# Latching relay for battery charging

#### 250ptm

Joined Jul 29, 2010
50
Apologies it should have read the LED’S need 0.03A (30Ma) not 0.3A.

Here are my calculations based on a $0.20c per kwh. Please correct me if this is barking mad! Full charge mode @ 6A: 6A*12v = 72W *3 hrs = 216W *52 weeks = 11.232kwh *$0.20c = $2.24 per year. Float charge mode @ 2A: 2A*12v = 24W*24hrs = 576W*365 = 210.24kwh *0.20c =$42.48 per year.

To keep the battery charged using the same amount of power it uses at 6A for 3 hrs once a week, the charger would have to consume 11.23kw over 365 days. That would give a charger output of:

11.23kw/365 days = 30W/24hrs = 1.28W/12v = 0.1A output.

Is 0.1A (100mA) output is enough to keep my battery topped up?

#### Just Another Sparky

Joined Dec 8, 2019
149
Your battery shouldn't be taking 24 watts to float. If the charger is pushing that much power into a charged battery then it's probably boiling away the electrolyte and killing your battery. Modern digital chargers and maintainers are notorious for this. Check the output voltage. 13.5V would be the upper limit for a safe floating voltage. 13.0-13.2 would be more comfortable. This number is also temperature dependent.

50-100mA would be a much more realistic floating current to see with a car battery connected to an ECU - \$1.00-2.00 annually at your given rate.

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#### LowQCab

Joined Nov 6, 2012
195
Did you actually MEASURE these numbers,
or are you going by the label on the Charge-Rate-Switch on the front of the Charger ????

2-Amps is a maximum "limit" on the Current,
not necessarily the Current that is actually being consumed by the Battery.

A 2-Amp "limit" is "supposed to" indicate that it is low enough to not boil the Battery if left unattended.

If the Output-Voltage is less than ~13.2,
and you have a Battery in good condition,
the Charging-Current will eventually go to almost zero.
Then you only have Charging-Circuit inefficiencies drawing any measurable power,
which "should be" so ridiculously low that you have to measure it
2 or 3 times to make sure you have obtained an accurate reading .

How are you measuring high Voltage AC Current ?
How are you measuring low Voltage DC Current ?
.
.
.

#### 250ptm

Joined Jul 29, 2010
50
Thanks, JAS, will check the v output of the charger, once I have the LED indication that the battery is full.

Hi lowQcab,
Ac current has not been measured yet, although I do have an AC clamp meter available. Low DC current measured with multimeter set to 10A. Perhaps I haven't waited long enough for the current to drop as its still charging! And yes the numbers i quoted above also agree with the mfg specs.

#### Flashy

Joined Jun 7, 2010
3
Thanks, JAS, will check the v output of the charger, once I have the LED indication that the battery is full.

Hi lowQcab,
Ac current has not been measured yet, although I do have an AC clamp meter available. Low DC current measured with multimeter set to 10A. Perhaps I haven't waited long enough for the current to drop as its still charging! And yes the numbers i quoted above also agree with the mfg specs.

#### Flashy

Joined Jun 7, 2010
3
Have you thought of a solar charger to keep the battery charge a Wall Mart product may do this I have one on my sons car that doesnt get used much it just plugs in the Gigar lighter socket and trickle charges. mine cost 5 Uk Pounds and is now 6 years old. Just stick it in the wind screen.
That's only the purchase cost no running cost and its green. if your car internal 12v outlet is live when the ignition is off your fine other wise use the crock clips that come with them directly to the battery.

#### du00000001

Joined Nov 10, 2020
15
@250ptm
Take your 10SQ050 Schottky diode and wire it into the positive connection between charger and battery. It might require a heat sink to get rid of the power dissipated during high current charging situations but should perform well in all other aspects. This will prevent reverse current flow nicely and will come simple and cheap.

But I'd suggest to use a Schottky diode in a TO-220 package (to ease attaching a heat sink) and rated not higher than 45 V.

(The magic of the 45 V rating: Schottky contacts cannot be manufactured to withstand >45 V. Any Schottky diode rated beyond this value is composed of 2 or more individual Schattky diodes in series, doubling/multiplying the voltage drop and the resulting power loss. An SBCT1040 and alike should nicely do.)

#### Hugh Riddle

Joined Jun 12, 2020
77
There can be good reasons for including a relay but some unexpected experiences (with integrated charging of low-power devices) prompt me to ask:
Do you want the relay to disconnect the solar panel at night? The arrangement shown leaves it latched on until the battery is totally flat.

#### Flashy

Joined Jun 7, 2010
3
@250ptm
Take your 10SQ050 Schottky diode and wire it into the positive connection between charger and battery. It might require a heat sink to get rid of the power dissipated during high current charging situations but should perform well in all other aspects. This will prevent reverse current flow nicely and will come simple and cheap.

But I'd suggest to use a Schottky diode in a TO-220 package (to ease attaching a heat sink) and rated not higher than 45 V.

(The magic of the 45 V rating: Schottky contacts cannot be manufactured to withstand >45 V. Any Schottky diode rated beyond this value is composed of 2 or more individual Schattky diodes in series, doubling/multiplying the voltage drop and the resulting power loss. An SBCT1040 and alike should nicely do.)
There can be good reasons for including a relay but some unexpected experiences (with integrated charging of low-power devices) prompt me to ask:
Do you want the relay to disconnect the solar panel at night? The arrangement shown leaves it latched on until the battery is totally flat.
The solar panel has an internal diode to prevent discharge at night. The solar cell will charge to full voltage as its capable of over 17 volts o/p. As the battery voltage rises this back EMF is in oposition to the battery voltage so its self regulating like the old chargers that used only ballast resistors for charge regilation.

#### Hugh Riddle

Joined Jun 12, 2020
77
The solar panel has an internal diode to prevent discharge at night. The solar cell will charge to full voltage as its capable of over 17 volts o/p. As the battery voltage rises this back EMF is in opposition to the battery voltage so its self regulating like the old chargers that used only ballast resistors for charge regulation.
I wasn't aware of any (non-photovoltaic) diode normally being incorporated into solar panels; at night, a panel anyway presents a string of reverse diodes which draw very little current. But, for the circuit shown, the battery holds the relay on indefinitely. In other words, the solar panel loses control of the relay (until the battery is disconnected or goes flat). That may be undesirable, for example when the panel is also used to sense dawn and dusk.

#### Hugh Riddle

Joined Jun 12, 2020
77
I wasn't aware of any (non-photovoltaic) diode normally being incorporated into solar panels; at night, a panel anyway presents a string of reverse diodes which draw very little current. But, for the circuit shown, the battery holds the relay on indefinitely. In other words, the solar panel loses control of the relay (until the battery is disconnected or goes flat). That may be undesirable, for example when the panel is also used to sense dawn and dusk.
Apologies, I must have slipped a mental cog. Of course, in the dark, a solar panel presents as a string of forward not reverse-biassed diodes to the battery although it only draws a small current and control problems can still arise.