Author Topic: Is there a way to safely charge a car battery with this equipment? If not, why?  (Read 1201 times)

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Offline epigramx

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I have a step-up module from the china shops on which I can adjust both the current(limit) and the voltage output and I can feed it with an old PC PSU's 12V (they're supposedly max 400W, both the module and PSU). Now, I wonder how can I figure out only with that equipment and a particular car battery and a multimeter(I also have a voltage + current monitoring device) how to charge it safely (under supervision). 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)?
« Last Edit: October 20, 2019, 09:32:52 am by epigramx »
 

Offline Yansi

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Re: What's the safest way to "ghetto"-charge a car battery?
« Reply #1 on: October 20, 2019, 09:18:15 am »
Use a proper charger.
 

Offline Simon

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Re: Is there a way to safely charge a car battery with this equipment?
« Reply #2 on: October 20, 2019, 09:32:50 am »
If you want to charge a battery right up 14.4V but don't leave it too long. You can float charge at around 13.5V. i would not try to put 400W or nearly 40A into a typical 40A car battery. i think the charge rate is usually arout 5-10A.

as suggested how about a comercial charger? they would be as cheap as the whole combo you have.
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Offline epigramx

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Re: Is there a way to safely charge a car battery with this equipment?
« Reply #3 on: October 20, 2019, 09:37:49 am »
If you want to charge a battery right up 14.4V but don't leave it too long. You can float charge at around 13.5V. i would not try to put 400W or nearly 40A into a typical 40A car battery. i think the charge rate is usually arout 5-10A.

as suggested how about a comercial charger? they would be as cheap as the whole combo you have.
It's also for the lols because I kept one that is not dead yet, and for general interest about the technology of battery charging (I don't need it for anything particular yet, so it's mainly general research, if it was for a real project, I'd probably try to find an always-on charging system anyway).
So you seem to imply that the voltage of the battery will remain less than 14.4V if I feed it with a 14.4V input (unless I misunderstood) and at that point it can be considered full (or, it sounds more logical to me that the voltage will/should remain 14.4V (if the source can manage it) but the current will drop below what is first drawn).
Regarding the current limit, I would never use the max of that module anyway (I doubt it can survive itself that way), so I guess it would help to research what current that battery model usually draws (unless 5-10A you mentioned is universally true).
« Last Edit: October 20, 2019, 09:40:43 am by epigramx »
 

Offline Yansi

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I do not think that you car alternator, let alone in cars equipped with start/stop system cares about your 5-10A charging limit, bhaha!
 

Offline Simon

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if you apply 14.4V the battery will charge to that voltage. once you remove a car battery from charging it will drop the valtage. A sitting battery with nothing connected will be at 12.6V when fully charged. the extra voltage you apply when charging is to make it charge. In a vehicle the battery is held at 13.5V
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Offline Simon

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I do not think that you car alternator, let alone in cars equipped with start/stop system cares about your 5-10A charging limit, bhaha!

that is for a manual charge. that is why the car alternator runs 13.5V not 14.4V
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Offline Yansi

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The thing is, alternator is not set for 13.5V.   Be pretty sure it can output over 14V.
 

Offline Simon

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It can output anything within the limits of the magnetics and regulator. Probably easily 28V.....
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Offline Yansi

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No, you don't understand, nor know.

Car alternator output is usually set to about 14.4V. There is no point in charging using 13.5 or 13.8V. That would take for ages to charge the battery.

You 5-10A charging current limit  is just a silly talking. Alternator is like "zero fucks given" to your current limits.
 

Offline Simon

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No, you don't understand, nor know.

Car alternator output is usually set to about 14.4V. There is no point in charging using 13.5 or 13.8V. That would take for ages to charge the battery.

You 5-10A charging current limit  is just a silly talking. Alternator is like "zero fucks given" to your current limits.

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!
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Offline Yansi

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Why not you go first then?

 

Offline Yansi

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Offline Simon

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i did! i recently measure the output of a bus! even though it has a ot to drive the alternator is set at 27V or 13.5V per battery. Same for a tank i measured the supply voltage on: 27V / 2 - 13.5V last time I measured my car: 13.5 to 13.8V.
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Offline T3sl4co1l

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FYI, last time I went through this (floating my car battery to top it up) I set it to 14.3V and went from about 20A initially down to a few 100 mA after 24 hours.  This is a little high for a proper (always-on) float, but fine for this duration, and as you can see there was very little if any overcharging.

Electrolysis starts at higher voltages; you can't really charge much faster (than whatever the peak rate is, C/2 or thereabouts?) because the plates load up with bubbles, greatly increasing resistance.  Dangerous, anyway (H2+O2).

OCV is around 13V and the extra voltage is required for charging, to overcome the overpotential of the electrode materials.  Basically the overpotential consumes charge efficiency: you have to charge so-and-so above the electrochemical potential to make anything happen, and the difference is lost as heat.

Which is what makes lithium ion so amazing, there's essentially no overpotential and the cell voltage is entirely due to concentration of ions in the electrodes.  So the charge and discharge efficiency is super high, it looks like a crummy nonlinear capacitor more than a cell.

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Offline RoGeorge

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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)?

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: [Select]
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

- 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/
« Last Edit: October 20, 2019, 11:28:10 am by RoGeorge »
 

Offline BradC

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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!

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.
 

Offline mikerj

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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!

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.
 

Offline Simon

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what vehicles use AGM batteries?
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Offline helius

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what vehicles use AGM batteries?
Motorcycles and scooters.
 

Offline Simon

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Wel they tend to be lower capacity so it's more vital to charge them up ready for the next start. I recently charged a collegue motorbike bttery after he left the lights on.
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Offline Yansi

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AGM or not, chemistry is the same, voltage is the same.

AGM is just Absorbed Glass Mat, meaning the acid solution is wicked into a woven glass fiber rag, so it does not splash and you get a freedom in orienting of the battery in space whichever way you want.

 

Offline Simon

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Well you charge you batteries as you damn well please then  :horse:
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Offline Yansi

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It is fun to watch you being angered when confronted with the truth.
 
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Offline Simon

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I'm not angry. I supplied my information above. i recently worked on a bus with a flat battery because of the electrical load we were putting on it. The alternators (yes 2 of them) were struggling at idle speeds and the battery voltage slowly fell. When I asked for the engine to be reved up hard the voltage spiked at 27.2V and immediately (millisecconds) settled at 27V despite the flat battery and load on the system. It could have gone higher but would not. This is a comercial vehicle. I think they know what they are doing........ You may have any opinion you like.....
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