How much to charge lithium packs

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bladecar
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How much to charge lithium packs

Post by bladecar » Thu, 04 Jul 2013, 20:32

Hi,

I see a newspaper article giving advice to avoid charging mobile phones to 100% and to not leave them on charge as it shortens the life of the battery.

A knowledgeable friend of mine believes 50% charge is the place to aim for when storing lithium batteries.

If you have an ev, should you be able to set your bms to charge fully to 50%, to 100% or somewhere in between by adjustment of a control depending on your needs and could that be done?

What effect does it have if you turn your charger off at 80% for the highest charge when it doesn't balance until 100%.
If you do this for 30 days and then charge the system until it shuts itself down, do you think there would be a significantly wider range of voltages within the batteries compared to always making the system fully charge?

I think this is something worth discussing.   :)

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Johny
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How much to charge lithium packs

Post by Johny » Thu, 04 Jul 2013, 20:39

My belief is that these reports and ideas for charging to 80% or less to extend battery life are intended for Lithium Ion or (Polymer) and Lithium Cobolt batteries (iMiev/LEAF) - not LiFePO4. While it is generally believed that LiFePO4 would suffer from being "floated" at 100%, most of the 384 cells I had in storage for 2 1/2 years were regulary charged to 100% whether they needed to be or not. A random sampling of my cells now 3 1/2 years later shows 100% capacity. Internal resistance of the pack as a whole is pretty much as quoted new.

I do not now keep the pack at 100% if I'm not using the car (off the road for other reasons). I charge to 100% just before use.

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acmotor
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How much to charge lithium packs

Post by acmotor » Thu, 04 Jul 2013, 23:34

Another way to look at it is.... (with my iMiEV)

In an EV, range is the number one limiting factor. Why would you charge to anything less than 100% ?
For many EV users it is more likely to be the discharged end of the pack that will do the most cell damage if regularly visited ? After all, if it is going flat, users are likely to by trying to ring the last breath of life from the pack. Commercial BMS is probably quite good being able to actually stop the EV ! but it is hammering the bottom end of the pack that kills many cells in conversions. Been there.

I have always charged to 100% if I am going to use my EV within the next few hours. But then I use up to 80% of capacity on each drive.
If I get 2500 instead of 3000 cycles life will that really make up for the #1 limiting factor of an EV.... the range ?
It will probably end up more than 10 years either way and I am hanging out for the next technology battery pack rather than thinking I will still be driving on the same battery pack in 10 years !!

Mitsubishi does not say the battery warranty is voided by charging to 100% or discharging to turtle so maybe the charge level is an issue in our minds and not the manufacturers ? Leaf has an 80% charge option that may suit folk who drive shorter distances or charge once a week ?
But Leaf and iMiEV both need 80% to do my run (I tested this myself) so there is no way I would only charge to 80% in the first place.

Mitsubishi does suggest not storing with low battery capacity as the BMS and some ECU functions are live and consuming power. Taking a lithium cell low in voltage or worse still reversing it is far worse than holding it at a high state of charge AFAIK.
Mitsubishi also suggests keeping the battery fully charged for days or weeks is not a good practice. No chance of that !
But hey, get out there and drive your EV ! Neither of these scenarios will present themselves then.

As long as I don't have to eat my words the day the warranty expires I'll be happy. Image
iMiEV MY12     110,230km in pure Electric and loving it !

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woody
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How much to charge lithium packs

Post by woody » Thu, 04 Jul 2013, 23:38

Aha but what do 100% and Turtle on your commercial EV dial correspond to? Turtle = 10%
100% = 90%?

90% of what? Volts, Amp-hours, Watt-hours?
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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acmotor
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How much to charge lithium packs

Post by acmotor » Fri, 05 Jul 2013, 00:08

I know what you are suggesting but.... there are 88 x 3.7V 50Ah cells that's 16.28kWh and pack capacity is claimed as 16kWh.
Measuring charge from external power meter confirms nearly 17kWh for full charge less charger loss 5-6% (specs claim 95% eff.) gives 16kW accessable kWh.

The energy meter has 16 segments that seem to read in very accurate 1kWh units. I expect the meter data is Ah originated but for lithium curve that would be flat in voltage so Wh implied ? I'd expect that to give some rate dependent errors, however data I've recorded and technical manual suggest 'energy in Wh' not simple Ah is displayed.
iMiEV MY12     110,230km in pure Electric and loving it !

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Simon
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How much to charge lithium packs

Post by Simon » Fri, 05 Jul 2013, 00:18

I wouldnt of thought battery longevity was an issue with phones with people upgrading every time a new model comes out!? :-)

I remember reading the my Nissan Leaf forums and some of the guys over there were only charging to 80% all the time and yet they were discharging the battery down to only one or two bars which seems silly to me. Why do deep discharges every day and not use the top 20%? I don't think Nissan intended this sort of behaviour when they recommended 80% charges to extend battery life!


Better off to just do 100% charges and time the recharge to finish when you are ready to drive.

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