MUS 7002
MUS 3300
MUS 4.3
MURS 7.0
CTX Battery Sense
MXS 5.0



Battery Knowledge

Charger Knowledge

Company and Products



Q. The lamp for charging is lit. When I press the MODE button a number of times or if I cut the power supply temporarily, the green lamp lights.
A. This is totally correct. The charger starts with a constant current until the battery is up to maximum voltage. The voltage is then locked while the current reduces. When the battery is fully charged, the current is low and the charger automatically switches to “green”. If you press the Mode button instead, the charger checks if the battery voltage exceeds around 12.9V in which case the charger considers the battery to be fully charged. The difference between these two methods of checking if the battery is fully charged is marginal.

Q. The charger switches quickly to maintenance charge without reaching capacity in the battery.
A. The behavior of the charger can be explained in two ways from the point of view of the battery: a. Specially sealed batteries that have lost fluid can behave in such a way that they increase in voltage but there is no capacity left. This should express itself in such a way that when you disconnect the charger, the battery remains at a relatively high voltage. This can be compared to a 50Ah battery having been converted to a 20Ah, for example. Measure the battery voltage with the charger disconnected. If the voltage remains at 13V or more, this is the likely explanation. The battery is dead and needs replacing. If it has lost fluid, the top of the battery’s plate surface may be oxidized and thereby destroyed. b. A sulphated battery, see Charger Academy – Sulphating can also behave in this way with the sulphated layer being insulating and allowing no current through. You can also measure the voltage here with the charger disconnected, but the voltage will now drop instead quickly down to 12V or lower. The charger should react by switching on and off frequently as the voltage drops so rapidly. Even here the chances of achieving a long service life for the battery are unlikely. A CTEK charger tries to “revive” such a battery with pulses, but even if it succeeds the battery will soon be dead.Both of these possible causes of failure could have been avoided using a CTEK battery charger connected during the off season.

Q. When I switch on the charger, the diodes flash alternately. What does this mean?
A. There may be several reasons: Poor contact. Move the connections a little. Check that Comfort Connect (or the white plastic contact on the charger lead) is properly assembled. Sulphated battery, see Charger Academy – Desulphation, is a likely cause and where CTEK chargers have an exclusive feature. A sulphated battery has a layer of insulating lead sulphate on the inside. It does not cover the inside fully as you can still detect voltage across the battery. What happens is that the charger sends out pulses of energy until the battery starts to accept the charge. Batteries that “will not accept a charge” can often be revived using this method. Let the battery charge for one hour. If the charger still flashes, this indicates that the battery is dead and needs to be replaced. If the yellow or green lamp lights, you have activated the battery and it will probably work, but it is nevertheless at the end of its service life. The MULTI US 7000, MULTI US 25000 and MULTI XT 14000 models alert the charger if desulphation has been in progress for more than four hours. Let it charge for a couple of days to boost the power, while being aware at the same time that the battery will soon be dead. There are coarse sulphate crystals which no charger in the world can break down.
* Fault in battery cell. If the battery started charging in the correct way, with the orange lamp, and then starts flashing, this is due to a break somewhere. The most likely explanation is that the connection has simply fallen off, but it could also be a break inside the battery. Turn the charger to OFF/STANDBY and then back to charge. If there is a break, the charge will not start.

Q. Why do you have to connect negative to ground and not to the battery?
A. All CTEK chargers observe the directives that apply in the EU. All battery chargers whatever the brand must be approved in line with the same standards. Many more basic chargers can cause the battery to emit a significant amount of oxy-hydrogen gas and also generate a spark when connecting. This represents a major risk of injury and it is therefore stipulated that the first terminal should be connected away from the battery in order to reduce the risk. A CTEK charger is spark free, has a minimal amount of gassing and can therefore be connected directly to the battery. However, we do not recommend this for approval reasons.

Q. Can I charge several batteries in parallel?
A. The charger has no problem whatsoever of charging or maintaining several batteries, as long as they are connected in parallel. The charger sees the parallel connected batteries as one large battery. The sum of the capacity has to be below the recommendation for your model. Please note that there could be a high current between batteries when they are connected together, i.e. use high quality wiring.

Q. Are CTEK chargers suitable for caravans and boats?
A. Yes, CTEK chargers work exceptionally well here. Particularly the MULTI US 7002 and MULTI US 25000 models which are well-suited for installing in a caravan or boat, although the MULTI US 3300/ZAFIR 90 models perform less well when you have parallel loads. The way for you to handle this is by either avoiding having any heavy parallel load or by shutting down the charger manually now and then. However, the fridge in the caravan will not cause any problems. Our other chargers have a built-in timer as security which thereby avoids the problem.

Q. Can I fit the charger permanently in my car?
A. This is perfectly ok in the coupé, but it is not approved for fitting under the bonnet. We recommend that you place the charger in the coupé and that you run 12V cables to a suitable point.

Q. Does the battery have to be disconnected from the vehicle when charging with a CTEK charger?
A. No, CTEK chargers cannot damage sensitive electronics so you do not need to disconnect the battery from the vehicle. However, particular attention must be paid when Boost/Recond mode is used as the voltage is 16V. If the voltage is below 16V, most manufacturers consider that everything is in order and CTEK chargers are well under this level, with the exception of Boost/Recond modes. Bear in mind, however, that high voltage shortens the service life of some components. A rule of thumb says that if you raise the voltage by 5 %, a light bulb will lose half of its service life, but normally the problem is not that great. If you have sensitive electronics where the manufacturer advises against higher voltages: Disconnect it!

Q. I have a COMFORT INDICATOR and would like some advise. When attached to my car battery, the red light flashes even after charging my battery with my CTEK. MULTI US 3300 and it is indicating green light (for fully charged). Is this correct? I thought the COMFORT INDICATOR would flash green?
A. The CTEK Comfort Indicator is what the name says: an Indicator. It provides a snap-shot of the system voltage and works best when the battery is at rest. The voltage drops down when a door opens or the lights is turned on and accordingly the indicator LED’s could change status from green to yellow and sometimes even to red. This is normal and nothing to worry about. The Comfort Indicator is working correctly, but can’t take into consideration these external activities. This should be seen as an additional benefit. The Comfort Indicator could alert that there’s some unexpected current drainage.

Battery Knowledge

Q. What do the abbreviations MF, GEL and AGM mean?
A. Gel, AGM, Vlies, MF, Calcium/Calcium and Hybrid are abbreviations for different types of lead-acid batteries

Q. Can you really connect and then forget about the charger?
A. Always check that the charger has switched to maintenance charge before leaving your charger unattended and connected for longer periods. If the charger has not switched to maintenance charge within 72 hours, the charger must be disconnected manually. If the charger has switched to maintenance charge, everything is as it should be and the battery is probably healthy and will work for a long time together with your CTEK charger. If the charger has not switched to maintenance charge after 48 hours and the green lamp is lit, this is a sign that something is wrong.

Possible causes:

  • A larger battery of an older type, known as an antimony battery behaves differently, see Charger Academy – Battery Types.
  • If major consumers are connected to the battery, charging will take a long time and the battery can even be overcharged.
  • The battery is sulphated from the start, see Battery Knowledge – Sulphating. In this case, charging will take longer as the higher internal resistance of the battery restricts the amount of current it can receive.
  • The battery is spent and needs replacing.

Q. What happens if I use the charger on batteries larger than you have recommended?
A. Using a small charger results in longer charge times. Sometimes this may be critical in which case you should use a larger charger. If you use the charger for maintenance charging only then you can get by using a really small charger. However, for larger batteries you need to be a little more careful, see next question.

Q. What does ripple mean and how does it affect the battery?
A. Current ripple is a measurement of how much alternating current the charger supplies. High current ripple causes high losses which costs money and generates heat in the battery. This heat mean that you must charge using a lower current to avoid gassing. Conventional chargers heat the battery up to four times as much as CTEK chargers. Current ripple also has other negative impacts, see Charger Academy – Current Ripple.

Voltage ripple describes the amount of alternating voltage that is mixed with the direct current output. This may cause voltage peaks that could damage sensitive electronics, see Charger Academy – Voltage Ripple.

Q. Can I charge GEL batteries?
A. GEL and all other types of lead-acid batteries can be charged using CTEK chargers with no problems whatsoever.

Q. I have an AGM battery. Do I charge it in snowflake mode?
A. It depends. Optima and Hawker recommend snowflake mode. If there is no indication on the battery, use car mode for batteries over 14Ah or MC mode for batteries under 14Ah.

Charger Knowledge

Q. Why does it take less time to charge with a MULTI US 3300 with just 3.6A compared to my old 11A charger?
A. Your old charger starts to charge with a high current that quickly drops. In addition, a lot of energy is wasted from the charger in just heating the battery. A CTEK charger starts charging from a very low voltage with virtually a constant current up until the battery has reached maximum voltage. The charger then continues with a constant voltage until the battery is fully charged.

Q. It takes a long time to charge.
A. If charging takes an unusually long time, this maybe because the battery is sulphated. A battery of this type has a higher internal resistance and will not accept a charge as quickly. However, CTEK chargers continue to charge at the rate the battery is able to receive.

Q. How drained can a battery be before it cannot be charged by a CTEK charger?
A. A battery is considered as drained when the voltage is below 10.5V. However, it can still emit energy down to 7-8 volts. Most models can handle batteries as low as 1.5V. The US 800 charges from 6V, while the XC 800 charges from 3V. The MULTI US 7000, MULTI US 25000 and MULTI XT 14000 models in supply mode can charge from zero in the battery. Note that some batteries can be damaged as a result of being too deeply discharged. The CTEK chargers try to revive these batteries through pulsing to reduce sulphating or through soft start (certain models). A battery that is below these levels is dead and needs to be replaced.

Q. Can I charge a 24V system with two serial connected 12V chargers?
A. Yes, this is perfectly ok and is actually preferable for the batteries. Connect a charger for each battery.

Q. What settings should I use?
A. It depends on the type, size and application of the battery. Read more under Battery Types & Settings for Each Charger.

Q. Does a CTEK charger remember the settings if you disconnect from the mains supply?
A. For most models the charger remembers the settings regardless of what you do, although for the MULTI US 3300 and ZAFIR 90 models the battery must have a voltage above 6V in order for the charger to remember the settings. Avoid placing a MULTI charger too close to an extremely strong electrical field such as an ignition system as this may cause the charger to lose the settings.

Q. Can you charge a frozen battery?
A. No, the battery must be thawed first. Note that the battery has been discharged previously otherwise it would not have frozen. Check the battery carefully for cracks.

Q. The MULTI US 25000/MULTI XT 14000 does not have the higher voltage recommended for brands like Optima. Is this a problem?
A. We recommend the higher voltage for these batteries. These chargers do not have this setting but can still be used.

Company and Products


Q. Where can I buy a charger? Can I buy directly from you?
A. Check for dealers. There are a number of options, and the nearest shop is usually not too far away. These shops have a number of distributors to choose from, see the tab Distributors. We do not sell directly to shops or consumers.

Q. Where can I get a copy of the user manual?
A. All the manuals can be downloaded from our website

Q. What’s the difference between Coulombi Chargers and CTEK?
A. CTEK SWEDEN AB was once called Coulombi Chargers. A Coulombi MULTI US 3300 is identical to a CTEK MULTI US 3300.

Q. A CTEK charger looks so small. How can I charge as quickly and effectively as other chargers?
A. CTEK chargers use the same technology as computers to keep size to a minimum and increase power. The charge will be well-controlled and provide a “clean” current.

Q. What’s the difference between MULTI US 3300 and ZAFIR 90?
A. ZAFIR 90’s performance is a little lower than the MULTI US 3300, which gives a longer charge time. Otherwise the functionality is identical.

Q. What charger should I choose?
A. It depends on the type of battery, the size of the battery, the application and how much you are prepared to spend.

Q. Can I charge my 6V batteries?
A. The XC 800 for 6V is excellent for both charging and maintaining 6 volt lead-acid batteries. Trying to charge a 6 volt battery with a 12 volt charger is extremely risky. NOTE that the XC 800 is not avalible for the US market.

Q. Are there CTEK chargers available for other battery types?
A. CTEK’s strategy is to manufacture the world’s smartest charger for lead-acid batteries, and this is sufficient for the time being. We have no chargers for other types of battery as of today.

Q. One supplier recommends the MULTI US 3300 only up to 75Ah, yet the box says 120Ah. Which is right?
A. The MULTI US 3300 works well up to 120Ah. However, the MULTI US 7020 or MULTI US 25000 are better choices above 75Ah with shorter charging times and more features. It is primarily a balance between price and performance

Q. I have a COMFORT INDICATOR and would like some advise. When attached to my car battery, the red light flashes even after charging my battery with my CTEK MULTI US 3300 and it is indicating green light (for fully charged). Is this correct? I thought the COMFORT INDICATOR would flash green?
A. The CTEK Comfort Indicator is what the name says: an Indicator. It provides a snap-shot of the system voltage and works best when the battery is at rest. The voltage drops down when a door opens or the lights is turned on and accordingly the indicator LED’s could change status from green to yellow and sometimes even to red. This is normal and nothing to worry about. The Comfort Indicator is working correctly, but can’t take into consideration these external activities. This should be seen as an additional benefit. The Comfort Indicator could alert that there’s some unexpected current drainage.


Q. What should be considered when choosing a battery charger for a boat?
A. You should consider 2 things:

1- The charger should be safe i.e. safe for you as a user, (CTEK’s chargers are non-sparking, reverse-polarity protected and short-circuit proof). It should also be safe for the battery (no risk of over charging and have safe maintenance charging). Last but not least, the charger should pose no risk of damaging the electronics in the boat. It is incredibly unfortunate to have to replace an expensive starter battery just because the battery has been shock charged with too high a voltage.
2- The size of the charger. If you consider this: What sizes are your batteries, how much do you drain them and how quickly do you need them fully charged again? Your needs and requirements will be the decisive factor when choosing the size of your charger.

It is easy to calculate how quick you can recharge a battery up to 80% of full capacity. The remaining 20% depends on the battery’s age and condition. If you take the size of the battery bank, for example 200 Ah and then calculate 80% of that (200 x 0.8 = 160 Ah). Divide 160 with the number of A the charger is supplying (25A for M300) (160/25 = approx. 6.5). This means that it will take up to 6.5 hours to charge from fully discharged to 80% of full capacity. It will take between 1 and 6.5 hours to charge the remaining 20% depending on the battery’s age and condition.
All in all, it will take up to 13 hours to charge from fully discharged to fully charged. If you do the calculation in this manner, you will get a maximum charging time. The battery will rarely be fully discharged, but the calculation will give you a good idea of whether the charger is of the right size or not.

Q. Some CTEK chargers are a lot cheaper than CTEK’s marine chargers. Is there any difference between marine chargers and other chargers?
A. As I mentioned earlier, CTEK works with many different platforms (sizes, power, charging steps, features, models for different applications) which, depending on the model, can vary in price. The quality of the charger and the charging is, however, just as good. Some chargers can be more expensive than others depending on the model and its requirements.

I don’t think that you can find an “unbelievably cheap” CTEK charger at any serious dealer. CTEK is doing its best in trying to maintain fair pricing regardless of industry.

Q. CTEK has a DC/DC charger that seems really good, but how does it work and when do I need one?
A. DC/DC means direct current to direct current and it charges from 12V to 12V.

The idea is that you should be able to use the current from the generator in the boat or from the solar cell more efficiently. There are some problems that the DC/DC charger can solve easily:

For example, a generator in a boat gives 14V. The battery does, however, really need 14.4V (depending on type and temperature but we are generalising a little here). This means that you are charging the battery, but not much or very efficiently.

The second problem is that when the voltage in the battery reaches the same level as the one in the generator, there will not be much current flow. The generator may be providing 60A, but when the voltage in the battery reaches 14V (may take a few minutes) the charging will only be a few A. It doesn’t matter if you drive until you’re out of petrol and don’t use any at all, the battery will NEVER be fully charged!

How does CTEK think? Imagine if you charge 14V via the IN connector and discharge 14.4V via the OUT button! And not just a few A but 20A. This would give you a fully charged battery. To this, we add CTEK’s intelligent charging steps and charge on full speed up to 80% of full capacity and then reduce the speed until it is fully charged. This means that you can spend more time out at sea, have a battery that is better charged when you get back and reduce the amount of time the boat has to be on shore power before the battery is fully charged again.

You can also connect a solar cell to get a more efficient charging without causing any damage to the battery.

Q. Are there any more tips that you can give boat owners looking for a hassle-free summer when it comes to battery charging?
A. Get a charger of the right size. It is better that it is too big rather than too small. Connect the charger when you leave the boat to ensure that it will be fully charged for your next trip. If possible, leave the battery on charge for the entire winter! If you have normal wet batteries (not GEL or AGM) remember to RECOND both when you take the boat out of water and put it back in. This will improve your chances of having a summer with working batteries so that you can enjoy your boat rather than worry about whether the batteries will last or not.

“Having owned several of another ‘popular’ brand of charger, the CTEK is far and above superior in every possible category. WELL worth the miniscule additional cost. ”