Lithium Batteries

[ChrisOlson]

This is the new weed whipper my wife got. It's fairly sturdy and I'm pretty impressed with its performance. It has a lot of power and saws off pretty big weeds with no problem. We had a 12V Toro before and this one has a lot more power. The motor in it must be a neo magnet motor like they use in model airplanes because it cogs like a bastard when you try to turn it by hand, almost like it has pistons in it running up against a compression stroke. That indicates to me some pretty strong magnets in it. And this one has a geared head. Our old Toro was direct drive and it lasted four years before the lower bearing went out of the motor. The Toro had VRLA battery in it, 3ah 12V, and even after four years the battery was still good and never seemed to lose any capacity. I'm hoping to get the same, or better, from this one:

[ChrisOlson]

Another thing to keep in mind is that unless you are a professional landscaper, a full year's usage is probably only going to be 30-100 cycles, and the product is only expected to last five years or so. This is a very different usage pattern than your off-grid whole-house batteries.

I think this is a good point too. We probably mow the lawn once a week (and subsequently use the trimmer) from about mid-May to end of June. Then it goes to 10 days or so in the heat of summer when the grass stops growing so fast. So it will actually be excessive to even get 30 cycles on it in a season of lawn mowing, I would say, because lawn mowing is pretty much done here by mid-September because the grass don't grow anymore after that.

They got some timers at Menards for only $4.99 that you can set for a one-time "on" for up to 6 hours. I'm going to guess that the charger that came with the whacker probably charges the battery to 100% so it has maximum run time. It says on the charger that it puts out 400 mA @ 20VDC and the battery says it is a 2ah 20V (40wh). After I used the whipper for about an hour the other day (I didn't time this exact) but it seems it took about 4 hours before the green light came on on the charger. I would assume if the battery was completely discharged it would take 5 hours (or a little better) to recharge it to 100%.

I would say our normal trimming can be done in a half hour after we mow the lawn in most cases. So if I understand the correct care of these batteries, what I'll do is use the trimmer and then keep tabs on how long it takes to get the light to go to green during charging. Probably try that a couple times to make sure it's the same. Then set the timer to a little shorter time that shuts the charger off somewhere in between when it transitions from orange to green (estimating 80% SOC). I'm pretty sure we won't pull it down to 20% SOC during normal use based on the fact that I ran it for an hour the other day on a full charge. So hopefully we'll be cycling it from like 40 or 50% to 80% every time we use it. Then in the winter I'll store it with the battery charged to only orange on the charger.

Then maybe long term we can defeat the battery selling scheme that I'm pretty sure is designed to make money for the company on these cordless weed whippers.

[pleppik]

Another thing to keep in mind is that unless you are a professional landscaper, a full year's usage is probably only going to be 30-100 cycles, and the product is only expected to last five years or so. This is a very different usage pattern than your off-grid whole-house batteries.

You've probably seen yard tools which come with a sticker along the lines of "Not for professional use." I assume that's because the manufacturer knows it can't stand up to 10 hours/day, 6 days/week and still last for the full warranty period.

[PNjunction]

The whacker does not have any sort of SOC indicator but the charger does. It has LED's that light up indicating SOC as it charges. The LED's go from red to orange to green. Since our trimming use pattern is pretty consistent whenever we mow our lawn I will figure out how long it takes to recharge after use. And buy one of those cheap timers that shuts the charger off when it's charged.

Well, Sunking is spot on about the charger. However, we can work with what little your tool has.

It is possible that the charger only charges the battery up to about 90%, not 100%, but we just don't know for sure. So, yes, a programmable timer or other manual device would be a good idea if it doesn't cycle back on every day.

For storage more than a week, or during non-seasonal use, what you could do is use the trimmer, place it back on the charger, and cut if off when the middle led lights up orange. That way you'll be storing it somewhere around the middle of the flat curve, much like when you received it initially.

This points out why I still use Pb/lead batteries for emergency standby use. You shouldn't float li-ion in a full state of charge constantly, and if you need full capacity in an emergency, that means one has to buy twice $$ what they normally would for Pb, which is maintained fully charged. Of course you can maintain li-ion at full charge, but after the emergency is over, cycle life is vastly reduced if put back into a cyclic application after that extended full-capacity storage condition. I love Lifepo4, but it is not a good fit for all applications.

Example:
You store a Li-ion battery at 50% SOC for emergency backup use. Since you don't want to take it below 80% DOD, that's only 30% usable emergency capacity before recharge.
Store a Pb/lead battery at 100% SOC and only take it down to 50% DOD (the normally prescribed cutoff point). That's 50% usable capacity in an emergency, a 20% increase over li-ion.

I know I'm getting off track from a trimmer application, but you can see how operationally, li-ion (in my case LiFepo4 sub-category) may not be the best fit in this situation.

[Sunking]

Sunking - thanks, do you know if they have to be recharged if used a little bit?

No, in fact they should never be stored at full charge. Depending on what pro you ask storage voltage is 40 to 60% SOC. I use a lot of LiPo batteries for my planes, and the charger I use has a storage charge/discharge setting. All I have to do is connect them, set for storage, and the unit will either discharge or charge them.

It is something you might want to check out Chris is getting a RC battery charger. They are slick. They require anything from 12 to 30 volt power input of sufficient power level and will charge any battery chemistry to any algorithm you want or pre-programmed algorithm. from a single cell up to as many as 10 Lithium cells (about 46 volts) Not only will they charge, but can discharge, form cells, exercise, and capacity test. With a power resistor you can discharge up to 1C for load and capacity test. Has temp compensation input for the Nickel chemistry to charge using delta temp bump, delta voltage bump, or both. They are slick my friend.

I use if for all my power tools, camera, and of course RC Toy batteries. It will even charge up a plain ole lead acid 12 volt or 24 volt battery with as much as 60 amps @ 12 volts and 30 amps @ 24 volts. There are quite a few manufactures and model to choose from in various ranges from a few hundred watts up to 2000 watts. Only draw back about them most all require a DC source power.

FWIW I have the I-Charger 3010. It is bi-directional meaning it charges from either direction of input to output, or output to input aka as Regenerative Discharge.

One more note for long term storage on lithium, charge/discharge to manufacture recommended storage voltage (about 50 to 60% SOC), and place them in the fridge, not freezer, fridge of around 38 to 50 degrees.

[ChrisOlson]

Ah Ok, it starts to make sense. What my wife relayed to me was that the salesman said the lithium whacker is supposed to be better than the SLA one we had before because the lithium one has full power right to the very end of the battery charge. After reading your description, I'll bet what people are doing is running them dead, then leaving the charger on continuously until next use. And that's why they sell so many batteries for 'em. That's why I asked because I remember some discussion among sailboaters at the marina that have lithium batteries in their yachts saying that if you have solar panels on your yacht you can't use a controller that "floats" your lithium battery or it will wreck it.

The whacker does not have any sort of SOC indicator but the charger does. It has LED's that light up indicating SOC as it charges. The LED's go from red to orange to green. Since our trimming use pattern is pretty consistent whenever we mow our lawn I will figure out how long it takes to recharge after use. And buy one of those cheap timers that shuts the charger off when it's charged. Hopefully we can prevent becoming one of their "battery victims". The battery is half the price of the whole tool. And the tool is well built. I surmise they make the tool good enough to last like 5 years (it has a 3 year warranty that does NOT cover the battery), then make really damn good money selling replacement batteries for these things.

[PNjunction]

Chris - because there are no real specs on the charging setup they use internally, and an inherent distrust of salesman that don't know diddly about Lithium, here is what you can do.

Due to the extremely flat charge/discharge curve, the rpm's should stay nearly constant down to about 80% DOD or so. If you just start to notice the rotational speed starting to slow down, STOP. You are DONE even if there seems to be some life left in it. At this stage, you are starting to fall down the extremely sharp discharge knee. Going there cuts life tremendously. Do not be tempted to trim the other half of the wall. Place it back on the charger. Like right now. Not after lunch or tomorrow, but right then and there.

We don't know for sure if the salesman was trying to say that this pack has an "LVC" or low-voltage-cutoff, or if he is just allowing people to let it die. Very bad. We are assuming that it doesn't have an LVC.

Li-ion batteries don't like to sit around at 100% charge either. That is one reason they come about half-way charged in retail. If you are not going to use the trimmer in the next few days, then about the most practical thing you can do without any monitoring, is after a full charge just run the trimmer for about 30 seconds or so just to get the charge off the top. In addition, once you have charged it fully, remove the charger. Worse than sitting around fully charged is sitting around fully charged with a constant voltage (but no current flowing) on it forever.

After a session, take the battery inside to someplace cooler than the garage and reinstall when trimming time comes around again. Fanatics may want to place it in a ziploc bag in the frig, and let it thaw out for an hour at least under ambient temps in the bag before using it.

We also don't know how long, or how well treated those batteries were in retail. Assume 3 year life whether you use it or not under best conditions. Possibly inquire about getting a battery directly from the manufacturer as it *may* be treated better than the trip and storage in retail outlets.

The biggest problem I saw with neighbors and friends using Lithium powered tools, is not fully charging it upon receipt, but using it immediately, and driving it down the discharge knee until dead before recharging. No wonder they had bad cycle life.

This is about the best I can recommend for a tool that has no charging specs, no monitoring, and possibly a salesman that simply doesn't know. Ask him if it contains an "LVC" without explaining what that is, and see if he has an answer.

In the past, these tools contained LiFepo4 cells from A123 and were a favorite for homebrewers to rip apart. Not sure today if those cells are A123, or maybe some less reputable brands - and possibly bad quality control and balance issues.

[ChrisOlson]

Sunking - thanks, do you know if they have to be recharged if used a little bit? Like say the battery is charged and I do some trimming around the fireplace a bit that uses some of the energy stored - is it OK to just leave it off the charger until it's ready to be charged after more use at a future date? Or does it have to be charged up right away like a lead-acid battery does to prevent it from sulfating?

Edit:
I should make my question more clear. I can only imagine that lithium batteries have a limited number of discharge/charge cycles before they are wore out, just like a lead-acid battery does. So I'm wondering if people are over-charging them and shortening their life, and that's why they have a rack of replacement batteries for them at the store. They cost $45 bucks for a little battery that you can hold in the palm of your hand.

The salesman at the store told my wife that when the battery goes dead the weed whacker just shuts off. It doesn't die out gradually like our old one did with a VLRA battery. Say the battery can hold 1 hour of run time (which seems pretty likely after using it). If we use the whacker for 20 minutes then it has 40 minutes of charge left in it. Should it be recharged right away, or use the remaining 40 minutes of charge before recharging? What will give the best cycle life for a lithium battery?

[Sunking]

Chris just remember 80/20.

You can discharge then down to 20% SOC. If possible charge them up only to 80%.

Having said that I bet the whacker battery has a low voltage cutoff at 20% SOC.

[ChrisOlson]

Guys, my wife bought a new cordless weed whipper that has a lithium battery on it. I never had anything with lithium battery before and wondering how deep you can discharge these before it damages it (if at all). The instruction booklet don't really say much about the battery other than to "charge it as soon as possible after use for longest life", and don't try to charge it at temperature below 40F.

I used the weed whipper today to do a bunch of trimming around our sheds and ran it probably and hour or so and it never did slow down as it discharged like our old one did that had a VRLA battery in it. Our old weed whipper had LED's on it that showed how much charge was left in the battery - this new one has nothing that shows SOC.

Hoping somebody can give me info on proper care of lithium battery packs, like how deep you can discharge them before damage happens. My wife said they had a whole rack of replacement batteries for it at the store with a box where you can return your old one for recycling. Since stores usually don't stock stuff they can't sell, I deduced this as meaning maybe the battery is a common replacement item and lots of folks are ruining them by not taking care of them properly.