Battery Safety

by James Hudak

1/25/2007


This message includes some basic information about battery types, and some tips on safety for each type. There are several new types of cells available now, and they CAN BE DANGEROUS IF IMPROPERLY USED. PLEASE: If any of you have questions, please ask them, and I'll be glad to help. I am no expert, but I study this stuff so that I can help people avoid fires, injuries and damage to property. Of particular note is the lithium cells discussed items 5 and 6 below. Please read the PDF document on safely using Lithium-Polymer cells. MANY of the safety items in this document do apply to other cell types, so please take a look at it.

 

GENERAL WARNINGS FOR ANY RECHARGEABLE CELLS

  • NEVER INTENTIONALLY SHORT CELLS
  • NEVER INTENTIONALLY OVERCHARGE CELLS
  • NEVER TRY TO SOLDER CELLS TOGETHER UNLESS YOU ARE EXPERIENCED IN PACK ASSEMBLY PRACTICES
  • NEVER EXCEED THE MAXIMUM CURRENT SPECS FOR ANY GIVEN CELL (for charging AND discharging)
  • NEVER LEAVE A BATTERY UNATTENDED DURING FAST/QUICK CHARGING CYCLES, PARTICULARLY FOR THE NEWER GENERATION CELLS
  • ALWAYSUSE THE MANUFACTURER'S DATA SHEET TO DETERMINE THE MAXIMUM RATINGS OF CHARGE/DISCHARGE CURRENT

***THINK OF ALL BATTERIES AS IF THEY ARE BOMBS FULL OF SHRAPNEL, TOXIC CHEMICALS, AND FLAMMABLE COMPOUNDS

COMMON RECHARGEABLE CELLS AND BASIC INFO

1) NiCd - Nickel Cadmium - about 1.25 V per cell - They've been around forever- but get little respect :-) Safety: These will "vent" in the event of serious overcharge and excessive current flow, and have a good "safety record" - hot gases can be harmful while venting, bursting is rare.

Guidelines: Most cells can be quick charged (15 minutes or less) with currents as high as 4x capacity ("C" x 10), charged at ("C"), or slow charged at ("C"/10). Maximum discharge rates are usually very high and some are as high as 20 to 50 x "C". Running peak currents of 100A through Sub-C cells is possible, and they are not damaged.

Issues: They automatically discharge during storage, and often get a "memory" effect, which requires "training" and special care.

Bonuses: They automatically rise in temperature when they near a full charged state - so TEMPERATURE is a handy indicator of when a cell is fully charged. They also have very low internal resistance which makes them a good choice for high current applications.

2) NiMH - Nickel Metal Hydride - 1.2 V per cell - these were supposed to replace NiCd, but they fall short in several places. They have found use in small household devices, like cordless phones and such.

Safety: They have a pressure vent for safety, and are similar to ni-cd in terms for durability and safety. The internal compounds are more "environmentally friendly" as well.

Guidelines: SOME manufacturers state maximum charge/discharge rates similar to NiCd, and some will state about 1/2 of those values, DO NOT OVERCHARGE:doing so can slowly degrade the capacity level. The temperature rise signal of a full NiCd does NOT apply to NiMh. A warm NiMH is already OVERCHARGED, and you should avoid letting them get to that state very often, CHECK with the manufacturer's specs for chargerates. Some chargers unfortunately are not sensitive enough to cut off early enough, unfortunately. DO NOT USE A NICD CHARGER FOR NIMH CELLS.The peak detection point is different for NiMH and NiCd.

Issues: The internal resistance is higher than NiCds, and they can't and shouldn't be run with high current levels. They also don't tolerate abuse as well as NiCd.

Bonuses: They have a less pronounced "memory" effect, and have higher energy density than nicd. Some can have 50to 100% more capacity for the same size/weight.

3) Lead-Acid (liquid) - 2V per cell -- It's your good old fashioned car battery. - these are typically plastic cases with several cells lined up - which weighs a ton.

Guidelines: Fairly high charge currents are OK for most large types, such as automotive ones.

Safety: Do not overcharge, if you do, it could "boil" the electrolyte, during charging. Hydrogen gas can be emitted which can be a explosion risk and sulfuric acid in it is dangerous as well.

Issues: Some require periodic re-filling of water/electrolyte solution, they are HEAVY, and can't be used in any alternate orientations,they also have greatly reduced capability in cold temperatures, most of these also have difficulty in deep-discharge conditions.

Bonuses: Very high currents are allowed during short bursts. They also can be used for moderately long-term storage, with less loss than other types. Cost per mAh is quite low, and availability is great.

4) Lead-Acid (gel cell) - 2V per cell, with similar chemistry to standard lead-acid cells, but it has gelled electrolyte and partially sealed case.

Guidelines: I suggest you try to keep the charge rate somewhat lower than standard lead-acid - which helps with the longevity of the cells.

Safety: In general, they are more safe than standard lead acid, with no gas emissions, and non-spillable electrolyte. Serious overcharge can still cause internal pressure buildup - causing venting or case distortion due to the pressure.

Issues: Repeated deep discharge cycle use could reduce the cell's life. Cold temperaturesreduce performance.

Bonuses: They have all the "good" characteristics of lead-acid, while solving many of the drawbacks. The non-spillable electrolyte and multi angle mounting makes them great for robotics. There are also some types of these that are in metal cylindrical form, which are great for smaller devices. They are available in much smaller sizes than standard Lead-Acid (liquid).

5) Lithium-Ion (cylindrical or prismatic case) - About 3.7V per cell (used in computers, phones, and now cordless tools) The case can be a metal cylinder or a metal rectangular block (prismatic).

Safety - OVERCHARGE and SHORTS can cause the cells to BURST and cause fire. These do have a safety vent, which is helpful, but you MUST treat them with greater care than older cell types.

SOLDERING/CONNECTIONS: NEVER. EVERTRY TO SOLDER DIRECTLY TO THE CELL CASE. The heat of soldering will likely kill the cell and possibly cause fire. Use solder tabs that are attached using spot welds. If your cells don't already have tabs it's, sometimes hard to get tabs welded on, unfortunately.

Charging: You MUST use a charger DESIGNED for Li-Ion cells AND the charger MUST be set to theCORRECT CELL COUNT (voltage). If the charger is improperly set, aFIRE is POSSIBLE. NEVER charge these unattended - especially at quick charge rates. I highly recommend that you use a FIRE RESISTANT surface to put the cells on during charging, and NEVER charge them inside your device. Plenty of RC airplanes got burned-up from people charging inside the plane.

Discharging: NEVER SHORT THESE CELLS - doing so can permanently damage the cell, and can cause bursts and / or fires. NEVER exceed the specifications for discharge current.

Issues: Internal pressures can cause bursting in some cases, while maximum currents are typically lower than similar cells of other types. Safety circuitry is typically necessary - and encouraged (which is why most notebook PC's have batteries costing $150 and up)

Bonuses: High energy density, almost no "memory" effects, and the metal case is much more resistant to puncture, dents and other external damage. These are also available in rectangular steel containers which are much easier to install in devices.

 

6) Lithium Polymer (solid) about 3.7V per cell - these are similar to the Lithium Ion, but the electrolyte is made as a solid polymer block - the "case" is nothing but a foil envelope with tabs

Safety - PLEASE READ THE ATTACHED PDF FILE FROM THUNDER POWER BATTERIES.Also, the foil envelope these are in is not very puncture-resistant, and the chemical makeup of the cell does make a fire a real possibility in the event of a puncture. These cells also SWELL during charging, and if overcharged, they can swell too much and burst. A small 2"x 3" x0.25"Li-Poly cell can swell by as much as 0.012" during normal charging. The electrolyte can burn when it contacts air, so never use a damaged cell. MOST OF THE FIRES FROM THESE CELLS ARE DUE TO USER ERROR, WITH THE REST DUE TO POOR-QUALITY CHARGERS AND HOOKUPS.

SOLDERING/CONNECTIONS: NEVER, EVER TRY TO SOLDER DIRECTLY TO THE CELL, OR THE FOIL TABS. The cell is typically in a aluminum foil packet, with a pair of tabs for connections. The positive tab can sometimesbe soldered, but the negative tab is usually aluminum and is nearly impossible to solder to. The manufactured packs typically use crimp or clamp style connections, or are heat sealed in production.

Charging: Just like Li-Ion, you MUST use a suitable charger in the right mode, and with the CORRECT CELL COUNT. ALWAYS stick to the manufacturer's specs on charging current.

Discharging: Never exceed the maximum current rating. Early ones could do around "C"x1, but newer ones can do as much as "C"x10 - but you must check before you use them that way.

Issues: Foil "Packets" are easy to dent and puncture and swell during use. The electrolyte is said to be flammable when exposed to air.

Bonuses: The energy density is among the best available, and the rectangular shape is easy to mount in devices. In many cases it's the best thing for light weight, maximum power. That's the reason that they are popular in electric RC aircraft.

7) Tadirian - (These are not very common in RC use, and not very common in consumer devices)- these have even higher energy density, but have serious drawbacks that place them in some specific applications. Few RC-type chargers can charge these, and I don't recommend that anyone use these cells. Military, automotive and space applications are where these are used commonly. The main drawback is that they have a VERY low maximum discharge rate, which makes them suitable for memory backup and real time clocks.


Published 1/25/2007