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I know that when charging a lead acid battery, there are 3 stages,

1.Constant Current

2.Topping

3.Float.

enter image description here

Referring to this image, I can see that there is some current, 1A in this case, driving the voltage to some voltage threshold, about 1.85 maybe, and then in stage 2 the current decays exponentially till float level is reached.

My questions are the following:

  1. How is the constant current decided? Why is it 1A in this case? How would I pick a current that will be safe?

  2. How is the decay in the topping current decided? Is current a function of the voltage here, or is there a specific timing for the decay?

  3. For float charge, again, how is the minimum current needed decided? At the end of the chart, is the current rising, or is that my imagination?

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1 Answer 1

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  1. Ideally, charging current 'is decided' by the manufacturer's recommendation--but really, it's a pretty well known electro-chemical reaction that has been extensively studied/tested to determine the best way to charge such batts. Most LA batts can be charge with a higher charge current, but a gentler charge is usually best. I know some people quick charge LA batts with 10A or higher, but I would never recommend that. Personally, I use a regulated bench power supply & charge at 2A or less. It takes longer, but I think it is less destructive to the batt plates.

  2. The 'decay' is not decided. What happens during the 'top charge'--which I prefer to think of as the 'saturation charge' stage is that the charge V is held at 14.1V and the batt takes as much charge as it is able to take. As the batt becomes saturated, it increasingly accepts less current all on it's own--you don't need to do anything except keep the V at 14.1V (on a typical 12V flooded-LA batt).

  3. Essentially, once the batt is fully charged (i.e. saturated), you do not want to continue to charge it @ 14.1V. Depending on the batt composition, continuing to charge at that rate simply causes unnecessary gassing (electrolysis of the electrolyte into H2 & O2) & unnecessary heating of the batt. Personally, when my charging current drops to ~300 mA, I lower the charging V to 13.5V. The reason I use 13.5V is that once the batt has rested for 24h, I want the resting V of the batt to be @ ~12.7V. If you set the float charge below 13.5, your resting batt V after 24h will likely be < 12.7V. It's not a big deal though. Some automatic chargers set the float V to 13.2V instead. Usually 13.2V is better for newish batts compared to oldish batts--which seem to need a bit more charge V.

hth, best regards!

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    \$\begingroup\$ The range of suitable charge currents depends on the battery capacity. If you have a large bank of batteries (for example in an off-grid setup) 10 Amperes will be much too low. But if you have a very small lead acid battery, 10 Amperes might be much too high. Also, the voltage thresholds depend on the battery temperature and battery type (gel vs AGM vs flooded). \$\endgroup\$
    – user57037
    Feb 21, 2016 at 19:20
  • \$\begingroup\$ I pointed to the batt manufacturer's recommendation first because that is the best source (if you can obtain it) of info for charging, maintaining, & troubleshooting the exact batt chemistry you are working with. I also used a 12V FLA batt for my answer because of its ubiquitousness. I agree that all of your points are also valid and a better generalization than what I provided. \$\endgroup\$
    – zeffur
    Feb 21, 2016 at 22:44
  • \$\begingroup\$ You provided a good answer. I just thought the OP should be aware of a couple of other points. \$\endgroup\$
    – user57037
    Feb 22, 2016 at 0:22

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