Ever since the first plastic wrapped multi-cell LiPo battery appeared in RC modelling applications, the lithium rechargeable battery has come a long way.
Why Lithium batteries are better
COMPARED TO LEAD BASED BATTERIES, LITHIUM BATTERIES HAVE THE FOLLOWING ADVANTAGES
- Substantial weight reduction (in the order of 60% for the same physical size).
- Nominal 13,2v battery voltage compared to 12,5v for lead batteries, means that effectively more power is available. The voltage discharge curve is much flatter.
- Low internal resistance, allowing very high currents in the case of engine starting applications. Engines spin much faster when starting.
- Extremely low self-discharge when not in use.
- Up to 4 times the number of discharge/recharge cycles makes them more cost effective. A lithium battery will typically last twice as long as a good AGM battery.
- Shorter recharge time due to 97% charge acceptance, compared to lead batteries at 75%. Lithium batteries can be fully recharged in 2 or 3 hours.
- Lithium batteries are far more environmentally friendly than toxic lead based batteries.
- Lithium battery performance is less affected by low ambient temperature.
Risks associated with lithium batteries
- Over charging of an unprotected battery will result in heat and possibly fire (remember the Boeing 767 lithium cobalt battery fires?)
Solution: Select a battery with an internal over charge limiting circuit, and the more stable LifePO4 type. Where possible employ a charger designed for lithium batteries.
- Even factory matched cells will become unbalanced in charge with respect to other cells in the pack after some time. This means that some cells will become over charged, even though the overall battery voltage has not yet reached the designed maximum. Heating will occur and battery life will be reduced. All RC modellers know this, and employ balancing chargers. But in many applications it is not practical to use a balancing charger which needs to plug into the individual cells.
Solution: Select a battery with internal cell balancing electronics.
- Short circuiting any battery can have often catastrophic results.
Solution: Batteries which are used for storage applications (ie not engine starting), should incorporate internal current limiting circuits.
Why are rechargeable lithium batteries so hard to find:
- Tales of kids burning down the house when recharging LiPo model batteries are rife.
- IATA regulations make it very difficult to ship batteries rated over 100Wh (12V7,5Ah)
- Battery distributers are content to just deliver short life, old and “safe” technology.
Why Li-Start lithium batteries?
- The internal Battery Management System of a Li-Start lithium storage battery prevents over charge and over discharge, ensures cell balancing and limits the short circuit current. A standard lead battery has none of this internal protection and neither do many other lithium batteries on the market. As soon as a manufacturer recommends the use of a special charger with a multi-pin connector for battery conditioning, the associated battery almost certainly has no internal BMS.
Even Li-Start engine starting batteries have internal cell balancing and over charge protection.
- Li-Start lithium storage batteries incorporate an internal BMS which simply disconnects the terminals internally before the battery becomes over discharged.
- Li-Start batteries employ mechanically stable wound cylindrical cells, as against the slightly more energy dense prismatic (flat) cells which can easily swell and burst the housing when abused. Swollen and hot prismatic cells inside a mobile phone is a common phenomenon.
- Li-Start engine starter batteries incorporate substantial solid terminals to accommodate the huge cranking currents.
- Li-Start batteries are closer to a fit and forget substitution of a lead battery, than other types.
Typical Li-Start BMS modules