How Hard Can We Charge Our Lead Acid Batteries?

charging-battery Of the many myths that surround them, the idea that lead acid batteries have a charge limit (maximum number of amps that can be pushed into them without damage) is probably the most prevalent.

What Really Matters

But we slayed that dragon in the first Ohm's Law chapter when we learned that even with a very powerful charging source we won't blow up our batteries as long as we don't exceed the manufacturer's maximum recommended acceptance voltage—typically around 14.4 volts (12 volt system). Why? Because lead acid batteries self limit current (amps) by raising their internal resistance—the harder we push them, the harder they push back. (A quick reminder, I'm writing about lead acid batteries only. Lithium batteries are much less forgiving.) By the way, not only will we not damage a decently-built battery by charging it quickly, testing at LifeLine battery has shown that charging at higher rates (amps) actually extends the life of their batteries—I suspect most others' too—since it reduces suphation. Another battery myth bites the dust...well, not quite.


Some battery manufacturers do specify quite low maximum current (amps) charge limits. (Thanks to Erik Snel and Enno for pointing this out.) I'm not sure why this is, given the way that lead acid batteries automatically raise their internal resistance. Perhaps the restrictions come from liability lawyers rather than engineers. Or maybe there are weaknesses that I don't know about in the construction of some batteries that makes them vulnerable to higher-charging currents. Whatever the case, I strongly recommend respecting the manufacturer's charging specifications—OK, covered my ass, let's move on.

Don't Buy Wimpy Batteries

This brings up an important buying guideline: don't buy batteries that have a charge current restriction under 30% of their rated capacity. Here's why:

Typical Cruising Recharge Time

Let's assume a 500 amp hour bank. Thirty percent is 150 amps, the output of a reasonably-sized externally-regulated alternator, capable of bringing the bank from 50% discharged to 80% (the point where charge rate tails off) in an hour, or about the time it takes to get the anchor up and motor out of an anchorage in the morning, or reverse the process in the evening. Do you really want to have to run the engine for two to three times longer to get to the same charge level? Didn't think so.


The other problem with unrealistically low charge current restrictions is that on a typical cruising boat there is no way to reliably honour them since very few voltage regulators have any way to convert to current (amp) regulators—they're called voltage regulators for a reason.

The Big Five

Now that we have all those preliminaries out of the way, let's get to the meat of the chapter: five things that really can damage our batteries, or even blow them up and start a fire. All are fairly rare, but it's best to be aware, particularly as multiple and more powerful charging sources are becoming common on cruising boats.

  1. One Simple Law That Makes Electrical Systems Easy to Understand
  2. How Batteries Charge (Multiple Charging Sources Too)
  3. How Hard Can We Charge Our Lead Acid Batteries?
  4. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  5. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  6. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  7. The Danger of Voltage Drops From High Current (Amp) Loads
  8. How Batteries Get Wrecked and What To Do About It
  9. 11 Steps To Better Battery Life
  10. 10 Tips To Buy And Install A Live-aboard’s Alternator
  11. Stupid Alternator Regulators Get Smarter…Finally
  12. Smart Chargers Are Not That Smart
  13. Equalizing Batteries, The Reality
  14. Battery Monitors, Part 1—Which Type Is Right For You?
  15. Battery Monitors, Part 2—Recommended Unit
  16. Battery Monitors, Part 3—Calibration and Use
  17. Do You Need A Generator?
  18. Efficient Generator-Based Electrical Systems For Yachts
  19. Battery Bank Size and Generator Run Time, A Case Study
  20. Battery Options, Part 1—Lithium
  21. Battery Options, Part 2—Lead Acid
  22. Lithium Ion Batteries Explained
  23. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  24. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  25. Q&A—Are Battery Desulphators a Good Idea?
  26. Renewable Power
  27. Wind Generators
  28. Solar Power
  29. Hydro Power
  30. Watt & Sea Hydro Generator Review

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 18 years, as co-editor/publisher of AAC.

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