DBG: Batteries for portable equipment
We rely heavily on batteries to power our portable equipment - flashlights, bat detectors, recorders, etc. Choosing the wrong type of battery can be expensive - and also leave you in the dark unexpectedly. Its important to match the type of battery you choose to the equipment it will be used in.
These can place a heavy drain on batteries. This graph shows the discharge characteristics of common AA size batteries when supplying a current of 2.0 Amps. You can see that the Alkaline cell goes flat very quickly, (about 20 min) while the output voltage of the rechargeable NiMH cell stays above 0.8V for over 1 hour. The Lithium (LiFeS2) cell does even better - but costs more and is not rechargeable. The capacity of a battery is given in Amp hours (Ah) - this is the length of time in hours that the cell will supply 1A. Suppose our torch has a single AA cell with a capacity of 2.0Ah and a voltage of 1.2V. That gives us total stored energy of 1.2 * 2.0 Wh or 2.4 Watt hours. We will only use about 80% of this i.e. 2Wh. So a 1W LED will run continuously for 2h on a single AA cell.
High power white LEDs require 4.3V and will stop working at about 3.2V. They often use a driver IC that converts the 1.2V to 4.8V (X4) so when the battery voltage falls to 3.2 / 4 = 0.8V the torch goes out. In any case you should never discharge rechargeable cells completely.
This figure shows how cells behave in less demanding applications. Your bat detector, radio, recorder etc could consume 100-200 mA. In these applications NiMH rechargeable or (expensive) LiFeS2 cells will provide much longer life. Of course the NiMH cells can also be recharged after use!
Approximate prices per AA cell at September 2015:
- Energizer E91 Alkaline £0.40
- Energizer NH15-2300 NiMH £0.75
- Energizer L91 Lithium LiFeS2 £1.50
Primary (Disposable) or Secondary (Rechargeable)
Batteries can be "primary" or "secondary"; Primary cells have the energy built into them at manufacture, and cannot be recharged as Secondary cells can. Here is a brief comparison of some of the more common types.
Primary Cells: not rechargeable There are three main types, the "heavy duty" Zinc Chloride batteries, the Alkaline cells such as Duracell Procell, and Lithium batteries.
Zinc Chloride : "Heavy duty" is not true now, as other modern types of AA cells store up to five times as much charge. They are OK in torches etc. for intermittent use, but don't last very long. They can start to leak corrosive material to the detriment of your equipment! Typical capacity said to be 400mAh. AVOID
Alkaline or Manganese-Alkaline batteries give good performance in low-drain applications such as smoke alarms, but are not well suited to higher drain applications such as torches. Despite manufacturers hype reviews claim there is little difference (except in price) between different manufacturers AA alkaline cells. Duracell Ultra M3 nominal capacity is quoted as 2600mAh.
Lithium cells out-perform alkaline AA batteries, do much better in demanding applications (e.g. digital cameras), and work over a greater range of temperature. You need to check the voltage is correct as they come in a wide range of voltages, from 1.5V to 3.7V. The L91 Energiser Ultimate Lithium AA cell (pdf data sheet) has a voltage of 1.5V and stores 3000mAh. Lithium cells are used where long life, reliability and dependability are more important than cost. Theyare especially well suited to equipment that is used infrequently and for short periods (eg remote controls) or not easily accessible (smoke alarms), or where their low weight is an advantage.
Secondary cells: These give much better service than primary cells in most high-drain applications
(Lithium L91 primary cell included for comparison purposes.)
% per month
|NiMH AA||1.2||2400||130||3||20 - 30%||Charge before use. Inexpensive, can provide lots of current, work when cold.|
|NiMH AAA cell||1.2||750||13||1||20 - 30%||Best not used to provide currents above 250mA|
|Hybrid NiMH AA||1.25||2000||130||4||1 - 2%||Slightly higher voltage means longer before cameras etc stop working, and can be recharged in any NiMH charger.|
|1.5||3000||15||4||negligible||Higher voltage means longer before cameras etc stop working. Not rechargeable, but have a very long shelf life - around 20 years. Some equipment designed for these batteries will not run from other types.|
|3.6||2600||47||1.5||5 - 10%||Good for cold climates. Light weight but expensive. Damaged by overcharging - must not be trickle charged. Need special charger.|
"gel cells" (not AA)
|2.0||X||HEAVY||VERY high||5% - 10%||Normally in packs to give 6V or 12V
Ideal for larger halogen lights etc. but heavy.
Some examples of modern batteries for torches
The new generation of "hybrid" NiMH cells are supplied precharged, have much longer "shelf life" than the older type, cost £1 - £2 each. Sold as GP ReCyCo, Uniross Hybrio, Varta "ready to use", Sanyo Eneloop, Vapex Instant.
18650 This is a lithium battery, type 18650. Its becoming very popular for torches since it has a voltage of 3.6 and capacity of 2400 mAH giving a total energy of nearly 9Wh as compared with 3Wh for NiMH or Alkaline AA cells. Don't worry about mixing it up with your AA's (14510) its bigger so it wont fit. Good ones are also also protected against over-charging and over-discharging which can be damaging to Li-Ion cells. The naming of these cells is from their size 18mm X 65mm. The last '0' confirms the shape is cylindrical.
26650 As torches with higher output become available a higher output battery is needed. The 26650 cell (26mm dia * 65mm long) with a capacity of up to 5AH can deliver peak currents of up to 20A safely. This cell is the newer LiMnNiCo or LiFePo which does not have the fire risk associated with some Li-Ion cells.
CR123A size 3.6V Li batteries are available in both rechargeable (600mAh) and disposable (1300 mAh) forms (quite cheap in bulk on ebay!) and are also called 16340 i.e. 16mm * 34.0mm Some torches allow you to put two of these in place of one 18650; the higher voltage gives a brighter light, but note the reduced capacity means much shorter use time.
PP3 type 9V batteries
Bat detectors and lots of other portable equipment use PP3 9V type batteries. I haven't been able to find rechargeable Lithium batteries in this category, but here is some information about currently available types.
|PP3 / MN1604||NiMH rechargeable||Alkaline disposable||Lithium disposable|
|Voltage||8.4 or 9.6V||9.0||9.0|
|Capacity||170 - 250mAh||550mAh||1200mAh|
|Self-discharge||20-30% per month||1- 2% per month||negligible|
|Typical price||£3 (170mAh) - £4 (250mAh)||
Duracell or Energizer from £2,
|Notes||Hybrid NiMH technology is also coming in here making these a viable alternative to disposable batteries.||Remember cheaper brands here (e.g. PIFCO) can give almost as good performance for much less cost.||can last over 10 years in low-drain application - ideal for smoke detectors etc. Fit and forget.|
Caring for your batteries
Forget the old rules about looking after NiCd batteries, they don't apply to NiMH batteries. Most important things to remember for NiMH cells are:
- DONT charge them in NiCD chargers - they cant tell when they are fully charged. However you can charge NiCd in most NiMH chargers.
- DONT let them become completely flat - NiMH cells need reconditioning after a deep discharge, and may never fully recover.
- DONT store them when discharged and remember even if not used they will need recharging every 6 months or so.
- DONT draw excessive current. NiMH batteries degrade rapidly if asked to supply current at more than half their capacity per hour (i.e. 1A for a 2Ah cell)
- NEVER put a fully charged battery into a charger - good chargers look for changes in voltage or temperature to tell them the battery is fully charged, and putting batteries back to "top up" will almost invariably result in shortening their life.
Keep your batteries in sets (use an indelible marker to label them) and dont mix new with old, or higher with lower capacities.
Keeping Li-Ion rechargeables and NiMH batteries in a cool place (the fridge - not the freezer) will reduce self-discharge.