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Charging strategies for flight packs and safety.

 

The mainstays in terms of battery types in use in RC modelling at the time of writing this article are Nickle Metal Hydride (Nimh), and Lithium Polymer (LiPo). If either of these battery types are misused, they can cause fire.

In the case of Nimh types, thermal runaway due to overcharging or physical cell damage will typically cause cells to get very hot and if allowed to continue can melt plastic casing, arc between adjacent cells, and flame directly. Should this happen while in contact with other materials serious fires can be started.

In the case of LiPo types, thermal runaway due to overcharging or physical cell damage will typically cause cells to get very hot and and if allowed to continue cells will flare. Should this happen while in contact with other materials serious fires can be started. The nature of a LiPo flare is quite different to the burning which can occur with Nimh types. A LiPo flare is a source of intense heat. Injurys caused by LiPo flares can be very serious indeed, even life changing.

Given these serious potential hazards, the BMFA has prepared some key documents which RC modellers everywhere can benefit from. The links to these documents are as follows:

BMFA. Safe and effective use of Lithium Polymer batteries in model flying.

BMFA 2012 Members handbook. Pages 24 to 27 for batteries.

It's easy to understand given the level of excellence of modern electronics that modellers using advanced digital chargers can feel that leaving packs on charge is safe. In the majority of cases this would be true. That said there are several possible failure modes that we know of:

1. The user could make a programming error in setting up his charger and so cause serious overcharging while being totally unaware of it.

2. Any electronic circuit can fail. For example we have seen delta peak circuits fail to stop the charging process causing serious over charging and heating. 

3. Soft or micro shorts can occur within battery casings which can lead to failure, even off charge.

All these failure modes can lead to fire.

Given the serious nature of the safety hazards involved the BMFA recommend for example in the case of LiPos that the charging rate should be 1C, and that the charging process should not be left unattended but do read the articles at the above links to get their full perspective on the matter.

All this said, safety during charging is the sole responsibility of the user. This is usually made clear by the disclaimers written on most chargers, e.g.

CAUTION.

NEVER LEAVE CHARGER UNATTENDED.

CHARGE IN FIRE SAFE AREA ONLY.

Given this, the user must decide what represents a largely safe method of chargeing which minimises risks.

Note: Although widely known, for clarity, some charging fundamentals are given here:

For a 2000 mAH Battery charged at 1C, the charging current is 2 amps.

For a 2000 mAH Battery charged at 2C, the charging current is 4 amps.

For a 2000 mAH Battery charged at 0.1C, the charging current is 0.2 amps.

The tables below show some typical methods of charging currently in use by RC model pilots:

Balance Charging LiPo Batteries at 1C

Considered best practice.

Pros

Cons

Key Safety Precautions

  1. When multiple chargers are used, all LiPo batteries needed for a flying session can be charged in one hour or less. This very convenient and makes staying with the process easy.
  2. With LiPo digital charging techniques, there should be no temperature rise during the 1C charging process. This means optimised battery life.
  1. Takes longer than charging at higher “C” numbers.
  2. Takes longer than none balanced charging.

 

 
  1. The user remains in attendance and monitors the process continually. Any unexpected failure can be spotted and dealt with as it happens.
  2. The user places the cells under charge in a metal box with a removable or hinged lid to ensure that any unexpected failures are contained. The user takes special care to ensure no short circuits can take place between the batteries' terminals and safety metal box.
  3. The charger(s) used are kept outside the closed box allowing the user to monitor the process at all times, the box only being opened at the end of the process.

 

Charging NiMH Batteries at 1C

Considered best practice.

Pros

Cons

Key Safety Precautions
  1. When multiple chargers are used, all NiMH batteries needed for a flying session can be charged in one hour or less. This is very convenient and makes staying with the process easy.
  2. Modern digital chargers allow adjustment of delta peak parameters. This means that any temperature rise that occurs during the 1C charging process can be kept at a minimum, thus prolonging battery life.
  1. Takes longer than charging at higher “C” numbers.

 

 
  1. The user remains in attendance and monitors the process continually. Any unexpected failure can be spotted and dealt with as it happens.
  2. The user places the cells under charge in a metal box with a removable or hinged lid to ensure that any unexpected failures are contained. The user takes special care to ensure no short circuits can take place between the batteries' terminals and safety metal box.
  3. The charger(s) used are kept outside the closed box allowing the user to monitor the process at all times, the box only being opened at the end of the process.

 

Trickle Charging NiMH Batteries at 0.1C

Pros

Cons

Key Safety Precautions
  1. No heat is generated during the charging process. This means optimised battery life.
  1. Because the charging process can take up to ten hours, the process is typically left unattended.
  2. Because the charging process can take up to ten hours, grabbing the opportunity for a quick visit to the field for a couple of flights is takes more planning.

 

 
  1. The user typically ensures that should the process run over time, heat generated in the batteries is insignificant. This can sometimes be tricky to get right, although dedicated trickle chargers usually avoid this.
  2. Where programmable chargers are used, the user must be very careful to ensure that charging current values are entered correctly, as the process is typically unattended. An extra zero error for example would lead to certain failure.

 

Balance Charging LiPo Batteries at >1C

Pros

Cons

Key Safety Precautions
  1. When multiple chargers are used all LiPo batteries needed for a flying session can be charged very quickly. This is very convenient and makes staying with the process easy.
  2. Because the charging process is so quick, grabbing the opportunity for a quick visit to the field for a couple of flights is relatively easy. the charging process is so quick, grabbing the opportunity for a quick visit to the field for a couple of flights is relatively easy.
  1. At charging rates greater than 1C heat is generated which can stress the batteries leading to shorter cell life.

 

 
  1. The user remains in attendance and monitors the process continually. Any unexpected failure can be spotted and dealt with as it happens.
  2. The user places the cells under charge in a metal box with a removable or hinged lid to ensure that any unexpected failures are contained. The user takes special care to ensure no short circuits can take place between the batteries' terminals and safety metal box.
  3. The charger(s) used are kept outside the closed box allowing the user to monitor the process at all times, the box only being opened at the end of the process.

 

 

Charging NiMH Batteries at >1C

Pros

Cons

Key Safety Precautions
  1. When multiple chargers are used all Nimh batteries needed for a flying session can be charged very quickly. This is very convenient and makes staying with the process easy.
  2. Because the charging process is so quick, grabbing the opportunity for a quick visit to the field for a couple of flights is relatively easy.
  1. At charging rates greater than 1C heat is generated which can stress the batteries leading to shorter cell life.

 

 
  1. The user remains in attendance and monitors the process continually. Any unexpected failure can be spotted and dealt with as it happens.
  2. The user places the cells under charge in a metal box with a removable or hinged lid to ensure that any unexpected failures are contained. The user takes special care to ensure no short circuits can take place between the batteries' terminals and safety metal box.
  3. The charger(s) used are kept outside the closed box allowing the user to monitor the process at all times, the box only being opened at the end of the process.

 

 

 

 

Details
Category: Safety: Batteries
Hits: 1963

Comments  

# Bob Hynes 2016-03-19 08:06
Revision History.
19/March/16 Added the safety text:
The user takes special care to ensure no short circuits can take place between the batteries' terminals and metal box battery enlosure. Contributed by Ian Jones.

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