Depth of discharge?

Fordulike

Esteemed Pedelecer
Feb 26, 2010
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Please excuse me if this sounds like a Harry and Lloyd question.
When people talk about depth of discharge for example 80%, does that mean 80% of power used or 80% remaining in the battery?
 

flecc

Member
Oct 25, 2006
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80% used where I'm concerned. I also speak of 20% or 80% of charge remaining for example.
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emissions-free

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Oct 24, 2009
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There's 2 terms commonly used, DOD, Depth of Discharge and SOC, State of Charge. DOD of 100% is completely discharged, no more power remaining. 100% SOC is fully charged and is related to temperature as well as voltage so it's not as simple as saying 100% SOC is 3.65V for LifePO4. The same charge voltage at lower temperatures is actually a higher SOC which is why charging in low temperatures is not recommended.

Keeping to say no more than 90% SOC for charging and discharging to no more than 80% will increase the lifecycle of all lithium based batteries as far as I know.
 

flecc

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Oct 25, 2006
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Keeping to say no more than 90% SOC for charging and discharging to no more than 80% will increase the lifecycle of all lithium based batteries as far as I know.
and all battery chemistries I know of. NiMh for example can have as much as a ten year life with more tightly centre banded charge states, and Toyota's Prius NiMh batteries have an 8 year warranty, working between 20% and 80% of charge.
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emissions-free

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Oct 24, 2009
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That's absolutely spot on and this is how packs in EVs can have such a long lifecycle. The only problem with actually trying to implement this on your average ebike is that although you can control the DOD it's not so easy to control the charging level. The BMS inside is fixed to certain values as is the charger. These are not overly high charge levels although the HVC on a typical LiFePO4 BMS is 3.9V whereas the recommended charge level is 3.65V so if you've got a bit of a weak cells it's likely to only get weaker if it hits HVC every time the pack is charged.

Charging in the cold is another big no no, as that 3.65V at -10degC is equivalent to a much higher charge voltage at 20degC. If you then throw in a weak cell which hits HVC into the bargain you can then see why things start to go bad.

The only way I can see to control the level is charge is to use a BMS that is either adjustable or fixed to lower Balance voltages and a charger set up to this lower voltage. The best solution IMO is a proper balance charger that does some form of singe cell charging and is adjustable. With this solution the BMS in the pack can be much simpler.
 

barrycoll

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Sep 14, 2009
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charge/discharge

very interesting...

thought I had picked up all the necessary info re lithium batteries by reading all posts avidly, and now we come to the importance of temperature to charging as well as storage..

I have been storing my wife's e Brompton 37v 10ah lithium in the fridge for the several weeks of non use....trying to store it with 2 charge lamps alight but no more or less, so minimally topping up after use
so when use is planned, can it be done 'from cold' as it were, or better to room temp the battery overnight....
and should a top up charge be added again before use, considering the time scale, and if so, can it be done from 'cold'???

the thought of Long Life battery is very appealing............but the complications seem many and varied!!!

thanks to All barrycoll
 

flecc

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Oct 25, 2006
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Definitely bring up to room temperature naturally first Barry, then a top up charge if the storage has been at below a 50% charge level.

I've got a laptop which I only use on it's lithium battery once or twice a year, and at over five years old when last used, it's capacity was little different from when new. When I take it out of use, I run it down to roughly around 30% of charge, then wrap it in clingfilm to seal against damp and store it at the back of the lowest shelf of the fridge, the lowest temperature point but still above freezing of course.

To bring it back into service, I take it out of the fridge the day before I need to charge it, immediately removing the clingfilm. Then I fully charge it.

The clingfilm is probably not important, but it's so easy with a smal laptop battery there's no point in not taking that precaution.
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flecc

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Oct 25, 2006
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Does this mean little and often with the occasional deep discharge?
Not quite, though it could do if the charge state could be accurately measured. The key thing for long life of a battery in constant use is avoiding the 20% at each end of the charge, using it only between 20% and 80% of charge.

However, that's not the whole story by far. Very important is the rate at which the content is used, regardless of the charge level, and our e-bike motors do tend to give batteries a rough time through drawing high currents, that reducing life. Its a problem of battery size and weight, versus the e-bike's current requirements. Ideally for long life a very large capacity battery should be used, but ideally from the weight and size point of view, an e-bike needs a small and light battery. Therefore the manufacturer has to choose a compromise, not perfect for either smallest size or longest life.

We have a practical example with the orbiting satellites. For obvious reasons their NiMh or lithium batteries have to last ten years, so they are specified at seven times the capacity needed to do the job. Each 24 hours they go into the dark phase out of sunlight and one seventh of the current is used then, this then being topped back up by the solar panels as they get back into sunlight. That way they easily last for the 3600 or so daily charges that are necessary.

Both the usage and charge in those satellites are at very constant and gentle rates, completely unlike our e-bikes where the current varies hugely, very heavy usage climbing hills but little used cruising on the flat.

You can see from all this that we have no hope of realistically doing very much to extend e-bike battery life beyond the natural type abilities. Therefore I think it best to just use the batteries without worrying too much for their welfare, accepting their limitations.

Deep discharge with lithium is only necessary for new battery conditioning when that's recommended, or when it's needed for meter re-calibration as on the Panasonic unit battery at times.
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Straylight

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Jan 31, 2009
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I wonder what batteries the Voyager probes were fitted with - they're still sending data back after 20yrs:eek: :)
 

flecc

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Oct 25, 2006
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I wonder what batteries the Voyager probes were fitted with - they're still sending data back after 20yrs:eek: :)
They use nuclear batteries. These use the radioactive emissions of isotopes to directly generate electricity in various ways, which can include thermal via material differentials. No charging from solar is necessary of course, or even possible in deep space. The only limitation is the decay set by half-lives. Strictly speaking they aren't truly batteries but generators.

They are extremely long lasting and have high energy density, but very low efficiency, sometimes under 1%.

We aren't likely to have "hot" seats on e-bikes any time soon though. :eek:
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Alex728

Esteemed Pedelecer
Dec 16, 2008
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They use nuclear batteries. These use the radioactive emissions of isotopes to directly generate electricity in various ways, which can include thermal via material differentials. No charging from solar is necessary of course, or even possible in deep space. The only limitation is the decay set by half-lives. Strictly speaking they aren't truly batteries but generators.

They are extremely long lasting and have high energy density, but very low efficiency, sometimes under 1%.

We aren't likely to have "hot" seats on e-bikes any time soon though. :eek:
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a few of them were (are?) used in Soviet/Russian lighthouses in remote areas. Following the fall of the USSR, some of these devices were decommisioned and fell into the hands of the conventional scrap metal markets by fair or foul means, with predictable unfortunate results.

One bunch of winter scavengers had even been warming themselves around the device before taking it to the scrapyard! they didn't even survive long enough to get arrested.. :eek: