rechargeable battery storage -- any thoughts?

idk. i keep mine in the barn, which also gets quite toasty. they have survived surprisingly well for a good number of years, but i don't have any scientific evidence.
if they get too hot or cold, the charger won't charge them.

my concern is keeping the lithium topped up all the time

 

I probably wouldn't worry about it. With Li-ion, just store them below fully charged, and charge up before use.

 

The rule I picked up very long ago was that for best longevity, store these with roughly a 40% charge. That rule appears here too:
BU-702: How to Store Batteries - Battery University

For the Li-Ion rechargeables that can be air shipped, IATA / FAA rules place a maximum of 30% state-of-charge. Though this is for vessel safety, not for product longevity:
BU-704a: Shipping Lithium-based Batteries by Air - Battery University

 

I was originally going to say store discharged, but that could imply having the battery dead. That may not be a good thing with Li-ion. Personally, I just put the batteries away after use. I don't have an easy way for checking the charge level. When needed, I'll charge them up before hand.

Ideally, you want things to last as long as possible, but replacement batteries for the more popular product lines can be quite inexpensive. The case of buying a new tool is cheaper than simply getting a replacement battery are gone.

 

Storage while fully discharged is not a good thing for Li-ion, it will accelerate a damage mechanism. And if the cells have a built-in battery protection circuit (with MOSFETs to disconnect the battery), a significant self-discharge after reaching the undervoltage threshold may make it difficult to wake up that protection circuit to allow recharge.

 

Unfortunately, the chargers and battery packs generally don't have an easy way for the user to know what the state of charge is for these power tools.

 

is state of charge measured in voltage?

 

I think there are various ways of measuring it. Most people don't want to pull out the multimeter to check a tool's battery, assuming they have the multimeter.

 

Not reliably. But that is often just what we have.

 

Some lithium batteries have percent of charge indicators. Kobalt uses 3 LED's that illuminate when a button is pushed to show pecent of charge.

Though probably not desireable two years ago I left the two Litium batteries on their chargers from November (last mower use) to April (1st mower use) I didn't see any issues. We have had the mower and batteries for almost 5 years and the batteries and chargers stay in our garage at all times which varies in temperature from a high in the 100's in August to sometimes single digits in January/February. Probably not ideal but heck, they are yard tools, they belong in the garage.

Our 2008 Prius hybrid batteries shared the same housing arrangement in the garage and they seemed fine with it - no proiblems in the 12 years we owned it (yes i know they are not lithium)

 

toyota recommends not leaving lithium fully charged for long periods of time. but they don't define 'long periods', so i keep mine around 1/3 to 1/2 most of the time, and drain to hybrid mode if going away.

 

A car traction battery is more likely to have a larger buffer in the state of charge range than ones for power tools.