The Offshore Voyaging Reference Site

Equalizing Batteries, The Reality

JHH5-12590 Fully charging your batteries after each discharge on a live-aboard cruising sailboat is simply not practical. Instead, most of us will cycle our batteries between 50 and 80% of their capacity. The bad news is that this will ruin your lead-acid batteries (regardless of type) in a distressingly short time due to sulphation.

The Solution

But there is an answer to this problem: regular equalization, a process where you deliberately overcharge your batteries for a specified time and voltage to remove the sulphation.

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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. 11 Steps To Better Lead Acid Battery Life
  36. How Hard Can We Charge Our Lead-Acid Batteries?
  37. How Lead Acid Batteries Get Wrecked and What To Do About It
  38. Equalizing Batteries, The Reality
  39. Renewable Power
  40. Wind Generators
  41. Solar Power
  42. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  43. Battery Monitors, Part 1—Which Type Is Right For You?
  44. Battery Monitors, Part 2—Recommended Unit
  45. Battery Monitors, Part 3—Calibration and Use
  46. Battery Containment—Part 1
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Nick Kats


I’m truly dumb when it comes to electricity, but I’m going to ask this – What about solar panels & windvanes? Won’t these top up the batteries? Solar panels & windvanes can be used at a marina – no need for shore power. My windvane is connected to a regulator to prevent overcharging of the batteries. My hard solar panel is small & is put out on deck when I’m away.

What am I missing here ? (!!!)



PS – The solar panel & the windvane terminate in clips (as in alligator clips, or jumper cables) which I can move from one bank of batteries to the other as needed. A brilliant idea suggested by an electrician friend.


Isn’t equalizing the batteries a completely separate issue from the regular charge/discharge cycle that we all put the batteries through regardless of the number and kinds of charging capabilities we may have?

Now I feel like I’m missing something in this ‘conversation’.

On the topic of equalization; what happens when the batteries have to be left aboard for the winter while the boat is laid up? We have a couple of solar panels that will keep them topped up but there is no way to equalize them every month. Will we just suffer the loss of life at some point?

Chris Witzgall


I have built one 24v 40ah LIFEpo4 lithium battery, and just bought a pre-made (DIY CLASS) 200ah 12v battery, which arrived today. I think these will be the standard batteries for many cruisers not too many years from now. Some highlights:

-little to no Pekert effect. 1c+ charging and 3c discharging allowed.
-1/2 of the weight
-5x the cycling ability
-no gassing, and no equalization
-works with most charging sources. Since they are designed to be charged up higher than most marine charging sources, current stops flowing before they reach full charge, but the % lost not charging fully is minuscule.
-Bulk charging extends up to 90%, and you can safely discharge 80% without grossly effecting the # of cycles. The cells I have in the Torqueedo pack are rated to 2000 cycles at 80% discharge, the Hipower brand cells in the 200ah 12v pack are rated to 1000 cycles at 100% discharge, I have not found their rating at 80% online yet. Thundersky brand cells are rated even higher.

Cost? You can put together what I bought, a 200ah 12v pack with a Battery management system to protect both the cells and your alternator, for about $1200 cell cost and $450 BMS cost. This one weighs about 80lbs in a 4d battery box, and is set to replace 4, t-105 wet cells at 445ah capacity.


Matt Marsh

Re. equalizing on solar/wind: With typical setups, I’d agree that equalizing from solar or wind power wouldn’t work. But I can’t think of any reason why a good buck/boost MPPT controller couldn’t be programmed to provide an equalization voltage from a solar or wind source. It should be just a matter of programming the unit’s microcontroller to allow a few different operating modes.

Re. LiFePO4: It’s interesting to see those costs, Chris- they seem to be coming down dramatically. The current interest in electric cars is causing a lot of money to be poured into battery development, and I would consider it quite reasonable to expect the option of choosing between a wide range of traction batteries using half a dozen different chemistries within six or eight years’ time.

Justin Godber

Justin again from Lifeline Batteries. Interesting posts. John, your article is right on about equalizing.
Re: Equalizing using solar and Wind. This can be done but it just needs to remain constant for the full 6-8 hours. This is usually the down fall to equalizing this way. If you can find a way to have it remain constant and still push good amperage then by all means use that source.

Re: LIFEpo4 or and Lithium Batteries for that matter. We have built and have been testing Lithium batteries for quite some time. You can see this link here where we have built one for the US Navy and got the contract for it:
You can also find a full sheet of the advantages and disadvantages here:
In summary here is the limitations to the technology that we have found.
•Requires protection circuit to maintain voltage and current within safe limits. Protection circuitry involves both additional hardware and software.
•On aircraft battery monitoring and alarms will be required for safe operation.
•Subject to aging, even if not in use – storage in a cool place at 40% charge reduces the aging effect.
•Transportation restrictions – shipment of larger batteries may be subject to regulatory control.
•Expensive to manufacture – about 40 percent higher in cost than nickel-cadmium.
•Not a fully mature chemistry – metals and chemicals are changing on a continuing basis.
•Extremely flammable electrolyte.

Although we are working very hard to provide top quality products we are also very concerned about safety. Right now the lithium cells are very dangerous and very volitale. If they get the wrong scenario you don’t want to be anywhere around that battery. Lithium Ion cells if ignited can and will burn at 1,100 degrees F. Not only do they burn at that temperature they also create their own oxygen at the battery plates, which means you cannot put out the fire. The cargo plane that just crashed in Dubai caught on fire because of the Lithium batteries aboard. Lithium batteries can spontaneously ignite if the air gets hot enough. That’s what they think happened. They think when it was sitting on the runway the cargo hold got up to 135 – 140 degrees and they self ignited.
We have come up with a non oxygen producing Lithium Ion but the electrolyte is still very, very flammable.
There are new shipping regulations going into effect January 2011 for Lithium Batteries and they are expected to cost the Lithium industry 1 billion dollars more annually. This means the price of Lithium is going up next year, not down. Not to mention they are now considered Hazardous Cargo even when shipping ground.
That still gives AGM batteries the best advantage.


The info on the current issues around Lithium batteries is very helpful. I wonder if you have also been testing the new TPPL (Thin Plate Pure Lead) chemistry/constructions, like the Odyssey batteries. In their own product literature they are described as AGMs, but they are actually quite different. TPPL batteries are said to have very high charge acceptance rates and handle very deep discharges. The chemistry is still lead acid, so it would seem like equalization would still be required, but maybe less frequently? Please comment if you’ve been exposed to those.

Were TPPL batteries an option you considered, and if so what put you off of them?

Thanks for this very helpful series of posts.

Chris Witzgall


You use the term Lithium batteries multiple times in your post. Knowing that there are multiple chemistries of Lithium cells, and they can be very different in their performance and safety characteristics, which ones are you specifically writing about?



So if I do not have access to shore power once a month, I should not go with AGMs, but with gel. I expect to have no shore power for montrhs at a time.


Hi John,
I believe a previous commentor tried to make this suggestion, but didn’t communicate it clearly. Let me try. If you have 2 house banks, could you use one bank to fully charge and equalize the other one (kind of like lifting yourself up by your shoelaces)? If each bank is 300 amps, to fully charge the offline bank from 85% to 100% would draw down the bank in use about 45 amps, plus some for inverter/charger inefficiency. To equalize add 3 amps per hour for 8 hours= 24 amps, plus inefficiencies. Say 80 or 90 amps out of the house bank in use. That is one hour of generator time with a 90 amp Iota charger ($400?) or a half hour if you stack 2 chargers. I hope there is a solution this simple…why won’t this work?


Hi John,
I am very interested in your research on this topic as I have a similar situation with a 47′ aluminum Van de Stadt based at a wilderness cabin in Alaska, nearest shore power 12 hours away.
AGM batteries are my first choice as I have an 8 kw generator that should be able to run at least 3 60amp chargers to take advantage of the high absorption rate of AGMs for bulk charging. Per my understanding of Justin’s post solar and wind are not consistent enough for a proper equalizing charge, but could charge the house bank in use.
After both battery banks were charged to 85% the generator could be shut down, and the house bank in use would be used to power an inverter and small charger that would complete the charge of the offline bank to 100% (@15A for 6 hours=90A) followed by the equalizing charge of 6 amps at 15.5 v for 6 or 8 hours (48A) Total 138A into the battery being equalized, plus inverter use & inefficiency. Maybe a 50% discharge (200A) of the in use L-16 house bank at most, assuming no extra generator run time for projects, or solar, wind, or alternator charge off the main engine (160A) which gets used a lot in Alaska. So lots of surplus power available at certain times. Worst case of 52 discharges a year if done weekly. According to the tables it seems like it would extend the life of the batteries, even with the extra discharge cycles factored in. At least in theory. A selector switch would be needed to choose the house bank in use to power the inverter, which would run the small charger. Another switch would be needed to direct the charger output to the offline bank being equalized. A small dedicated charger for each bank may be even simpler. I will sketch it out and hopefully see what I have overlooked. Seems like it could be kept very simple. (I have lots of ideas, and am happy if only 1 in 10 is good and worth pursuing. And the only dumb question is the one you didn’t ask.) Any suggestions are appreciated.

Joe Casey

John, a number of years have passed since you worked with Lifeline to equalize your AGMs amd modify your charging methods. What was the result? Are still using the same AGMs? Also, if you are still in contact with Justin (and he is watching this site) I would love to hear his opinion about equalizing East Penn (Deka) AGMs. The factory says no, but it sounds like the ‘conventional guidance’ that Justin referred to.

BTW, we are on our second set of AGMs since 2002 but unlike you have been able to take advantage of ground charging resources. Our experience suggests that on-board generators are not necessarily the answer.



Great info! The equalization makes sense and it looks like a real battery saver. My question pertains to a 48VDC (or 24 VDC) system which is comprised of multiple batteries in series. Do you need to equalize each battery individually? Or does this happen “naturally” based on all batteries are in series together.
Thanks, great website.


Hi Chris,

Seems no-one wants to answer your question so here goes;

It would not be necessary to equalise separately, the voltages applicable to the 12v system can just be multiplied for series connected batteries eg;

12v > 15.5
24v > 31.0
48v > 62.0


Conor Smith


Thank you for a very useful article.

I was wondering if there is any instance or reason you are aware of where a continuous battery maintainer/desulfater would be bad for AGM batteries? Unfortunately we live on shore power a lot right now, and I wanted to buy a maintainer for our AGM starting batteries to live on, such as this one:

The distributor said I did not need it, and actually said to not get it. Do you know of any reason why a maintainer/desulfater for AGM batteries, hooked up full time to AGMs, would be harmful? (Full River AGM batteries)

Thanks in Advance!


Hi John.

We are in the process of designing the electrical system in our new boat. This obviously includes the choice of battery type. While discussing the possible choice of AGM’s, the following question came up. Why is it that different companies have opposing guidelines with regard to equalizing, although it is the same technology (or at least that is what you would assume). Lifeline (the brand you use) says you need to equalize. Trojan specifically indicates in their maintenance tips never to equalize AGM’s ( At the boat show I got different answers from different dealers. Nothing conclusive.

This issue is important notably regarding the need for shore power. We would have relatively low usage (+/- 120 Ah), the possibility for a fairly large bank (500 Ah), a good dose of solar on the hard top (400 W, and more when at anchor) and a large alternator on the engine (175 Amps). Therefore it looks that we might get around with these charging sources for our daily usage. However, if we need to equalize, we now need to add shore power. Quiet a substantial investment (cable, plug, 2 chargers, a galvanic isolator, …).

Maybe the people from Lifeline can shed some light on this?

Many thanks


Hi John.
Your statement ‘we should simply never install a battery on a cruising boat that can’t be equalized.’, helps clarifying things.
In summary. Equalization is a must. Choose only those products from a given company for which equalization is permitted or a must. While the charge source for equalizing does not need to be shore power, can be more tricky if using solar. I will look into this last aspect.



I have talked to Lifeline about equalizing from solar specifically and they did not recommend it. Basically it was an issue of enough light for long enough. They recommend at least 6-8 hours continuous for equalizing and that is starting off at a battery at 100%! Not, 4 hrs today, 4hrs tomorrow.
Given that you have a relatively small power demand, and a pretty diverse and capable charing mix, it sounds like equalizing may be every 2nd or 3rd month, as you may re-charge your batteries pretty often.
Have you considered not hard-wiring a shore power connection for your relatively seldom equalization needs? I wrote previously about using a power max battery charger, with manual voltage adjustment, for equalizing and I was thinking, if you just put some alligator clips on the DC output, and keep the supplied cord with plug for AC input, you have a simple, equalizing solution for under 200 bucks and no installation requirements.



John, Conor.
Thank you for the further clarifications. I will look into this a bit further.
Shore power moves back to the top of the list. All things considered, tested and proven solutions often turn out to be less expensive than experimenting with ‘big black clouds’.

Robert H Andrew

John, in a comment back in 2010 you wrote:
“The bottom line is that if you want long life from lead acid batteries you have to come up with a way to fully charge them regularly. If you can’t do that, as most liveaboards can’t, then regular equalization is the next best thing.”

So, what constitutes “regularly?” If through solar or a long day of motoring, batteries are fully charged once a week, is that “regular”?

Regarding using solar to equalize, the concern seems to be that it is too unreliable because a constant current is needed over a relatively long period of time for a battery bank of any real size. I would think that “partial equalizations” on successive days would still be beneficial, but maybe I don’t understand how the equalization actually works.

Jack Gill

Seems like the link is not good, I get the below response.
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Rob Ramsey

Hi John! Love your website … tons of useful info! I have a question. Given that a battery bank should ideally be charged to 100% and given that the last 20% takes a long time what would you think of the following suggestion: have two battery banks (let’s say A and B). One (A) for use, the other (B) for being charged (and disconnected from my grid). The latter (B) would be charged with the engine or a genny and then fully charged with solar panels over a day or so. Should get them up to 100%. The other bank (A) is being used. When the charging bank (B) is full I would switch banks (and charging genny/engine & solar). The discharged bank (A) will now be charged with genny/engine and solar panels and the full bank (B) will be used. I would only need 300-400 AH per 24 hrs so having two banks of three 100AH batteries should do the job. What do you think of that idea? Cheap enough (I can get a 105AH battery for $120 or so) and easy to make a switch or even MacGyver one to switch automatically.

Chris Geganto

John, This is a simple one but when you actually go through the process of equalizing your lifelines should you turn the breaker at the main panel off if you are in a position to do so. Will this help prevent any sensitive electronics (i.e. lights, refrigerator, etc..) from being damaged by the high voltage? Just curious on the details as it was recommended by my battery supplier that I equalize my Lifelines after returning from a 6 month trip cruising the Bahamas. I started seeing my voltage drop to around 12.6 -12.7 after fully charging the batteries. They would never get back up to 12.8V

Thanks for your help.

Chris Geganto

John, I want to start by saying that you provide a huge amount of value in your posts as the are the perfect balance of theory and practical application. For people who have never equalized batteries or reprogrammed a battery monitor you really put the mind at ease. I wanted to let you know that i equalized my Lifelines and all went well. While my batteries were not in bad shape i will start this good practice. I just wanted to tell you what i saw in case someone else like me reads this post.

I used my Truecharge 40+ equalization mode and also had to “trick” it to thinking batteries were flooded. The Charge put out 15.73V with about 1.83amp of current. I equalized for three hours. The batteries never felt hot to the touch nor did they discharge any gas. I was nervous at first but then quickly realized that I had made all the necessary precautions by taking the kids off the boat, turning off the main DC panel, and opening up the boat.

Thanks again for all you do. Hopefully someone else reading your blog will find this comment useful when trying to equalize for the first time.

Dick Stevenson

Hi John and all,
After decades of using high quality gel cells and been generally quite pleased, I am seeing the end of my present set which 6-7 years old and saw a fair amount of neglect and some abuse (down to 10+v for undetermined periods for ex) when Alchemy was abandoned in Newfoundland for 1 ½ years because of covid. One of the things I have appreciated about gels was their forgived-ness: as shown by the fact that we have been using the above mentioned set so far this season.
For replacement, I have been thinking of AGMs: in part because they seem to be dominating the recreational market and, now, in casual inquiry, seem to find that East Penn gel cells are hard to come by (my last set, this present set, are Sonnenschein bought in the UK).
In preparation, I have read your reports on AGMs and have some questions.
Before doing so, I would suggest that your statement that batteries that can’t be equalized have no place on a cruising boat be temporized: my gel cell banks last ~~6 years or so and, during some years, rarely see shore power. In contrast to AGMs, they seem not to need to be fully charged with regularity. And I have been pleased that they have bounced back, seemingly unaffected, from the occasional abuses I have inflicted on them.
One of my concerns about AGMs is that they like (perhaps need) to be fully charged with some regularity. My house bank is 6 group 31 gels (~~570ah). Your regimen for equalizing is intimidating both in equipment needed (new battery charger for one), the need to find shore power for periods of time, possibly re-wiring my 6-battery system into separate banks as I live aboard, etc.
Casual research seems to indicate that many AGM battery makers do not suggest equalizing.
My questions are: How much of a longevity hit would one experience with say, Lifeline AGM batteries, if they were fully charged (not equalized) once per month on shore power on occasional marina visits? Would you suspect there to be a hit on overall battery capacity (in other words, would there be a “set” that would decrease the quantity of amps available)? And lastly, are there AGMs that do not suggest equalizing worth looking at.
Thanks very much for your thoughts, My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
As always, I appreciate your thoughts and agree with your appraisals. I will likely stick with gels as they have been a workable compromise: fairly good life, forgiving and no maintenance. As I get older, the no-maintenance aspects of life on board gets increasingly appealing.
BTW, further calls around for East Penn/Deka gels discovers that NAPA batteries are East Penn re-branded. East Penn can’t deliver for months while NAPA can deliver in days. Weird world. I neglected to ask about their AGMs.
My best, Dick

Dick Stevenson

Hi John,
Good to know.
From what you have written, I suspect any of the AGMs that are unable to be equalized would suffer a similar fate. I further suspect that this issue with AGMs is masked by how few live off their batteries and who are in marinas regularly with its shore power and so fully charge their AGMs.
When last in the US (15 years ago) I had good luck with East Penn gels. I hope they have maintained that quality.
My best, Dick 

Stephen Hinchman

John – I am wondering how carbon foam batteries (Firefly) might affect one’s equalization strategy. From the Firefly manual:

“Restoration Charge
As stated, Firefly Batteries can operate in a partial state of charge for long periods of time without sustaining any permanent damage. The usable capacity will decrease, however, with each cycle within a partial state of charge, up to a point. In order to regain the full original capacity and in some cases more, it is necessary to perform a restoration charge. To perform the restoration charge: charge the G31 to 14.4V or the 4V/450AH to 4.8V and continue to charge until the current drops to 0.5 A on the G31 or 2A on a 4V/450AH and apply a float charge of 13.5V(4.5V for 4V model) for 24 hours. Fully discharge the battery to 10.5V (G31) or 3.5V (4V/450AH), and then repeat the same charge cycle.”

So a much longer equalization cycle (4x) but at a lower (float) voltage. And you have to do it 2 days in a row. Knowing that you have very positive comments on carbon foam elsewhere in this e-book, does this requirement affect your thoughts on Firefly vs standard AGM.

I am wondering if my existing Heart Interface Freedom 10 (circa 1994) charger/inverter is all I need and how I might do a restoration charge once a month in practice. Seems like shore power with my existing equipment would cover both load and restoration (float) charge needs without need to separate the batteries while not sleeping or leaving the boat, but it would take at least 3 days at the dock to do the cycle. (Not cheap and not want I want to do, but could get lots of chores done)

5-8 hours to recharge depleted battery back to 100%

24 hours at float voltage

5-10 hours to draw back down to 50%

5-8 hours to charge back to 100%

24 hours at float voltage

Anyone have real world experience with restoration charging of Firefly carbon foam?

Thx- Steve (btw, this site is an absolute gem).

Stephen Hinchman

Practical Sailor testing concluded-yes, a while back. See

Eric Klem

Hi John,

Just followed the link over from your latest charging article and noticed that your conclusions on equalization may not apply to everyone depending on the specific battery manufacturer’s recommendation. In our case, our Trojan T105’s only recommend a 2 hour equalization but at a whopping 16.2 volts. This is a short enough period for us that we just shut everything down when fully charged and not underway at some point and do it, no multiple days or having to split the bank in 2 (I still do like what you do with 2 separate house banks but generally paralleled for other reasons). And we find that our 140W solar panel has no problem completing this task if it is sunny and towards the middle of the day on 4X GC2’s assuming we started fully charged so no need for shore power either. The limited sample size of equalization recommendations I have read seem to suggest that FLA allow for higher voltage shorter duration equalization than AGM but this is too small of a sample size to say definitively.


Robert Andrew

Eric, I have a similar battery and solar set up. What is the rest of your charging system? How do you accomplish the equalization exactly? Bob Andrew

Eric Klem

Hi Robert,

Our boat has 2 charge sources, an externally regulated alternator and a solar panel. To equalize, we get to full charge at anchor, typically due to having had good sun for a day or two but sometimes due to motoring then switch our main battery switch off and turn on the equalization on our solar controller and it takes care of the rest. Our 9 year old charge controller died a few weeks ago (solder melted out of a thru connector on a PCB) and I replaced it with a Victron SmartSolar. I have generally been quite happy with the unit but I can’t seem to get it to equalize the way I want so I will reread the manual this week and make sure it isn’t operator error and if I can’t fix it next weekend, it will be time for a call to customer service. It certainly is capable of doing what I want from a hardware standpoint, it just seems like the settings I program for equalization are not getting saved. If something were to happen like a squall line were to move in, I would just turn off the equalization and turn the battery switch back on and complete it another time.

Both charge sources go to the house bank and then the start battery is charged by a charging relay. All batteries are FLA so when the house bank gets equalized, so does the start battery.