 | Home > In Car Electronics > CTEK and TMAX Split Charge |
 
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| d-digby Member Since: 02 Aug 2013 Location: New Forest Posts: 12   
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Hi Jack
Not sure is this solves your problem but I used a CTEK MXS 5.0 on my Defender without any issues so if there are model differences between the CTEK 5 and 7 it might be a cause. Barry |
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29th Feb 2024 2:01pm |
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| Bluest Member Since: 23 Apr 2016 Location: Lancashire Posts: 4248 
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I have this problem, different make of charger, but same type of thing. It's only a 3.8 amp max smart charger which I think is the crux of the issue. As my starting battery is charged, voltage rises to the point my voltage sensing relay (VSR) combines the 2 batteries. At that point the second battery drags down the overall combined voltage and the current of the charger isn't enough to compensate, so the VSR disconnects the 2 batteries. I then end up in a cycle of the VSR connecting and disconnecting.
The less than ideal solution for me is to disconnect the VSR sensing wire and charge the 2 batteries separately. I keep meaning to put a switch in the VSR sensing wire so I don't need to physically disconnect it from the battery terminal. A better solution would be to get a VSR that has the function to force it to combine both batteries and then do that when you are charging. I thought your T-MAxx might have that function, but I guess not from your comments. Another option would be a charger with a higher current rating such that the combined voltage doesn't drop below the cut-off voltage, but I don't know how many amps you would need. Also smart chargers do funny things with the battery voltage once they think the battery is full, so may not fully solve the issue. It would be a case of trial and error. 2007 110 TDCi Station Wagon XS |
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29th Feb 2024 5:09pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1105  
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As Bluest said.
I have an MXS 10 and a NL split charge and this on-off happens if both batteries have a very different SOC and one is deeply discharged or when both are really down. Happened only two or three times in years. I installed a switch in the ground wire to the split charge electronics, to disable it. I also have CTEK connectors wired to each battery. I would then charge the deep discharged battery/ies separate. |
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1st Mar 2024 8:13pm |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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Hi all
This has come back around after not using the car for a while and two flat batteries So I think I’ve got a few options Option 1 Stick a switch in the ctek ground and treat each battery independently. Add a ctek connector to each Option 2 Use two relays Pass the ground through normally closed on them both Connect trigger to the ctek connectors Then when either ctek is plugged in the relay would break the ground? Any thoughts on option 2? |
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28th Feb 2025 11:09pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1105
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If I understand correctly you want to put two relays in series to switch off the split charge control when either gets power from one of the CTEK connectors? An auto-disconnect feature?
Nice idea, but the connectors have to be connected to the batteries, of course, so both relays will always get power from them, not just when you plug the charger in. Unless I totally misunderstood. Diagram? |
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28th Feb 2025 11:22pm |
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| bear100 Member Since: 22 Mar 2010 Location: South Wales Posts: 1959  
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i had similar issues with an MXS 5, when on an expedition the aux battery would be used to run the fridge, lights etc.
When plugged into a mains supply the charger would first as (its supposed to do) charge the main battery then once satisfied or after the delay the relay would open to charge aux battery, but the voltage would share between the 2 batteries and the relay would close again because the aux battery dropped the voltage. I have fitted on my cubby box a row of carling switches, initially i had one CTEK charge point which was connected to the main battery but since added a second for the aux battery which solved the problem. Since adding the second charge point i found an MXS10 on sale (black Friday deal very cheap) and now use this which has no issues dealing with both batteries on the original charge point. next step I'm going to upgrade the system from the National lunar to a Victron B2B charger with a PV input and swap the Optimas for Odyssey batteries 2023 Defender L663 V8 2016 Range Rover Autobiography 4.4 TDV8 (gone) 2010 110 XS Utility 2.4TDCI 2010 Range Rover Sport TDV8 (gone) 2007 Discovery HSE TDV6 (gone) 1993 110 csw 200 tdi (gone) 1994 90 HT 300 tdi (gone) 1994 discovery 300tdi (gone) 90 hybrid 3.5 v8 (gone) Range rover bobtail 3.5 v8 (gone) |
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1st Mar 2025 9:05am |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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 Click image to enlarge
No you are right, the battery will keep it closed unless I use a diode Switch would be easier lol I am right though I think that I could then charge both batteries at the same time? |
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2nd Mar 2025 8:17pm |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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Thanks, I assume you broke the link when charging both at the same time? |
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2nd Mar 2025 8:19pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1105
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Since the relays are NC, they will both be *open* all the time, so your TMAX will never connect the batteries (I think the CTEK in the upper right of your diagram should read TMAX?) Not sure where you'd see a diode here?
"then"? sorry |
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2nd Mar 2025 8:43pm |
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| bear100 Member Since: 22 Mar 2010 Location: South Wales Posts: 1959
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I only used one of the charge points at a time, the main battery is always charged 2023 Defender L663 V8 2016 Range Rover Autobiography 4.4 TDV8 (gone) 2010 110 XS Utility 2.4TDCI 2010 Range Rover Sport TDV8 (gone) 2007 Discovery HSE TDV6 (gone) 1993 110 csw 200 tdi (gone) 1994 90 HT 300 tdi (gone) 1994 discovery 300tdi (gone) 90 hybrid 3.5 v8 (gone) Range rover bobtail 3.5 v8 (gone) |
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2nd Mar 2025 10:00pm |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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Sorry - Struggled to function right over the weekend 
Yep that should say TMAX. I was thinking diode between relay and battery, so charge current can flow to battery but battery can't trigger relay? |
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3rd Mar 2025 10:15pm |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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Something like this
https://www.altecautomotive.co.uk/10-amp-i...5466-p.asp |
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3rd Mar 2025 10:27pm |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 78
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Click image to enlarge |
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3rd Mar 2025 10:30pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1105
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Theoretically that should work, however...
The silicon diode you linked to has a forward voltage of 0.9V which means you'll loose 0.9V of the charger's voltage - that's a lot, given that both the charger and the TMAX operate on various exact voltage levels to correctly charge the batteries. Also at 7A charge current the diode will give off 6W of heat (0.9V*7A), so IMHO it needs a heat sink. If you'd go for a CTEK MXS 10, which I'd recommend to charge both batteries at the same time, you'll generate 9W of heat. A Schottky diode (e.g. 20A, 100V) has only around 0.4V but that's still 3..4W. Lastly if a diode fails, e.g. due to heat, two things can happen. Schottkys usually fail short, causing the relay to engange. A typical 12V car relay needs around 100..150mA which can drain your battery in a few weeks. Given the low current the TMAX needs for its control circuit you could use a non-automotive relay, google 'reed relay', which need only 10..15mA. They're also much smaller. If a diode fails open, you wont be able to charge that battery. |
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4th Mar 2025 8:16am |
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