Electronics > Beginners
Is there a way to safely charge a car battery with this equipment? If not, why?
RoGeorge:
--- Quote from: epigramx on October 20, 2019, 09:16:16 am ---Say, can I just set the module to 14.4V and the current limit to something high-ish and then monitor if the current it draws is dropping, or is there another way (if there is even a way)?
--- End quote ---
Yes, it's possible, but you must:
- put a series diode in series with the battery (a high current diode, I put a double diode in parallel). This is a must, because if there is a power outage, then the battery will start pumping current back into the PSU. Depending on the PSU model, pumping current into the PSU might be dangerous or not, but better safe than sorry. Put a series diode.
- limit the current, e.g. for the first few hours put a car light bulb in series with the battery. After the battery voltage is 12-13V, the light bulb can be removed, but keep the protection diode in series with the battery.
- the 14.4V top charging voltage (measured on the battery) depends with battery temperature, I am using this table:
--- Code: ---Lead Acid generic specs
-----------------------
At a comfortable temperature of 20°C (68°F), gassing starts at charge voltage of 2.415V/cell. When going to 20°C (0°F), the gassing threshold rises to 2.97V/cell.
Battery status -40°C (-40°F) -20°C (-4°F) 0°C (32°F) 25°C (77°F) 40°C (104°F)
Voltage limit on recharge 17.1 16.2 15.3 14.7 14.1
Float voltage at full charge 15.3 14.7 14.1 13.8 13.5
or lower or lower or lower or lower or lower
gasing at 20*C for V >= 14.4V
nominal, no load = 12.6V
floating charge = 13.5...13.8V
--- End code ---
- do not let it unsupervised, and be careful with wiring, because a car battery can easily melt very thick wires and start a fire when short-circuited.
Later edit:
https://www.eevblog.com/forum/chat/charging-my-car-battery/
BradC:
--- Quote from: Simon on October 20, 2019, 10:22:55 am ---why does the alternator need to charge the battery that much? the battery is only used to start the engine and if the power demand is in excess of the alternator. The alternator will have a current limit but that is not what controls the battery charging it is to protect the alternator. The charge is controlled with the voltage and hene it is NOT 14.4V, go and measure your own vehicle if you don't beleive me!
--- End quote ---
Well, that was interesting. My 1989 Volvo setpoint at 21 degrees C is 14.3V. My 2008 Landrover starts with a setpoint from the ECU of at 15.3V! (real voltage from the alternator while putting 80A into the battery ~14V) and tapers back to 13.8 once the battery hits > 14.2V. A pair of Volvo TAMD70B diesels are set to 28.6V (14.3/battery) at 21C. Our Volkswagen Golf V (2005) starts at 14.4 and winds back to 13.9 once up to temperature (after 30 mins driving).
Calcium based batteries can be charged extremely hard. The alternator in the Landrover can put 80A into the battery after a particularly cold and hard start and the battery just laps it up until the charge current tapers off before the ECU turns the voltage down.
So there you go. Real data from 3 vehicles and a vessel. Initial charging current on all of the batteries is > 50A. The Landrover is particularly brutal but it's also a pretty big battery.
mikerj:
--- Quote from: Simon on October 20, 2019, 10:22:55 am ---why does the alternator need to charge the battery that much? the battery is only used to start the engine and if the power demand is in excess of the alternator. The alternator will have a current limit but that is not what controls the battery charging it is to protect the alternator. The charge is controlled with the voltage and hene it is NOT 14.4V, go and measure your own vehicle if you don't beleive me!
--- End quote ---
Pretty much all modern cars will regulate charging voltage to 14.2-14.5v, and even higher than this on start/stop equipped vehicles that use AGM batteries.
Simon:
what vehicles use AGM batteries?
helius:
--- Quote from: Simon on October 20, 2019, 05:05:14 pm ---what vehicles use AGM batteries?
--- End quote ---
Motorcycles and scooters.
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