Opti-power fully charged at 4.18v per cell

[dav20043]

Ashley's explanation looks correct to me based on previous knowledge? What part looks backwards, Dave??

In My experience you have lower voltage when the lipos are cold and higher when they are hot which is why every winter your low voltage protection cuts in early even though batteries have been charged at home in the warm. The cold lipo charge reduces the cv level to prevent over charging of the packs and the voltage rising when it warms through use or higher ambient temps.This is also how it is easy to over discharge packs as when they are hot of the model the voltage is higher and then the voltage drops as the pack cools below what would be your safety cut off point.

[Ashley Davis]

Unfortunately Dave you've made the mistake of confusing voltage with capacity, let me explain .......

The reason your low voltage protection cuts in early is because your lipo capacity has dropped. So your 3000mah pack is now only 2000mah in the cold. This means that when you run it it will run for less time and not only that but the load your are placing on the pack hasn't changed. So when you run your heli it's like putting a pack in it that is too small, you get a bigger voltage drop under load and it runs for a lot less time. So to some extent you are correct about lower voltage but this is purely down to capacity loss and running under load.

However, the effect is completely opposite on a pack that isn't under load and has been fully charged. Let's say you have your 3000mah pack charged up at 20 degrees C and you walk it outside to freezing temperatures. The same capacity loss occurs as described above but you now have 3000mah of charge in a 2000mah pack, so what happens is it goes significantly over voltage just like you charged it too much, this damages the pack. If you fly the pack that over voltage condition won't last though because as decribed above you are going to put too big a load on it for it's effective capacity at freezing temperatures and you will see poor performance and hit the voltage cutoff early.

I hope that helps explain it

If you want to prove this... connect a voltage meter / balance checker to one of your packs and walk outside into the cold...... you will see a voltage rise, it happens almost immediately you won't have to wait around for the pack to cool, they are really quite sensitive.

[Ashley Davis]

Something else I was going to add was that the effect of the above is also increased due to the effect of temperature on the internals of a lithium pack. Low temperature reduces electron mobility and slows the chemical reaction, which leads to a higher IR. With higher IR you loose even more capacity as the pack struggles to release charge and becomes inefficient, further leading to lower voltages under load on top of the problems caused by capacity loss. At extreme low temperatures (-20 and beyond) lithium plating of the anode occurs and this is usually permanent damage.

[dav20043]

Something else I was going to add was that the effect of the above is also increased due to the effect of temperature on the internals of a lithium pack. Low temperature reduces electron mobility and slows the chemical reaction, which leads to a higher IR. With higher IR you loose even more capacity as the pack struggles to release charge and becomes inefficient, further leading to lower voltages under load on top of the problems caused by capacity loss. At extreme low temperatures (-20 and beyond) lithium plating of the anode occurs and this is usually permanent damage.

so in short you have lower voltages when cold due to hi IR and decreased capacity this is why it charges to a lower voltage so when the pack warms it won't go over 4.2 per cell which is the reverse of protecting from a warm to cold environment

When you take a lithium pack from warm to cold it will suffer a drop in capacity. This causes a rise in voltage as the charged state will effectively increase as the capacity decreases. This results in damage. If you move a cold pack to a warm room it's capacity will increase with the temperature rise and the voltage drops as a result, no damage should occur due to this.

The reason I believe (and this is my best guess) the PL8 only charges to 4.1v in the cold is that the expectation is that you have flown the pack and are now field charging in the cold. The pack if it's flown may well be quite warm inside but the ambient temperature outside is cold. This creates a dangerous scenario where by the pack has it's full effective capacity due to it's internal heat but if the charger charges it to full capacity then it is quite obvious that it will cool either during or after charge and this could lead to an over charge condition. Therefore the PL8 takes the decision to only charge to 4.1V in the cold and avoids this particular scenario.
The other interesting scenario is the automatic 1C/2C/3C charging on the Cellpro range, these calculate the pack capacity by measuring voltage changes based on current. I suspect this process may yield incorrect results in the cold whereby again it may be safe to only charge to a lower termination voltage.

[Ashley Davis]

so in short you have lower voltages when cold due to hi IR and decreased capacity this is why it charges to a lower voltage so when the pack warms it won't go over 4.2 per cell which is the reverse of protecting from a warm to cold environment

um, not exactly, I think you may be trying to oversimplifying it, or I'm not reading your response in the right way.

Please read my first post on this that you said was backwards, then read my second and third posts. It's not massively complicated but it should make sense if you read all three posts. All three posts work around the same principles of temperature affecting capacity, voltage is affected by the capacity change not the temperature change.

The bottom line is you shouldn't charge packs fully in the warm and then take them into the cold but you can fully charge packs in the cold and take them into the warm. However, see my first post as to why the PL8 has a safety charge feature for in the cold.

[dav20043]

um, not exactly, I think you may be trying to oversimplifying it, or I'm not reading your response in the right way.

Please read my first post on this that you said was backwards, then read my second and third posts. It's not massively complicated but it should make sense if you read all three posts. All three posts work around the same principles of temperature affecting capacity, voltage is affected by the capacity change not the temperature change.

The bottom line is you shouldn't charge packs fully in the warm and then take them into the cold but you can fully charge packs in the cold and take them into the warm. However, see my first post as to why the PL8 has a safety charge feature for in the cold.

Hi Ash

with the risk of being pedantic i think you charge packs fully in the warm and less in the cold this is how the cold protection charge works and what i said you had backwards

[Stuart]

Right my head hurts your both right.....lol lets say in the winter pack your electric heli away and get a nitro one.....

[Ashley Davis]

OK Dave, by all means do as you please with your packs, however, I'll state one last time that charging fully in the warm and taking them into a cold environment will irreversibly damage your packs.

[cjcj1949]

I thought it was generally accepted that packs should be stored cool, especially when fully charged?? Perhaps nothing about Lipos is generally agreed?? Except that they work better in the warm.

This is all off the topic.

[Ashley Davis]

Nothing wrong with storing packs cool, the pack will degrade less at lower temperature. The key is not to charge it warm and then store it cool.

At freezing you get something like 2% loss of capacity over 1 year when the pack is charged to 40% of capacity. Charge the pack to full capacity and store at zero degrees and that loss will increase to around 6%. Do the same at 25 Centigrade and the loss over 1 year for 40% charge is about 4% of capacity, storing fully charged will lose you about 20% of the pack capacity.

So, definitely don't store packs fully charged, much better to store them at 40% of capacity and storing them cool will further decrease degradation.

[cjcj1949]

Well at least storage is agreed.20% capacity loss is the usual definition of a dead battery.

[Mark_T]

I thought it was generally accepted that packs should be stored cool, especially when fully charged??

Stored cool, yes
Stored fully charged, no
Cool and fully charged, definitely not.

I've never seen anything else other than cool and around 3.8V advised for long term storage.

[scallybert]

[...]The bottom line is you shouldn't charge packs fully in the warm and then take them into the cold

Yep...

but you can fully charge packs in the cold and take them into the warm. However, see my first post as to why the PL8 has a safety charge feature for in the cold.

I do like that theory, but I'm not sufficiently convinced by it to believe it's ok to fully charge in the cold & move to warm.

I think I'd like CellPro to say why the charger behaves like it does in cold ambients, and Andy's promised research would be good.

[cjcj1949]

As I believe it, there are two things that kill lipos when not being used. Their temperature and their charge level. Keeping them cool and around 3.8V is best. There seem two opinions on what to do with fully charged packs, either keep them warm or keep them cool, unless they were charged cool. I only charge before using. I'm sure its ok to keep them charged for a while before using, but if I do that I'll forget and leave them charged when a good day turns out to be wet, or whatever

[NEAGRON]

Gentlemen,this is turning out to be a really good post,as its been very informative about lipo's,certainly increased my knowledge base about them,I really think there should be a sticky made by one or two of the guys that have posted on here,to give a guide on good lipo chargeing practices for us that dont know.How about it Stuart,and I suggest the author's be Andy&Ashley,your's Marty.