Safe to charge lipo over 4.2 volts if resting voltage under 4.2?

I have asked this question before and was told that I can charge lipo over 4.2 volts as long as the resting voltage is under 4.2 volts.

For example, while charging a Lipo at a high rate the voltage will spike to maybe 4.25 volts under charge but when you stop charge the lipo will have a restring voltage of only 4.19 volts.

In another forum I was having a discussion, when I use regenerative breaking the battery would spike over the max resting voltage by a decent amount. I was told I shouldn't do this.

My question is it OK to go over the max rated voltage of a battery when charging /regen, as long as the resting voltage doesn't exceed the max rated voltage of the battery? Thanks.

Offroader said:

My question is it OK to go over the max rated voltage of a battery when charging /regen, as long as the resting voltage doesn't exceed the max rated voltage of the battery? Thanks.

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Short answer - no. You can never exceed the max battery voltage of a li-ion. (Well, you can, but bad things happen, ranging from slight loss of battery life to fire.)

Longer answer - it is sometimes OK to exceed the max rated voltage AT THE CHARGER in order to compensate for resistive losses. For example, if you are charging at 1 amp, and you have .2 ohms in your wiring (including all resistances up to the cell itself) then it is safe to charge a single cell at 4.4 volts - because you are going to lose .2 volts in the wiring. I believe the Satiator charger does this. This can reduce charge times. However, when you do this, you must monitor current and reduce the overvoltage as current decreases; you can't just say "oh! great! I can go to 4.4 volts." You have to do the math.

No. Just no.

Resting voltage is always below 4.2v. There is very little capacity to gain above 4.1 even. 4.2v is the limit, not a target. Staying under that is always better.

LiPo doesn't have a concept of a surface charge like Lead Acid, so I wonder if you are really charging over 4.2v per cell, or just seeing it due to resistance, as Billvon said.

The reality is, 4.2v/cell isn't a magic number. Its an industry best practice to balance between cycle life and capacity (best for manufacturers, not owners). It's basically the highest voltage you can put in while getting acceptable life spans. You'd get 8-10x more life charging to just 3.9v per cell, but then the advertised capacity and energy density won't be as good.

Some batteries are rated to charge to 4.35v called "LiHV" or "HVLi". There is nothing conclusive I can find, but there are a lot of trusted punters out there who are suggesting it is the same chemistry and manufacturing technique, but higher quality control. In some RC races, people deliberately overcharge their packs for higher voltages and allowing them to use slightly smaller batteries, but they tend to then retire those packs immediately to practice grade.

The long and the short of it is, it's probably okay for some definitions of "okay". I would not expect your packs to last long regening while the battery is nearly full, but will your packs catch fire? Probably not.