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Compass Marine How To | all galleries >> Welcome To >> Do I Need A Solar Charge Controller ?? > 400Ah Bank Just 0.1A To Maintain 14.4V !!!
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400Ah Bank Just 0.1A To Maintain 14.4V !!!

PHOTO: In this photo I have two Trojan L16 6V batteries wired in series for a 12V 400Ah bank. The charger is set for 14.4V and the batteries were allowed to become chock full. Once full all they needed, in order to not over shoot 14.4V, is 0.1A...

In the video below I have two group 27 batteries, in parallel, consisting of 160 Ah's at the 20 hour rate. The batteries had been recently equalized, specific gravity checked and tested. They were in excellent health for their age. Once equalized and fully charged they were left in float mode over night. Room temp in my shop was about 72F. These batteries were used but represent a good real world scenario for batteries that have been in use.

As you will see in the video below the current accepted or needed to maintain 14.4V, on these full batteries is just 0.2A.


Two tenths of an amp is all these batteries will take when this full at 14.4V without over shooting 14.4V. Remember though, once full, they should be reduced to a float voltage and not held at 14.4V.

These batteries will taking this 0.1A - 0.2A for a few days days if given the 14.4 volts to do so but if held long enough the current needed to maintain 14,4V can drop even lower. Of course by now we are into severly over-charging the batteries. Maintaning a battery, that is already full, for multiple hours per day at absorption level, is simply not good for the batteries.

With an accurate enough charger or power supply, that can deliver steady mA current levels, they will accept roughly 0.02 - 0.08A at 13.6V or a 13.6V float voltage.

Most switch-mode chargers don't have the low amperage accuracy to do this so they will begin to "pulse" on and off when they get to low current levels. For older feroresonant chargers they begin pulsing on and off at much higher currents to maintain a voltage.

The actual acceptance rate of a full battery is a far cry from the often misguided and incorrect information spread around the net that;

"A full battery will accept 2% of its capacity indefinitely."

The above statement is simply misguided bovine-dung. This wives tale likely stems from resetting Ah counting battery monitors when a bank is accepting less than 2% of "C" or 2% of bank Ah capacity. It never meant the battery is 100% full at 2% of Ah capacity in current or that it would keep accepting that much current indefinitely. All it meant was that for an Ah counter, this is a good enough spot, on a boat or other off-grid application, to deem the bank "full". It was simply meant as a resetting-point an Ah counting battery monitors.


The general wisdom floating around the net is that a battery bank will accept 2% of its Ah rating, when full, and can do so indefinitely. This = FALSE.

What this guidance really means is that with some deep cycle flooded batteries there is really no need to continue charging, at absorption voltages, once the bank is accepting less than 2% of its Ah capacity in charge current, at absorption voltage. It does not mean this all the bank will continue to accept, in current, when 100% full..

AGM batteries, such as those made by Lifeline, are considered full when the net accepted current, at absorption voltage, drops to 0.5% of Ah capacity. Other manufacturers such as Odyssey & Deka/East Penn claim that 0.3% in accepted current would be a 100% full battery at absorption voltage if using absorption return-amperage as a definition of "full". Again, this is not chock-full, just full enough to stop pushing them at absorption levels and switch to a float voltage.

I suspect this 2% misunderstanding is where companies like West Marine advise that a solar panel with 1.5% of amp hour capacity in current potential does not need a controller. D'oh...! Bad advice....

Sadly there is a lot of information out there suggesting that a solar panel of 10% of Ah capacity, in wattage (10W panel for a 100Ah battery), or 1.5% of Ah capacity in amperage (1.5A panel for a 100Ah battery) does not need a charge controller?

It's All in How You Use the Bank:

Sure, in some cases where batteries are used or cycled daily, or even sometimes every third or fourth day, this unregulated charging can work and could be a sort of truth.

Conversely, when batteries are left to sit for weeks at a time, charged via a solar panel with no voltage regulation, it can actually become a dangerous situation. Battery health can suffer and this incomplete advice quickly becomes an untruth.

Many boats sit for days or multiple weeks between use with all loads turned OFF. In these situations the batteries can still get to 100% full even with just a 10% of Ah capacity panel in wattage, and can certainly over-charge with 1.5% of Ah capacity in panel amperage. Remember the 400Ah Trojan bank pictured above needed just 0.1A to maintain 14.4V.

1.5% of Ah capacity in current is a 6A panel for that 400Ah bank! When the the batteries eventually get to "full", and they eventually will with solar, they will have the voltage pushed well beyond the safe zone or far beyond where it where it should be. After a suitable absorption period batteries should always drop to safe float voltage level.

Having charged hundreds & hundreds of batteries in my shop, and watched the accepted current at varying voltages, these comments always made me cringe. The other day the question came up again and I decided to use my bench top power supply to illustrate this.

It should be noted that I have had four (EDIT: scratch that now FIVE) customers destroy banks using the misguided advice they found on the net. A bass boat trolling battery = cooked, a 6V golf cart bank = cooked, an expensive AGM battery bank = cooked, a 315Ah bank on a power boat = cooked & a group 24 battery on a center console.. Every one of these panels was smaller than the West Marine advice and smaller than the 10% of Ah's in wattage yet the batteries were still MURDERED by controller-less solar panels.

Even a diminutive 10W solar panel can produce about 0.59 -0.6A in good sun. We already saw a 12W panel producing as much as 0.72A and pushing a 220Ah bank of Lifeline AGM batteries to over 15V.. Ouch!!

12V nominal panels have voltages from 16V to 18+V. Even with a small panel if the bank is left on charge for multiple days or weeks at a time, with no loads on, as is the case with many boats, you can over charge your batteries if you are not careful.


(The quote below is as of 8/11/2012 they will likely change that statement when they realize they were giving potentially destructive information.)

QUOTE = WEST MARINE ADVISOR: "Do you need a charge controller?

As a general rule panels that produce less than 1.5% of a battery’s rated capacity in amp hours don’t require regulation. This means that a 1.5A panel is the largest you should use without a regulator on a 100-amp-hour battery. Regulators should generally be used any time you have two or more large panels connected to your batteries."

If we translate that advice to watts:

*West Advisor is suggesting that approx 25% in watts/Ah capacity is safe

*They are also saying that unless you have "two or more large panels" connected to your batteries you will be safe without a controller??

What is "large"? Who defines "large"?

Unfortunately many boaters put a lot of trust in the West Marine Advisor articles. generally speaking they are pretty good and fairly well researched. Sometimes they just miss the mark. Boat owners often blindly trust what the West Advisor articles say, and then do as they say.. I know this for sure because one of my customers did exactly this. Guess what? HE DESTROYED HIS BATTERIES!!! Thanks West Marine..... (head bonk)

While the difference from 0.2A, what the battery bank above is willing to accept at 14.4V, to 0.6A, what a 10W panel is capable of, may not sound like much, the difference between what the batteries actually need at FLOAT, which is in the range of 0.02A to 0.08A, this increase can be quite a dramatic increase. It is actually a HUGE increase, and a potentially BATTERY DESTROYING increase...

A 10W panel can produce, about 0.6A. This is actually a 637% increase in current from a float current of 0.08A that the batteries "accept" to maintain 13.6V. When an unregulated panel gets the batteries approach full it will simply cause the voltage to continue rising. Holding them there will cause electrolyte to gas off and can also cause premature positive plate erosion..


The general guidelines are sometimes stated that; a panel wattage of 10% of Ah capacity or 10% of "C" or less would not need a controller. 10% of 160Ah would be a 16W panel but a 16W panel could produce nearly 1A of charge current even more than a 10W panel. Of course the "suggestions" for controller-less solar are all over the map so who is to know what to believe? This is why I felt compelled to make these videos and share real world examples not some pulled from thin air or shoot from the hip advice. I have absolutely no clue how the West Advisor screwed the pooch so badly on that advice, but they did.

EDIT: 3/6/2013 - The West Marine catalog just came and the 1.5% of Ah capacity in panel current still stands.

You think I am kidding about the advice being all over the map...?

*West Marine = 1.5% in current of the 20 hour rate

*Nigel Calder = 0.5% in current of the 20 hour rate

*Don Casey = 0.3% in current of the 20 hour rate

Everywhere you turn on the net there are formula's for controller-less solar. They rarely if ever agree.. Who are you to believe? West Marine? Calder? Casey? Norther Arizona Wind Sun etc. etc. etc.........? I really wonder if any of them has completed the simple experiments I have just shown?

Kudos to Don Casey for getting it the closest...! he is the official winner of a pat on the back Still, his advice may be a bit off the mark on some banks.

*400Ah Flooded Bank = 0.1A to maintain 14.4V - 0.3% = 1.2A Panel

*220Ah AGM Bank = .7A to go over 15.2V - 0.3% = .66A Panel

*125Ah Flooded Bank = .7A to go over 15.5V - 0.3% = 0.38A Panel

*160Ah Flooded Bank = .6A to hit 15.0V - 0.3% = 0.5A Panel

The best guidance I can give on the subject is this:

If your panel really does not need a controller it is simply a big fat waste of money and is really capable of doing little to keep your bank healthy anyway.

If your panel can do something IT NEEDS A CONTROLLER if it will ever be left unattended.

Sorry for the rant..... (wink)

As always, don't just take my word for it, do your own research, set up the same test I have done and you will see it for yourself... Be safe, and be kind to your batteries!

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Eco-Ants 28-Mar-2017 20:10
Thanks for the very well documented article.
The internet is full of unproven opinions and wacky ideas, it's great to find real facts correctly documented.
Thanks again,