[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.
Hi
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
Hi Lars Erik,
Given that you say the alternator company adjusted it for a higher voltage it sounds to me as if it has an internal voltage regulator. Assuming that’s right, I would recommend changing to an external programmable regulator, even if that means changing the alternator. The best option is the WS 500 from Wakespeed: https://www.morganscloud.com/2020/07/04/stupid-alternator-regulators-get-smarter-finally/
More on how to choose an alternator here: https://www.morganscloud.com/2013/11/06/10-tips-to-buy-and-install-a-liveaboards-alternator/
Even if the present regulator is external, if you have to turn on a bunch of loads to get it to the correct absorption voltage it’s not smart or consistent enough to treat your batteries properly, which will definitely shorten their lives.
The Wakespeed will measure the current going into the batteries (only regulator that does) and then, when they are fully charged, cut back to float voltage for the remaining time you are motoring. Way kinder to the batteries to the point that longer battery life will, over time, pay for the regulator and probably the new alternator too.
Hi John
Thanks for your reply. I will consider your recommendations.
Already have a Victron shunt.
You are right. The regulator is internal.
John
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?
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?
Hi Steven,
That’s easy, assuming you are referring to shore power charges: pretty much anything from Victron. I also hear good things about Mastervolt, but know less about them.
That said, I do have a new Victron charger on the J/109 that I will write more about at some point: https://www.morganscloud.com/jhhtips/victron-chargers-rock/
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. https://www.victronenergy.com/chargers/phoenix-smart-ip43-charger?_ga=2.111165324.969479737.1661034727-1340857872.1660924159
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.
Hi Stein,
I agree with most of that, thanks. However, it’s not true that all AGM batteries should not be equalized. Lifeline actually require equalization of their AGM’s for long life, and the Victron AGM’s I’m currently evaluating on our boat allow it too, and my early testing indicates they are better for it.
More on equalization here: https://www.morganscloud.com/2010/08/02/agm-battery-test-part-1/
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.
Hi Robert,
While the Wakespeed is better, a Balmar programmed as advised by Rod is a perfectly acceptable solution for robust lead acid batteries like yours.
And yes, what you are seeing is the Balmar cutting back output as the alternator heats up. That’s a good thing, since otherwise the alternator would be toast in short order. Most any alternator will exhibit this behaviour when charging a big bank, but I agree that yours sounds excessive.
Is it by any chance the alternator that came with the engine that you had modified for external regulation? The reason I ask is that OEM alternators are not intended for continues duty and so tend to heat up quickly.
The other possibility is that due to the ratios between the drive pulleys the alternator is not spinning fast enough, particularly at low engine RPM. This can result in overheating because the fan is not moving enough air.
But the most likely is, as you say, engine space temperature is too high. The answer to this is an air blower bringing in outside air. A PITA to do, but worth it because a hot engine space is bad for your engine as well as alternator.
More on alternators here: https://www.morganscloud.com/2013/11/06/10-tips-to-buy-and-install-a-liveaboards-alternator/
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.
Hi Ramon,
If memory serves, there have been discussions in the comments here from time time. The consensus is that they are generally not a better alternative to generally available lithium iron phosphate, although some disagree. I’m not qualified to opine on that, but what I would say is that my guess is if they were better a main stream company like Victron or Mastervolt would be selling them.
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.
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 (https://lifelinebatteries.com/wp-content/uploads/2015/12/Internal_Resistance_and_Short_Circut_Current_of_Lifeline_Batteries.pdf). 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: https://batteryuniversity.com/article/bu-902-how-to-measure-internal-resistance
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.
Hi Dan,
Thanks for the corrections, you are, of course right.
I do delve into ohms law bin a bit more detail in another chapter. That said, no question that I have simplified the heck out of all of this, and possible oversimplified, but on the other hand when writing these I always need to simplify as much as possible, otherwise those who need the information the most won’t read it.
So the question I always ask is will a small inaccuracy effect the outcome for a lay person working on a battery. In this case the only point I’m really trying to get across is that the internal resistance of a battery is really low and will increased with state of charge and decrease with increased temperature.
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.
Mark
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 ?
Hi Mark,
Sorry to hear that. Scary indeed. I would recommend a complete trace out of the system since there are just so many stupid mistakes in many (maybe most) yacht electrical systems that could cause this, or other problems.
That said, that sounds like a single defective battery, maybe with an internal short. Of course that still begs the question of what caused the battery to go bad.
So far I’m happy with the Victrons but I have not really stressed them yet, so that’s not very meaningful.
At the moment the only batteries I can endorse for sure based on long first hand experience are AGMs from Lifeline, although you do have to manage them against sulphating, but we have a lot on how to do that in the Online Book.
Late question: I run Trojan 6 volts ganged to created about 450 amp hour 12 volt house bank. Seoarate Group 27 starting battery. BlueSea relays used as charging combiners. The tach on my new Beta 35 seems occasionally to stop working. I believe it may be when my Balmar regulator drops to float. I was sure that Trojan recommended 13.9 float voltage, but when I went to the spec sheets, it says 13.5. Any suggestions to prevent my alternator driven tach from dropping off as the regulator moves to float. The tach stops working for several minutes, but usually comes back on. It is enough that in about 330 engine hours, the tach hour meter is several hours behind my hard wired hour meter. Thanks.
Hi Terence,
Are you running the tack sense wire through the regulator? If memory serves, the Balmar regulator does not use RPM to make charging decisions but it does have tach contacts. So if I’m right about that (check the manual) how about just connecting the tack sense wire direct to the tach from the alternator? If nothing else, running the wire direct will clarify if the regulator is causing the problem.
I guess the other option is that as the voltage drops from 14.4 (acceptance) the current being suppled by the alternator drops right off so the pulses die too, and then as the residual charge on the battery plates drop off the pulses start again. Not a lot I can think of to do about that, other than increase the float voltage and see if that helps. On that note, I would doubt that floating the batteries at say 13.8, which may help, would hurt them.
Another experiment that would be interesting is when the tach fails, turn on a good big load, say >30 amps, and see if the tach comes back.