Constant current / constant voltage 12V battery charger questions?

[Chris Wilson]

I need to have a permanently connected mains powered battery charger on a 12V lead acid battery for a 20kW diesel standby, auto start generator. Ones marketed specifically for generators have a high price, can any "battery maintainer" type chargers be used on a battery where the alternator on the generator's engine will start making output and charging when it is fired up? (I.E, back feeding the trickle charger, if I put a hefty diode in one of the leads between charger and battery?) I realize there would the be the voltage drop across the diode to consider.

Or on a conventional constant current / constant voltage charger would that diode stop it "seeing" the state of the battery it is charging?

I was thinking of building this, 2 Amps might be enough to keep the battery topped up and just running the 100mA or so the control panel draws in standby mode?


https://www.engineersgarage.com/des...ge-limited-current-charger-for-ups-part-2-17/

Thanks.

 

[Ramussons]

All you need is a unit to keep the battery in float mode since the charging will be handled by the engine when it runs.
You will need 13.8 volts for float condition, not 12.
You can use a P channel MOSFET as an isolation diode that does not have much diode drop.
And if you are reasonably comfortable in electronics repair, you can modify a surplus computer ATX PSU to give you that 13.8 volts. It can also function as a "charger" since it can source 15 to 20 Amps at that voltage.

 

[crutschow]

You will need 13.8 volts for float condition, not 12.

2.25V per cell is the Battery University recommendation at 25°C, making it 13.5V for a 12V lead-acid battery.

You can use a P channel MOSFET as an isolation diode that does not have much diode drop.

A single MOSFET can be used for reverse bias protection, but it requires additional circuitry to act as a low-drop diode in this case, since there is always the same polarity voltage at both the source and drain, keeping it always conducting otherwise.

 

[crutschow]

I was thinking of building this,

That should work as long as you tweak the voltage to give 13.5V battery float voltage at the diode output.

 

[crutschow]

On second thought, since I assume the generator and its battery are subject to significant outdoor temperature variations, you should consider temperature compensation of the charging voltage.
This from the Battery University website:

"Heat is the worst enemy of batteries, including lead acid. Adding temperature compensation on a lead acid charger to adjust for temperature variations is said to prolong battery life by up to 15 percent. The recommended compensation is a 3mV drop per cell for every degree Celsius rise in temperature. If the float voltage is set to 2.30V/cell at 25°C (77°F), the voltage should read 2.27V/cell at 35°C (95°F). Going colder, the voltage should be 2.33V/cell at 15°C (59°F). These 10°C adjustments represent 30mV change."

Below is the LTspice sim of an LM317 circuit to do that.
It uses a TL431 programmable reference to control the output voltage, along with two diodes, whose forward voltage drop change with temperature generates the desired -18mV/°C output voltage change.

The LM317 here is only used essentially as a "smart" transistor with current limiting and over-temperature protection, since the TL431 controls the LM317 ADJ pin voltage to regulate the output voltage.
Resistor Rlim determines the charge-current limit, so a separate LM317 for that isn't required.

D3 protects the LM317 if the battery is connected without charger power.

Trimpot U3 allows adjustment of the float voltage, which is typically set to 13.5-13.8V @ 25°C.

The sim below shows a -383mV change in output voltage for a 20°C rise in temperature, reasonably close to the desired 360mV (60mV/cell).