Do I need a diode using a bench power supply for charging battery ?

[Trader]

low the diode voltage drop is, it is still non-zero. It wouldn't be much of a problem if it was only non-zero (just crank up the CV setting, and you're fine, right?), but it's also not constant and depends on current, thus making it impossible to set a precise final voltage, unless the PSU has a dedicated set of wires to sense it right at the battery terminals.

The Lead-Acid Battery, usually is fully charged when the current stabilizes at 0.01C (my battery is 7Ah, so doesn't decrease more than 70mA).

I verified that my 1N5822 from 3mA to 100mA has basically the same drop-voltage 290mV, so I set up the charging voltage to 7.79V, and when full-charged, the voltage will be up to 7.5V (I'm doing a Cycle charge [7.2V to 7.5V], not a Float charge [6.9V]).

But I found that using an Ideal Diode (2 MOSFETs with a control circuit around them) could be better.

It will definitely be better. Please share the circuit you have in mind (or maybe even tested?). I would be interested to try one myself.

I didn't develop one yet but look for an "ideal diode" on eBay/Aliexpress, there are a lot of models, and cheap (just 2 MOSFETs, capacitors, very simple).

[Trader]

About using a Battery Charger instead of my PSU:  I'm a hobbyist, I like to explore and learn, the PSU is fine if you respect the already cited rules:

- do not exceed max allowed voltage per cell (and min voltage for discharge);
- do not exceed max allowed current, both charge and discharge;
- make sure that the temperature stays within the allowed range and that the battery charged at a low ambient temperature is not then stored at a much higher temperature being still fully charged.

In fact, with a PSU I can have much more control of the Voltage and Current, and since I just do this eventually, isn't necessary to buy a 6V/12V/24V charger, and I guess that even those chargers are not so good in following those rules.

[shapirus]

I didn't develop one yet but look for an "ideal diode" on eBay/Aliexpress, there are a lot of models, and cheap (just 2 MOSFETs, capacitors, very simple).

meh, thanks. Moral: whenever you want to DIY something, rest assured, AliExpress is always ready to spoil the fun

[Trader]

And a little more general info on this topic from KeySight:
https://edadocs.software.keysight.com/kkbopen/what-can-cause-a-power-supply-output-voltage-to-exceed-its-setting-583423316.html

It would be nice if you read your own links...

It is possible for an external source of power (such as a battery, charged capacitor, inductor with changing current, or another power supply) to cause the voltage to go above the setting. The OVP will respond to this condition as outlined above. If the external power source can provide more current than the rating of the power supply and an SCR circuit is used in the power supply, it is prudent to put a fuse in series with the external source of power to prevent damage to the power supply SCR and/or output circuit from excessive current.

TLDR: nothing about diode. Use a FUSE.

Another question for you, what do you think the current will be when the output capacitor with ESR=X and voltage=Y is connected to a battery with ESR=Z and voltage=Å? Hint: the fact that you set the current limit at 4A is meaningless. Question: what do you think happens to your 4A rated diode? Do you know how to interpret SOA curves?

This is interesting. I'm using a Diode to avoid the back feeding when turning the PSU off with a battery connected to it, other posts on this thread related to burning PSUs because of the Back-Feed.

I'm sure the fuse won't protect the backfeeding, and about the current, I just measure the resistance of my PSU when off and it's 920 ohms (maybe there is a correct way to measure its impedance), but in a 24V battery this is just 26mA, so, the fuse will be useless and the diode can handle it very well, even a surge.

This is an interesting topic, I won't buy a charger because most of them are crap, maybe I can develop a circuit to charge batteries with limited voltage and current (like the PSU), and this back-feeding issue must be handled.

(https://www.st.com/resource/en/datasheet/1n5822.pdf)

[blackdog]

Hi,

I also keep in mind this is a topic in the beginners section of this forum.

Safety has many faces, also with Power Supply's.
I develop Power Supply's and try to think about what can go wrong if the user has a moment of not thinking before doing. 

A beginner usually does not have a professional Power Supply from KeySight which I have shown some info from.
The average hobbyist does not have a thick Thyristor across the output of his Power Supply and does not have all those safety measures that the fancy Power Supply's from Harrison, HP, KeySight and some others, usually all have.

If you are going to charge with e.g. a current of say 4-Ampere then you are not going to take a diode just that can handle 4-Ampere, then you choose something that can handle say 20-Ampere or more
and again think about the reverse voltage of the diode.

It is often beneficial to use a dual Schottky diode to keep the voltage drop low across the diode.
Also consider cooling of the diode at higher currents.
Again, so also keep in mind the reverse voltage of your diode!

The series diode is primarily intended to ensure that you do not send energy back into the Power Supply to do damage.
If you still want to use your power supply to charge batteries, then including a good fuse in series is an extra layer of safety.

Example
Below is a not complete description of how to connect and charge a car battery for charging.
The point is to give you a direction on how to carry it out, additions I would love to hear.

Measure the voltage of the battery you want to charge e.g. a car battery.
This measures e.g. 11.3V, you use a thick Schottky diode(heatsink?) and you use a fuse suitable for your application.

Turn on your Power Supply and set it to e.g. 12V for this car battery and also set the maximum current you want to charge it with, then connect the plus pole in the Power Supply last.

This way is to keep the difference between the battery voltage and the Power Supply Voltage as small as possible so that large surge currents do not flow,
if your Power Supply has a large capacitor across its output terminals, this is common with smps power supplies.

Slowly increase the voltage of the Power Supply until it charges with the current you want, but also measure the voltage at the battery terminals.
Be sure not to exceed the maximum voltage for the battery you want to charge.

Make sure you can quickly disconnect the battery or Power Supply if calamities occur.
If your battery is charging, don't hang out with the neighbors for dinner. 
Check the temperature, charge current and the battery voltage regularly.
Battery voltage is different from de Power Supply voltage if using a diode and fuse!

Kind regards,
Bram

[shapirus]

If you are going to charge with e.g. a current of say 4-Ampere then you are not going to take a diode just that can handle 4-Ampere, then you choose something that can handle say 20-Ampere or more
and again think about the reverse voltage of the diode.

It is often beneficial to use a dual Schottky diode to keep the voltage drop low across the diode.
Also consider cooling of the diode at higher currents.

This should be generalized for ALL components, not just diodes, as follows:

- read the datasheet for the component which you are going to use in your circuit and make sure that you: a) understand what it says; b) keep the component within the recommended operating conditions range, including those boring thermal requirements.

[Vovk_Z]

as a side note, FWIW, I have yet to see a PSU (at least of the CC/CV capable ones) that would be damaged by backfeeding from the battery.

You may easily find such PSUs with a simple test. Take a powerful enough and charged battery (e.g. >=12 Ah) and set the PSU output voltage much lower than 12-13 VDC, e.g. 3-5 VDC (lower voltage like 1-2 VDC may work even better).

[shapirus]

You may easily find such PSUs with a simple test. Take a powerful enough and charged battery (e.g. >=12 Ah) and set the PSU output voltage much lower than 12-13 VDC, e.g. 3-5 VDC (lower voltage like 1-2 VDC may work even better).

Maybe. I didn't think of this scenario. What I did test was to keep the battery connected to the PSU's output terminals with the input terminals being unpowered while the PSU's output voltage was set to the battery charge end / float value (e.g., 13.6 V for a 4s LiFePO4 battery).

In practice this is what happens when a battery is being charged, or, having reached the CV level, is staying in buffer mode, and the PSU/charger input gets powered off, e.g., in case of mains power outage. Worst case that I have seen so far when this happened was ~50 mA draining from the battery back into the charger.

[Trader]

as a side note, FWIW, I have yet to see a PSU (at least of the CC/CV capable ones) that would be damaged by backfeeding from the battery.

You may easily find such PSUs with a simple test. Take a powerful enough and charged battery (e.g. >=12 Ah) and set the PSU output voltage much lower than 12-13 VDC, e.g. 3-5 VDC (lower voltage like 1-2 VDC may work even better).

This isn't so clear to me, how can I verify if the PSU has back-feeding protection, measuring the current in reverse? But why this reverse current wouldn't damage the PSU?

[shapirus]

But why this reverse current wouldn't damage the PSU?

Why would it? It could, potentially (I have never seen this, but it doesn't mean it can't happen), but sometimes this current will, for example, simply light up a LED or something. It's actually an interesting question, I wonder where those 50 mA went in that PSU/charger that I mentioned. It didn't seem that this current did any harm, but still it was dissipated in something beyond lighting up that LED. Could also be in poorly chosen (too low) resistors in the feedback divider, for example. Maybe something else too.

[nightfire]

Example
Below is a not complete description of how to connect and charge a car battery for charging.
The point is to give you a direction on how to carry it out, additions I would love to hear.

Measure the voltage of the battery you want to charge e.g. a car battery.
This measures e.g. 11.3V, you use a thick Schottky diode(heatsink?) and you use a fuse suitable for your application.

Turn on your Power Supply and set it to e.g. 12V for this car battery and also set the maximum current you want to charge it with, then connect the plus pole in the Power Supply last.

Question: Does "plus pole last" mean that the intended sequence would be:
1) minus pole of battery
2) minus pole of power supply
3) plus pole on battery
4) plus pole on power supply

I mean, in car charging or giving starting help the minus should get connected last, if I remember correctly- would love to understand the underlying mechanisms in the PSU!

[Trader]

I just bought these cheap Ideal Diodes: https://s.click.aliexpress.com/e/_DCVtEoD

One of the reviewers said:

"Several voltage drop tests

1st module:
28v 3a-115mv
24V 2.75a-109mv
20V 2.5A-98mv
12v 1.9a-75mv
12v 3.76a-156mv
9V 3.2a-140mv
9V 1.6a-67mv
5v 1.17a-56mv
5v 2.38a-116mv

2nd module:
9V 3.38a-200mv
12v 1.9a-100mV
24V 2.75a-120mv

At 0-2a do not warm at all, at 2-3a barely warm. In general, I recommend. Top for your money."

I guess the Drop Voltage is related to Current, not Voltage, so, seems that ~1A is ~50mV.  I'll use ~0.5A, maybe the drop will be ~25mV, what's great for me.

[shapirus]

I just bought these cheap Ideal Diodes: https://s.click.aliexpress.com/e/_DCVtEoD

Beware, there are two kind of "ideal diodes": one protects agains reverse polarity, the other also blocks reverse current. From what I have found in terms of circuits, the latter is a little tricky to implement with discrete components, and there are ICs which do the job of controlling the switches. Looking at the photos of what you bought, I think that it's the first type. When they arrive, check what they actually do and whether they block reverse current. If they do, let us know, I'll then order some too and probably try to reverse-engineer them, since they use a minimal number of components which is interesting.