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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 '18 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 '18 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\$ – Peter Green Oct 1 '18 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 '18 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\$ – Peter Bennett Oct 1 '18 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\$ – Peter Bennett Oct 1 '18 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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