Battery charger guide

A battery charger is not just a battery charger. There are very big differences in battery chargers. Both in how it charges, how long lifetime it has, if it fits your batteries, Capacity, roughness, and much more.

When choosing a new battery charger for your Offgrid installation (In Boat, Caravan, Motorhome or other). Then there are a lot of things you need to be aware of.
Your batteries are probably the most expensive and most important item in your Power installation. Unfortunately, they are also the most vulnerable.
Therefore, make sure that all the equipment that charges and discharges your batteries does it without damaging the batteries. It’s simply good economy.

Here I will review the things you have to have thought through and taken a decision about and there are a lot more than you would expect 🙂
Since we are Mastervolt distributor, the review will relate slightly to Mastervolt’s products, but the review is universal and it is the same that applies to all battery chargers.

Battery types and charge data

Lead / acid batteries

You need to know what type of batteries to charge. Not all battery chargers have multiple profiles to charge different battery types, but they must have it.
GEL, AGM and starter batteries (Lithium we handle a little farther down this page) should not be charged alike. Do not charge GEL and GEL batteries the same way either (the same applies to all other types of batteries).
Various manufacturers of eg. GEL batteries, have different charging data and therefore it is not enough that you set up your charger to charge a GEL battery. The charger should have exactly the data that is available for the GEL battery you are using. Of course, it is better to charge on a GEL battery in the general GEL setting than to charge in the general AGM setting, but the GEL setting is not enough!

* Experience tip! *
Always charge your batteries as soon as possible after a discharge, as sulfate (Lead / Acid batteries) will otherwise remain on the cathode and the anode.
After 4-5 days of discharge, the battery can not recover 100% again and its capacity and service life have been cut.

Charger and battery must fit together and they do, if for example you have a Mastervolt battery charger and Mastervolt batteries. In that case, you can simply choose the battery type on the dip switches and then all is done. This is because the charging profiles in Mastervolt Battery Chargers are set by default to the exact data of the Mastervolt batteries.
Now we all know that there is many other battery manufacturers than Mastervolt and therefore a Mastervolt battery charger can also be set to exactly the battery data you need for excatly your batteries. After setting battery data, Mastervolt battery chargers then treats other manufacturers’ batteries as well as it was one of Mastervolt’s own batteries.
To change the charging profile of the Mastervolt battery charger, log on the battery charger with a PC and the MasterAdjust software. We can also help you make the change.

Lithium Batteries

Lithium batteries are more difficult to handle for a battery charger. For you as a user, there are great advantages of Lithium. You can read more about it in our battery guide.
You must chose your lithium battery setup as a single solution. You cannot just put a lithium battery and a charger together. It’s not possible! Do not go there!
Lithium must be charged and, in particular, monitored and secured, in an other way than a Lead/Acid battery. You can not just replace your Lead/Acid Battery with a Lithium, without relating to the rest of the system. It will at least mean your lithium battery will die, but it may also be worse than that 🙁
Lithium, contrary to Lead / Acid batteries, will not stop charging when the battery is full. If charging is offered, the lithium battery will receive until it cracks. obvious the Lithium won’t survive that and therefore lithium batteries must have advanced electronics built in, that understand when the battery is full/empty and the battery charger should be able to receive this information from the battery.
In addition, there must be a Latching relay that disconnects the battery before it’s drained too much, because you can not “reboot” a lithium battery in the same way as a Lead / Acid battery.
On the other hand, a lithium battery can be charged very quickly and only has a short absortion phase (about 15 minutes). Lead / acid is in absortion for about 4 hours and during that time only a small current are charged but, for example, your generator must run anyway, just to supply a little charge current.
Mastervolt was the first to market a decent and integrated lithium solution and it has given Mastervolt a great amount of experience in handling Lithium batteries.
Mastervolt MLI-Ultra and MLI-E have built-in advanced technology in the batteries equipped with Masterbus communication for battery chargers, solar regulators, MasterShunts, Displays, etc. The master bus is the “secret” of why Mastervolt’s Lithium solution is the market’s best and most durable solution.

For almost any Offgrid battery installation, the professionally correct choice will be a Lithium solution!

Charge from flat

There will with almost guarantee be a day where your battery will be completely discharged. You may have forgotten to turn off something before you left the installation. When you come back a few days later, your battery is flat and has to be recharged. Of course, it is not good for the battery 🤔, but can the battery charger do the job? Most can not. The reason is that most battery chargers need to use min. approx 8 volts on the battery to “get started”.
It’s simply cheaper to make the electronics so that it depends on the approx. 8 volts to start charging. Mastervolt has chosen not to use that method and can therefore charge from 0 volts on the battery (not applicable to the Easy chargers).
A Lithium installation is rescued by a properly configured Latching relay.

Temperature sensor

The battery temperature is important in terms of how to charge your battery. The voltages to be charged with the battery must be adjusted, depending on the battery temperature. This is necessary to prevent your battery from being charged too much or too little and, for both, this is bad for your battery.
Therefore, there is a temperature sensor for all Mastervolt battery chargers (Except for the small Easy Chargers).
Lithium does not have the same temperature challenges as Lead/Acid and therefore the temperature sensor is not necessary.

Voltage sensor.

A little in the same ball bame, also the charging voltage (volts) should be adjusted according to the actual voltage of your batteries and not what the battery charger think the voltage is on the battery.
Because you have a loss in the cables that connects your charger with the batteries (you have :)) and even worse if you have a diode splitter between them (not recommended).
Since the voltage loss occurs in the transport path and varies with current and cable thicknesses/lengths, the battery charger does not have the correct data to adjust by default. Therefore, there is a Voltage Drop Compensation input on the Mastervolt battery charger (again not the very small ones).
The input is connected directly to the battery terminals, parallel to the main cables. Since there is no current in these cables, there is also no voltage drop and the charger now knows what to adjust.

Galvanic isolated.

Especially in Boats, but also in other contexts, a galvanic isolation is necessary between the battery charger power input (eg. 230 volts AC) and the output to the batteries.
In a boat it’s among other things, to avoid galvanic corrosion, which can ultimately sink the boat, but there may also be other reasons for wanting a galvanic insulation. All Mastervolt battery chargers are galvanic insulated

* Experience tip! *
If you have to leave your batteries without charger connected, leave it fully charged and as cool as possible.
When the battery is cool, there will be less self-discharge.

Charging

3 step + charging

There are many battery chargers on the market. and there are different theories about what is best for your battery. It is from battery manufacturers that one should listen to how they want their batteries charged and it is the common perception that there are 3 phases and in some cases one more.
On the market today you can find battery chargers with more than the 3 steps, but they are not required. It’s a bit like changing a 6-speed gearbox in your car with a 12-step … It does not really make a difference.

The 3 steps required are stated in DIN 41773 and you can read more about it on Wikipedia :

  1.     Bulk phase, where the battery charger leans all the power available in the battery. This will last until 80% charge is achieved (Lithium to 99%). Then starts the …
  2.     Absortion phase, where charging continues, but with a decreasing current. This is done to avoid overheating the battery (Lithium is only very short in Absortion since Lithium does not have the same temperature problems).
  3.     The float phase occurs when the battery is almost fully charged and will bring the battery the remaining way up to fully charged. Then the float phase will maintain the battery fully charged, also over long periods. It is best that your battery charger is on all the time while the batteries are in storage. However, it requires that your charger is built for it (all Mastervolt battery chargers are)

There is as described a step more or one + more. It’s every 12 days when some battery types need a “shake” and the charger will automatically do that if the battery types needs it

Ripple power

This is a very overlooked point when talking about battery chargers.
The ripple current is how much current (ampere) fluctuates, thus rising and falling. It is not the voltage (Volt) that fluctuates, but the underlying current.
Battery manufacturers typically prescribe that the ripple current should not be more than 5%. If the power fluctuates too much, it will heat the batteries (Lead/Acid batteries especially) and damage the battery.
But many manufacturers of battery chargers “forget” to restrict the ripple current sufficiently. Unfortunately, it’s not abnormal to see ripple currents of up to 50% …. Even reputable brands that go for quality chargers often have ripple currents of 15+%.
Mastervolt chargers all have less than 5% ripple power.

Power supply

Your battery charger is usually not “just” a battery charger. The equipment connected to your entire DC circuit, will receive power directly from your battery charger while charging your battery. Therefore, the DC that comes out of your battery charger should be of such high quality that your equipment are not harmed.
The best battery chargers act as power supply for DC, even without battery connected (Inductive loads usually requires a battery connection).

Parallel-connected battery chargers

Mastervolt has battery chargers up to 100 Amp per charger. But if your need is 250, 500 or more amp, then the battery charger can be connected in parallel operation.
You can also get a 3 phase battery charger for 400 volts or you can set 3 x 1 phased devices to each phase of a 400 volt installation.
The important thing is that your 230/400 volt AC supply can supply the power needed, then the chargers will convert it to charging your batteries.

Battery Bank

You do not need to use a single battery, you can make a battery bank of several batteries.
There may be 2 reasons to make one or more batteries banks

  1.     Series coupled batteries to raise system voltage.
        Eg. you can set 2 X 12 Volt, 200 Ampere hours (Ah) batteries in series and achieve a 24 volt, 200 Ah system.
    When you put batteries in series, you get a higher system voltage, but you do not get any change in your capacity.
    2 x 12 volts, 200 Ah in series therefore provides a 24 volt, 200 Ah battery bank.
  2.     Parallel coupled batteries to increase capacity
        2 x 12 volts, 200 Ah in parallel coupling is still 12 volts, but a 400 Ah battery bank.
    That’s a doubling of your capacity, but the same system voltage.

You can also combine serial and parallel in the same battery bank.
If you have 4 pcs. 12 volts, 200 Ah, you can connect 2 pcs. in series to achieve 24 volts and then do the same with the 2 other batteries.
Then you have 2 pcs. 24 volts, 200 Ah battery bank.
They can then be connected in parallel and achieve 1 battery bank of 24 volts, 400 Ah.

You must max. connect 4 batteries in parallel, otherwise there will be an imbalance in the system.
So 4 pcs 12 volt, 200 Ah batteries provide 1 x 12 volts, 800 Ah battery bank in parallel coupling.
Your 12 volt, 800 Ah battery bank you may want to serie connect with another 12 volt, 800 Ah battery bank.
Then you have a 24 volt, 800 Ah battery
Then you might think that it does not give more capacity? But it does. Not measured in Ah, but in the Watt hour(Wh) that you can pull out of the system.
Should we convert to Wh, then we simply simulate the system voltage with Ah and get Wh.
12V x 800Ah = 9,600 Wh
24V x 800Ah = 19,200 Wh

Therefore, in larger systems it is an advantage to raise the system voltage.

But read much more about batteries in our battery guide
What size should the battery charger be?

It has years back been said that your battery charger should be your battery capacity in Ah/10, but it was before there were modern battery chargers on the market that meet all of the above factors. In addition, the rule also forgot that the consumption that is on the DC circuit while charging goes from charging power.
Therefore, today’s rule of thumb is that your charger should be 25% of the total Ah of your battery bank  (do not include start-up batteries, which is max. 20%).

* Experience tip! *
There are many people who have purchased a poor quality Battery charger and who end up throwing out the too cheap battery charger (lost money), then buying the charger they should have bought in the first place.
This is an important choice as all your equipment, from navigation, security and other devices, is dependent on your power.

Hard environment and certifications.

Where should the battery charger be mounted?

Battery chargers are often something that’s mounted in a engine room, under a bench, inside a closet or elsewhere where it’s out of the way. It’s also the right way to do that, just remember that such a charger releases heat and reduces performance at high temperatures.
Most brands on the market define their battery charger specifications at 25 degrees, but as performance usually drops fast at rising temperatures, it’s an important point to keep an eye on.
Mastervolt indicates their specs generally at 40 degrees while the Mass Charger series is specified at 45 degrees.

For most commercial purposes, DNV and Germanisch Lloyd, Lloyd approvals are usually required and can be delivered with the largest Mastervolt Mass battery chargers.

It is usually in solutions for use on the sea and in mobile installations that the devices must fill, weigh and consume as little as possible. It is best achieved using High Frequency Technology (HF), instead of the more old-fashioned transformer-based technology. A Transformer “buzz”, heats and alarms more than HF. The heat is energy that is converted and thus lowering efficiency.
HF is silent and much more efficient to convert energy.
All Mastervolt chargers are HF based and have a noise level of less than 48 dBA @ 1mtr. The sound comes from the fan, but it only runs when needed.

The Mass Series is for the very hard environment and high performance demands, and is designed to run 24/7 at full load.
Chargemaster is the solution to Lesure vessels, mobile recreation solutions and semi professional solutions.

To de mounted in an engine compartment, a battery charger must be approved for IP23. All Mastervolt chargers are at least IP 23. There are a lot of chargers, also from reputable brands on the market today, which only achieve IP 21 and that’s not enough.
You must remember that for IP approvals and the like to be applicable, the charger must be mounted in upright position or as explained in the manual

Communication between devices

Something that in the future, will be a greater and greater part of getting a proper power system to perform, is communication between the units in the power set-up.
Both because it is more comfortable as owner to get more options, but also the automatic communication between the devices.
As an owner, it’s great to be able to collect the information from all the power units in one and the same display and even get them featured on the Nmea2000 network displays. It’s also great to be able to access the system via the internet and always see the status of the battery, sun/wind production and check if everything is okay.
But it’s even more important that your power devices can talk to each other and exchange information about all the parameters that can optimize the energy in the batteries, extend their lifespan and intervene before something goes wrong.
Eg. it is important that a Lithium battery can tell the solar controller that it should stop charging when the battery is full. Alternatively, the solar cell controller would continue and the battery would be damage.
Similarly, the battery or battery charger can tell the display that the battery status is low and an alarm must be triggered. The battery charger may also ask a generator to start to recharge the battery automatically.
If there is access to the system over the Internet, the Mastervolt installer is ready to solve problems with setup/ fault finding without having a man on site.

Communication between devices becomes more and more important. Mastervolt has solved it with their Masterbus, as well as facilitating communication with other Mastervolt devices, it also bridges to many other communication platforms like CanBus, ModBus, Nmea2000, CanOpen, etc.

Now you should be able to figure out what battery charger you need, but if you still are in doubt, feel free to contact Energig and get our input. You can write to us here or call us at +45 70 500 999