# “lithium for buffered usage in UPS”, or floating-charge lithium accumulator

For lead acid there are typically two paradigms of charging (or use-scenarios).

• First one is deep cycles, when you exaust 40-60% of capacity (not less than 12.4V), then charge it to 14.4 with impulse-charger, and start to discharge and use it again. Lifetime is measured in "cycles with x% DoD, before capacity fells more than twice"

• Another one is buffered use (for example in UPS backup) - you set 13.8V on rail, and keep that voltage forever, performing "floating charge". And battery will take just the current needed to keep itself in tip top condition. In case of power failure battery could stay long, release most of its energy, ready to deploy like 80% or more, waiting before power will be restored. Lifetime is measured in "Years under 13.6V without discharging, before capacity fells more than twice".

That is well known, and I even found very good and detailed report with graphs on lead acids. Later on I will provide a link here on that material (if I could remember it).

Now I am trying to find some information for li-ion. And only what I typically could find is a CC-CV, for a Li-ion specific subtype. Typical algorithm advices (that are typically met) are as follows:

• Charge it for 4.2V with current=1C (they mean "battery capacity in AH, divided by 1 hour"), that charges to 80% within hour.
• Keep it under 4.2V until current falls under 0.1C, that charges to 99.9% within 2-3 hours

And I learned that by heart to the day :)

What I am trying to find - is an information about "float charging" lithium accumulator of any type - LiIon/LiFePO/LiAnything. Different types have different cell voltage: for example LiFePO CC-CV ranges fot 3.2-3.6V; But Li-ion are higher - 3.6-4.2V; Maybe there's others..

What voltage should I choose to keep battery under voltage source safely for years, without disconnecting?

And for an expert a good answer is a graph series - "How does chosen voltage and ambient temperature affects capacity over time, for various lithium battery types". Cant really find that on the net, it feels like no one ever used lithium for ups needs.

• No one uses lithium for UPSes because lithium batteries can't safely be float-charged, as far as I'm aware. – Hearth Jan 24 at 15:30
• @Hearth Could you please provide a link, where you get this info, so that I could be aware too (and close question, as being irrelevant/impossible) – xakepp35 Jan 24 at 15:32
• Unfortunately I can't, as this is just something I've learned at some point in the past. I don't remember where I learned it--which is why I said "as far as I'm aware", because I could be wrong. – Hearth Jan 24 at 15:35
• @Hearth For now I only can see that typical lead acid gel vrla is priced below 2.5 usd per 1AH, (or below 20 cent per WH). On the contrast, li ion is at least twice as more expensive - goes from 40 cents per WH. From my perspective, that's the main reason why they are not used for UPS - usually backup bank requires large capacity, and that's comes to be over-costly - "you may put 2x capacity or more for that price, and lead-acid UPS hardware is more common" and most people just opts for it. Also lithium is considered more dangerous, to store high amount of highly-flammable stuff at home. – xakepp35 Jan 24 at 15:41
• "it feels like no one ever used lithium for ups needs" - what, other than Tesla in their Powerwall and Powerpack products? I'm sure there are others, but the reality is that for the average small UPS weight and size are less important than cost. – Finbarr Jan 24 at 16:26