Do I Need A Solar Charge Controller?

I get asked this question quite often and the answer is almost always a resounding, yes. There are always caveats to everything however.

Last summer I finally had the opportunity to set this up for demonstration with a small 12W solar panel and two different battery banks. It then took me a full nine months to get this article up on Marine-How-To. Sheesh, sorry for slacking.

The Battery Banks:

  • Bank #1 = 220Ah Lifeline 6V AGM
  • Bank #2 = 125 Ah Marine Maxx Group 31 Flooded Lead Acid
  • Solar Panel = 12W

Both battery banks were charged with the 12W panel and a Morningstar PWM solar controller until they were full.

The controller was then removed and the panel allowed to feed the banks unregulated. The solar panel was left flat, not angled towards the sun, just like it would be on a boat.

Getting these banks full did not happen over night, and does not happen in short order. I purposely drew each bank down to approx 60% SOC before doing this. With batteries full the solar controller was removed from the circuit and the small 12W panel eventually brought both banks over 15V.

  • The panel is just 10% of the 125Ah bank, in wattage
  • The panel is just 5.5% of the 220Ah bank, in wattage

A 12W Solar Panel Over Charging a 125Ah Deep Cycle & a 220Ah Deep Cycle Battery:

As can be seen even a diminutive 12W solar panel can push a 220Ah bank of Lifeline AGM batteries well over 15 volts!

Do you need a solar controller? The answer is almost always a resounding YES!

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, as shown, is set for 14.4V and the batteries were allowed to become full, by holding 14.4V longer that one should. Once full, like an unregulated solar panel can do, the power supply/chargers accepted current at 14.4V was just 0.1A. In other-words all this bank needed, in order to not over-shoot 14.4V, was just 0.1A of charge current. If your solar panel can produce more than 0.1A, as can be seen here, you can over-charge even a massive 400Ah bank such as these Trojan L-16’s..

In the video below I have two well used group 27 batteries, in parallel, consisting of 160 Ah’s at the 20 hour rate (when new). The batteries had been recently equalized, specific gravity checked and impedance tested. They were in decent 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 charge acceptance rate or CAR or needed to maintain 14.4V is just 0.2A.

Two tenths of an amp is all the current these batteries can accept when full and limiting voltage to 14.4V, without over shooting 14.4V.

However once full they should be reduced to a float voltage and not held at 14.4V!

These batteries will remain taking this 0.1A – 0.2A for days or weeks at a time if given the 14.4 volts to do so. Doing this is not good for them. With an accurate enough charger or power supply, that can deliver steady mA current levels, they will accept 0.02 – 0.08A continuously at a 13.6V float voltage.

Most larger switch-mode chargers don’t have the low current resolution accuracy to do this so they “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.

This charge acceptance rate or CAR is a far cry from the often misguided and incorrect information spread around the net that says; “A full battery will take 2% of its Ah capacity in charge current indefinitely.“.

This information is incorrect and likely stems from resetting battery monitors when a bank is taking less than 2% of its Ah capacity in charging current. It does not mean the battery is 100% full at 2% of capacity in CAR, it just means that this is a good enough spot on a cruising boat to call full enough for resetting a battery monitor.


The general wisdom floating around the net, and I have no idea where it stems from, is that a battery bank will accept 2% of its Ah rating when full and will do so indefinitely. THIS IS COMPLETE BUNK!

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

AGM batteries, such as those made by Lifeline & Odyssey, are considered full when the CAR, at absorption voltage, drops to 0.5% of Ah capacity. (Source Lifeline Technical Manual & K. Jana EnerSys/Odyssey Battery)

AGM batteries made by Deka / East Penn, are considered full when the CAR, at absorption voltage (14.4 -14.6V) , drops to 0.3% of Ah capacity.  For a 105 Ah group 31 AGM this is 0.315A for the battery to be considered full (Source R. Jones East Penn / Deka)

Again, this is not chock-full like unregulated solar can do 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 readers 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?

Sure, in some cases where batteries are used or cycled daily, or even sometimes every third or fourth day, this unregulated charging could 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 become a dangerous situation. Battery health can suffer and this misguided 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 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 and far beyond where it where it should be, which is a 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. It should be noted that I have had five customers now destroy banks using the misguided advice they found on the net regarding controller-less solar.

  • A bass boat flooded trolling battery = FRIED
  • A 6V golf cart bank on a sailboat = FRIED
  • A 440 Ah AGM  trawler battery bank = FRIED
  • A 315Ah flooded bank on a power boat = FRIED
  • A 210 Ah AGM bank on a sailboat = FRIED

Every one of these panels was smaller than the West Marine Advisor advice and smaller than the 10% of Ah capacity 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 AGM bank to over 15V. Not good.

Solar Panel Voltages:

12V nominal solar 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. This is why they make and sell solar controllers to limit the voltage of the PV array to the batteries safe level.


The quote below was taken on 8/11/2012.


“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 the West Advisor advice into watts:

  • The West Advisor is suggesting that approx 25% in watts/Ah capacity is safe
  • The West Advisor is suggesting that unless you have “two or more large panels” connected to your batteries you will be safe without a controller

What are “large panels“?

Who defines “large panels“?

Unfortunately many boaters put a lot of trust in the West Marine Advisor articles. Generally speaking they are very 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! He is the 6V golf cart batteries on a sailboat from above. Thanks West Marine….. Not…. (head bonk)

While the difference from 0.2A, what the battery bank in the video 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 needs 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 to 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

Of course 10% of 160Ah in wattage would be a 16W panel. A 16W panel could produce nearly 1A of charge current which is even more than a 10W panel.

In reality 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.

PHOTO: A screen capture taken from the West Advisor section of the West Marine web site on 4/20/2015. Yes, they are still giving this HORRIBLE advice…. D’oh….

Did you think I was kidding about this advice being all over the map…?

  • West Marine = 1.5% in current of the 20 hour rate is safe
  • Nigel Calder = 0.5% in current of the 20 hour rate is safe
  • Don Casey = 0.3% in current of the 20 hour rate is safe

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.

Let’s review the banks we’ve seen in this article using Don Casey’s “0.3% in current is safe“.

  • 400Ah Flooded Bank = 0.1A to maintain 14.4V – 0.3% = 1.2A Panel
  • 220Ah AGM Bank = .7A to exceed 15.2V – 0.3% = .66A Panel
  • 125Ah Flooded Bank = .7A to exceed 15.5V – 0.3% = 0.38A Panel
  • 160Ah Flooded Bank = .6A to achieve 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.

* If your panel can actually charge your batteries, IT NEEDS A CONTROLLER especially if it will ever be left unattended.

Destroyed 6V Bank

As I previously mentioned a customer destroyed his 6V bank in one winter. When I found the batteries in the spring the positive posts & case were all heaved up and distorted. The batteries had almost no water in them, and when filled, would not take a charge and hold it.

In the fall these batteries had passed all tests with flying colors, the cases were not distorted and they had been full of electrolyte. I charged, equalized & disconnected them then left them for the winters nap. The owner came aboard a few weeks later and connected a small controller-less solar panel after reading the West Marine article. He murdered his batteries in the dead of winter in Maine with controller-less solar.

Oops, a $500.00 mistake!

Sorry for the rant….. (wink)

As always, don’t just take my word for it, do your own research or better yet set up the same test I have done and you will see it for yourself…

Be safe, and be kind to your batteries!