The Offshore Voyaging Reference Site

How Hard Can We Charge Our Lead-Acid Batteries?


[We originally published this chapter back in 2016, but I have completely rewritten it to better mesh with recent chapters, so we are republishing it.]

One of the worst myths about lead-acid batteries is that charging them slowly is good for them. Let’s deal with that and then dig into how hard we can charge them.

In the first Ohm’s Law chapter, we learned that, even with a powerful charging source, we generally 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.

I’m writing about lead-acid batteries only. Lithium batteries are much less forgiving.

But even more importantly, not only will we not damage a decently-built battery by charging it quickly, testing at LifeLine has shown that charging at higher rates (amps) actually extends the life of their batteries—I suspect most others’, too—since it reduces sulphation.

Another battery myth bites the dust.


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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. Lithium Buyer’s Guide—Budget: Economy Options
  36. 11 Steps To Better Lead Acid Battery Life
  37. How Hard Can We Charge Our Lead-Acid Batteries?
  38. How Lead Acid Batteries Get Wrecked and What To Do About It
  39. Equalizing Batteries, The Reality
  40. Renewable Power
  41. Wind Generators
  42. Solar Power
  43. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  44. Battery Monitors, Part 1—Which Type Is Right For You?
  45. Battery Monitors, Part 2—Recommended Unit
  46. Battery Monitors, Part 3—Calibration and Use
  47. Battery Containment—Part 1
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Lars Erik Karlsen

Interesting article.
As you know, cruising in higer latitudes often mean motoring all day long, because of lack of wind. Leaving port with fully charged batteries soon results in charging volt of 14,5 or 14,6 in my boat.Will this ruin my batteries if running all day like this?
I try to avoid this by putting all inside lights and navigation lights on to get charging down 14,4 or a bit lower
The batteribank is 400 amp, starting battery excluded.
The alternator is 80 amp.
Some years ago I send the alternator to company who adjusted it so its far more efficient than standard.
Before this was done it never reached 14,4 volt.
Keep up the good work.
Best regards

Lars Erik Karlsen

Hi John
Thanks for your reply. I will consider your recommendations.
Already have a Victron shunt.
You are right. The regulator is internal.

David Popken


Great stuff as usual. I have question regarding equalization. Our system consists of a two bank FLA. A four Trojan 6v house bank and a single 12v start battery. Rarely does either bank get depleted to 50%. Typically, the house bank might get down to 60-75% before being put back to 100% or close. In five years of this typical use, I have never felt the need to equalize, but maybe it’s something one should do occasionally regardless of the usage profile. Thoughts?

Steven Schapera

John, these articles are hugely informative – thank you. I don’t seem to find much on your site that references any chargers that you recommend. Can you please either point me in the right direction, or consider the topic for a future article?

Stein Varjord

Hi Stephen,

There are a lot of competent companies building good chargers nowadays. There are also some not so good ones, but by sticking to the established brands and doing some investigation on each model, you can probably get the right stuff. Many now go for a combined inverter and charger. If you need both, that might be a good option, but I don’t like it. I want a charge bus that is separate from all consumers. That’s not possible when the inverter is the same as the charger.

I like the Victron brand. They are good at making user friendly good equipment. They make fancy inverter chargers, but as mentioned I prefer one of their charger series, the Victron Phoenix Smart IP43 charger.

They are rugged and compact. They can charge more than one bank. The right versions can use both 120V 60 Hz and 240V 50 Hz, with some fault margin to go. They automatically detect it and adapt. Just plug in anywhere. There are 12V and 24V versions. They all have Bluetooth and all programming and monitoring can be done in an easy to use app, where you can also see your other Victron Smart devices, like the MPPT chargers or the BMV 712 battery monitor. If you want to have it all on a fancy screen, it can all talk to the Victron Cerbo GX and some other options they have. That also has WiFi, so you can connect and monitor your systems from anywhere. Loads of possibilities, if you want to nerd out. For me, the important point is that it’s easy to know and adjust how it’s doing its job.

The important issue is that every detail in the charge cycle can be adjusted. This means there will never be a battery it cannot perfectly adapt to. I think this one capacity is a must for any charger. Lots of new chargers, also Victron models, don’t have this, or it’s too inconvenient. The reason I insist on this is that the presets for different battery types most chargers have are usually not good enough.

For FLA the standard preset is OK. For AGM some might be sketchy, especially that some can start equalization far too easily. AGM and gel must NEVER be equalised. For gel, also the voltage is often wrong, and the timing is “always” wrong. All the Lithium charge presets I’ve seen will harm lithium cell life significantly. As battery tech develops, these problems will grow.

Conclusion: Always use manually set charge parameters, conservatively adapted to the battery data sheet. It’s way easier than it sounds. No need to be a nerd.

The mentioned Phoenix chargers are (for 12V) only 50 Amp and 30 Amp. (Half that for 24V, of course). For most long distance cruisers, that’s not much. However, they can be put in parallel, as many as you want. That works quite ok right away, but these can also communicate with each other, via Bluetooth or a single data cable (VE net / NMEA 2000 / Ethernet), to operate as one big charger. If you’re on a weak shore power, you can unplug one or more to avoid tripping a fuse. This also gives you nice redundancy, of course.

I’m not saying that this is the best charger, but I like it and I think it’s useful to compare its abilities to other chargers that might look interesting.

Robert Andrew

Charging systems is a topic I’ve learned much about over the years from you and Rod Collins and have made changes to my system based on that input. My current system is a 4 Trojan 6V house system wired as a single 440AH 12V bank plus a separate starting battery (all connected with a Blue Seas master switch allowing cross connect), a 100 A alternator wired directly to the house bank (with a Blue Seas combiner to charge the engine battery) and a Balmar “smart” (I know I know, not smart) external regulator (programmed per Rod Collins), plus a Victron battery monitor with shunt. I realize you recommend the Wakespeed and if I hadn’t just installed the Balmar when you first made that suggestion, I would get the Wakespeed, but I’m not sure it would matter for my questions which relate to alternators.
When you discuss sizing charging systems and battery banks you start with alternator output that is essentially the same as the rated output.  The batteries will not accept that rate at all levels of battery charge, but it has always been my understanding that an alternator running at 100% of its rated output for any length of time will fail in short order. So, my first question is, what is best practice in your opinion on max alternator output?
With respect to my own experience, I rarely get more than about 65A from my 100A alternator (though batteries are rarely below 50% charge); in addition, after not very much running time, it will cut back to about 25A and then after 5 minutes or so go back to a higher level and will continue to cycle in this manner (with a declining maximum output rate). I’ve never gotten an answer from Balmar on this behavior, but other sources who seem knowledgeable say this pattern is the temperature monitor on the alternator limiting the output. Does this seem reasonable after such a short running time? Any suggestion for a setting on an alternator temp monitor (or a knowledgeable source for one?) My only other thought is to try and get more cooling air to the engine room.    

Ramon Pla

Does anyone have knowledge or experience with nickel-iron batteries? My understanding is that they have comparable capacity and performance to FLA but are more tolerant of hard charging and can be fully discharged without being damaged.

Ramon Pla

Thanks – I should’ve done a better search first. It looks like weight is the biggest issue: more than double an AGM or gel installation of the same capacity according to Iron Edison specs.

Even with the improvement of DoD to >80% I could add an entire additional AGM/gel battery to my setup for the same weight.

At least there is progress being made on developing a sealed version.

Dan Beresford

Hi John,

I am a relatively new AAC member and I am totally blown away with the depth and breadth of information on this site. I have an electrical engineering degree and have practiced for many years in circuit design and electronic product development (mostly avionics), so these sections on electrical systems are very interesting to me and I have learned a lot in a short period of time.  

In the Temperature Effect section of this article, the calculation of battery internal resistance is incorrect; dividing the charging voltage by charging current does not give us battery internal resistance. To calculate/estimate battery internal resistance using the outlined parameters (and taking a simplistic approach), we must know the voltage of the battery under charge. For example, given the 14.4V charging voltage, let’s assume that the battery open circuit voltage is 12.2V (or pick any other value representing state of charge). We must then take the difference between these two voltages and this is the voltage that the internal resistance “sees”. So, 14.4V – 12.2V = 2.2V, then divide by 100A and we have an internal resistance of 0.022 Ohm (22 milliohm). This is a simplistic view, but it is the correct application of Ohms law in this scenario. Below is a simplified battery electrical model (dotted outline) along with a charging source.

The value calculated in this article (0.114 ohms) is too high. For example, internal resistance will limit battery short circuit current and cold cranking current (CCA) performance. For a 12V battery with 0.114 ohms internal resistance, short circuit current would be 12V/0.114ohms = 105A, and CCA will be much less. There is a picture of a Victron AGM battery earlier in the article that has a 550 CCA spec, so internal resistance for that battery cannot be more than about 20 milliohms and probably in the range of 2 milliohms. I don’t see Victron specs for battery internal resistance, but Lifeline publish data ( You can see internal resistance for this series of deep cycle batteries ranging from 0.23 to 6.43 milliohms and the corresponding short circuit current.

For more comprehensive discussion on internal resistance/impedance measurement, here is a source:

Also note that as the charge on the battery increases, charge current decreases (due to Ohms Law) and as the battery voltage approaches the charge voltage, then the charge current approaches zero. Ohms law gives us the following:

Icharge = (Vcharge-Vbattery)/Rint

With Vcharge and Rint fixed (simplified case), then Icharge decreases with increasing battery voltage (i.e. charge). (Yes, Rint might vary with state of charge, but this is not what limits charging current as battery SOC approaches 100%.)

Again, thanks for the work that you do and all of the great information on this site.

Battery Model.jpg
Mark Wilson

Having just yesterday cooked one of my sealed lead acid service batteries I can say its a scary experience. (Here’s a tip: if you smell rotten eggs closely inspect each of your batteries. Disconnect them one at a time and measure and compare voltage across the bank – I did none of these things quickly enough.)

I bought the boat this year and according to the broker the batteries were “recent”. I should have been suspicious that the dates had not been recorded on top of the batteries.

Initial theories for the failure are either imbalance of battery sizes and manufacturers across the bank or malfunction of the quite old Victron charger/inverter. I have read your thoughts on this combination, John.

Both charger/inverter and batteries were on my future improvements list. But as they had been working fine so far they were relatively low down that list.

Yet again, I am reminded of the scary things that can happen on boats even before you even cast off your lines.


PS Another warning sign I missed was the unusually high number of amps being pumped into what should have been close to full batteries.

PPS Any early thoughts on your new Victron AGM battery in the J 109 ?