2 battery questions. Charging below zero and LED stays on.

[HittheroadJ]

I have SLA batteries at the moment. I am considering getting a LiFePo4 battery (this would involve new charger etc. of course).

I have 2 questions.

a. I usually charge outdoors/on the bike. The shed has electricity. In winter I take the battery inside to charge. SLA's have recommended not to charge below 10 centigrade. The LiFePo4 is recommended to charge from 0 centigrade upwards. If I buy a big LiFePo4 battery and attached it to the bike, how bad is it for the battery to be charged at, say -5 centigrade? Will it damage the battery?

b. I fitted timers to the chargers and have them switch off after a couple of hours. I have noticed that the LED's on the chargers stay green when the charger is disconnected. This indicates that the current from the battery to the charger keeps the wee LED light on. From this perspective it is better to physically unplug the charger. But the LED seems to use very little power. I'd hate to have to go out every night. How bad is it to leave he charger plugged in?

Thanks for the help.

 

[flecc]

a) The battery case and cell casings in the battery provide an insulating effect so it takes a long time for the cell contents to drop to external air temperatures. The battery in use during the day before the charging will have gained warmth from use, and possibly from higher air temperature if any. Since the charge commences then after the use presumably, and the charging process raises the internal battery temperature, I can't see this is a problem in practice. If you wanted to charge after the battery had been out of use for a while, that could be a problem of course.

b) I'm sure a BMS in an LiFePO4 battery couldn't allow any current to flow into a charger in the way the SLA battery/charger combination does, so not a problem. However, if you intend using a home constructed battery such as with A123 cells without a BMS, there could be a problem.
.

 

[Dynamic Position]

Consumer grade Lithium-ion batteries should not be charged below 0°C (32°F). Although the packs appear to be charging normally at freezing temperatures, the cell impedance goes up and the acceptance of the ions on the anode is drastically reduced.

What is most troubling is the plating of metallic Lithium that occurs on the anode. The higher the charge rate, the more pronounced the plating will be. A prolonged charge at cold temperatures will eventually compromise the safety of the pack. The plating is permanent and no amount of cycling can reverse this effect. Unknown to the user, such a battery will become more vulnerable to failure if subjected to impact, crush or high rate charging. Venting with flame could be the result.

Quality chargers reduce the charge current at cold temperatures and avert a charge altogether below 0°C (32°F). When charging a cold battery, allow the pack to warm up before putting it into the charger. Discharging a Lithium-ion battery at cold temperature does not cause any harm. The lower performance will only be noticeable while the pack is dwelling in the cold state.

There are specialty Lithium-ion batteries that allow low-temperature charging. These packs are made for military and aerospace applications. The lower viscosity of the electrolyte of these batteries may reduce the rate capability and decrease the cycle life. Beside lower performance, the high purchase price is another concern for the consumer.

Hittheroad confirms LiFePo4 User Instructions recommend charging at 0 degrees Centigrade upwards. I would guess that the LifePo charger would not function because it will have a built in temperature sensor and CMS to prevent operation at ambient temperatures outwith recommended range?

We will need to wait and see how this recent cold snap will affect electric car batteries at road side charging points could present problems? It is just as well e-bike batteries can be taken indoors for charging.

 

[HittheroadJ]

Thanks for your insights, I did not know about plating. Taking the bike indoors to charge could cause the "rolling-pin" effect and serious headaches, though.

I was hoping to make a DIY box for a battery, but it looks like I'll have to invent a detachable one (less headache than the rolling-pin effect, I hope).

cheers,
Y

Consumer grade Lithium-ion batteries should not be charged below 0°C (32°F). Although the packs appear to be charging normally at freezing temperatures, the cell impedance goes up and the acceptance of the ions on the anode is drastically reduced.

What is most troubling is the plating of metallic Lithium that occurs on the anode. The higher the charge rate, the more pronounced the plating will be. A prolonged charge at cold temperatures will eventually compromise the safety of the pack. The plating is permanent and no amount of cycling can reverse this effect. Unknown to the user, such a battery will become more vulnerable to failure if subjected to impact, crush or high rate charging. Venting with flame could be the result.

Quality chargers reduce the charge current at cold temperatures and avert a charge altogether below 0°C (32°F). When charging a cold battery, allow the pack to warm up before putting it into the charger. Discharging a Lithium-ion battery at cold temperature does not cause any harm. The lower performance will only be noticeable while the pack is dwelling in the cold state.

There are specialty Lithium-ion batteries that allow low-temperature charging. These packs are made for military and aerospace applications. The lower viscosity of the electrolyte of these batteries may reduce the rate capability and decrease the cycle life. Beside lower performance, the high purchase price is another concern for the consumer.

Hittheroad confirms LiFePo4 User Instructions recommend charging at 0 degrees Centigrade upwards. I would guess that the LifePo charger would not function because it will have a built in temperature sensor and CMS to prevent operation at ambient temperatures outwith recommended range?

We will need to wait and see how this recent cold snap will affect electric car batteries at road side charging points could present problems? It is just as well e-bike batteries can be taken indoors for charging.

 

[HittheroadJ]

Thanks Flecc.

I may charge on arrival and the use and charging process should keep the battery warm enough. Time-clock would switch off after a while. Unfortunately I would still have to make a detachable battery casing, so I can also charge when the battery has gone cold as well.

I would just use the supplied BMS, So I could just let a timer switch the charging off.

Thanks again.
Great.

a) The battery case and cell casings in the battery provide an insulating effect so it takes a long time for the cell contents to drop to external air temperatures. The battery in use during the day before the charging will have gained warmth from use, and possibly from higher air temperature if any. Since the charge commences then after the use presumably, and the charging process raises the internal battery temperature, I can't see this is a problem in practice. If you wanted to charge after the battery had been out of use for a while, that could be a problem of course.

b) I'm sure a BMS in an LiFePO4 battery couldn't allow any current to flow into a charger in the way the SLA battery/charger combination does, so not a problem. However, if you intend using a home constructed battery such as with A123 cells without a BMS, there could be a problem.
.

 

[Dynamic Position]

Thanks for your insights, I did not know about plating. Taking the bike indoors to charge could cause the "rolling-pin" effect and serious headaches, though.

I was hoping to make a DIY box for a battery, but it looks like I'll have to invent a detachable one (less headache than the rolling-pin effect, I hope).

cheers,
Y

You could always use an electric heater in your shed while you charge the battery. Expensive Though!