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Let's assume a lead acid car battery (12V, 50Ah, 250A output).

According to BatteryUniversity article BU-403:

The charge time is 12–16 hours and up to 36–48 hours for large stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 8–10 hours; however, without full topping charge. Lead acid is sluggish and cannot be charged as quickly as other battery systems.

So the charging rate is no more than C/12. But car batteries (usually 6 cells) can discharge at often beyond 200A. 250A in my example. For a 50 Ah battery, this would mean discharging at a rate of 5C.

Does that mean, that a lead-acid battery can be discharged at least 60 times faster than it can be charged?

Or have I misunderstood something?

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    \$\begingroup\$ Some cars need 800A and trucks can require 1000A or more at 24v... However, newer designs have also helped lower the current requirements. \$\endgroup\$
    – Solar Mike
    Oct 1, 2018 at 16:45

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You are entirely correct. The issue comes down to the fact that it's pretty easy to overcharge a lead-acid, but normally you try not to "over-discharge". So you have to be careful about the last stages of charging. This is not (exactly) true for discharging, but that is only true because you don't want to completely discharge lead-acid anyways. Unless you have a deep-cycle battery, you don't want to pull more than about 50% of the available charge out when discharging. If you do, you'll severely reduce the battery life.

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  • \$\begingroup\$ But this answer electronics.stackexchange.com/a/398854/174733 claims 70 to 100 A of charging ampèrage for a short time, which would mean around 2C of charging rate. Have I confused something? Or is it true that the charging rate can peak at 100A but needs to be no more than 2.5A (C/16) once the high terminal voltage is reached? \$\endgroup\$
    – neverMind9
    Oct 1, 2018 at 20:01
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    \$\begingroup\$ Yes in the early stages of charging you can charge quickly, but then you reach your voltage limit and the current drops, so it takes a long time to finish the job. \$\endgroup\$ Oct 1, 2018 at 20:50
  • \$\begingroup\$ I see, @PeterGreen. The article stoneacre.co.uk/car-service-and-repairs-q-and-a/… claims that charging a car battery with the generator takes just 30 minutes of driving. But the Battery University article, if I am not mistaken, suggests that the maximum charging rate is C/12 at all. \$\endgroup\$
    – neverMind9
    Oct 1, 2018 at 21:08
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    \$\begingroup\$ @neverMind9: Starting a car engine does take a very high current, but only for a few seconds, so the battery is only discharged by a small amount.That small discharge can be replenished with a modest current in several minutes - much less than the 30 minutes your article claims. I often drive less than 15 minutes at a time, and don't need to use an external charger to keep the battery charged. \$\endgroup\$ Oct 1, 2018 at 22:29
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    \$\begingroup\$ @neverMind9: When cruising on my boat I may sit at anchor for a couple of days, and discharge my house battery to 50% or so - then I'll want to charge at a high rate while I motor a few hours to the next anchorage, to get the batteries full again. The standby batteries in a telephone exchange can be charged at a very low rate because they will likely have weeks to recharge after a power failure, before the next one. Different requirements for different situations. \$\endgroup\$ Oct 1, 2018 at 22:37
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You said '..a lead-acid battery can be discharged at least 60 times faster than it can be charged?" In general that may be true of a particular charging system, but it varies depending on the charging system and the charge profiles used.

Your premise is far too simplistic to cover all situations. Battery University presents quite reasonable information, but it does not usually provide highly accurate technical details that cover all situations. The main premise of the statement in BU-403 you referred is to "Learn how to optimize charging conditions to extend service life". But the charging profile presented is only one of many possible options.

For example the design of many car alternator charging system is typically a simple current limited (not CC) and CV profile. You discharge the battery at perhaps 200-800A when starting the vehicle but charge the battery at perhaps 70-100A once it's running. The current profile drops when the battery terminal voltage rises but the profile is simple. Here the discharge/charge ratio may only be 10:1, at least for a short time.

If you read relevant information on smart charging profiles you may get a better picture of the situation. Start with something like this from TI.

If you want in depth detail for extending battery life you may find this paper on VRLA EV use of use. This uses a ZDV profile to ensure the minimum of overcharging, but maintaining fully charged terminal voltage.

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For use in boats and RVs, I've seen C/5 recommended as a maximum charge rate, but lower rates are probably kinder to the battery. For marine and RV use, you want to charge the battery as fast as practical (without damage), but for stationary use (UPS and similar applications), you usually have lots of time between discharges, so a slower charge rate is practical.

Engine starting batteries are made to deliver very large currents for a short time. Look at the Cold Cranking Amps (CCA) rating of a starting battery for examples. The diesel engine on my boat requires a starting battery with a 900 A or greater CCA rating, if I recall correctly.

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In short, you are right, batteries can discharge faster than charge. But in terms of efficiency of use, the matter is not the same.

The way the question as initially done does not consider some very important topics, as battery is essentially a (rechargeable) source of electrical energy. These are some relevant points to consider:

  • How much is the charging current of a Lead Acid battery?
  • How much is the maximum continuous discharge current of a L.A. battery?
  • How much is the maximum intermittent discharge current?
  • How long the battery can deliver a given discharging current?
  • How efficiently the energy can be converted during a discharge?
  • Addendum: How a car battery is charged by an alternator-based charging system?

Most of this questions I have already answered in this post #1, but I will present some discussions here, using your data as an hypothetical example:

Battery = 12V.
Capacity = 50Ah.
Cold Cranking Amps: CCA = 250A.

A safe charging current is limited to 15%~20% of C during the bulk charging stage, as I illustrated in this other post #2, on the first picture.
So, I.ch < 10A.

For an AGM SLA battery a safe continuous discharging current is 3C. The AGM data I found in that post had a maximum intermittent (5s) current of 15C, where the CCA (discharging for 30s) was about 55% of that absolute maximum, or CCA = 8C, when maximum continuous was 3C.
On the other hand, your (hypothetical) battery had CCA = 5C. I would then guess this is a deep-discharge battery with no further data available.
In this case, my guess for the maximum continuous discharge current would be 3C/8C x 1C = 18~20A.

Obviously, you could discharge at CCA = 250A, but just for up to 30s and the voltage would drop as low as 7.2V.
It can be useful for a cranking motor running for 10~20s, but to use it to power an inverter, probably the electronics would disconnect sooner, by 10V. Please check that first link (#1) for further details.

An important concept not spoken is the amount of energy = Voltage x Current x Time or
Energy (Wh) = Voltage (V) x Capacity (Ah).
Stored energy as “V x C” in your case is 12 x 50Ah = 600Wh. This value is valid for a discharge rate of 0.05C = in 20 hours.
Discharging an AGM at C/20 = 2.5A <-> you get 100% C.
Discharging an AGM at C/1 = 50A <-> you get 70% C.
Discharging at AGM maximum of 3C = 150A <-> you get 42% C.

In your case, your battery’s maximum continuous discharge was estimated as 20A, and it last just 42% C; time = (42% x 50)/20 = 1.05 = just 1 hour!

When discharging rate become less efficient?
From the Graph on Figure 2 of the post #2, it seems that discharge rate should be limited to C/5 or the battery will loose actual capacity “%C” more drastically.
Again, extrapolating that manufacturer’s data to other batteries - until we have more exact data, it is the best guess we have.

A curious observation from the available data:
Both continuous current limits - the maximum charge current and maximum discharge current are the same, 10A = C/5.

Manufacturers have declared intermittent discharge current limits as that 15C max, or the CCA; but they do not provide information (at consumer level) about intermittent charging current limits (correspondingly similar to the mentioned limits of 30s or 5s).
I could speculate that some manufacturers design their batteries to accept intermittent charging excursions higher than C/5, as the ones caused by alternator charging under variable engine rotation.

Another point is:
How much energy is used and how deep the battery is discharged for starting an engine for as long as 30s?
Let’s assume the CCA efficiency is even lower than 42%, for instance, just half of it, at 20%:
Energy = (V x A x Time)/ Efficiency E = 12V x 250A x (30s/3600)H / 20% = 125Wh.
This 125Wh is about 20% of the rated energy of 600Wh.
So, due to this intense cranking effort, the battery discharged to 80% State of Charge and battery voltage will not drop too much.

Recharging by the car’s alternator:
As the battery may be in a car, alternator will recharge promptly, topping off with 10A~20A (or more) along several minutes (or hours on a trip), with variable charging capacity mostly due to engine’s variable speed/RPM, until voltage reaches nominal about 14.4V.
As a general estimative, for every Hour with I.ch = 10A, the alternator sends “12V” x 10A x 1h = 120Wh nominal, somehow recharging in 1h @ 10A the energy used in that long engine cranking of 30s. Complete charging time may be shorter if maximum initial charging current is higher and if actual cranking time is usually as short as 5~10s.
The actual charging voltage is indeed higher (13.x up to 14.4V), but here I assumed it being used to compensate the electrochemical inefficiencies; that is why I used the same nominal 12V.
The exact charging voltage is compensated by the ambient temperature - this alternator regulating IC has a graph illustrating the Charging Voltage versus operating temperature:

enter image description here

Addendum:

How a car battery is charged by an alternator-based charging system?
I found an interesting article discussing several details about the charging in more realistic situations,as variable rotation speed (RPM). Here is a collage of some figures in this article, where the colored curve graphs represent charging system response varies significantly along different engine speeds - 750 rpm, 1500 rpm and 3000 rpm.

enter image description here

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