The Offshore Voyaging Reference Site

How Batteries Charge (Multiple Charging Sources Too)

In the last chapter I explained Ohm’s Law, that wonderfully elegant and simple relationship between three variables that allows us to clearly understand just about any problem we have with our boat’s electrical system.

Now let’s look at some practical applications: how batteries charge and voltage regulators work—two of the most misunderstood pieces of gear on a voyaging boat.

To make this easy, we are going to use lead-acid batteries in all or our examples. Yes, I know, you want lithium. That’s fine, we will get to those in a later chapter and the stuff we learn in this chapter will make understanding lithium systems way easier.

First we need to do one of those algebra trickery things (that we all slept through in school) so Ohm’s Law will get us the answer for any one variable as long as we know two others:
ohms-law- That is:

  • Amps equal volts divided by ohms.
  • Ohms equal volts divided by amps.
  • Volts equal amps multiplied by ohms.

Real World

Now let’s say:

  • Our system is 12 volt.
  • Our lead-acid battery bank has 400 amp hours capacity and is half discharged.
  • We are sailing along using 20 amps for various loads.
  • We have 200 watts of solar panels and it’s sunny so they are putting out 100% of their capacity—unlikely, but it doesn’t matter for our purposes.
  • We have a 100 amp alternator on the engine.
  • Both the solar panels and the alternator have regulators.

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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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Marc Dacey

Well done, John. I think this is going into the log book to edify the rest of the crew.


Hi John
Good article.
There is one thing that I do not understand. Where du you get the 120A from in point four under “Get charged”? Or ist this rounded up from 116.66A?


Hi John
A small point. Watt hours or kilowatt hours are a measure of the amount of work done or energy expended. Power is measure of work done in a period of time or Work divided by Time. So multipying it by Time , the two “Times”cancel out and you are left with Work.


Hi John
I was just trying to help your Science teacher to stop revolving by pointing out a small thing that in no way detracts from a really useful Article . You are right that demystifying seemingly unnecessarily complicated electrical systems by the application of some simple science basics will benefit us all.

Thanks for the valuable article

David Branyon

I would vote to fix it because it is at the root of some other misconceptions. In the true engineering sense, kW-hr is a ridiculous term, similar to saying I filled up my boat with 20 gals/hr-hrs at the fuel pump. It’s just gallons. For energy, it’s just joules (or kJ or MJ). A joule is a Watt-second so that is the right term, but I acknowledge that multiplying/dividing by 3600 is hard and kW-hrs are here to stay so…

But the other confusions driven by this lack of understanding are things like “I need 100 kW-hrs but I only have a 50 kW engine so I have to get a bigger engine” and other foolishness. Right up there with people saying they have a “500 amp battery” when they mean amp-hr. But power as a *rate* of energy flow is just as important a distinction as amps vs. coulombs, therefore my vote.

Great site!


Hi John, I know it would add a small complication to the equations, but I think you’ve assumed the solar panels are operating at 100% efficiency. In reality, its about 80% I think.

richard s (s/v lakota)

just wondering what happens to the amps from the alternator and panels when the batteries are fully charged…i’ve read they are ‘sloughed’ off but i don’t understand what that means as the amps need to go somewhere just as if they are water…cheers


Hi John,
“inserts progressively more resistance” would mean that this is a heat generation issue. So we would be wise to put the solar(wind regulators at a spot where they can get rid of the heat, or we might be in trouble.
Right assumption?


Hi Ernst,
Spot on. I was PM on a very complex mega-sailboat and recall a point where the owner decided to replace the machinist mate’s personal clothes storage locker with a device called a Helionetics phase converter that could accept almost any form of electrical input from a lightning bolt (lol) to 220v AC and output DC in 12v, 24v 50 cycle, or 60 cycle. When I asked what size AC unit we should specify to overcome the resultant heat the Famous Designers of the vessel looked at me as if I were crazy.

Alex Borodin

Hi Ernest,

I realize I’m a few years late to the party and yet…

I don’t think your assumption is right. As the resistance goes up, the current through the regulator goes down and the dissipated power goes down as well (P=U*I). In the extreme, if you disconnect the leads from the solar panels, the resistance between them is effectively infinite, the current is zero, and you get absolutely no problems with the heat generation.



Thanks for the great article that even a mere woodworker like me can understand. I particularly like the hydraulic theory of electricity as it is always the one I’ve fallen back upon when the water got too deep for my limited technical knowledge!


The funny thing is that electrons flow in the opposite direction of the current.
Oops! Spilled the beans. Sorry, couldn’t help myself! 🙂

Bill Attwood

Hi Ernest.
My SuperWind 300 generator has a dump resistor attached to the regulator. Excess and unwanted energy is transferred to this.

Erik Snel

Good introductory article.
I find one aspect of charging missing: batteries have a maximum charging capacity. If this capacity is exceeded, things may become quite hairy.
Therefore, when adding charging capacity by using parallel charging sources (alternator, wind, solar, …) is would be a good idea to check the max charging capacity of the battery bank and add batteries in parallel if necessary!


that’s what regulators are for. When the bank gets full the internal resistance rises, and therefore the regulator needs more voltage to pump the same amount of current (“amperage”) into the batteries. At a certain point (which depends on the type of battery, wet, gel, LiFePO4) th regulator reduces the voltage.
For smart charg regulators there are 3 steps: (1) bulk where the regulator tries to keep the voltage at its peak (14.6 for lead, and 14.0 to 14.2 for Li), (2) Absorption where the voltage is reduced a to only a bit above the normal cell voltage (13.8 e.g.) to top off, and (3) Float, where the voltage is kept at the cells maximum (12.8) to keep the cell fully charged.

Usually for Li batteries (3) Float should be avoided as the cells have only a very small tendency for self discharging, and when you leave your bolat they like to sleep at 50%, other than lead packs that hibernate best when kept at 100% and need (3) Float.

I found a very good article on Lithium packs here:

Erik Snel

The regulator, as you well describe, is mainly for capping the charge when the batteries reach full (and smart ones do a lot more when conditioning the batteries).
But the battery bank may be 50% empty and still have a maximum charging capacity. Even if the charger regulates this, it would not be wise to have a charging capacity higher that the max charging capacity of the battery bank, as you will be converting a lot of energy into heat…

Erik Snel

Hi John,

That’s interesting, as the spec’s of my batteries (Sonnenschein gf-06-180v) clearly state a maximum charging capacity in Amperes…
On the other hand, your logic following Ohm’s law seems flawless, the Amps going in will depend on the battery banks resistance and the voltage (in case of a limitless charging source). But than why the max charging capacity stated for my batteries?

Erik Snel

Hi John,

For this type of batterySonnenschein specifies a max charging Amperage of 30A per 100 Ah battery capacity. Interestingly, they specify different Ampf for different kind of batteries, ranging from 15A to 30A…


Hi everybody
Batteries do not have a maximum charging capacity as John says. This dos not mean that you should charge your 100Ah bank with 1000A. You could do so but only for a few seconds. After this the internal resistance would rise very fast and the regulator will reduce the current. If you do not use a regulator the battery will die.
Batteries do however have a recommended charging current. This is ca. 10-20% of the capacity in Ah for open heavy duty lead acid batteries. More for the modern types like AGM and Li. This would mean ca. 10 to 20A for our 100Ah bank. Regularly charging with more will reduce the life of the batteries.


Hi John
Lifline Batteries seem to be very special in several regards. The manual you are linking to is for AGM batteries which can accept a higher Current than flooded batteries. I doubt these recommendations can be expanded to other batteries, especially not flooded ones. Nevertheless Lifline is specifying 500%C as maximum charging current which is extremely high even for AGMs. They also recommend conditioning/equalizing which should after common knowledge kill any konventional AGM battery.
Trojan is specifying 13%C in their user guide for flooded batteries like mine and strongly discourages equalization for their AGM batteries.

Eric Klem

Hi John,

Reading this and the previous articles has gotten me thinking about how complicated electrical systems should be (sorry for being slightly off topic). Looking at what I can see of your setup from the pictures, it appears that you have a robust system that is designed to have multiple charging sources and good redundancy. For the type of cruising you do, I would consider this to be an appropriate level of complication.

I have seen many electrical systems that are far more complicated and harder to troubleshoot on boats that should be far simpler than yours. Ours is significantly simpler but we don’t have several items like a generator. I believe that part of this is a sort of “over-engineering” that can occur when people try to deal with all theoretical eventualities. In my experience, this is especially true of people who buy a boat without a lot of previous experience and then proceed to set about refitting it to be the best bluewater cruiser ever without having ever cruised that boat before. To avoid this, I would recommend doing a sort of fault tree type analysis of your system and seeing what actually adds value and what is a belt and suspenders where the suspenders don’t work without the belt.

When we bought our current boat, I redid the critical parts of the electrical system and nothing else and we cruised it coastal for a year before I did any real work. What the plan for the electrical system looked like at the beginning of that year and the end of that season were totally different as I adjusted what I thought was appropriate and how we would use our boat. In the end, we have a much simpler system than I had originally planned on that is more robust. For example, we eliminated our shorepower setup completely as we realized that we would never use it once the solar was installed since the only time we could plug in was when we are hauled out and the solar covered that. On our first boat, we made a lot of changes (not just electrical) before the first year and ended up finding that many of them were not optimal because we didn’t really understand the boat and us yet.



Eric, I find myself in the very same predicament as you did before dropping loads of money into upgrading your electrical system. We are getting ready to set sail this fall and i was wondering if you could give me a few more details on what you did. i am looking to hear about what you started out with and then what you ended with.

I have a Beneteau 423 with a 6KW genset, 40A xantrex charger, 150W solar, 80A alternator with about 250AH flooded batteries that are old and need to be replaced. I am going to replace with 2 4D lifeline to give me 400AH. Because of this change i need to upgrade my charging configuration.

Curious to see what you did before i spend thousands!


Eric Klem

Hi Chris,

John is correct on both accounts, how we currently use our boat and how applicable our system may be to you. To take our system offshore, we would need to increase the generation capabilities but otherwise it would be fine. My comment was more based upon my experience sailing a wide range of boats both for work and fun, both coastal and offshore, and seeing an amazing range of electrical systems. Some systems started more complex than necessary (trying to deal with every eventuality) while others got modified in strange ways over the years, very few of them were as simple as they should have been. I always try to remember that the happiest people in any given anchorage are often on the simplest boat, while they may have to do things more manually, their reliability is really high.

My best suggestion would be to start as John suggests with the energy budget. Then, if it were me, I would create a schematic of the electrical system I would design if starting from scratch, then create a second one of what is currently there. By comparing these 2 schematics, you can figure out how to modify your existing system to do everything it needs to but have a minimum of extra complication. How you handle fault tolerance is another thing you will need to decide although the best way to start is always to not have something that could have a fault.

Good luck.


Damon Lane

A comment and a question:

I think solar panels can curtail their output without a resistor. The charge controller may open circuit them because they don’t seem to mind being in the sun with their terminals disconnected, but I’m not sure what’s going on inside them.

I have a four battery system that is charged in series by solar panels and a battery monitor with a shunt. That seemed to work well until I added a dual pro AC charger that charges each battery individually. The battery monitor can’t see this and can’t be wired so that it could. I think I’ll have to manually sync the monitor to 100% state of charge whenever I use the AC charger because it doesn’t see those amps going into the batteries. Other ideas? That wouldn’t be so bad, but today I saw overvoltage, so I’ll have to figure that out.


As I understand “charges each battery individually” I believe the chargers feed wires go directly to the battery terminals, bypassing the common negative, and thus bypassing the shunt. To my understanding this shouldn’t be necessary at all.

However this raises a question in my head – charging batteries separately – doesn’t that increasebank imbalance, requiring extra caution to have the battery bank equalized (esp. in 24V systems with 2x12V in series)?

Damon Lane

Thank you two. The solar PV charger has two leads and charges at 48V. The AC charger has eight leads: four sets, 12V each. The shunt, the PV negative, and one of the AC negatives are at the system negative, let’s say 0V. The shunt is installed like yours, except for the AC charger. The other AC negatives are at 12V, 24V, and 36V, which is why I don’t think they can go through the shunt.

I thought the AC charger would correct any imbalance the batteries got being charged in series. Because in series I thought difference in internal resistance would mean they get different amounts of charging.

Damon Lane

Hi John, I have four Concorde SunXtender 1080T group 31 AGM batteries wired in series to provide 48V to the electric motor. The shunt is on the negative of the side of this battery series. I have a DC/DC converter that provides 12V for the rest of the boat’s systems. A Midnite KID charger controller provides solar charging to the system and was the sole means of charging for a few years. Now that the mooring I used to rent was removed to expand the docks, I have a slip and so added an Dual Pro SS4 AC charger to take advantage of the electrical service on the dock I am paying for. Maybe I should have gotten a 48V AC charger. This charger has four sets of 12V leads (it is really four chargers in one case). The reason I don’t think I can put all the negatives where the shunt is, is because the batteries are wired together in series, so the negative of the second battery from the negative terminal is 12V higher than the negative of the first battery, and so on up the series. The AC charger doesn’t care which 12V potential it is filling, so when connected to the battery terminals it charges them all: 0-12, 12-24, 24-36, and 36-48. However, if I moved the negatives to the system negative the AC charger would see 12, 24, 36, and 48 Volt differences because of the series wiring. I had a friend with an electric boat that motored in series and charged in parallel with complex switches that dealt with an issue similar to this, because if I removed the wires that connect the batteries in series when charging with the AC charger, putting the negatives all through the shunt would work.


Ah. If I understand you correctly you have an 48V bank comprised of 4 12V units. And you have 4 chargers charging each unit seperately, right?

John, do you have an idea how this could be solved? Having a shunt on each unit negative, and somehow summing the current up (how the heck could this be done…) ?

Damon Lane

Yes that is right. Neat idea with four shunts! I think I will relegate the AC charger to backup as it was meant to be and reset the monitor when the AC charger finishes. A perfect solution isn’t necessary for me and my lake boat, but I threw it out there as a system that doesn’t easily conform the concepts in the post. And as you may have guessed, the boat has an electric motor, so you can add 48v charging challenges as another knock on electric propulsion.

Matt Boney

John, one error or omission – a battery is fully charged ONLY when the current is 0.5% of the Ah Capacity – AT THE ABSORPTION VOLTAGE OF 14.4v.

The trouble is you never see this voltage and current combination because the charger will have dropped to FLOAT, at a much lower voltage, well before the battery is fully charged.

To see the charging current at 14.4v the charger will have to be forced back into Absorption Mode by completely turning it off and back on which will raise it back up to 14.4v. When it has stabilised check again to see what the charge current is. It will be very much higher. My Victron charger switches to FLOAT at only 85% full and the current into my LIFELINES drops from 60 amps to 15 amps at 13.2v!!!

Matt Boney

Thanks John – but I really don’t understand your comments!

“…that does not change the fact that a battery is only fully charged when the criteria I specify is met”…

All regulators are stupid – with very very few exceptions – so how do YOU guarantee full charge? You really don’t make it clear.

If you programme the charger to stay at 14.4v for say 10 hours to GUARANTEE the charge current reduces to 0.5% of Ah Capacity then surely that is over-cooking the batteries at the gassing voltage? Too long at this voltage causes excessive gassing – not good for sealed batteries – but then I agree they shouldn’t be on a cruising boat if they can’t be EQ’d. Most battery manufacturers specify 14.4v or higher up to a charge current of 2% of the Ah Capacity, and THEN to finish in FLOAT. This may take another 12 hours or more as battery acceptance falls.

I hope I’m not being pedantic – but 0.5% of Ah capacity isn’t fully charged at FLOAT voltages. That has to be be made absolutely clear.

There is than a whole new Online Book on what is “100% fully charged”, and why this must be reached very 3-4 weeks to avoid sulfation – if not the batteries MUST be Equalised.


At first, it was not really easy to measure amps flowing over the various wires in my boats. Out of solar panel regulator, out of alternator, out of batteries…
Adding shunt and gauge to measure this is very costly.

I purchased a 100$ volt / amp meter with a clamp so that it can be used to measure amps simply by clamping around the wire. This is using induced current in the clamp loop to measure the actual current running in the measured wire. Best tool I ever bought for electrical measurement.

Since then, I have learned a lot around my electrical circuits and devices. This allows me building some reference numbers, and can figure out much more what is going on. Here are 3 examples:
-Alternator for instance, rated 80 amps can nearly deliver 50 amps with a 50% discharged batteries.
-Fridge compressor was supposed to be 2 speed, high with alternator on when sensing high voltage (14.4v), or low while on batteries only (sensing 12.8v or less). It actually draw 4.5 amps, what ever the situation is.
-There is 1 feet long cable, gauge 8 AWG taking all the return from the service battery bank, sometime 50 amps…
Getting all that info is helping me planning for the next move.


Regarding the alternator you should feel lucky – your alternator is sitting half idle and will never complain or overheat and cook itself. Probably thanks to your regulator.

Marc Dacey

A company called Mastech makes a decent one for about $60 Canadian, for those who weren’t left a Fluke in a relative’s will.


Note: when buying a clamp on amp meter, make sure that it can measure AC and DC current. Many, especially the cheaper on can measure AC and DC voltage but only AC current. Not much use on a sailboat…

Philippe Meloni

I dont know if it the right post for it, but do you know this product:
An interesting conversation on the same subject :

Thank you!

Danny Blake

Hi John
Last week I did a stupid thing and left my engine room lights on (not LED then, are now!) then 2nd error forgot to isolate batteries, came back after 2 days to realize but too late my domestic bank (new in 2015) was down to 8 volts measured on my balmar smart gauge.
I have now charged fully but have noticed they do not hold their charge overnight with fridge loads etc as they did before. Have I completely done them over? or is there any hope?
The bank is 4no 140 amp lead acid truck type batteries.
Thanks for any help you can give

Danny Blake

Hi John
Thanks for the swift reply and thanks for the extra info links,


Forgive me for my sycophantic comment – as I’ve said previously, your website is providing so much of the info needed to turn a great old cruiser (1986 Pedrick ‘41) into a successful long-distance traveler. I’ve bought and read Chen/Cheseau’s book on route design; now I’m getting through your book on battery charging. As an engineer, we’re trained to truly understand a subject (the difference between an A and a C in school) so my approach to this boat is to understand all aspects that are within my reach – from heavy wx drogues to propeller/engine/transmission match to energy management. Your work is walking me through much of this, helping to keep my family safe and the sailing successful. For what it’s worth, you are making a difference to this small family preparing to launch from Bayfield, Wisconsin. If we ever meet, the first round is on me.
Chris Hartnett
s/v Morning Winds


I am currently in the process of upgrading my battery system and there are simply too many options for configurations. Here is where I am. I am looking to update 235Ah Flooded batteries with 400Ah Lifeline bank. My alternator is the stock 80A yanmar and I have a 40A Xantrex charger. I am getting ready to move aboard my boat for the next 3-4 years with my family. The update to the alternator, charger, and battery bank will run me about $4K. By the Way I also have a 6Kw Genset. Here is what I am thinking:

100A Mastervolt Charger
120A Balmar alternator with belt kit and regulator
2 x 200AH Lifeline AGMs.

As I read your post I keep asking myself do I need all of this. What is the minimum viable solution for cruising. Do I upgrade the bank and alternator and keep the 40A charger? Is there another configuration? Any advice would be great.


Klaus Bonde

Hello John

It has been pleasure to read all your stuff on electrical, batteries and charging. I all ready did have Lifeline / Optima batteries installed by previous quality conscious owner and they lasted for almost 15 years ( Lifeline confirmed serial number). The house batteries consist of 2 groups controlled by a “ 1-2-both” switch always positioned in “ both”
The old huge Xantrex charger and Link 2000 did however leave for another electronic world and I have invested in Victron Charger/ inverter and Victron 700/712 battery monitors
I believe that you also have had Link 2000 and now use Victron , so this is where I really need advise.

The old Link 2000 had a double shunt, “ U “ formed with common ground ( -) and the 2 load sides connecting to load from the 2 house battery groups. The shunts provided by Victron is 2 single 500 amp. I cannot figure out how to connect these new shunts to get a accurate reading for battery monitoring.
The Victron shunts has RJ11 cable connections ,- could I just strip the wires and connect the old shunts to the 2 new Victron 700 series?
Or could I make a “ U / V “ form out of the Victron shunts and then connect to to the 2 Victron 700?
Or put in other way how do you get 2 Victron 700 to work together with 2 house battery groups with “ 1-2-both” setup??

Also changing the 100amp Balmer alternator to a 160amp Balmer , now with Serpentine belt and MaxCharge 614. Can you connect temperature monitoring on both house groups without confusing MaxCharge ?

I am aware that this is a very specific subject but I could imagine that others have 2 groups and would like to monitor.

Thanks for any help

Berst regards
iSea / J-46


Thanks for a well-written and helpful piece. Although I consider myself reasonably intelligent, I still feel a bit (OK, a lot) unsure about all the electrical stuff. Can you recommend a book that would be a good introduction to marine electrical? Thanks

Robert H Andrew

John, I’ve been reading your books on electrical systems along with the many comments as I think I may need to make some changes to my system (4 6V Trojan T-105s in a 12V configuration, about 450 AH total, 100 A alternator and 200 W solar). Coastal cruising primarily, but I am finding that I need to run the engine way more than I would like (2+ hours a day) .

My first question relates to solar – you talk about 200 W of output going into the battery resulting in 16.67 A. The spec on my panels says that output voltage is 17.5 V and so the maximum current output is around 11 A. I asked a manufacturer’s rep how that would relate to charging a 12 V system and he said you just look at the current (11 A) and how long the charging time is. This would imply quite a bit less going into the battery than 200 W I think. It does not make sense to me and perhaps this rep isn’t really all that knowledgeable. I would be interested in your opinion.

My second question relates to alternator output (and if you address this somewhere else, I apologize and please direct me there). I rarely get more than 60 A output from my 100 A alternator – I realize this could be a regulator issue, but couldn’t it also be related to the RPM of the alternator, which is determined by the pulley configuration? Since the engine will run over a wide range of RPM that has to be a factor in the design of the charging system I would think, but I’ve never seen any discussion of that.

Appreciate your thoughts as always.

Bob Andrew

Robert H Andrew

John, thanks for your helpful info as always. I’ve now read much more about the ins and outs of solar chargers and I think the way forward is clear. Regarding the alternator, I have more specs on what the output should be and you are right that it cannot be the RPM (output is indicated to be 35A at engine idle and 90A at 2000 RPM) . I have spoken with the engine reps (Beta Marine) and while they have been very helpful on engine related issues, I am not getting anywhere so far with the alternator/regulator situation. I think it’s a black box to them and all they are telling me is that batteries can’t be charged too quickly, but at 50% capacity and resting voltage of 12V I think I should get close to the maximum alternator output properly regulated, not just 60A (and declining from there after 10-15 minutes running)? So, I may have to look elsewhere to get this right, Looking over your info on this topic and all the comments it seems that Balmar is the only real supplier of the type of regulator that is needed. Are there others that I have missed? In case it would be of any use, here is a link to a pdf on the Beta alternator set up:

Thanks as always for your input. Bob Andrew

Roger Walters

In this chapter, you use the term “shunt” but don’t explain what a “Shunt” is or does. You also say it is usually on the negative side?? Could you clear this up for me?

Ernest E Vogelsinger

Hi RWalters, there is a good explanation on shunts on wikipedia:
Basically a shunt is a means to measure current entering or leaving the battery bank to always have a most realistic image of your energy balance.
Technically it is done using a resistor causing a (very small) voltage drop, and using Ohms law you can calculate the amperage (current going in/out) from this. The backside of a shunt is that it uses energy by itself, but normally quite below 1 watt.

Ee Kiat Goh

Hi John, thanks for your guidiance through your webpage. I am beginning to understand this complex subject better. On this point under heading Naming Names, “And started acceptance charging in which both regulators keep the voltage at 14.4 and the amps drop off as the battery’s internal resistance increases.” Now, what determines how much amp is provided by the alternator and solar panels at the acceptance stage? Is there a way to maximize the full charge of the solar panel at this stage?

Ee Kiat Goh

Hi John, sorry my question was not clear. I am not trying to increase the charge rate. Lets try again. At acceptance stage. Both the alternator and solar maintains the voltage at 14.4V. The alternator has an output of 100A and the solar has an output of 16.7A. However the battery only will accept 100A (say, the internal resistance has reached a stage that dictates that). Is there a way that I can have the solar provides 16.7A and the alternator provides 83.3A totalling 100A. That will make the alternator work less and the Solar out maximized for the rest of the charging.