A battery cell contains chemistry able to store electrons. Regular batteries use chemistries that cannot be recharged. Rechargeable batteries can be re-charged thousands of times.
A number of different chemistries are in use today that vary considerably in price, energy density, and other specs. Let’s first take a look at use cases, then closer investigate the specific advantages and disadvantages of each chemistry.
Use Cases
In DIY scenarios, these chemistries are worth mentioning:
- Lead-Acid: due to its low cost per Wh, commonly used in model making when weight does not matter (i.e. ship modelling)
- NiCad: Not widely used anymore due to its toxic Cadmium ingredient.
- *NiMH: Used for rugged environments, i.e. in solar lamps (that must sustain high operating temperatures)
- LiIon: commonly used to power all kinds of small devices, due to their high energy density
- LiPo: highly popular due to their flexible sizes that can be easily fit into even small devices.
- LiFePo4/LFP: used whenever large amounts of energy are required (and size and weight is secondary). Also used to directly power 3.3V microcontrollers (without the need for a voltage regulator) in low-power devices.
Chemistry Types
Batteries behave differently based on their chemistry:
| Battery Type | Cost per Wh | Life Span | C-Rate | Energy Density (Wh/kg) | Depth of Discharge | Self-Discharge/month | Nominal cell voltage | Operating Temperature (Celsius) |
|---|---|---|---|---|---|---|---|---|
| Lead-Acid | €0.09 | 3-12 years | 2-3 | 30-50 | 50% | 10-15% | 2.2V | -40 to +55 |
| NiCd | €1.05-€1.31 | 1500 cycles | 15 | 40-60 | 80% | 20% | 1.2V | -40 to +70 |
| NiMH | €1.30-€1.80 | 180-1000 cycles | 1 | 70-100 | 80% | 100% (4%/day), less with modern NiMH | 1.2V | -30 to +100 |
| Li-Ion | €0.28-€0.50 | 500-1000 cycles | 1 | 150-200 | 80-90% | 1-5% | 3.6V | -20 to +60 |
| LiPo | €0.59-€0.80 | 300-500 cycles | 1 | 150-250 | 80-90% | 1-5% | 3.7V | -20 to +60 |
| LiFePo4 | €0.08-€0.45 | 2000+ cycles | 1 | 90-120 | 80-90% | 1-5% | 3.2V | -20 to +45 |
A few notes:
- Price: Cost is just an estimate and is affected by the typical battery sizes. NiCd and NiMH are typically small batteries with a higher relative cost. LiFePo4 are typically very large capacity batteries with a relatively low cost per Wh.
- Life Span: for lead-acid, life span is a time span due to the internal chemistry (which degrades over time). Lithium-based chemistries are relatively unaffected by time and degrade only during charge/discharge-cycles. All battery types degrade prematurely when discharged above the Depth of Discharge level due to irreversible chemical and structural changes that occur at these low voltage levels. High charge currents further reduce battery life.
- Operating Temperature: Lithium-based batteries must not be charged below 0 Celsius (but can be discharged down to -20 Celsius). Nickel-based batteries are among the most rugged chemistries and work well both in extremely cold conditions as well as in full sunshine.
Self-Discharge
All batteries are affected by self-discharge, regardless of technology. The rate of self-discharge varies greatly, though.
Primary cells (non-rechargeable batteries) such as lithium-metal and alkaline retain the stored energy best, and can be kept in storage for many years.
Rechargeable batteries constantly lose charge. The rate is affected by their state of charge (highest losses when fully charged, then tapering off) and environment (discharge doubles for every 10C temperature rise). High cycle count and aging also increase self-discharge of all systems.
A fully charged LiIon battery may loose 6% per month at 0 Celsius, yet 35% at 60 Celsius environmental temperature. At just 50% charge, the discharge is 2% at 0 Celsius and 15% at 60 Celsius.
Nickel-based batteries lose as much as 10–15% of their capacity in the first 24 hours after charge, then 10–15% per month. These batteries need recharging before use when placed on a shelf for a few weeks.
Risks And Permanent Damage
Modern rechargeable batteries are robust and can be used for many thousand cycles. There are just two mechanisms that can permanently damage the battery:
Over-Charge
When a battery is charged with too high currents, or too high voltages, it over-charges: the supplied energy cannot be absorbed and instead turns into heat. In addition, chemical processes can produce gases (like oxygen) that build up pressure and can lead to bulking, fire, and even explosion. That’s why you should only use trustworthy chargers that were designed for the battery chemistry you use:
- Do not use LiIon chargers for LiFePo4 batteries, or car (lead acid) chargers for LiIon.
- Do not use generic constant current power supplies (let alone regular DC power sources without regulation)
- Do not charge batteries with currents higher than necessary. Do not use quick chargers if you have the time for a regular charge. Never charge a battery above 1C (its capacity in Ampere, i.e. never charge a 100Ah battery with more than 100A).
Over-Discharge
Most battery types can be permanently damaged when over-discharged. Below a certain voltage, inside the battery irreversible chemical reactions take place. For lead-acid batteries, discharging below 50% can already start sulfation. Lithium-based batteries experience similarly destructive reactions when they are discharged below 10% capacity.
Storage
If you do not use batteries for an extended period of time, self-discharge can lead to over-discharge and permanent damage. To safely store batteries for extended periods of time, you therefore need to make sure that natural self-discharge cannot excessively discharge the battery:
- Cool Environment: since self-discharge is affected by temperature, store batteries in a cool place (0-15 Celsius)
- Fully Charge: while you shouldn’t routinely charge your batteries to 100%, a battery should be fully charged when you put it in storage. This way, natural self-discharge will take much longer until critical over-discharge voltages are reached.
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(content created Oct 17, 2024)