Charger extends lead-acid-battery life
A circuit that properly charges sealed lead-acid batteries ensures long, trouble-free service.
Fran Hoffart, National Semiconductor Corp, Santa Clara, CA; Edited by Paul Rako and Fran Granville -- EDN, December 1, 2011
Originally published in the February 7, 1985, issue of EDN
A circuit that properly charges sealed lead-acid batteries ensures long, trouble-free service. Fig 1 is one such circuit; it provides the correct temperature-compensated charge voltage for batteries having from one to as many as 12 cells, regardless of the number of cells being charged.
The LM301A compares the voltage
drop across R1 with an 18-mV
reference set by R2. The comparator’s
output controls the voltage regulator,
forcing it to produce the lower float
voltage when the battery-charging current
passing through R1 goes below 180
mA. The 150-mV difference between
the charge and float voltages is set by
the ratio of R3 to R4. The LEDs show
the state of the circuit.
Temperature compensation helps
prevent overcharging, particularly
when a battery undergoes wide temperature
changes while being charged.
The LM334 temperature sensor should
be placed near or on the battery to
decrease the charging voltage by 4
mV/°C for each cell. Because batteries
need more temperature compensation
at lower temperatures, change R5 to
30Ω for a TC of −5 mV/°C per cell if
your application will see temperatures
below −20°C.When the circuit charges more than six cells, the additional voltage across the LM334 increases self-heating, so use a small heat sink and increase the resistance of R6. Likewise, use higher resistances in series with the LEDs to avoid overloading the LM301A.
The charger’s input voltage must be filtered dc that is at least 3V higher than the maximum required output voltage: approximately 2.5V per cell. Choose a regulator for the maximum current needed: LM317 for 1.5A, LM350 for 3A, or LM338 for 5A. At 25°C and with no output load, adjust R7 for a VOUT of 7.05V, and adjust R8 for a VOUT of 14.1V.
Talkback
-
It's unusual to see a Nat Semi circuit with errors, but J S is right, the float voltage should be 13.5-.6V for a 12V battery not the 14.1 given. It is misleading to say the bulk charge phase is constant current; a current limited voltage source is closer.
Einar Abell - 2011-29-12 01:04:13 PST -
Actually this circuit has a good chance of shortening battery life. A 100Ah SLA can have a float current of 200mA at 25oC if it has endured only partial charging or the stockist has not performed a six month refresh charge before delivery.
A new 100Ah battery fully charged should have a float current of around 30mA to cancel losses. The float voltage stated of 2.35Vpc is too high. The bulk charge voltage is normally set at around 2.4Vpc.
Manufacturers recommend a float voltage of 2.27Vpc any higher or lower could damage to the battery. Most chargers now use a constant current for the bulk charge phase switching to constant voltage to finish off.
A good reference guide for lead acid batteries
is the Exide Handbook for Stationary Lead-Acid Batteries, Part 1.
The temperature compensation circuit is over the top
and given the vintage of this circuit I'm surprised they never used a silicon resistor. Also checkout Linear Technology Application Notes AN51 and AN66 for similar circuits from the same era.
John Straker - 2011-7-12 09:15:13 PST -
Why would you do this with discrete components when there are dedicated IC chips that can do this at lower cost, complexity, and added protection?
Just one example is www.ti.com/product/bq24450
Karen
Karen Nakamura - 2011-6-12 18:03:22 PST -
It is true that turning the supply off with the power connected would cause some problems. One simple solution would be to add a diode in series with the output, and recalibrate. There are undoubtedly other solutions, but hey, this is indeed a classic. Can you still buy LM301s?
William Ketel - 2011-5-12 14:46:56 PST -
Just don't turn the power off with the battery connected!
Alan Crawford - 2011-5-12 09:36:15 PST
