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Old 11-29-2012, 09:39 PM   #259
_cy_ OP
Beastly Adventurer
 
Joined: Sep 2011
Location: Tulsa, Oklahoma
Oddometer: 5,938
a common question .... will a standard lead acid battery charger properly charge my LiFePO4 battery?

answer is it depends on the charger. for not all lead acid battery chargers work the same. it's best to use a charger designed specifically to charge LiFePO4 batteries. But one can improvise and use a charger designed to charge lead acid by carefully monitoring charge. then remove when charge gets close to full. being careful not to exceed max voltage of 14.4v for LiFePO4 batteries.

Cellpro Powerlab 8 is what I'm using. A favorite of the RC world. considered the most powerful/versatile of all hobby chargers with software to track charge cycle and generate graphs.

here's my charge station with DVP-2212 (22amp) and HP 6263B regulated power supply


below are the three stages for lead acid charging which works fine with LiFePO4 batteries. not recommended are chargers with an equalization feature which could overcharge/damage LiFePO4 batteries. best lead acid chargers without extra features, like Schauer shown below.

some battery tenders don't actually terminate charge when a certain voltage is reached. but instead continues to deliver a small milliamp charge. this could result in an overcharge condition and damage your LiFePO4 battery.

for most bikes with LiFePO4, battery tenders are not needed or desired. discharge rates are under 1% per month. if no parasitic drain exist on bike, then no tender will be needed.

here's a chart showing LiFePO4 battery's discharge profile. reading is for per cell, 4x 3.6v per cell = 14.4v full charge for LiFePo4 battery. max charge of 3.6v per cell or 14.4v

so long as lead/acid battery charger used doesn't contain an equalization mode and doesn't exceed 14.8v. it should work fine. note equalization (controlled over charge) is also called desulfation mode. note equalization is also not recommended for AGM or Gel cell batteries.




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Three Stage Battery Charging for Lead Acid

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125 F, or an AGM or GEL (valve regulated) battery over 100 F.
The ABSORPTION stage (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged.
The FLOAT stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.

Equalization
is essentially a controlled over charge. The electrolyte in a wet battery can stratify over time, if not cycled occasionally. In equalization, the voltage is brought up above typical peak charging voltage (to 15 to 16 volts in a 12 volt system) well into the gassing stage, and held for a fixed (but limited) period.

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LiFePO4 uses constant current (CC) and constant voltage (CV) and consists of three charging phases: pre-charge; fast-charge CC; and CV.

In the pre-charge phase, the battery is charged at a low rate for testing, if the battery is internally shorted when the cell voltage is below 0.5 V.

Fast-charge current is applied to charge the battery quickly. Its charging rate can be up to 10C rate, which is much higher than the traditional LI-Ion battery without additional degradation. The charger enters to the CV mode when the battery reaches a voltage regulation limit (typical of 3.6 V/cell). During the CV mode, the charge current exponentially drops to a pre-defined termination level where the battery is fully charged and the charging is terminated. Since the LiFePO4 battery has much lower internal resistance, its charging time is much shorter than the Li-Ion battery.
While the LiFePO4 is much safer than the Li-Ion battery, a fast charge safety timer is usually required to prevent charging a dead battery for an excessively long period. The LiFePO4 battery can be overcharged to 4 V without safety issues, even though it is specified to charge to 3.6 V. However, the energy stored in the battery between 3.6 V and 4 V is very limited. From the discharge curve in Figure 1, the voltage drop is very fast at the beginning of the discharge period. This demonstrates that the battery does not store much energy at higher voltages.
Most of the battery energy is stored near the battery voltage between 3.0 V and 3.4 V for 1C-5C discharge rates. It does not give much benefit to charge the battery higher than 3.6 V though it does not degrade the battery. The voltage difference between rechargeable voltage threshold and battery charge voltage should be around 200 mV, since it takes a few seconds to drop the battery voltage from 3.6 V to 3.5 V. Although the LiFePO4 battery has excellent and stable high temperatures, it is still preferable to monitor its temperature to improve safety.
LiFePO4 battery charge profile.

_cy_ screwed with this post 12-11-2012 at 06:39 AM
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