How Hard Can We Charge Our Lead Acid Batteries?


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 Temperature Effect

Battery charging is not 100% efficient. Some of those amps get lost. But wait, as our old buddy Sir Isaac pointed out, you can’t just lose energy, it must go somewhere, and it does: heat. So if we charge a battery hard it will heat up. And as a battery heats up, its internal resistance drops.

For example, if our voltage is at 14.4, and we look at a meter measuring the current going into the battery and that reads 100 amps, Ohm’s Law tells us that the internal resistance of said battery is just 0.144 ohms. And, folks, that’s not very much.

(You can play with that using this calculator—no need to sprain our brains doing this stuff by hand, I say.)

If we reduce that internal resistance by just a tiny bit due to heating of the battery, say just one hundredth of an ohm, the amps go up to 108, and then the battery heats up a bit more and the resistance drops and the amps go up, and if this goes on long enough you get thermal runaway, a fancy name for some seriously bad shit.

Fortunately, most cruising yachts have quite big batteries in relation to their charging sources, so this is not usually a problem. But it is still good practice to control powerful charging sources (big alternators particularly) with voltage regulators that have temperature probes that attach to the batteries, so that the regulator can reduce the acceptance voltage as the battery heats up.

(The other benefit is that when it’s cold the regulator will increase the acceptance voltage a bit for more efficient charging.)

#2 Recovery From Total Discharge

As I wrote above, “lead acid batteries self limit current (amps) by raising their internal resistance”. But there is an exception to this:

If we discharge our battery to the point that the at-rest voltage is below the battery’s operating range—let’s say below 11 volts—by, for example, having a load on and leaving the boat for a considerable period, our poor batteries get so low that they don’t have the strength to push back. Now, if we immediately hit ’em hard with a big charger, bad stuff can happen, including overheating and buckling the plates.

So in cases where we have screwed up and over-discharged, it’s important to recharge at no more than 5% of the battery’s rated capacity. For an 8D battery that would be about 10 amps. This may present a problem on a boat with a big charger and alternator since most regulators have no way to control amps. In this case I’m thinking the best bet is to go buy a small automotive charger and use it to bring the battery back up to say 12 volts before hitting it with the boat’s chargers.

#3 Paralleling Two Unbalanced Banks

I’m a big fan of splitting the house bank into two, connected by a 1-2-Both switch. There are many advantages (that I will detail in a future chapter) but there’s a danger too (nothing’s perfect, particularly around boats).

And that is, if we make the mistake of over-discharging (detailed in #2 above) just one of the two banks and then parallel the batteries by switching to “Both” (say to charge), things can get very exciting very quickly when the charged battery drives all the amps it has through the discharged one.

Given this risk, as a general rule on Morgan’s Cloud we keep the switch at “Both”, unless we are separating the banks for a specific purpose, such as desulfating by equalizing, as explained in this chapter.

Having said that, it’s important to understand that there will only be a problem if one of the batteries is grossly over-discharged. In normal circumstances, with one battery say discharged to 50% of capacity and the other full, the full battery will just charge the discharged one until they reach equilibrium.

#4 Regulator Failure

While we can say that most batteries do not have a current (amps) charging limit, that only applies if we don’t exceed the manufacturer’s recommended temperature-compensated (see #1) acceptance voltage. If for some reason the regulator fails, particularly with a big alternator, charge voltage can climb to scary levels very quickly.

And since we know that Ohm’s Law states that amps are equal to volts divided by resistance, that means that amps will go up too—not good. Therefore it’s a really good idea to have an over-voltage alarm to warn us of this situation before bad stuff happens. On Morgan’s Cloud we have an alarm on our sailing instruments set to 15 volts (12 volt system).

#5 Misplaced Sense Wire

This one results in the same problem as number #4 (over voltage) but is caused by a very easy-to-make installation mistake. Many alternator voltage regulator manuals will tell us to connect the sense wire directly to the battery positive pole. And that’s a good idea too.

But what if you have two banks and the battery selector switch is set to the other bank? You got it. The regulator senses the battery that is not being charged and therefore cranks the output of the alternator to the max…this will not end well.

Therefore, if you have a battery bank selector switch, make sure you ignore the manual and connect the voltage regulator sense wire to the common terminal of said switch.

Bottom line, never install a charging source in such a way that there is any conceivable way that the sense wire and the output could become electrically separated.

Bonus Tip—Melt a Tool Today

It’s not to do with charging but it’s worth knowing that when a battery is not being charged its internal resistance is much lower (only a couple of thousandths of an ohm), which is why dropping a screwdriver across the terminals of a battery gets right exciting.

For example, a LifeLine 8D battery is capable of producing 7300 amps when shorted out. Now let’s use some of our new-found knowledge from the last chapter: 7300 amps multiplied by say 6 volts (volts will drop when the battery is shorted) equals…yikes…wow…43.5 kilowatts. That’s the equivalent of the heat produced by 29 household portable electric heaters. 

Moral of the story: Fuse your batteries and make sure the terminals are properly covered to conform with ABYC standards.


There you have it. A well-built lead acid battery is a pretty robust piece of kit, but it’s best not to kick it when it’s down, nor should we forget that some pretty scary power lurks within.

Thank You

A big thank you to Justin Godber, a deeply experienced principal at Concorde Battery Corporation, makers of LifeLine batteries, who has, over the last few years, been kind enough to answer my questions about batteries with infinite patience.


If you have any questions on the above, please leave a comment, but please only after you have read this chapter and this one.


And yes, I’m more than a little partial to Justin’s LifeLine batteries because, not only will they happily lap up high charge currents (amps), they are also the only sealed (AGM or Gell) batteries I know of that can be equalized.

Further, you should be aware that Justin has very kindly provided us with two sets of batteries for testing on Morgan’s Cloud at zero cost. One set six years ago, that have done very well for us, and a new and much larger set this summer. (More details on both in future chapters.)


  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 Lead Acid Batteries Get Wrecked and What To Do About It
  9. 11 Steps To Better Lead Acid Battery Life
  10. 10 Tips To Install An Alternator
  11. Stupid Alternator Regulators Get Smarter…Finally
  12. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  13. Smart Chargers Are Not That Smart
  14. Equalizing Batteries, The Reality
  15. Battery Monitors, Part 1—Which Type Is Right For You?
  16. Battery Monitors, Part 2—Recommended Unit
  17. Battery Monitors, Part 3—Calibration and Use
  18. Do You Need A Generator?
  19. Efficient Generator-Based Electrical Systems For Yachts
  20. Battery Bank Size and Generator Run Time, A Case Study
  21. Battery Options, Part 1—Lithium
  22. Battery Options, Part 2—Lead Acid
  23. Why Lithium Battery Load Dumps Matter
  24. 8 Tips To Prevent Lithium Battery Load Dumps
  25. Lithium Ion Batteries Explained
  26. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  27. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  28. Q&A—Are Battery Desulphators a Good Idea?
  29. Renewable Power
  30. Wind Generators
  31. Solar Power
  32. Hydro Power
  33. Watt & Sea Hydro Generator Review
  34. A Simple, Efficient and Inexpensive¹ 12 or 24 Volt DC Electrical System
  35. 8 Checks To Stop Our DC Electrical System From Burning Our Boat

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 25 years, first in yachting magazines and, for the last 20 years, as co-editor/publisher of AAC.

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