Remaining battery capacity - from volts or display?

[DynatechFan]

I am still learning my way with my ebay sourced kit, it came with the very common KT LCD3 display so I am hoping this is an answerable and useful (to others) thread - basically my question is about gauging how much power is left

The Shimano Steps set up I used to use gave me a fairly precise % and miles remaining, whereas LCD3 only offers about 5 levels of battery (see below), ie full, 3/4, 1/2, 1/4 and panic! It also allows you to see battery voltage, which might be more useful. It shows 41.2V for my battery freshly charged

Can anyone advise, what % levels do the various full, 3/4 etc come on at? My commute ends just as the display drops to 1/2 - but is that really 50%?

Alternatively, in a 36V system what battery voltages correlate with 100%, 75% 50% 25%? Or isnt it that simple?

 

[malkie0831]

There are several curves and tables on the web that show the voltage of Lipo’s vs the state of charge. Most are related to RC packs rather than 18650 cells, though if you know the manufacturer of the cells in your pack you should be able to see a data sheet for them.
Here is a typical chart, use the 1S column and multiply by the number of series cells in your pack.

For a 36v (or 10S) pack,
75% is about 39.8v,
50% is about 38.4v
25% is about 37.5v.
Much “expert” opinion on this topic says for longevity keep the pack between 20% and 95% charge levels.
An added complication concerns the accuracy of the voltmeter reading in your display. It will only be roughly calibrated, and may not see the full battery voltage due to the cables and other components between the display and the battery.

 

[DynatechFan]

outstanding, thanks for that makie0831. The battery seller claims Samsung 18650 LiNiCoMnO2 cells so I can go look for that

Accepting that the accuracy of the display may be an issue at least I can now have a much clearer sense of the battery health from the voltage. A mate of mine has techy stuff like meters so I will do some comparisons

 

[anotherkiwi]

The best way is to use a Watt meter which shows mAh used. The second best way is experience.

Your battery doesn't charge to 42v, why? Is it used? I have a used 10.4 Ah bottle battery which despite heavy use still charges to 41.5v.

 

[DynatechFan]

Yep, its a new battery. The LCD3 shows 41.2V so possibly not quite accurate as per comments above, it is also -2 deg out there right now . . .

 

[anotherkiwi]

Must be luck of the draw, my LCD-3s are both spot on when it comes to voltage compared to the Watt meter voltage. I now rely on display voltage rather than LiPo alarms - more practical to have it there under your nose rather than relying on a beep from the depth of a pannier in heavy traffic...

 

[Deleted member 4366]

We can't make a general rule for battery voltage and remaining capacity because different batteries behave differently. The main thing to be aware of is that they tend to go down faster as the voltage goes down, and they can suddenly go down very quickly, when any spec. sheets show that they shouldn't.

Voltage changes continuously with load. The LCD3 has an adjustable damping factor (P5), which dampens out the change on the display, so what you see, isn't the actual voltage. It's a moving average.

You could have 25% capacity left, but then you go up a steep hill with full power. That could make enough sag to hit the low voltage cut-off, so you could be effectively empty with 25% remaining.

I have a battery tester, so I can make discharge graphs. They often show anomalous behaviour, which I guess is because the cells are not genuine or first quality.

The only thing you can do is observe the voltage and mileage over a few rides when you do some deep discharges, then you will learn about your own battery's behaviour. How other people's ones behave is to an extent irrelevant.

 

[anotherkiwi]

Yes, I forgot I have P5 set to 0 "real-time voltage", "smart power" mode sucks...

 

[DynatechFan]

okay, got me a little meter, so can be a bit more factual

Charger shows 42.0 volts, fully charged the battery is 41.5 volts - so the LCD 3 is reading slightly low

Should I push the battery (I unplug charger as soon as it goes green) or contact the seller - to be fair it performs within parameters I would expect - but I am no expert

 

[NJS]

okay, got me a little meter, so can be a bit more factual

Charger shows 42.0 volts, fully charged the battery is 41.5 volts - so the LCD 3 is reading slightly low

Should I push the battery (I unplug charger as soon as it goes green) or contact the seller - to be fair it performs within parameters I would expect - but I am no expert

I've read -- here and elsewhere -- that 41.5V (for 10s, 4.15 per cell) is a better maximum voltage for the life expectancy of 18650 cells; and many chargers cut off at that point.

How are you measuring the 42V? Does your charger have a voltmeter integral? If so, does it show partial volts? (Thinking maybe its just rounding up to the nearest whole volt.)

 

[Nealh]

okay, got me a little meter, so can be a bit more factual

Charger shows 42.0 volts, fully charged the battery is 41.5 volts - so the LCD 3 is reading slightly low

Should I push the battery (I unplug charger as soon as it goes green) or contact the seller - to be fair it performs within parameters I would expect - but I am no expert

When the charger goes green it does so as it or the bms says its fully charged, by this it reads that some cells groups have reached max voltage (42v) so will stop, the battery may be a little unbalanced so leaving plugged in once the charger has gone green may allow balancing to continue though at a very slow pace. The final balancing s rate is about 50milliamps so leaving it an hour or two may help. As long as you see 41.5 - 42v there should be little to worry about and you may see the voltage fluctuate between these two figures then settle down once balancing has been achieved, it may take 2 or 3 charges. There is no damage done by leaving it on charge as the bms / charger switch's off.

 

[Nealh]

okay, got me a little meter, so can be a bit more factual

Charger shows 42.0 volts, fully charged the battery is 41.5 volts - so the LCD 3 is reading slightly low

My meter agrees with lcd3 voltage, the meter was tested on newly charged PF & HE2 cells giving a spot on 4.2v.

 

[DynatechFan]

Done some more tinkering, left battery on charge for 90 mins after green light and it now shows 41.7V.

A single 10.5 mile fast, hilly return commute takes it to 37.5V - so I guess the plan will be to recharge every trip - currently that is approx 3 hours so plan to set up a timer for 3hr 30 mins and check full voltage in the mornings

 

[anotherkiwi]

240 minutes more it might show 42v...