C-Tek D250 SE and lithium charging - Expert required

Submitted: Thursday, Mar 10, 2022 at 16:45
ThreadID: 143374 Views:6517 Replies:5 FollowUps:14
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I have a friend with one of these. Its seems to have a strange profile, when compared to how my Redarc stuff works

This set up has a Victron meter as well, so we can see whats happening.

When charing the battery up (Lithium 100AH) if will charge it to 100%. Once it gets to 100% it will stop charging, and the appliances will start to bring the battery back down. The C-tek does not continue to supply any power, the battery will drop by 10% + before the C-Tek decides to charge again. If you disconnect the C-Tek it will start to charge again.

C-Tek tell us this is the way it should work. The issue is, when trying to be fully charge right at the end of the day with your panels, you could be as much as 10 to 15% down, as the C-teck has decided not to charge.. if this is the way they work, its stupid..

My redarc just keeps the battery at 100% and supplies the power the appliances need as well. Right up to the point solar is no longer supplying any power.

Can any experts on C-Tek comment on how these chargers should work, is C-tek correct in stating this is how they work?

PS: The Standard non Lithium version of this charger we have, charges "normally" as I would expect, always keeping the battery at 100%.. We just used the AGM profile and that seem to work for now.

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Reply By: Frank P (NSW) - Thursday, Mar 10, 2022 at 17:22

Thursday, Mar 10, 2022 at 17:22

I'm no expert but I've been reading what they say. I also talk to my lithium battery supplier, EV-Power in WA, who supplied my Karavan batteries.

He says what Ctek says. IE, lithium batteries in service should be charged to 100% then stop the charge, use them a bit and resume the charge. A properly configured lithium charger will do that. Even better, if capacity and useage allows it, limit charge to 80-90%. This is what battery electric vehicles do - they have an unused and unaccessable top and bottom buffer so that even when the gauge says 100%, the battery is not at 100% and when it says 0% the battery is not dead flat - both of which influence longevity.

For longevity they should not be kept at 100%, especially in hot weather, so leaving a trickle on them to keep them at 100% is not ideal.

I know there will be differing points of view - the Redarc vs Ctek philosophies is a good example.

There won't be catastrophic failure if kept at 100% but longevity will be reduced, according to my sources who I consider expert and reliable. Longevity is possibly more relevant when considering the high purchase cost of Li batteries - depends on the depth of your pockets, I suppose :-)

For long term storage they should be stored at 50-70% IIRC. That's what I do with my Karavan batteries.

Love your vids, BTW

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Follow Up By: Gbc.. - Thursday, Mar 10, 2022 at 19:04

Thursday, Mar 10, 2022 at 19:04
Everything I have read agrees with you. Lithium at 100% isn’t great.
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Reply By: Allan B (Sunshine Coast) - Thursday, Mar 10, 2022 at 19:24

Thursday, Mar 10, 2022 at 19:24
Hi JJ,
You say that you are basing your observations on a "Victron meter". If this meter has not been correctly programmed for your particular battery then you may be seeing incorrect information.
For example, the default setting (as supplied) for a Victron BMV-700 is 200Ah. You say the battery is "Lithium 100AH" so if the Victron is programmed at 200Ah ( or any value other than 100Ah) you will not be reading the correct state of the battery.
This could be the reason for your "strange profile".

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Follow Up By: JJAdv - Thursday, Mar 10, 2022 at 19:43

Thursday, Mar 10, 2022 at 19:43
Its correctly programmed.

Its the behaviour of the Ctek, and how its charging that is in question.
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Follow Up By: Frank P (NSW) - Thursday, Mar 10, 2022 at 21:10

Thursday, Mar 10, 2022 at 21:10
From the advice I have received from multiple experet sources, I believe the Ctek behaviour is preferred for Lithium batteries.

Slightly off topic here, but....
This even extends to phones, laptops, cameras, etc, that use lithium batteries. You will get longer battery life if you don't plug them in overnight and leave them sitting at 100% on a charger for however long. There are apps and devices that help you limit the charge if you want to go down that route. Chargie uses a device between your charger and your phone/tablet, plus an app to stop the charge at the desired level. AccuBattery simply uses an app to alert you when you've reached the desired level of charge but will continue to charge if you ignore it.

With so many devices having built-in, non-removable batteries, technologies like this might prevent an otherwise unnecessary expensive device upgrade due to battery failure.

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Follow Up By: JJAdv - Thursday, Mar 10, 2022 at 21:40

Thursday, Mar 10, 2022 at 21:40
Yes, I am very aware of the best ways to get the best out of lithium, most modern lithium batteries will take care of this for you for the most part..

The question..

Is this behaviour normal for this charger? If it is I will change it.. if not I will get it replaced as it must be faulty..
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Follow Up By: Frank P (NSW) - Friday, Mar 11, 2022 at 09:47

Friday, Mar 11, 2022 at 09:47
G'day JJ,
I looked up the owner's manual for the Ctek. ( Link) It says that on its lithium profile after charging to 100% it will maintain the battery between 95% and 100%. So the short answer to your question, I think, is that the Ctek is working properly.

Now, why does the Victron show 85-90% when the Ctek cuts in, when Ctek says it should be 95%?

I think that is due to discrepancies caused by the differing methods of sensing battery state of charge (SOC). The Ctek does it by sensing battery voltage. The Victron is a coulomb counter.

It is difficult to accurately assess SOC in a lithium battery by measuring battery voltage, owing to the very flat discharge curve displayed by lithium batteries. Typically, a 12V lithium battery will hold 13.2-13.3 volts for a large part of its operating range. To estimate SOC using this method the Ctek has to measure hundredths of a volt or smaller and even then the result is approximate.

The Victron battery meter is a coulomb counter, a far more accurate way of measuring SOC. You probably know this, but for those that don't, a properly set up coulomb-counting battery meter will know how many "particles" of electricity (coulombs) are in a fully charged battery. It then counts those going out (discharge) and those coming in (charging) to arrive at a relatively accurate SOC.

Given measurement errors and other unknowns like possible parasitic discharge able to be measured by the Victron but not by the Ctek, it doesn't surprise me that there is a 5-10% difference between the two.


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Follow Up By: JJAdv - Friday, Mar 11, 2022 at 13:47

Friday, Mar 11, 2022 at 13:47
This actually tells me is not working correctly as this one will go down to 85% or so before charging again.. if it were 95% this would be what I expect.

We have tested the Victron and it counts out the 100 AH correctly. The battery BMS is also a major part of the charging, and will turn off the charging when the battery is full. The charger does not have to make this decision. You can charge a Lithium battery with a simple power supply if you wanted to. The BMS will shut down charging when its decides its full.

If the profile of the CTEK is to allow the battery to get to 85% before charging again, this isnt going to work, and it will be changed out for a Redarc

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Follow Up By: Bazooka - Friday, Mar 11, 2022 at 16:20

Friday, Mar 11, 2022 at 16:20
Can't help with the answer JJ but would have thought the 100% charging issue would be taken care of by the battery's BMS as you say above. CTEK's setup might assume that not all BMS's will do that job properly so they'll err on the side of caution?

Most LFP experts I've read/listened to/watched suggest that squeezing the last little bit of charge into the LFP cells is detrimental to their health over the long term, hence any decent BMS should prevent that from occurring. As Will Prowse has said and demonstrated many times when testing 12V LFPs, cell voltages will appear to be high when the battery (with BMS) is fully charged but they soon settle back to a voltage which is slightly under "full". Thus the BMS is doing its job and the charger shouldn't have to be concerned.
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Follow Up By: JJAdv - Saturday, Mar 12, 2022 at 00:51

Saturday, Mar 12, 2022 at 00:51
I have tried multiple other chargers and they all work the same. The CTEK is the very odd one out..

Letting the battery get to 85% before charging is just not required, and if that’s what CTEK are doing it’s stupid and not required, and I wonder how they manage to sell chargers..

It seems the answer is to bin the CTEK and get a quality charger..
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Follow Up By: Bob Y. - Qld - Saturday, Mar 12, 2022 at 08:45

Saturday, Mar 12, 2022 at 08:45
A quality charger to replace the Ctek would be an Enerdrive DC2DC 40A charger. Perhaps a little cheaper than a Redarc, and charging rates can be fine tuned by the user, as well as being able to handle all battery types. Perhaps a bit bulkier than a Redarc?


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Follow Up By: JJAdv - Saturday, Mar 12, 2022 at 11:42

Saturday, Mar 12, 2022 at 11:42
Will look them up.. thnks for the tip.
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Reply By: Member - LeighW - Saturday, Mar 12, 2022 at 09:47

Saturday, Mar 12, 2022 at 09:47
Generally accepted that a Lithium charger should charge to whatever level you have set it for then turn off and not start charging again until battery voltage has dropped to the predefined level.

In practice manufacturers have gone generally one of two ways, they either do the above or they go into a power supply mode otherwise generally know as float, the float voltage will be a little lower than the float voltage for a AGM for existence. They do this as the problem becomes what at what voltage do you trigger restarting the charging, it is difficult with Lithium's to detect when 2% for instance has been taken out of the battery, can be done by monitoring Ah in and out but that adds extra cost to the charger.

In reality the power supply mode for cycling daily is just as good as the batteries want remain at 100% for long periods of time, only becomes an issue if floating 24/7. Also ensures battery is at 100% when the sun goes down which may be critical in small battery setups. If you have a lot of over capacity and you want to ensure maximum life of the batteries then may be the no float method is best for you.

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Follow Up By: JJAdv - Saturday, Mar 12, 2022 at 11:46

Saturday, Mar 12, 2022 at 11:46
OK, but the question is "is this behavior the normal behavior for a CTEK" as no other charger we have tried works like this. All other chargers will all keep the battery at 100%. The BMS and the battery design will be such that the cells are never actually at 100%.. even if the meters state 100%..

Just like a Tesla does..

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Follow Up By: Member - LeighW - Saturday, Mar 12, 2022 at 20:58

Saturday, Mar 12, 2022 at 20:58
Early Enerdrive DCDC chargers charge till they reached the set parameters then turned off. Restarted charging when battery dropped to around 13.3V so that type of behaviour is not unique. Their current version goes to power supply mode.

According to the CTEK manual charger goes into float mode for 2 hours then switches to pulse mode whereby it gives a pulse a 1 hour pulse charge and an auto pulse 10 days. I assume that means when the battery drops to a certain level it will go into charge mode for an hour. If battery not used then it will give a one hour every 10 days.

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Reply By: Member - Andrew W14 - Saturday, Mar 12, 2022 at 10:59

Saturday, Mar 12, 2022 at 10:59
I have a full Enerdrive system, 3000w watt Inverter/Charger, Enerdrive DC-DC, 300 ah Enerdrive LiFePo4. Set up correctly by Enerdrive, on mains it charges to 100% and stays there on float as long as connected to mains.
Due to oft repeated claims as above that you should allow a Lithium to fully charge then turn off I questioned Enerdrive at length about this. They were very adamant in their claim that on their system maintaining 100% was the correct method. Their argument was that having spent hundreds of thousands of dollars on developing their Lithium battery system, why would I prefer to listen to an unqualified ‘ expert’ or individual? To me it was a persuasive argument. So far after 2.5 years full time use my batteries still perform as new.
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Follow Up By: JJAdv - Saturday, Mar 12, 2022 at 11:49

Saturday, Mar 12, 2022 at 11:49
I agree.. the battery and system design is such that you can just use them like a normal battery and keep them at 100%, which is probably only really 85 to 90% of the actual cell capacity.

After trying a hand full of chargers, they all charge to 100% then maintain this even while appliances are using power.

The CTECH doesn't, at least the one I have. So my question is, is this the way the CTEK works? or do I have a issue with the charger? If it is the way the CTEK works, then it's useless and will be replaced..

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Follow Up By: Member - Bigfish - Sunday, Mar 13, 2022 at 17:14

Sunday, Mar 13, 2022 at 17:14
And yet there are lithium battery manufacturers who say do not keep at 100% if in storage. If being used constantly 100% is fine.
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Reply By: Member - Captain (WA) - Sunday, Mar 13, 2022 at 19:02

Sunday, Mar 13, 2022 at 19:02
It seems the C-tek is doing what its designed to do, but as you rightly pointed out, you can lose up to 10% of battery capacity if the solar drops off while at the lower end of its deadband.

The Redarc BMS 30 has two modes, Touring and Storage. In Touring mode (lithium profile), it keeps the batteries at 100% capacity, ready for use. But in storage mode, it switches off at 100% and stay off until less than 90%.

The C-tek profile seems to be very similair to the Redarc Storage mode and maximises battery life. But when you are using your van and want maximum power available, the C-tek seems to prioritise battery life over maximum power capacity.

Not sure why C-tek only has the one mode, the Redarc with its Storage and Touring modes seems to have typical useage patterns covered much better.

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