Care and Feeding of Your New Lithium Ion Battery

Rich CarrollPast President, Fox Valley Electric Auto Assoc.

Copies of this presentation in .odp, .ppt, and .pdf format can be found at:

http://rc.to/NSB

Lithium Battery Primer

As an overview:

● Lithium Batteries are about 4 or 4.2 Volts per cell

● In most applications, cells are packaged together into a 'battery'

● When cells are wired in series, voltage is additive, so 3 cells of 4 volts in series gives 12 volts

● When cells are wired in parallel, amperage is additive, so 3 cells rated at 1500 mAh in parallel would be rated at 4500 mAh

Several different Lithium substrates are used:

● Lithium ion

● Lithium Iron Phosphate

● Lithium Iron Yttrium Phosphate

● Lithium Iron Cobalt Phosphate

● Other “proprietary combinations”

Cycle life as a function ofdepth of discharge

A partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.1

Depth of discharge

Discharge cycles

100% DoD 300 – 500

50% DoD 1,200 – 1,500

25% DoD 2,000 – 2,500

10% DoD 3,750 – 4,700

Cycle life as a function ofdepth of discharge

A partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.

Depth of discharge

Discharge cycles

100% DoD 300 – 500

50% DoD 1,200 – 1,500

25% DoD 2,000 – 2,500

10% DoD 3,750 – 4,700

Estimated recoverable capacity when storing Li-ion for one year at various temperatures

Elevated temperature hastens capacity loss. The capacity cannot be restored. Not all Li-ion systems behave the same. 1

Temperature 60% charge

100% charge

0°C 98% 94%

25°C 96% 80%

40°C 85% 65%

60°C 75% 60%(after 3 months)

Estimated recoverable capacity when storing Li-ion for one year at various temperatures

Elevated temperature hastens capacity loss. The capacity cannot be restored. Not all Li-ion systems behave the same.

Temperature 60% charge

100% charge

0°C 98% 94%

25°C 96% 80%

40°C 85% 65%

60°C 75% 60%(after 3 months)

Discharge cycles and capacityas a function of charge

Every 0.1V drop below 4.20V/cell doubles the cycle; the retained capacity drops accordingly.

Raising the voltage above 4.20V/cell stresses the battery and compromises safety.

Summary: battery life can be extended by not charging to full charge.

Charge level (V/cell)

Discharge cycles

Capacity at full charge

[4.30] [150 – 250] [110%]

4.20 300 – 500 100%

4.10 600 – 1,000 90%

4.00 1,200 – 2,000 70%

3.92 2,400 – 4,000 50%

Lithium Batteries in VehiclesHere are some things they won't teach you: ● Amperage related to acceleration● Power (Watts = Amps x Volts) is related to overcoming

friction● Students won't have a choice of amps or volts, they are

pre-selected● Torque of an electric motor is high at virtually all speeds,

from motor speeds of 2 RPM to over 2000 RPM● Speed of a DC motor is directly related to voltage● Plan to have the electric motor at top (peak) RPM at or

near the end of the track

Lithium Battery Secrets● Lithium Ion delivers the most power from a full

charge (at maximum possible voltage)● Lithium Ion tends to deliver the most power

about 30-35º C (about 86 – 95º F)● These cells can be left on the charger at

100% SOC, as long as theyas long as they are not in an are not in an enclosed area and they are kept below 40º enclosed area and they are kept below 40º C (104º F) C (104º F) 2

● Occasionally (once every 30 cycles) cycle from 100% SOC to 10-20% SOC. Otherwise, short cycles are suggested, with recharge each time to 100% SOC 2

Lithium Battery Secrets

● Lithium Ion energy delivery slows down at lower temps and with use. This means the battery may be able to deliver full power at first, but have slightly reduced power after a couple of seconds. (Due to the chemistry of the battery cell not fully reacting immediately.)● This is shown by a voltage 'sag'● Voltage will go down significantly, but come back over time

Lithium Battery Sag Secret

● Vcell

= 4.20 at the starting line● V

cell = 4.17 as the vehicle starts

● Vcell

= 3.60 at the finish line● V

cell = 4.06 fifteen minutes later without charging

● Vcell

= 4.09 thirty minutes later without charging

Lithium Batteries Voltage vs. SOC

● Between 100% SOC and 90%, there is considerable variation in battery voltage

● Between 90% SOC and 10%, there is very little variation in battery voltage

● Between 10% SOC and 0%, there is consierable variation in battery voltage

Voltage vs. State of Charge

Voltage vs. State of Charge

● Vcell

= 4.20 when fully charged● V

cell = drops to 3.6 at about 90% SOC

● Vcell

= drops about 0.003V to 0.005 V for each % change in SOC down to about 10% SOC ● V

cell = after 10% SOC, voltage plummets to 2.50

● Don't let cell get lower than 3.4 V.

Safety

● DC voltages less than 50 are considered non-lethal● Your cell will be around 4 volts● Cells too hot to hold in your closed hand are unsafe. Temperature ~ 50º C (122º F)● Any rapid charge or discharge will warm battery● If the battery is not constrained, it may bulge or warp● Warped batteries means internal plates can touch which can be a 'short'● Lithium Ion cells can have 'thermal runaway'

Footnotes

1 http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries2 http://batterycare.net/en/guide.html 3 http://johndayautomotivelectronics.com/?p=517

Copies of this presentation in .odp, .ppt, and .pdf format can be found at:

http://rc.to/ANL