Energy measurement

[JamesC]

I have started to use one of these energy monitors from Maplin (£9-99) to measure the energy that is put back into the battery after a ride.

If it shows, for example, that 400 watt-hour went through the charger to refill the battery, what is a reasonable figure for the efficiency of a charger to gauge what energy the battery provided on that ride ?

Next question of course is to determine how much energy, in total, the ride required - distance, hilliness, wind, time - in order to see the balance between rider and battery.

How do I work out the total energy that is needed for a journey ?

For the present, I will return to a given route every so often, and hope to find that just a little less energy came from the battery !!

Thanks for your help

James

 

[Ian]

I've been using one for sometime now James, I bought mine when the price fell to £14.99! I usually find consumption with my NiMh Torq (From the mains) is about 10Wh per mile and that it takes about 420Wh to recharge a nominally 324Wh battery from flat. My Sprint with the same batteries is less economical at 13-14 Wh per mile, perhaps because its more powerful motor encourages a lazier riding style.

I also use the meter to check the consumption of just about everything, the biggest consumer in the house is the fridge-freezer, pulling it a couple of inches further from the wall cut its consumption by over 10%, and that has by now paid for the cost of the meter.

 

[Tiberius]

An interesting idea; it doesn't tell you what is actually going in and out of the battery, but it does tell you about the battery/charger combination.

I don't know the charger efficiency, but when charging its delivering about 100 W to the battery, and the size of the heatsink suggests its wasting less than 50 W as heat, so that would indicate more than 66 %. That's in line with your 420 Whr figure for a full charge. The efficiency when it switches to trickle charge would probably be worse.

I've got some appliances that that meter would be good for. Maybe I'm better off not knowing that how much that new dehumidifier in the bike shed uses.

Nick

 

[flecc]

I also use one of these meters, and my findings on the bikes are in line with Ian's.

I don't bother with the fridge and freezer consumption since in an early bit of environmentalism in 1970 I reconstructed my property's internal layout to re-use the heat given off from their radiators amongst other things.
.

 

[JamesC]

I've been using one for sometime now James, I bought mine when the price fell to £14.99! I usually find consumption with my NiMh Torq (From the mains) is about 10Wh per mile and that it takes about 420Wh to recharge a nominally 324Wh battery from flat. My Sprint with the same batteries is less economical at 13-14 Wh per mile, perhaps because its more powerful motor encourages a lazier riding style.

Thanks for the feedback everyone.

As expected, I am less effective than Ian and Flecc (or can I claim lower efficiency with Li-ion ? ) using about 16 Wh per mile on the Torq.

BUT what difference would you expect to see when the work is being shared by 2 batteries with a diode in place ?

When I add together the replenishment of the 2 batteries and look at Wh per mile, my contribution appears to have plummeted and the batteries are taking more Wh from the chargers than was the case with a single battery.

Alternatively, in these first 2 or 3 recharges, the new batteries are still conditioning and some extra capacity is becoming available to the chargers.
Does this happen in practice ?

James

 

[flecc]

Yes, there's a definite gain in capacity on those first charges. I've also noted a gain in ability to issue current during those, from which I'd infer that the battery will gain a greater willingness to take up the available charge, possibly speeding the charge a little.
.

 

[Ian]

As expected, I am less effective than Ian and Flecc (or can I claim lower efficiency with Li-ion ? ) using about 16 Wh per mile on the Torq.

My figures were from last year. I've took the Torq out for the first good ride for many weeks today and disappointingly got a yellow at 16 miles, a far lower figure than I've seen before, it could be me out of practice or it could be that particular battery losing capacity. I'm currently discharging it to see just how much is left, and I'll have to get back in the saddle more to work up a decent fitness level before spring.

 

[coops]

The difference between your 16Wh per mile and Ian's 10Wh per mile (from last year) could be due to several different factors James. Speed and hilliness of terrain probably the main ones - Ian has said he tends to take his time, averaging 11-13mph at times, and his Leicestershire terrain is quite level. While my area is also flat on the whole, if I rode mainly at 17-18mph or so I'd get a similar figure to your 16Wh per mile, but closer to Ian's 10Wh at 15mph or less, and higher consumption the 'slopier' the route I take .

Tyre pressure & rider input would also be factors, aswell as outside temperature for battery performance.

I'd say the easiest way to estimate the total energy for a ride is to estimate your average rider input x ride time & add to the total energy used from the battery given the outside temperature (minus efficiency losses if you want the total energy needed, not total used ) - there are too many variables otherwise and its always going to be an estimate anyway .

For average rider input I guesstimate roughly what speed my pedalling would take me on legpower alone and deduce the power output from that e.g. 10mph ~50W or so etc.

Thanks for the tip-off on the good price for that energy meter in maplins .

Stuart.

 

[frank9755]

Thanks for the tip-off on the good price for that energy meter in maplins .

Yes, thanks from me too, James - I've just ordered one!

 

[JamesC]

Thanks for the ideas to assess energy input.

I must now admit to some foolishness - I have not been consistent in reading the energy input to the battery as soon as the green light appears on the charger to say that charging is complete.

On occasion, charging has completed when I am not about, and it might be a few hours before I switch off and disconnect.

What surprises me is the number of watt-hours that clock up AFTER the green light.

The accuracy of my figures has been made worse by sharing the work between 2 batteries (half the previous usage), then charging each battery separately with the same amount of overrun after the green light.


Does any of the trickle charge after the green light actually go into the battery ?


I suspect that it just keeps the charger nice and warm in my cool shed.

Example:
20 mile ride, 10 miles each battery.
170 watt-hrs to green light goes into battery by evening.
By next morning, meter has gone up from 170 watt-hrs to 310 watt-hrs on trickle charge (ezee Li-ion).

This was done as a check - much safer to switch off as soon as charge is complete, but I am sure that many, like me, will charge overnight.


JamesC

 

[Ian]

What surprises me is the number of watt-hours that clock up AFTER the green light.

The accuracy of my figures has been made worse by sharing the work between 2 batteries (half the previous usage), then charging each battery separately with the same amount of overrun after the green light.


Does any of the trickle charge after the green light actually go into the battery ?


I suspect that it just keeps the charger nice and warm in my cool shed.

James, as a NiMh user I have never made any measurements with Li batteries (The only one I ever had was secondhand and unserviceable).

I can confirm that with NiMh the green light is not a trickle charge, when the LED turns green no current flows to the battery and the charger consumes a mere 5W from the mains.

The Li charger works differently in that charge termination detection and switching is done internally within the battery by the BMS, the charger simply serving as a dumb power supply, the colour of the LED being controlled by the current being drawn.

The amount of power drawn after the LED turns green does seem high, especially as trickle charge is not normal practice with Li types. The BMS will be drawing a quiescent current but this will only be a few milliamps and doesn't explain the high figure.

On possible explanation when charging 2 batteries is that the charger detects the reduction in current when the BMS in the first battery terminates the charge, this switches the LED to green but the second battery continues to draw current until its own BMS detects full charge and switches off.

Another possible explanation is that cell balancing may take place after the LED turns green, the BMS giving extra charge to any individual cells that require it.

Or finally perhaps the battery has a built in heater to keep in comfy in cool sheds.

 

[JamesC]

On possible explanation when charging 2 batteries is that the charger detects the reduction in current when the BMS in the first battery terminates the charge, this switches the LED to green but the second battery continues to draw current until its own BMS detects full charge and switches off.

Another possible explanation is that cell balancing may take place after the LED turns green, the BMS giving extra charge to any individual cells that require it.

Or finally perhaps the battery has a built in heater to keep in comfy in cool sheds.

Ian
Thanks for confirming that the Li-ion charger has no final trickle, but I should have mentioned that I am removing one of the batteries from the bike to be absoloutely sure there is no interaction between the two during charging.

Furthermore, I am impressed at how completely even they are - 2 separate batteries charging separately and always a differential of only 10 watt-hrs between them.

So battery 1 - 170 watt-hr rising to 310
and battery 2 - 180 watt-hr rising to 320

I will check again next time I charge, but I think I recall a current of 60 mA at 240v AC being consumed when the green light is on, and after say 8-10 hours there is gentle warmth from the charger.

So the extra watt-hrs after the green light is about right:
240 x 0.06 x 10 = 144 watt-hrs

but none of that goes into the battery !

Definitely need to look at a time switch to shut off the chargers, or check before bed !

James

 

[Ian]

So the extra watt-hrs after the green light is about right:
240 x 0.06 x 10 = 144 watt-hrs

James, I'm afraid with measurements of AC power such as the mains things are not that simple and V x A doesn't usually equal watts! There is another factor in the equation known as power factor which arises from the fact that with alternating current the current and voltage are not necessarily entirely in phase with each other and only the in-phase portion is converted to power.

The Maplin energy monitor will measure power factor (PF) and it will also measure Volt-Amps or VA which is the product of V x A as well as watts which is V x A x PF.

When using the meter it is best to its facility to measure watts or watt/hours directly rather than trying to calculate them from the current and voltage.

I have checked my NiMh charger and found that after the green LED comes on it is drawing 70mA from the mains but only using 5W, the anomaly being explained by a very low power factor of 0.3 which is typical of this kind of device on no load. It's likely your actual wasted watt hours are a more realistic 40 when the power factor is considered.

PS. your domestic electricity meter measures actual watts so low power factors don't cost you money.

 

[JamesC]

When using the meter it is best to its facility to measure watts or watt/hours directly rather than trying to calculate them from the current and voltage.

Ian, thanks for explaining the effect of Power Factor when working with mains electricity. Thank goodness that batteries are more straight forward.

Back on the topic of measuring the energy that has been taken from the battery by using a mains plug-in power monitor during recharge, I can see that comparisons are only correct if the charger is switched off when the green light comes on.

James