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I'm looking to make a car battery charger for learning purposes. I have read through this this post and this post.

While I am still confused about certain things, it seems generally agreed upon that a safe charging current is 15 to 20% of the battery's capacity. For the battery I have in front of me right now (60Ah) that would represent 9 to 12A.

I've connected the battery to a bench power supply, and getting it to 10A seems to take about 17V. This seems quite in excess of even the car's ~14.4V when turned on.

To be clear, I know that constant-current bulk charging is only meant to happen to a certain point, and that the higher voltage will damage the battery if it leads to overcharging.

So, if I'm doing constant-current charging on a lead-acid battery, should I be concerned about reaching a voltage that is too high for the battery?

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    \$\begingroup\$ I am concerned that your linked questions already contain the information I put in my answer. I would strongly recommend you spend a LOT more time reading and learning before attempting something like this. \$\endgroup\$
    – Mark Omo
    Commented Dec 16, 2023 at 0:32
  • \$\begingroup\$ @MarkOmo Well I read a few other things before that and I was quite confused because the "constant current" terms kept coming up, and to me that meant that (essentially by definition) you let the current become whatever it wants. Since many places mentioned 15-20% C for charging and my mostly dead battery wasn't reaching it with 14.4v (the usual voltage with the alternator running), I tested for a brief moment to see what it would take to get it to 10A and that's when I started thinking there may be some holes in what I thought I knew. Hence my making this post. \$\endgroup\$
    – TrisT
    Commented Dec 16, 2023 at 0:42
  • \$\begingroup\$ I think you made it click, but I'm still leaving room for the chance of having interpreted it wrong. Which is why I've been (and will continue to be) careful as I don't aim to endanger my health. Doing it outside and keeping some distance. \$\endgroup\$
    – TrisT
    Commented Dec 16, 2023 at 0:44
  • \$\begingroup\$ made a mistake I can no longer edit. should be: "you let the voltage become whatever it wants" \$\endgroup\$
    – TrisT
    Commented Dec 16, 2023 at 0:48

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YES you absolutely should be worried about exceeding the voltage rating of the battery!

Overcharging Lead Acid batteries will damage them and can cause Hydrogen and Oxygen gas to form, leading to an explosion risk.

You should never, under any circumstances, provide a voltage higher than the rated peak voltage! A charging curve limits the current into the battery until the voltage rises to the peak battery voltage. Then, the voltage is limited to the peak voltage until the current drops (to 3-5% of the C rate for lead acid batteries).

Charge stages of a lead acid battery

Standard "12V" Lead-acid batteries are six cells; the peak charge voltage is between 13.8 and 14.7V (at 25C, this value is temperature dependent); however prolonged time at this voltage will cause damage. After the current reaches the cutoff point (3-5% of the C rate of the cell) the voltage should be lowered to 13.5V to 13.8V (the "float voltage").

Diagram from the excellent Battery University. Read there article on Lead Acid charging for excellent detailed information BU-403: Charging Lead Acid. Please don't attempt to build a charger or anything like it until you understand in detail the considerations and the risks; you can seriously injure yourself or others.

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  • \$\begingroup\$ I was about to ask some questions, and your perfect edit answered all of them! So if I limit voltage to some value within [13.8v, 14.7v] and current to some value within [6A, 12A], that's essentially both of the first steps. After that, the amperage will go down 'on it's own' (the battery simply pulls less current). I should look to detect the cutoff point and subsequently lower the voltage to enter this maintenance-like float stage. Is this correct? \$\endgroup\$
    – TrisT
    Commented Dec 16, 2023 at 0:45
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If somehow you managed to get the battery to 17Vdc...EVER, that battery is no good anymore. The charge current would be so high, it would explode, or the battery is just so high resistance internally, it would never be able to supply much current. Again, dead. Between the 2 options, it would certainly be high internal resistance.

Know the charge voltage/current curve (as shown above) for whatever chemistry battery you are using, BEFORE you either draw current, or charge ANY battery. E.g., LiPo batteries are are very unforgiving, and if you make them mad, they can burst into flames, or blow up, or both. Find YouTube vids to see just how mad they can get.

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Well there is something called an 'equalizing' or 'conditioning' charge where voltages higher than normal are applied to a lead acid battery. This is done to equalize all the cells and also is used as a desulfating process. For these processes they recommend applying between 16 and 17 volts. The electrolyte will boil at these voltage level but that's the desired effect and one only need to top off each cell with distilled water. Some recommend this treatment be done every month or so to help keep your batteries in good condition.⁹

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