Battery opinions wanted..

[Morgen 3Eman]

Hi Folks,

I am thinking about how to care for my LiFePO 48 Volt GM battery over the winter.  From posts that I have read, it is best to not allow the battery to fully discharge, and frequent short recharges seem to be a not bad thing.   I expect that I will not be riding my Morgen very much from October thru mid April, as that is the really rainy season in the Portland Oregon area. 

Here is a scheme that just occcured to me:  Just plug in the GM battery charger, turn it on and leave it on all winter.  As the battery self discharges, the charger senses the voltage drop, turns itself on, and tops off the charge.  And the cycle repeats all winter long.   The problem is that I don't really know if :  A.  The battery charger will restart as the battery voltage drops after a charge.  B.  Will it really  be a good thing for the battery, bad for it, or just a waste of time?

So what do you good people think?  And if someone can point me to some data, that would be really super nice.


TTFN,
Dennis

[pirontas]

Here is some good info:
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

Quote from that article:

Let’s look at real-life situations and examine what stresses lithium-ion batteries encounter. Most packs last three to five years. Environmental conditions, and not cycling alone, are a key ingredient to longevity, and the worst situation is keeping a fully charged battery at elevated temperatures. This is the case when running a laptop off the power grid. Under these conditions, a battery will typically last for about two years, whether cycled or not. The pack does not die suddenly but will give lower runtimes with aging.

Even more stressful is leaving a battery in a hot car, especially if exposed to the sun. When not in use, store the battery in a cool place. For long-term storage, manufacturers recommend a 40 percent charge. This allows for some self-discharge while still retaining sufficient charge to keep the protection circuit active. Finding the ideal state-of-charge is not easy; this would require a discharge with appropriate cut-off. Do not worry too much about the state-of-charge; a cool and dry place is more important than SoC. Read more about How to Store Batteries.

Of course they're talking about storing it for a year or longer. Maybe the best thing is to keep it in a closet where there are no heat vents so it will be a little cooler, and run a dehumidifier. I've read elsewhere it's a good idea to top off the charge once a month for short term storage.

Keeping it plugged to the charger all the time could be dangerous and may not help at all. Earlier this year we had a storm and power was knocked out when lightning hit a nearby transformer. The CPU in my computer got fried from the surge even though I was using a surge protector. Just goes to show things aren't perfect and stuff happens. I try not to charge my battery when there's a storm. But anyway that's just one potential problem. I'm not sure I trust my charger to avoid overcharging the battery when being connected for a long time, or to not malfunction and start a fire. It's also possible it won't recognize if the battery has discharged slightly and could be topped off. It may get stuck in that state where it saw the battery was fully charged and it's doing nothing more than waiting to be unplugged.

[Morgen 3Eman]

Hi Pirontas

Thanks for the great information.  My garage is pretty cool this time of year, and it appears all I have to do is plug it in for an hour or so every couple of weeks to keep it always partly charged, never fully charged or fully discharged.. 

I sure came up with a bad, bad, bad idea this time!  I think I'd better read some more.

Thanks again,

TTFN,
Dennis

[pirontas]

No problem Dennis, glad I could help. I bet the garage would be a good place as long as it stays above freezing.

[Kirk]

   Lithium cells should be stored at their nominal voltage, rather then fully charged. For instance LiPo cells should be stored at 3.7V rather then their fully charged state of 4.2V. If you store them fully charged you'll lose some of the cell longevity. Storing them at full charge for a year according to one of the maniacs on the ES forum, can lose you as much as 20% of the life of the battery. For short term Winter storage it wouldn't be so much of a hit probably. LifeP04 batts are pretty sturdy, maybe a 5% hit on the life of the battery storing it at full charge for that amount of time. Just a guess, YMMV
   Not sure that I would store them in the garage in Portland over the Winter. Seems like in recent years there have been some pretty cold times there. Out on the coast where it only gets down to freezing about every 15 years or so, it might not be a bad idea.

[Morgen 3Eman]

Hi Kirk,

To start out, all I know about chemistry is that you put salt on potatoes.   Well, maybe a little more than that, but I'm new to using hi tech batteries.  So please bear with me.

I'm afraid I don't understand what you are telling me.  I thought the GM "48V" battery consisted of 16 cells, and they were charged by the GM charger to 58VDC, or just under 3.7 VDC/cell.   Thus, they never reached 4+VDC, and the problem of capacity reduction from high charge stress was non-existent.  From the  site Pirontas pointed me to, letting them self discharge down to about 50 VDC before  re-charging is a good way to extend capacity lifetime.  Keeping them in a cool place is another good idea.   

Are you saying the battery should be slightly discharged  immediately after charging it will extend its capacity lifetime?

I have noticed that within a few hours after charging the battery to 58VDC the output voltage drops down to 54-55 VDC. Then it drops about .4-.5VDC/week .   Isn't this normal?  Or is this indicative of a cell failure ?

Regarding my garage temperature, I do oil paintings out there during the winter, and it seldom drops into the low 50's.  So i'm not too worried about it being too cold for the LiFePO4 cells. 

TTFN,
Dennis

[Cornelius]

I'm with Kirk on this one; Long time storage of Lithium batteries should be at their rated voltage (3.6/3.7V for LiCo and LiMn, and 3.3V for LiFePo).

Dennis;
If your 48V GM pack has 16 cells and the charger are 58V, then each cell gets max 3.625V at End-of-charge, which is a nice compromise between capacity and longlivety for LiFePo4 cells. LiFePo should have a max charge of 3.650V, and those 0.025V you're missing, results in maybe 1-2% capacity, but you gain a more assured life for your cells.
Charging LiFePo's at 3.70V + are a guaranteed cell-life-killer; charging at 4.0V++ are to play with your life and your enviroment, and will most certainly destroy your cells..

(All Rechargeable Lithium cells today have a thermal vent that pops when overheating, to prevent an explosion. Cobalt-based cells (like in your laptop and mobile phone) does this at around 150deg. C., and LiMn etc. and LiFePo, around 250Deg. C.) They also have an over-discharge-fuse that snaps if shorted etc.
Both of thos things above renders the cell useless afterwards.

No need to discharge newly charged cells; just do not charge them to the absolute max charging voltage. That way, they'll keep their long life.

Regarding self-discharge; I think Lithium have a 5% loss the first day, then 1-2% self-discharge/week.

Most Lithium cells today handles cold temperatures well, except that they'll have lower capacity, that is. But they should Never be charged below freezing!