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looking after your batteries

Introduction

  • each battery type has unique characteristics and optimised ways to look after them to ensure their life is extended as long as possible, and that the risk of them exploding (this is especially a risk with lithium ion batteries) is reduced
  • life span was particularly a problem with the older technologies such as nickel cadmium (NiCd) and nickel metal hydride (NiMH) as well as the old heavy lead acid batteries.

Lithium ion batteries

rechargeable lithium ion battery cells

  • Lithium ion cells are generally 3.6V or 3.7V nominal voltage chemistries
  • LiFePO4 cells have a nominal voltage of 3.2V and are safer and generally better performing than Li ion batteries
  • a 12V battery usually consists of 4 of the above cells connected in series and regulated by a battery management system (BMS) which helps to ensure the some level of charge is in each cell as well as having safety features to prevent overcharging, overdischarging, etc.

cylindrical

  • 32600 cell - similar size to D cell
  • 26650 cell - used in 12V battery packs as 3.2V 3.4Ah
  • 26500 cell - simiar size to C cell
  • 21700 cell
    • 21mm diam x 70mm long allowing 3000-5000mAh capacities
  • 20700 cell
    • 20mm diam x 70mm long which provides ~50% more volume hence capacity than a 18650 battery
    • 3.7V Li ion
    • 3.2V LiFePO4
  • 18650 cell
    • 18mm diam by 65mm long;
    • nominal 3.7V Li ion:
      • 4 are used to create a “12V” battery but nominal voltage is really 14.8V
      • 300 to 500 charging cycles, usually take 1-2hrs to charge per 1000mAh to 4.2V
    • 3.2V LiFePO4
      • 4 are used to create a “12V” battery but nominal voltage is really 12.8V but fully charged is around 13.8-14V
    • used in many devices
    • max. capacity is around 3500mAh
    • some are flat top, some are button top
    • some are “Power cells / High Drain” for much higher currents (around 30A instead of 5-10A) such as would be needed in power drills
    • smart protected“ cells include overcharge protection and over discharge protection
  • 14500 - similar size to AA cell
  • 10440 - similar size to AAA cell

prolonging the life of your lithium batteries

  • use the correct charger
    • an incorrect charger may cause serious overheating within minutes
  • avoid “rapid” chargers”
    • these will overheat the battery and reduce lifespan
  • don't drain them to zero
    • doing this may make it very difficult to regain maximum capacity and hold charge
    • it increases the risk of dendrites of lithium ions forming which results in increasing internal resistance for that cell which can lead to overheating, reduced charging of the other cells and internal short-circuits resulting in reduced charge capacity
  • don't keep them perpetually charging 24×7
    • remove them from the charger once they are fully charged
    • keeping them on a charger risks overheating, swelling and explosion risk
  • supervise them whilst charging
    • don't charge them overnight while you sleep in case they overheat and burn your house down
    • ideally charge them in the same room, or at least have a smoke detector in that room
  • don't store them fully charged
    • this can dramatically shorten their life span
    • it is better to store them at 50% if they are being stored for long periods
  • don't overheat them
    • keep them out of your hot car
    • don't charge them straight away if they are feeling warm
  • don't start charging them while they are hot
    • this will only make them hotter and risk damage to the cells
  • don't charge them while they are below zero deg C
    • most lithium batteries do not tolerate charging at sub-zero temperatures
  • remove from device when not being used
    • devices often leak current even when turned off leading to battery becoming drained
  • optimising a LiFePO4 battery rated at 2000-5000 cycles by keeping it cool, maintaining charge-discharge cycles between 75% and 25%, and avoiding high charge/discharge rates should prolong the battery life out to 20,000 cycles or perhaps 50yrs albeit with reduced capacity of perhaps 60% (in contrast AGM lead acid batteries will generally last 5-8yrs at best)

storage conditions

Recoverable battery capacity after storage for 1 yr at various temperatures and charge - avoid temperatures above 30°C (86°F)

ambient temperature stored at 40% charge stored at 100% charge
0degC 98% 94%
25degC 96% 80%
40degC 85% 65%
60degC 75% 60% after 3 months

avoid over heating battery

  • heat degrades lithium batteries
  • avoid high charge/discharge rates
  • avoid charging in hot environments

avoid fully charging the battery

  • charging to 100% requires overcoming greater resistance which creates heat and reduces battery life, perhaps the best compromise for most lithium ion batteries is to charge to 85% if you are not needing the full capacity for your task
  • note that LiFePO4 batteries handle over-charging better than the lithium ion batteries in the table below
cell charge voltage for a rated 4.2V cell discharge cycles available stored energy
4.3V over-charged 150-200 110%
4.25V 200-350 105-110%
4.20V 300-500 100%
4.15V 400-700 90-95%
4.10V 600-1000 85-90%
4.05V 850-1500 80-85%
4.00V 1200-2000 70-75%
3.9V 2400-4000 60-65%

optimise the charge-discharge bandwidth

  • this is the degree you allow the battery to discharge to and the degree to you charge it back up to
  • most lithium ion batteries do best at around 50% charge
  • discharging too much results in dendrite formation which can create internal short circuits which result in increased temperature during charging and may result in one cell within the battery being at different voltages during charge which also means greater heat
  • note that LiFePO4 batteries will do better than the lithium ion batteries in the table below
charge-discharge bandwidth at 20degC % total capacity after 4000 cycles
75-65% (only 10% used between charges) 94%
75-45% (only 30% used between charges) 91%
75-25% (only 50% used between charges) 88%
85-25% (only 60% used between charges) 86%
100-50% (only 50% used between charges) 83%
100-40% (only 60% used between charges) 82%
100-25% (only 75% used between charges) 79%

state of charge vs voltage

  • most 12V batteries do not have a state of charge (SOC) meter to show how much charge is in the battery
  • this can be very approximately estimated by the voltage
  • a much more accurate method is to install a SOC meter with a shunt device
  • some batteries are “smart” and have a dedicated module to display SOC without need for a shunt device such as:

LiFePO4 12V batteries

discharge voltage at 0A load discharge voltage under load Approx. SOC Approx. Ah for 100Ah battery
14V 13.6V 100% 100Ah
13.8V 13.4V 99% 99Ah
13.6V 13.35V 98% 98Ah
13.4V 13.3V 90% 90Ah
13.3V 13.2V 70% 70Ah
13.2V 13.1V 40% 40Ah
13.1V 13.05V 35% 35Ah
13.0V 13.0V 30% 30Ah
12.9V 12.9V 20% 20Ah
12.8V 12.8V 17% 17Ah
12.5V 12.4V 14% 14Ah
12.0V 12.0V 9% 9Ah
11.7V 11.7V 0% flat

NiMH batteries

  • one can expect 2000 charge/discharge cycles out of a standard NiMH battery
  • use the correct SMART charger
    • Most chargers made since about 2005 charge both NiMH and NiCd batteries. But some newer chargers charge only NiMH, not NiCd.
    • Any NiMH charger will charge both regular NiMH batteries and LSD NiMH's (Low Self Discharge NiMH).
    • Old NiCd chargers can't tell when to stop charging a NiMH battery
    • Use a smart charger! Smart chargers charge each battery separately, giving each one exactly as much energy as it needs and stops charging once it is full.
    • If your NiMH charger manages each cell separately, you can mix NiMh and NiCD, different sizes, and semi-fresh and nearly dead batteries at the same time. If it doesn't you can't. If you insist on mixing them anyway, some of your batteries will be undercharged and some will be overcharged.
    • NiZn batteries require a special charger. NiMH and NiCd chargers will not work.
  • Trickle Charging is the safest method
    • charge at the lowest possible rate that will keep your overall charge time BELOW 20 hours
    • Most would suggest to aim for 4-5 hours - a 4-5-hour charge means about a 500 mAh charge for most 2000mAh AA's, and 200 mAh for most lower capacity AAA's.
    • If the battery charges too slowly, the charger might miss the voltage change and not stop charging.
  • don't keep them perpetually charging 24×7
    • remove them from the charger once they are fully charged, excessive charging reduces battery life
    • Smart chargers will stop charging when charge completed HOWEVER, most chargers give a “trickle” charge after charging is finished. Some chargers give too much of a trickle, and even those which give a reasonable trickle can still overcharge if the batteries are left in the charger too long.
  • almost fully discharge battery prior to charging
    • NiCd and, to a lesser extent, NiMH batteries have a “memory” issue
    • You can recharge at any degree of depletion (you don't have to wait until the battery is mostly discharged), but the sooner you recharge the better, because you'll get more cycle life out of the battery.
    • for a 1.5V battery, it is better to re-charge at 1.1V than wait until 1.0V
    • failing to deplete the battery each time reduces its capacity
    • Newer NiMHs can be restored by “exercising” the battery (fully charging and discharging the battery a few times)
  • there are methods to REVIVE “dead” batteries that won't charge

NiCd batteries

  • use the correct charger
    • Most chargers made since about 2005 charge both NiMH and NiCd batteries. But some newer chargers charge only NiMH, not NiCd.
  • almost fully discharge battery prior to charging
    • NiCd have a major “memory” issue
    • failing to deplete the battery each time reduces its capacity
    • ideally the aim is to charge once the battery is ALMOST fully depleted (voltage will be around 1.0-1.1V for a 1.5V battery)
  • Dispose battery safely - cadmium is TOXIC
photo/batteries.txt · Last modified: 2021/10/28 22:24 by gary1