LM393 based battery maintainer question

Thread Starter

Rob1959

Joined Oct 30, 2023
6
This is probably really simple, so here goes. I am trying to build a trickle charger / maintainer for a 12 volt (14.2) lead acid battery that powers my shortwave gear. It is based on the LM393 dual comparator. It is currently configured with comparator 1 to turn on a LED when the battery voltage drops to 13.8v, and comparator 2 to turn on a different LED when the battery voltage reaches 14.2v. That is what I have selected to be the range of the maintenance charge.

I had intended to use optocouplers in place of the LEDs to control a mosfet for charging, but right at the crossover voltage the LEDs are unstable and i wonder about false triggering. I am using voltage divider resistors, 3.3k - 5k pot - 3.3k for fine tuning the trip points. I have 10k pull up resistors on the output pins and .01 ceramic caps across the inverting and noninvting pins of each comparator.

Also, am I looking at utilizing the output in the wrong way? Can I just tie the base of a pnp transistor to the comparator and use a pull up resistor. Any suggestions would be appreciated.
 

LowQCab

Joined Nov 6, 2012
4,065
What type of Lead-Acid-Battery do You have ?,
a small Sealed-Lead-Acid-Battery ?,
or an Automotive-style-Starter-Battery ?

Your Charging-Voltage is probably too high.

How long will the Battery run your Gear without a Charger ?

How much Peak, and Average, Current does your Gear draw ?

Without knowing what You actually need,
I would recommend simply purchasing an inexpensive Battery-Maintainer from Wal-Mart.

If You want to build your own Maintainer, with plenty of Current potential
check out the attached Schematics / PDFs.

Notes :

Charging for Cycle Operation

Cyclic applications generally require that the
recharging be done in a relatively short time.

The initial charge current, however, must not exceed
0.30 x C amps, (for small Sealed Lead-Acid Batteries).

Just as battery voltage drops during discharge,
it slowly rises during charge.

Full charge is determined by voltage and inflowing current.

When, at a charge voltage of 2.45 volts/cell, (14.7V),
the current accepted by the battery drops to less than
0.01 x C amps (1% of rated capacity), the battery is fully charged,
and the charger should be disconnected,
or switched to a float voltage of 2.25 to 2.30 volts/cell, (13.8V).

The voltage should not be allowed to rise above 2.45 volts/cell, (14.7).
---------------------------------------------------------------------------------------

Charging for Standby Operation

Standby applications generally do not require that the
battery be charged as fast or as frequently as in cycle operation.

However, the battery must be kept constantly charged to replace the energy
that is expended due to internal loss, and deterioration of the battery.

Although these losses are very low in Power-Sonic batteries,
they must be replaced at the rate the battery self discharges;
but at the same time,
the battery must not be given more than these losses or it will be overcharged.

To accomplish this,
a constant voltage method of charging called “float charging” is used.

The recommended constant float voltage is 2.30 volts per cell, (13.8V).

Maintaining this float voltage will allow the battery to define its own
current level, and remain fully charged,
without having to disconnect the charger from the battery.

The trickle current for a fully charged battery,
floating at the recommended charge voltage,
will typically hover around the O.OO1C rate,
(10mA for a 10AH battery, for example).

The float charger is basically a constant voltage power supply.

As in cycle chargers, care must be exercised not to exceed
the initial charge current of 0.30 x C amperes.



.
Lead-Acid Temp Graph .png
.Adjustable Current-Voltage Regulator .png
.Temp Compensated Maintenance Charger .png
 

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Last edited:

Ian0

Joined Aug 7, 2020
9,804
This is probably really simple, so here goes. I am trying to build a trickle charger / maintainer for a 12 volt (14.2) lead acid battery that powers my shortwave gear. It is based on the LM393 dual comparator. It is currently configured with comparator 1 to turn on a LED when the battery voltage drops to 13.8v, and comparator 2 to turn on a different LED when the battery voltage reaches 14.2v. That is what I have selected to be the range of the maintenance charge.

I had intended to use optocouplers in place of the LEDs to control a mosfet for charging, but right at the crossover voltage the LEDs are unstable and i wonder about false triggering. I am using voltage divider resistors, 3.3k - 5k pot - 3.3k for fine tuning the trip points. I have 10k pull up resistors on the output pins and .01 ceramic caps across the inverting and noninvting pins of each comparator.

Also, am I looking at utilizing the output in the wrong way? Can I just tie the base of a pnp transistor to the comparator and use a pull up resistor. Any suggestions would be appreciated.
How about a circuit diagram?
What limits the current from the charger?
I agree with @LowQCab that the float voltage is a bit high, but I think you do need to charge it to 14.5V every time it is discharged by a significant amount.
Charging it to 13.8V on an intermittent basis will keep the battery in good health and prolong its life over simply keeping it at 13.8V float forever.
 

LowQCab

Joined Nov 6, 2012
4,065
A Charger that is regulated to a "Maintenance" level Voltage,
and yet at the same time,
capable of supplying the full-Current-demands of the Load, is the best way to go.
But, this also means that the Battery must be capable of withstanding this "Full-Current" during Charging,
a small "Sealed-Lead-Acid-Battery" may not have that capability,
however, an automotive-style, "Sealed-Lead-Acid-Battery", like an "Optima" brand Battery
will handle almost any set of circumstances, and hundreds of Amps, but they're a bit pricey.

This arrangement will mean that the Battery will act more like a gigantic Capacitor,
actually providing Power only during Peak-Demands,
and stabilizing the Power-Supply Voltage.

We still have not been informed as to the detailed character of the Load,
so any recommendations would be completely speculative.

Are You trying to run a ~1000-Watt-Linear ???,
or a ~5-Watt CB-Radio ?
.
.
.
 
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