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Charging 12V Batteries: Best Practices

Alan Knight — Sun, 11 Sep 2011 18:33:26 +0000

This is a guest article written by Alan Knight, one of the more frequent contributors to this website.

What Happens to Batteries when you Overcharge them?

Do not attempt to charge a 12v battery with anything other than a 12v charger, whether it is a flooded unit, Sealed Lead Acid (SLA), gel or AGM. SLA and AGM batteries require a charge rate of about 1/10th of its Amp-hour rating. If you charge at a higher rate the battery can build up excessive heat and start to ruin.

In a Gel unit, airways will form over time in the gel silica. The gel hardens when it gets too hot and once it hardens it has a reduced ability to transform soft lead sulphate back to sulphuric acid thus reducing the surface area that the plates contact with the electrolyte. This means less of the plate can take part in the discharge - recharge chemical reaction.

An AGM battery will over heat and expel the electrolyte. The electrolyte transforms into gas and is able to escape through the one way valve and reduce the unit to scrap as it boils due to no liquid to cool it down.

A flooded unit will also overheat and buckle the end plates. This is clearly visible through a bulging on the side of the unit, although this will take a few months of constant over charging to achieve as batteries charging voltages are graded to a minimum and not a maximum.

What Type of Battery Charger Should You Use?

Use a three or five stage charger which incorporates an automatic 15v-16v equalization charge and your battery will last for years longer than its warranty. Stay away from the cheap 4-6 Amp chargers since these are unregulated and can destroy your battery very quickly.

When charging allow the electrolyte to mix by bubbling. Contrary to some literature, this is not the sign of a defective battery. It is the reason that there are vents on batteries - to allow any gas pressures that may build up during the process to escape without causing any explosions.

How to Make 36 Volts from 12 Volt Batteries

Robin — Tue, 06 Apr 2010 03:13:17 +0000

Recently George asked in the comments on our article about connecting batteries in series and parallel at the same time how he could make 36 volts from 12 volt batteries, while at the same time make the capacity (amp hours) add together so that each 12 Ah cell would combine to make at 36 Ah total battery. Here is what he wrote:

I have three 12V12Ah batteries. I need 36V36Ah.Is the following thought correct ?

I connect all the plus together, then all the minus together. The result is: in parallel I would have 12V and 36Ah.

Then I connect from first battery plus to second battery minus and from the second plus to the third battery minus. This would be series and I achieve the 36V.

Thanks for your help Robin.

And the answer is no, it doesn't work quite like that. In isolation George is correct. If all he wanted to do is make 36 Ah at 12V then his first idea would work. Three 12 volt batteries in parallel makes 12V @ 36 Ah. And then if he took those three batteries, disconnected them, and reconnected them in series like in his second idea, he would have 36V, but only at 12 Ah.

But if you need to do both at the same time, it gets a little more complex. Take a look at the article and study the diagram. In the example we needed 24 Volts and 200 Ah. We had four 12 Volt batteries, each with 100 Ah. We took two batteries in parallel to make one big battery with 12 V @ 200 Ah, did it again to make another big battery with 12 V @ 200 Ah, then took those two batteries and put them together in series to make 24 Volts at 200 Ah.

George, what you need to do is get (brace yourself) ... NINE batteries. Take three of them, put them together in parallel to make one big battery with 12 Volts and 36 Ah, then do it twice more to make 2 more BIG batteries at 12 Volts and 36 Ah. Take all three of your big batteries, and put them together in series to make a HUGE 36 volt battery, with 36 Ah. Like in the last article, I've drawn a simplified diagram that shows how to do it.

So this might seem a bit much, especially since you're only working with 36Ah in total. You're probably better off just buying three 12 volt batteries with the minimum capacity you need - you could also look into getting a 36 volt cell as well. Either way, the concept above stands. I hope this is useful to you.

24 Volt Load on an Existing 12 Volt System with a Battery Selector

Robin — Wed, 24 Mar 2010 03:39:07 +0000

We received a question from Michael Correale today asking for some help on wiring his 24 volt horn to his 12 volt boat electrical system. Here is what he wrote:

Ok, I am that guy who had to go and buy the 24 volt horn for my boat with a pre-existing 12 volt system. I am unsure how to get 24 volts to the horn switch without sending 24 volts to everything and frying a lot of $$$$ in electronics.

My idea is this: I would take a hot and ground directly from each battery and run it to the horn area. At this point I would connect the positive from one to the negative of the other leaving the other ground to be the 24 volt connections. My main issue is (considering that it will work) can the 24 volts go to the other stuff on my boat when the switch is on both batteries.

Michael, it's a good question. The answer is NO, DON'T DO IT!

If you have two batteries right now with a battery selector switch, such as a Blue Sea Systems 9001E shown in the image above, you cannot do what you're asking. The battery switch is designed so that you can choose to run 12 volts from either battery, or 12 volts from both batteries. In this case the switch connects the batteries together in parallel. The negatives on your batteries will already be connected together.

If you were to take the positive from one and the negative from the other and connect them, while at the same time the negatives from both batteries are connected (or at least both connected to ground) then you will short one of the batteries out. This will very quickly melt your battery, or melt the wires, or... melt something!

A better approach would be to install a transformer to step up the voltage to 24 volts from 12. These can be fairly cheap at an electronics store, but since you are in a marine environment it's worth getting a more expensive one designed to withstand the corrosion.

Alternatively, you could always go buy a 12 volt horn! And Michael, if you are still reading and I've somehow missed your point, clarify it below in the comments and I'll take another stab at it. Thanks for asking!

Which Battery Post Do You Connect First: Positive or Negative?

Robin — Sat, 12 Sep 2009 03:38:25 +0000

Someone came here the other day looking for the answer to this question - which battery terminal should you connect first. It's a great question. We're talking about automotive batteries, and whether you connect the positive terminal or the negative terminal first.

You've got the hood open to your car, you've just done some electrical work of some kind and the batteries disconnected. Now it's time to reconnect them, and you want to make sure you continue to work with the best and safest practices.

The Short Answer

The short answer? Connect the positive first and the negative second. This way you eliminate the risk of creating a short while you're connecting them. If you want to be sure then read below for the caveats - there are situations where this doesn't apply and/or you can damage equipment. In the meantime, for most of us, here's an explanation of why:

Short Circuits in Car Batteries are BAD!

Imagine you've connected the negative first, and now you're on to the positive side. You slip the battery clamp over the positive terminal. Maybe there's a tiny spark as your radio memory draws that tiny bit of power. But no worries, that little spark has so little current running through it that nothing happens. Next you take your half-inch wrench or 12 mm or whatever it is wherever you live. You start tightening the clamp tightly to the post. As you turn the wrench around it comes close to the body of the car and suddenly the wrench slips off the nut and slams against the body while still touching the the positive terminal.

Don't try this at home kids! If a circuit is completed (it might not due to the insulative properties of the car paint) but if the circuit is completed you'll have a great spark show on your hands, and the point of greatest heat will loose. You could melt a hole in your car body, you could melt the battery post (most likely being the softest metal), you could melt and deform your wrench, and of course you could burn yourself.

For most of us, I think we can agree this situation sucks. If you had not connected the negative terminal first, then the body would not be connected to the battery system, and the short would not have been possible.

Connection Sequence of a Car Battery

The car body is connected to the negative terminal, the negative terminal is connected to the positive terminal (and in between has harnessed the electrical potential in the battery), the positive terminal is connected to the electrical system, and each item in the system is connected to the car body. This is a complete circuit.

In the short circuit example above you actually "shortened the circuit". You cut out the electrical system, and just connected the battery straight to the body. If the negative terminal was not connected, then the circuit would have remained incomplete, and you would not have had a short.

Clear? If not, ask in the comments section below and I'll try to clarify. And the moral is connect the positive first, then connect the negative. Go ahead and slip with the wrench all you want when connecting the negative - it's already connected to the car body, and you won't create a short circuit.

The Caveats

Here are a few reasons why you might connect the positive terminal first rather than the negative terminal.

You've got a positive ground vehicle. Like an older Series Land Rover - yes you know who you are! The electrical systems in those cars are backwards to most of the rest of the cars in the world. Yes, the English had their reasons. If you've got a positive grounded car, then just do it in the opposite way.
You've got equipment that needs the ground to be connected last. There are some pieces of specialized electronics (poorly made I'll admit) that need to be connected in a certain order or they blow internal fuses. If you've got them, you probably know all about it and you also probably curse yourself for not spending a bit more on a better engineering product!

Any more? Leave a comment below if you know of any!

What is an Amp Hour and How to Calculate Battery Capacity

Robin — Wed, 26 Aug 2009 05:23:07 +0000

An Amp-Hour (ampere-hour, Ah) is a way of describing a battery's capacity - how long it will run before it is drained down. But I can be be more specific: the amp-hour rating for a given battery is the maximum amperage that can be drawn continuously until the battery is completely discharged, flat, empty, drained, dead, over a specific time period.

How Are Amp Hours Determined?

Battery manufacturers complete tests on their batteries to give them an Amp-Hour rating. A typical time period for a test is 20 hours, but it varies - batteries are tested over different periods, such as 24 hours, 75 hours, even 100 hours. But as you'll see below, a 100 Amp-hour battery tested over a 100 hour period (i.e. 1 amp drawn for 100 hours) will not have the same capacity as a 100 Amp-hour battery tested over a 20 hour period (i.e. 5 amps drawn for 20 hours). It turns out that if you drew 1 amp continuously from the 20 hour test battery, it would last for 110-120% longer than on the 100 hour test battery. I'll (try to) explain that later.

Let's Do an Example Calculation

Suppose you have a 100 Amp-hour battery, tested over a 20 hour period. 100 Amp-hours divided by 20 hours = 5 amps. That means that the battery manufacturer claims the battery can sustain a 5 amp load for 20 hours until the battery is completely dead.

That's great, but now put it into realistic terms. For starters you don't want to drain a battery to it's completely flat, empty, dead state as it will significantly reduce the life of the battery. A good rule of thumb (battery type depending) is that you should only drain a battery down to 40% of it's original capacity, or in other words you've only got 60% of the battery to use. This number should be stated by the manufacturer in the documentation.

Back to the 100 Ah, 5 Amp load example: We said you had 20 hours of time when drawing 5 amps, but in fact you only have 60% of that time, so 20 hours x 60% = 12 hours.

Up until now, what we have said is true because we have used the same amperage over the same time period at which the battery was originally tested (20 hour rate). So what if you took your 100 amp-hour battery, but wanted to draw 10 amps from it until it was at the safe discharge level (60%)?

Do the math: 100 Ah / 10 Amps x 60% = 6 hours, right?

Wrong.

The More Amperage You Draw, the Lower the Battery Capacity

That's right, the effective amp hours available drops with the more amps you draw. All of a sudden that 100 Amp-hour battery is not what it says it is. Ok, back to the numbers, lets try to keep this clear:

Using the 100 Ah, 20 hour test battery, at 5 amps draw, you get the full 100% of the battery rating (that's how the battery was tested in the first place, so you should get all 100%!). But step up the amperage to 10 Amps, and you will lower the capacity of the battery by about 10% (or 90% remaining).

That makes 100 Ah x 90% = 90 Ah / 10 Amps draw = 9 hours x 60% (max safe discharge number) = 5.4 hours. And that's quite a difference from the 6 hours you thought you had.

Where Did I get the 90%?

I'm going to stop here and leave you hanging - except to say that there's a relationship defined called Peukert's Equation, and that's where the approximate 90% comes from. The Peukert Equation quantifies the above phenomenon so that you can predict how much time you'll actually have on a battery given a specific discharge rate.

What's Next - How About A Helpful Tool?

My guess is that if you're still reading this your about ready to start figuring out how many batteries you need, and you could really use a little help with the calculation. You might have gone to the Wikipedia site above to investigate what Peukert had to say, but exponents aren't really your thing, and you can't find your calculator anyway.

Connecting Batteries in Series and Parallel at the Same Time

Robin — Fri, 14 Aug 2009 05:53:39 +0000

A few days ago Larry Amato from Amato Consulting replied to our previous article that describes how to create 24 volts from two 12 volt automotive batteries. He continued our thought process by showing that you could take four automotive batteries and connect them in both series and parallel at the same time to achieve a 24 volt battery bank with double the capacity of a single 12 volt battery.

His application was for a solar installations where capacity is usually important, along with a typical requirement for 24 volts which is a common voltage on solar panel systems.

He describes how to do it using a matrix - I had to take a pencil to write it down to follow what he was saying. Then I had to think about it for a bit, so I figured some you might have had to think about it for a bit too...hence this article. I'll try to give some more detail to break it all down. Here is what he said:

My need was 24VDC power with 200 amp hours.

I purchased 4 each 12VDC AGM 110 amp hour batteries. Picture the 4 batteries in 2 rows (R) x 2 columns (C).
1) R1C1 positive is connected to R1C2 positive.
2) R2C1 negative is connected to R2C2 negative.
3) R1C1 negative is connected to R2C1 positive.
4) R1C2 negative is connected to R2C2 positive.
5) R1C1 positive and R2C1 negative provide 24VDC power with 220 amp hours.

I take the R1C1 positive and R2C1 negative and run through a 24VDC Photovoltaic Charge/Controller which is connected Solar Panels with output to my various 24VDC devices including Programmable Controller, Instrumentation, and Spread Spectrum Radio.

Like I said, you need to draw it out. And for your benefit, I've done that (click to enlarge):

On the left is what Larry describes, and on the right is what I would have done. I've gotta say, Larry's way is WAY better. Less wires, less complex. So let's talk about it.

"My need was 24VDC power with 200 amp hours"

24 volts from 12 volt batteries: if you read our last article then you know that when you connect batteries in series, the voltages add together. So Larry took two of his batteries and connected the positive and the negatives together. Then he did it for the other pair. On the diagram, each pair is vertical - the negative of a top battery is connected to a positive of the battery directly below it. And so now there are two 24 volt battery banks, side by side.

On to the second criteria, 200 amp hours.

It's pretty tough to find an automotive battery that stores much more than 100 amp hours. In this case Larry found some 110 amp hour AGM batteries. But how do you get 200 amp hours? I describe this in detail in the last article, and if you've read it you know that if you connect batteries in parallel the amperages add together.

And so he added the amperages of the two battery banks together by connecting the positives on each bank and connecting the negatives on each bank. Presto, 100 amps plus 100 amps is 200 amps.

Lastly by taking a lead from the positive and a lead from the negative, he's got a 24 volt system, with the full capacity of 200 amp hours in reserve. On the image above you'll see what I call "our method". While it does the same thing, it's more complex, and after seeing how Larry does it, I'm converted. See if you can follow the differences - but really, they each do the same thing.

A few paragraphs ago I mentioned the AGM batteries that Larry purchased for his application. AGM battery - what is that?

What are Advanced Glass Mat Batteries?

AGM stands for Advanced Glass Mat, and is a type of deep cycle battery. Battery types is a whole other article that we need to write, but if you are reading this and you care to contribute, go ahead and leave a comment below. Hey, if you want to write a full blown article and give some good detail on what an Advanced Glass Mat battery is, go to our article submission page and do so - we come with benefits!

On to the second criteria - 24 volts. You also know from the last article that when you connect batteries in series, the voltages add together. So Larry then took his two 12 volt battery banks, and connected them together in series - in other words the negative of one bank to the positive of the other.

How to Connect Two 12 Volt Batteries in Series for 24 Volts

Robin — Sat, 08 Aug 2009 19:26:11 +0000

On our post about installing a 24 volt battery equalizer, Parker Sharpe asked in the comments section how to wire two 12 volt batteries to make 24 volts.

The Short Answer

Connect the negative terminal of one battery to the positive terminal of the second battery. You will be left with two battery terminals free of the total four. One will be positive on one battery, and one will be negative on the other battery. Take the free positive terminal and connect it to the power lead on your 24 volt device. Connect the negative to the ground connection of your 24 volt device. That's it!

The Longer Answer

It is called wiring the batteries in series, as opposed to in parallel. Here is the difference:

To connect batteries in parallel, connect the negative of each battery to the negative of the next battery. The same goes for the positive terminals - connect the positive terminals together. Here's what you get: the combined voltage of the batteries averages out, but the combined available amperage is additive. For example, two 12 volt batteries connected in parallel gives a combined average voltage of 12 volts. And if one of the batteries is capable of delivering 400 amps and the other can deliver 600 amps, the combined amperage is 1000 amps.

What does the combined average voltage mean? Here is another example. Say you have one 12 volt battery and one 6 volt battery. If you combined them in parallel you would have a combined average voltage of 12+6 / 2 = 9 volts. And most people will be connecting 12 volt automotive batteries together. So the combined average voltage is 12+12 / 2 = 12 volts. Get it?

Why Connect Batteries in Parallel?

In the examples above you keep the voltage the same (you don't normally connect batteries that are not of the same voltage), but you get more available amperage. This translates into the ability to do any of the following:

run your equipment longer (i.e. car stero)
run a more powerful piece of equipment (i.e. a large starter)
run equipment at colder temperatures (say starting your car in the middle of winter).

Why Connect Batteries in Series?

And this leads you to the next question, why connect batteries in series in the first place?

Since the voltage is additive when you connect batteries in series, you will be able to run equipment that is designed with a different voltage. The example that Parker asked about was for his 24 volt troller motor for his boat.

A lot of marine equipment is often 24 volts. And this presents a problem for boats that have a 12 volt starter on the main motor, and the owner has purchased a simple trolling motor that came off-the-shelf for 24 volts. Now you have two systems on your boat - a 12 volt main system for your main motor and boat accessories including the charging system, and a 24 volt system for running the trolling motor.

All of a sudden the whole problem gets a lot more complex. The question is how to connect batteries in series and parallel at the same time? The short answer, is no you can't. The soluton is another article.

But in the short term you can use the brute force method: buy two 12 volt batteries, connect them in series to make 24 volts, and run your troller directly from them. Eventually the troller will drain the batteries, and you'll have to quit, go to shore, hump the batteries back home, connect them to a charger to recharge them, and bring them back to the boat on your next fishing trip. What a hassle, but it would work (make sure you've still got your main motor to get home or else bring a paddle!).

How to Charge Two 12 Volt Batteries with One Charger

Robin — Sun, 18 May 2008 01:13:07 +0000

I've seen quite a few people coming to this site with search queries identical to this article's title. Since there is nothing explicitly written anywhere on how to charge two 12 volt batteries with a single 12 volt battery charger, here you go. Really, it's quite simple, but there are a few "best practices" to keep in mind.

The first thing you need to do is to take the batteries out of the vehicle. It's not actually a strict requirement, but it's a very good thing to do. You don't need to mess up your vehicle electrical system, and if you've got two batteries, who knows how they're connected, what they power, if you have a charging isolator, or other complications.

Now that the batteries are safely on your workbench, connect them together in parallel. Take a set of jumper cables and connect the positive terminal of one battery to the positive terminal of the other battery. Do the same for the two negative terminals. Remember to keep things simple - the red goes to positive, the black goes to negative.

By connecting the two batteries in parallel, we are allowing the charge between them to equalize - if one battery is charged more than the other, then the excess charge in one will be driven to the other, until the charge (measured by voltage) is exactly the same on each battery.

Be sure not to connect a completely dead battery to a fully charged battery. The amount of differential voltage can tend to send a large amount of current (amperage) to the dead battery, causing a significant amount of heat to build up. This is a bad thing and should be avoided. If you have a completely dead battery, charge it on its own before connecting the two together.

Finally you need to connect your battery charger. Now here's the trick: don't just connect the charger directly to one battery. Instead, connect the positive lead from the charger to the positive terminal on one battery, then connect the negative lead on the charger to the negative terminal on the second battery. This is a small detail, and in truth makes only a bit of difference.

Turn on the charger, and let it do its thing. Remember to select the correct amperage on your charger for you battery type (although not all chargers give you the choice), and also remember that you'll need twice the time to charge two batteries in parallel as you would need to charge a single battery.

We hope this helps - have more questions? Ask below and we'll see what we can do!

How to Repair a Car Battery

Robin — Sat, 24 Nov 2007 02:28:07 +0000

You are sitting beside your home, bed, kitchen, protection, transportation, and expedition vehicle all rolled into one, on a remote beach in the Baja. You hear a sizzling sound coming from the engine compartment. The truck has been parked for at least day. Hmm. You open the hood and take a look around. After a short inspection, you see a pile of white crystals around one of your battery trays. Hmm again, with a bit of uh-oh. You disconnect the ground lead, and the sizzling stops. Another look reveals the battery tie-down bolt has vibrated off. You've got a hole in your car battery; apparently washboard roads ARE good for something.

This happened to us - and like many places on the Baja, there was nobody around, and a walk would have taken far too long. This is how we got our truck going again.

...it's a true story, and was one of many that were less than ideal...

Let me preface this with a little "don't try this at home." What we had to do was dangerous and definitely a last resort. When you're all alone you have to be creative, but you must never sacrifice your safety. Remember expedition travel can be dangerous, and requires all of your wits and guts about you.

The Fix

Battery plastic can be welded (melted) together. Here we describe how to weld the battery hole closed.

If you have the handle to your battery, chances are it's the same type or at least compatible plastic with the battery case. The handle serves as your welding rod. We always have propane and a small torch tip with us in our box of tools, so we figured we would be able to weld up the hole.

Batteries produce hydrogen gas. Hydrogen gas is very explosive, and this is what makes the fix so dangerous. Blowing a battery up is bad for the battery, but if you're anywhere near it you can get sulphuric acid on you, which can result in serious chemical burns. The following points describe the safest way to fix a hole in your battery:

Wear protective clothing. Put on pants, long sleeves, and find some glasses or something to protect your eyes. Wear gloves.
Place the battery on the ground, with the hole facing up. Place a board or something between you and the hole, leaving just enough room so you can reach around and see what you are doing.
Clean and dry the battery around the hole with a rag.
Cut a piece of the plastic handle that is just larger than the hole.
Heat up the donor piece so that it is melted on the side that will go against the battery.
While the plastic is still hot and melted, place it over the hole. It will partially bond with the battery.
Now comes the dangerous part: you have to complete the bond. Do this by taking a section of the handle and melting the end. Quickly, lightly feather the battery around the edges of the donor piece to heat up the area. Dab the heated area with the melted handle end. You have to go back and forth a bit to keep the handle section melted, while keeping the battery plastic just hot. During this stage you run a serious risk of igniting hydrogen gas, blowing up the battery, and burning yourself.
Once you are satisfied that the hole is closed, turn off the torch and turn the battery right side up. If it's not a sealed unit, fill it up with water. Replace it in the vehicle, cross your fingers, and hope that it starts.

That's it. This fix worked for us - it's a true story, and was one of many that were less than ideal. However, it got us running again, and as soon as we came to a town with supplies we replaced the battery with a new one.

24 Volt Battery Equalizer Install

Robin — Thu, 22 Nov 2007 07:33:03 +0000

Over time, different batteries will vary slightly in their charge. The 24 volt alternator in a Land Cruiser makes sure that there is always 24 volts, but it doesn't care how that voltage is distributed. Since the system is made of two 12 volt batteries in series, the assumption is that each battery will be receiving an equal charge from the alternator. But, if one battery has a slightly higher charge, say 13 volts, then the other battery will only be receiving 11 volts. This overcharges the one battery, and undercharges the other. If this continues, eventually both batteries will be cooked.

The solution is to use a charge equalizer (battery equalizer). This is a unit that constantly works to ensure that both batteries are equal. It works on a voltage differential - if it senses a voltage difference between the two batteries, then current is directed from the higher battery to the lower battery. As the differential approaches zero, so does the current.

A charge equalizer works especially well if you are tapping 12 volts to run accessories such as a radio, fridge, or laptop. With the vehicle turned off, the accessories can run, draining the 12 volt battery. Meanwhile, the charge equalizer supplements the drain by taking charge from the high side battery to the low side one.

We bought our equalizer from G&S Cruiser Parts. This unit is made by Solar Converters, who offer excellent support. We paid $220 CAD for the 20 amp model (20 amp @ 12 volts).

Installation

There are 4 wires coming out of the box: black (grnd), white (12 volt), and red (24 volt), and an LED light that indicates the internal fuse health. The red one gets connected to the 24 volt terminal of the high side battery (where the starter lead is connected). The white one gets connected to the 12 volt terminal of the low side battery, and the ground is connected to the negative terminal of the low side battery. Connect the wires directly to the battery terminals; do not connect the ground to the frame. The unit needs to see the actual voltages coming directly from the batteries.

There are two internal fuses in the unit. One blows if the amperage going through the equalizer is too high. The other one blows if the ground wire is disconnected from the battery while either of the two hot wires are still connected. Any time I do any work on the truck, and have to disconnect a battery, the hot wires (white and red) must be unplugged from the charge equalizer.

The unit should be installed in the engine compartment at a convenient location. Inevitably the fuses will blow at some point, and you need to be able to remove the top of it to change them. I put mine on my drivers fender apron. I cut the LED wire and lengthened it to run back to the dash. I drilled a small hole to press-fit the light in.

A Note on the LED Light: There are two fuses, but the LED only indicates one of the fuse's health. This fuse is the one that blows if the ground wire is disconnected while the 12v or 24v lead are still connected to the batteries. When the LED light is off, then this fuse is blown. If the other fuse blows due to an overload of amperage there is no indication. If you suspect that it has blown, you can confirm it by turning the unit off, putting a voltmeter across each battery and measuring the voltage. Turn the unit back on, and re-check the voltage. You will likely see a small change with the unit on, and if so then the fuse is ok.

Setting the Voltage

There is a potentiometer inside the equalizer. You need to change the balance of the equalizer by setting the potentiometer. To do this, both batteries must be fully charged.

Connect the 12v lead to the battery and measure the voltage of the 24v lead to ground. It should be around 24-26 volts. Set the potentiometer to around 25.
Disconnect the 12v lead, and connect the 24v lead. Measure the 12v lead voltage. It should be half that of the 24v lead. If not, set the potentiometer so that it is.
You need to iterate this a few times until the 12v lead is half the voltage of the 24v lead.