. discharged 3.2V. Your BMS should not allow your battery pack to drop below 3.2V/cell and will cut supply if you put a large demand pulling voltage below this level.

Depending on your battery pack voltage 36V/48/52V, you can work out the cut-off voltage or run your battery flat to see when BMS cuts out.

As your battery nears 3.2V/cell then with careful control by not putting sudden demand you can prolong use a little longer.

My wife's ebike will not restart once BMS has cut power. It needs recharging for a little while to restore battery function.
 
OP
OP
ericmark

ericmark

Senior Member
Oh not so good, that means pushing me to limit also, think I'll use cross my fingers method. Under normal situation would go to venue to use the bike, but the British DVLA has decided at 70 I need to renew licence, and since I have a paper licence and no valid passport now been 2 months without a driving licence, other than my Falklands one which raises insurance problems. And wife also has a problem, seen off to update address and never got back, and police say she has no licence.

Never rains but pours, local train service only has one station open so I can only go half way to local town and not get off the train, it is a heritage railway and most only go on it for a ride.

So the e-bike is now only real transport.
 

Pale Rider

Legendary Member
I've had Bosch biked for more than 10 years and the battery capacity indicators seem to be more reliable than on most other bikes.

Experience is just as important, knowing your bike and wherever possible, knowing your route.

The OP's bike is not the best quality, so a certain amount of eccentricity about the electrics is to be expected.
 
OP
OP
ericmark

ericmark

Senior Member
Well today Welshpool and back while daughter visiting so rescue possible and it did the 16 mile return run without really a problem. I did not use maximum assistance, and the energy meter shows took 4.5 hours to recharge so seems I was close to limit, I thing from fully discharged to full should take assuming 16 Ah around 6 hours at a 120 watt charge rate.

The problem is the last 1/2 mile is the steepest bit, and the last 400 yards the worst, if I had use of a car then walking home to get car would be no problem, but I don't have use of a car.
 

gbb

Legendary Member
Location
Peterborough
As Drago suggests, stay close to home when pushing the boundaries of battery life, as they get to say 10%, my battery at least tends to deplete very quickly. You should avoid completely draining a battery anyway but the only way to satisfy yourself of its ability is to push a bit further mileage wise
 

Legomutton

Senior Member
Bearing in mind that it's a fairly sophisticated BMS (allegedly) that does the state of charge thing, I'd speculate it meters the charge going in, factoring in a charge inefficiency assumption, and also the power out - whether this is just current, or watts, I can't guess. When full charge is reached it would reset to 100%. This was how the battery monitoring system I once had on a boat worked (I believe that worked on current/amps).

Quite how this relates to the 5 charge lights I'm not sure. Does your display give a %age reading? Mine just has the lights. People seem to think it's non-linear but I think that might be function of it averagely 'overreading' - people see 3 lights and assume 60% charge for example, but when that 3rd light goes out it could logically be at 40%. Similarly 5 lights could mean 100% or 80.1%, even if the lights are working as logic suggests. or it could just be the voltage drop that messes it up if it's counting amps in/out.
 

Stul

Veteran
I have the Bosch system on my bike, and it gives a "range left" figure, but this is a waste of time IMHO. eg went out yesterday fully charged with it showing 88 mile range left (in ECO), probably based on my last trip which was a bit hilly as it normally reports 95 miles when "full".

During the ride it dropped down to 41 miles range left after a particularly steep hill, but when I got home it had returned to saying "88 miles left".

I would rather just have a "%" remaining as per my OH Panasonic system.

(I assume there is no way you can program the Bosch system to show this as well?)

I find the 5 bars to be fairly consistent and normally deliver between 15 and 25 miles "per bar" depending on the terrain covered.
 
OP
OP
ericmark

ericmark

Senior Member
1623158652218.png
This display to left, but also three lights where the key fits to right and on the 16 mile run one light went out. So seems loads in reserve.
 

Pale Rider

Legendary Member
I have the Bosch system on my bike, and it gives a "range left" figure, but this is a waste of time IMHO. eg went out yesterday fully charged with it showing 88 mile range left (in ECO), probably based on my last trip which was a bit hilly as it normally reports 95 miles when "full".

During the ride it dropped down to 41 miles range left after a particularly steep hill, but when I got home it had returned to saying "88 miles left".

I would rather just have a "%" remaining as per my OH Panasonic system.

(I assume there is no way you can program the Bosch system to show this as well?)

I find the 5 bars to be fairly consistent and normally deliver between 15 and 25 miles "per bar" depending on the terrain covered.

The bar gauge on Bosch bikes is fairly reliable, the mileage left function much less so.

I once started a long gentle decent on the C2C in County Durham with a full battery.

I managed to get mileage left to read more than 200 miles, which it would have been if I could have carried on descending for that far.
 
Top Bottom