When to initially charge a backup battery

[Snoop]

A quick question for the battery science nerds. ….

If I were to buy a second battery with a purpose of being an eventual replacement to my current battery (ie not intending to use it to extend my range), and assuming the battery arrives uncharged, would it be best to leave it “as is” until I am ready to start using it, or should I charge it as soon as it arrives and maintain at 70 percent-ish?

I know I can wait to buy the battery down the road, but this question is if I decide to buy it now.

Thx!

 

["A"]

I would charge it to full, ride it down to at least 20% (if not lower) to make sure it holds capacity then charge back to 80-90% for storage.

 

[m@Robertson]

Its common best practice for a Li-ion pack (Li-NMC is what an ebike pack usually is, to be technically specific) to be best stored at around 40-50%.

storage voltage for li ion packs - Google Search

Scroll down to the Lithium section here for a good shortie explanation.

BU-702: How to Store Batteries

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batteryuniversity.com

Something I once heard worth repeating: Puncture a battery charged to 100% and it goes BOOM. Puncture a battery charged to 60% and it goes fizzzzz. Not very scientific but you get the idea.

Also a not-so-bad best practice for a brand-new pack that I got from a major ebike vendor: For a new battery pack, run its charge up to 100% - don't let it sit at that charge state! - and then go ride it normally. After the ride, charge it to 100% and let it balance the cells. Usually that means leaving it for say 2 hrs at 100%. Do this for your first six normal-use charge cycles. After that you can drain it back down to a normal storage state. Probably 80% if you are using it, 50% if storing. The purpose of doing this is to ensure the cells in all the cell groups are balanced, as you can't say for sure how the manufacturer matched up the cells or charged them initially.

Going deep on depth of discharge is a potential issue for an Li-NMC pack. Not if going down to 20% but more than that is potentially bad for it. Scroll down to the Depth of Discharge section here for a bit more.

BU-808: How to Prolong Lithium-based Batteries

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batteryuniversity.com

or

depth of discharge effect on li-nmc packs - Google Search

Also, to get your most out of that investment, I would not let the pack sit and do nothing for extended periods. I have seen way too many posts over the years from people who somehow wound up with a dead battery for some reason. I'd suggest switching the batteries monthly so each of them gets some wear, but also each only wears down half as much.