Thinking out loud about battery metering

torrent99

Esteemed Pedelecer
Nov 14, 2008
395
36
Highgate, London
Just had an idea about battery metering...

One of the indications of the battery discharging is that the voltage sags under load.

The degree of voltage sag (for a given battery) is dependent both upon the charge state of the battery, and the load it is under.

So would a battery meter that measured both the voltage and the current and then compared against a notional "table" of charge statuses produce better results than the common "just measure the voltage" type of meters?? (Maybe it could be made more useful by either recording a rolling average or a peak "sample and hold" type arrangement)

Discuss.

Cheers

Steve
 
Last edited:

john

Esteemed Pedelecer
Nov 1, 2007
531
0
Manchester
Just had an idea about battery metering...

One of the indications of the battery discharging is that the voltage sags under load.

The degree of voltage sag (for a given battery) is dependent both upon the charge state of the battery, and the load it is under.

So would a battery meter that measured both the voltage and the current and then compared against a notional "table" of charge statuses produce better results than the common "just measure the voltage" type of meters?? (Maybe it could be made more useful by either recording a rolling average or a peak "sample and hold" type arrangement)

Discuss.

Cheers

Steve
Yes. You might want to measure temperature too. Sounds quite complicated. I believe that the meters on Lithium batteries measure charge in and out to report an accurate state. There are IC's you can get for this if you wanted to build a circuit and your battery doesn't already have one.
 

torrent99

Esteemed Pedelecer
Nov 14, 2008
395
36
Highgate, London
Yes. You might want to measure temperature too. Sounds quite complicated. I believe that the meters on Lithium batteries measure charge in and out to report an accurate state. There are IC's you can get for this if you wanted to build a circuit and your battery doesn't already have one.
Really? Sounds interesting! Do you know of any part numbers?
 

flecc

Member
Oct 25, 2006
52,763
30,349
I don't think the sophistication is worth the bother of more complex methods, since accuracy still wouldn't be achieved in practice. Integral battery meters need a fully discharged baseline to work from and this is established when the battery is first conditioned with full charge and discharge cycles. This then drifts out with use and can only be re-established by emptying the battery on the bike about every 30 charges which is not good for lithium batteries.

Also we have devices which measure current in-out and can act as rather more accurate meters, like the Cycle Analyst or the less expensive WattsUp, and they have the advantage of handlebar readout.

Personally I prefer just using the zeroing feature of my cycle computers trip recorders and judging the battery state by known mileage capability per charge, since most e-bike routes used are well known and understood by owners like myself.
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b-twinelec

Just Joined
Dec 26, 2008
3
0
Battery stored energy

On the topic of battery autonomy.

A) With a cycle analyst, it is possible to obtain a reading of the average Wh/mi consumption. This being dependent of the effort put in and the resistance encountered (hills and wind). But I was wondering if I can simply evaluate autonomy by saying:
If in average I consume a typical 10 wh/mi.
 If I have a 36*15Ah= 540 Wh battery pack, then I have a theoretical autonomy of 54 miles.
 Whereas if I had a 24V system with 14 Ah batteries, then my autonomy would be 24*14=336 Wh/ 10 = 33 miles.
Is the above valid?

B) My second query is that I have been told that a 36 V 15 Ah LSA had the same energy stored as a 36V 10 Ah Li Ion. Could I get an explanation as to why is this the case?

Regards,
 

flecc

Member
Oct 25, 2006
52,763
30,349
That autonomy idea seems valid, but it would be a bit to crude to place a high dependence on it, given the day to day variation in circumstances, temperature, variations in throttle use, wind speeds and directions etc.

The answer to (B) is that lead acid batteries cannot give up all their stored charge in the way that other types can. The amount they can give up varies somewhat with the discharge rate and with our quite high rates only about half to two thirds of the charge content is recoverable, so a larger battery is required for a given usable capacity.
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