As I wrote in the first post in this series on the care and feeding of batteries, and particularly AGM batteries, on voyaging live-aboard sailboats, charging them properly is complicated by the fact that some, perhaps most, battery chargers are, despite the claims made for them, not capable of charging any battery properly. Or at least not without a lot of user intervention.
The basic problem is that batteries are fully charged only when the current (amperage) that the battery is absorbing at the acceptance voltage (typically about 14.4 volts) reaches 1-2% of the battery’s total capacity. In other words, a typical 8D AGM battery is fully charged when the charge current drops below about 2.5 amps.
A Dirty Little Secret
But the dirty little secret of AC chargers is that, while they can monitor how many amps they are supplying at their output, if you are using any of those amps for anything other than charging (as you almost certainly will be on a cruising live-aboard sailboat) the charger has no way to know what part of its output current the battery is getting and what part is powering loads (say lights). Therefore there is no way for the charger to detect when the battery is fully charged.
The practical result of this is that if you have any appreciable loads on while charging, the charger will stay at the acceptance voltage (about 14.4 volts) long after the battery is fully charged, instead of dropping to the correct float charge rate (about 13.5 volts).
Don’t underestimate this problem, particularly if your batteries are sealed like AGM or gell cells. We are pretty sure that the early demise of one set of our AGM batteries was because we, when working on the boat while she was hauled and on shore power, over a cold winter, left the Espar diesel heater on for weeks. This meant that the batteries were being overcharged whenever the heater ran—probably near a hundred hours over that winter.
A Real World Work Around
On Morgan’s Cloud we now work around this problem by keeping our two chargers (see the last post in this series for our system) directly connected to the batteries on and in Bulk/Absorption/Float mode only until the batteries are fully charged. A state that we manually determine using our Link 2000, which accurately measures the actual amps going into the batteries. Once we see the actual charge amps drop below 2.3 amps on each battery (1% of capacity), we turn off the direct connect chargers and turn on our third charger which we have set in fixed mode (13.5 volts).
Wait, its gets worse. Our Xantrex chargers have a graph in their manual showing that they will supply their full output for as long as the voltage remains below the absorption level, as they should. But they don’t, or at least ours don’t. As the battery voltage rises during charging, their output tails off way before the absorption voltage is reached.
This is the third instance (see the last post in the series for the other two) where a product supplied to us by Xantrex has not performed as specified, so we can’t recommend Xantrex products.
Tips For Designing An AC Charging System:
- See if you can find a charger that has a method to measure actual amps going into the battery. This requires a shunt on the battery input. It is interesting to note that the old Heart Interface/Link 2000 system, once sold by Xantrex, had exactly this capability. The gotcha is that, at least as far as I know, the Link only works with the older inverter chargers, which are not true sine wave and don’t come with the temperature sensors required for AGM and gell batteries—Catch 22.
- Failing the above, you need to set up a system, like ours, that allows you to both monitor the actual amps going into the battery, and force the charger into float mode when they are fully charged.
- Make sure that the charger you buy really does provide its full output (amperage) until the voltage at the battery reaches adsorption (typically about 14.4 volts) since, if it does not, charging will take longer than it should. Worse still, testing at LifeLine Batteries has shown that their batteries actually last a shorter time when charged at low rates.
- Install a system that includes actual temperature monitoring by the chargers at each battery. Not only will this make all your batteries last longer, you need this feature to allow safe equalisation, particularly of AGM batteries.
So you think that’s a pain in the neck. Wait till you find out what this dirty little secret means for alternator regulators. That will be the next post.
Have a question, a better idea of how to manage this problem, or best of all, experience with a charger that does not exhibit it? Please leave a comment.
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{ 17 comments… read them below or add one }
Good morning John, the storm went East of us here in Stonnington. I see you will get the full brunt of what is left of Earl. Just rain here, wish you the same. I’ve continued my practice of lead/lagging my two house banks even though plugged into the boatyard. I have the same chargers as you and recently completed conditioning of all four banks that have these dedicated chargers. I experienced some of the same frustrating revelations as you. I can isolate my loads and condition banks while the batteries are at rest so it wasn’t quite as challenging as you write. Justin instructs us to condition at 15.5 volts (room temp) for eight hours. The chargers as you know cycle off at six hours so one must set the alarm and re-introduce the cycle for two more hours. Also my chargers must be programed at 15.7 volts because three of the four held that voltage for the cycle even though the battery temperatures varied ten degrees (F) or so. My battery temperature didn’t rise appreciably so the pre-programmed conditioning voltage was a non issue. One charger refused to enter conditioning mode even when fooled (flooded batt pos), actually it adopted the mode but refused to come above 13.7VDC. I replaced the charger with the spare and all is well again. I agree with your idea that this is an expensive process when away from the dock. I can expect the generator to run 16 hrs or more for this monthly exercise for each house bank. I do not count amps as you know but rather rely on voltage to determine battery condition. I think the amp counting is too complicated and subject to error and it’s one more thing for this old man to manage.
I look forward to your research on alternator charging as this may be my Achilles heel here on Sea Return. We do not use a three step regulator and charge every battery through a Mastervolt battery isolator, hitting all with as much as 14.2 vdc. For the house bank we keep the switch on all while alternator charging because we charge through the switch for the combined house bank. Not all the banks are under load so we may be heating some more than we should but everything stays topped up while underway. With the new engine install we plan to place two 90A alternators on the engine feeding that same Mastervolt battery combiner simultaneously. This deviates from my long time practice of avoiding belt drives but David (ace machinist at Billings) will engineer the two dual belt drives which should be a durable work of art. I also believe in not combining components in a single case so that will begin to explain the two sine wave, one square wave, inverters and four Statpower and one Newmar charger. Good luck with your continued research we should all benefit in the end.
Hi Bob,
The eye went through here, but only as a tropical storm and no damage done on MC.
Great comment, thanks again for sharing your experience.
We elected to use a single larger alternator and carry a spare to keep the clutter on the engine down.
The single alternator charges all the batteries through an automated solenoid that gangs them all together once the voltage is over 13.5. This always worked well on the old engine.
I would still recommend a digital amp meter to measure the amount of charge going into the batteries as the voltage will come up to 14.4 long before the batteries are fully charged. While our meter does count amps but, as you say, not terribly accurately, the big payoff is that it accurately measures the actual amount going into the battery.
Dear John,
Getting lots of practice for storm prep here on Cape Cod.
Not complaining!
I guess I may have missed something in your battery discussion. I have two smaller Rolls. I use an inexpensive Sears 110 charger. It tells me what the outgoing current is. Couldn’t you isolate one battery at a time to charge and use the others to run the ship until that one was fully charged then switch to the next…
Very much enjoy your site.
While it is on our minds, it might be a good time to review hurricane prep. New England Ropes has a new mooring system utilizing Endura line. They call it the Cyclone Mooring. The Endura line attaches to the vessel and the usual nylon line at the other end which attaches to the mooring ball.
Cheers, Eric
Hi Eric and Sue,
We will be doing more on hurricane prep later in the month.
Yes, you could do as you suggest on the batteries, but you would be constantly cycling the batteries while on shorepower and using up battery lifetime in so doing.
With our system, detailed above, we bring all the batteries up to fully charged as soon as we plug in, and then switch one of our three chargers to float at 13.5 volts and turn the other two off. This keeps all the batteries fully charged without overcharging and gassing off electrolyte, which is a finite resource in an AGM battery.
Hi, Have you looked at ambient temprature ?
We are getting ready to go to the Arctic with a 24v only boat and would suggest keeping batteries warm with a diesel heating system. How say?
David
david@drlogistics.org
Hi David,
We have never had a problem with low battery capacity due to cold. But then, generally, we don’t let the temp in the boat go below 50F, more for the sake of the crew than the batteries!
However, you do want to make sure that all your charging systems are automatically temperature compensated since when the batteries are cold you need to use a higher voltage to properly charge them.
GPL-L16T Marine Battery, I plan on using 2 of these 6volt bats on my 38 year old 20′ Ensenada. I am going to spend a year or two on the ICW starting next year 2011 in May.
I was wanting to know what is the best way to charge these bats as a 12 v system or isolate and charge as 6 v.
These are 400 amp. hr bats each. Will the system be 800 amp when serried to have my 12 volts? Also, I only have room for one solar panel, I’m thinking 235 watts, what kind of charge controller will have a 3 step charge control for AGM batteries?
Any info on this set up will help.
Thanks Rick
Hi Rick,
To answer your questions:
1). It will be best to install the batteries and charge them in series at 12 volts.
2). The bank will be 400 amp hours at 12 volts.
3). I don’t have any first hand experience with controllers for solar panels. Anyone else out the able to help?
Hi John,
I’ve installed a new Mastervolt based system on our new boat. This has the advantage that it ties all chargers, inverters, shunts etc together with (optional) remote control panel(s). The trick here is that you can tell the charger to interface with the shunt so that it knows how much charge actually goes into the battery.
Maybe this is a solution for your woes.
Hi Kees,
Yes we looked at Mastervolt. As you say, good equipment. The problem is that their 115 volt chargers top out at 50 amps, so we would need three of them–ouch!
We still think that the world needs a really good three stage charger that can output a total of say 150 amps at 12 volts, senses charge current from a shunt and does not include an inverter.
Take a look at Ample Power systems in Seattle. They have 55 amp chargers that can be stacked (they also provide a rack) for a total of 165 amps charging capacity. Their Smart Charge Manager and Alternator Regulator also seem to do all the things you are talking about: temperature regulated charging and equalizing. Check it out. Love to hear what you think.
Just to add a good source, I have a Magnum charger/inverter, an optional shunt for monitoring the battery is available, ME-BMK. I don’t have it yet but will be looking into it more. If I understand this discussion right; putting the shunt in at the battery (before any loads) the charger will have accurate data for controlling the charging cycle.
Hi Andy,
Thanks for the pointer. I have heard good things about the Magnum and you are right, putting the shunt in should allow the charger to properly sense when the batteries reach full charge.
My problem with the Magnum is that they only make inverter/chargers and I think that this is just putting too many eggs in one basket. I really prefer to see the inverter and charger(s) as two separate units. See this post for why.
Hi John
Having had yet another round of battery replacements (a saga similar to your own), i did some searching and found your site this time. You have some interesting points here, especially how the TrueCharge does not know what is actually going to the battery (the net charge of its output). I have been studying my own and realized the same thing, also that the charging profile for AGM is defective and drops amperage too early. Originally i thought it was due to the defective battery in the bank, but having read your article, clearly this is a wider issue.
So anyway the reason i am posting a comment, is that i am considering what to do next in my set up. Something has to change if i do not want another set of anchor weights….
So i have had some thoughts that i am going to put into practice which should alleviate the problems.
Firstly regarding the TrueCharge(ers). As the main load on the battery bank when being charged by them is the house trunk circuit itself, i am considering the following to get the charger to work correctly:
Re-route the circuit to run through a high amperage DPDT 12v relay, leaving it on the NC side as default. The input to NC side being the house bank, via existing distribution point. Now the interesting part. Wire the NO side of the relay to have a dedicated TrueCharge programmed to output fixed 13.5Vdc as the input, and the house circuit again as the output. Wire the coil to a relay controlled by the actual charging TrueCharge (so a 120Vac relay, switching a dedicated low amp 12Vdc control signal to the large relay).
The net effect is to simultaneously switch the house load to a power supply when the charging sequence starts and switch it back again whenever the charging TrueCharge goes offline.
This provides an automated way to ensure correct charging (so you do not have to muck the counting Amps input) and guarantees a correct charge (well sort of, certainly better than before).
Now for myself i am integrating this with a primary solar system, which makes the switching simpler for me. However as you have a 3rd TrueCharge it is on the other hand simpler for you (i do not have a spare one, so have to do some other reconfiguring to make this work in my case).
Anyway i thought i would offer this up to you and see if the idea might help you, and also let you know if you want further information or ideas to email me directly. Also as a way of saying thank you for being so generous with your experiences.
In addition to the sort of problems you have outlined i also have to solve how to stop the load on the batteries from drawing down the voltage too quickly when the solar system switches to divert. In my opinion the rapid voltage drop caused by the load is being misread by the solar regulator as a battery needing more charge. So solving this problem led me to a more integrated solution with the TrueChargers, basically as i have outlined above.
anyway best of luck, and stay warm
David
Hi David,
An interesting idea on the TrueChargers. However, it is a bit more complicated than I like for an offshore sailboat. Relays are notorious for failing around salt air.
The other issue is that your system, as I understand it, would mix 12 volts and 120 volts on the same relay and that makes me really nervous.
If it were me, I would either get a new charger(s) from Ample Power or Mastervolt with a shunt and controller that does it right or do as we do and go with the cheapskate (comparatively) option and install another TrueCharger.
Hi John
Well i designed a little more elaborate system as my primary power source is solar/wind. So i already have a 12Vdc based switching system to control the various aspects of charging. Therefore adding another relay to shift the house load during charging from solar/truecharge/ or engine is easy. I am in the middle of doing it now in fact. Certainly a much cheaper solution than a new shunt based charger.
I use the NTE R04 series high amperage relays for large loads. They have been in operation for 5 years now, must switch hundreds of time a day when the batteries are charged. I am a long term cruiser, been on the hook for 9 years now. I have another set that have been in place for a different purpose for 10 years, no problems with them either. No rust on any of them either.
Unlike the small “automotive” style relays, these are durable and long lasting, even in the engine room near thru holes.
Anyway it was just a suggestion to give you a totally automated way to achieve your goal. I will let you know if i find anything new about it.
As for switching a 12Vdc load with a 120Vac coil, to be fair that is what a lot of systems on your boat do, including the TrueCharge and probably your fridge and inverters. Just seal the relay in a suitable conduit box and clearly mark the cables, black/white for AC and perhaps Orange or Purple as the low voltage slave signal to divert the load. Run that to a separate area to do the actual 12V work.
I already have one as on the rare occasions i use the TrueCharge it must automatically divert the solar and wind generated charge or the batteries would be ruined. I learned quickly that it is too risky to have to remember to turn off all the stuff as i do it so infrequently. This way the job is done correctly every time.
Anyway enjoy the cruising life, i certainly do
David
Hi John,
I had built up a DC system on my Endeavour 35 OEM using Lifeline GPL-4CT’s for the house bank and a 27 as a starter battery system, upgraded to a Truecharge 40 battery charger w/bat. temp sensor, a HO alternator w/3 stage regulator, and Echo charger 6 years ago. At 4 1/2 years the house bank showed a loss of capacity which I felt was short for life expectancy.
After conversing with Xantrex and Lifeline my conclusion was that the batteries were undercharged and have being pursuing this line as I rework the charging methods and settings.
I am stuck where the boat sits in its slip a month at a time with the charger in float mode and the voltage at 13.2vdc and the temperature in the 30-50 deg range. It would seem that there is enough time to top up the batteries to 100%. What seems absent is whether the battery charger actually does temperature compensation as the temperature drops below 70 deg. at the battery. Does anyone have any data that the temp. probe for the Truecharge 40 works going lower? All that I can gather is that the temp probe is for hi-temp thermal shut down.
Jim