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Old 22-04-2024, 13:33   #1
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DC TO DC Charger Question

I have a LifePO house bank (400AH) and 2 x small (24AH) AGM start batteries. (2 engines)



With my old AGM house bank (600AH) no start batteries, I would run my engines when anchoring that would also maintain good voltage on my windlass. While the Lithiums can keep up, the drain seems unnecessary as I would have my engines running anyway.


So I have installed a 40A DC to DC charger.


The DC - DC needs a 12volt feed to initiate charging. The manual says connect this through the Ign switch of the engine. BUT my thoughts are this, if the Lithium bank is low then by using the Ign switch as a feed this would instantly apply a 40A load to the start battery circuit, thus reducing its ability to start the engine.


1. Should I add a manual switch into the ign circuit feed to the DC-DC and only switch it on after the engine is running? I could of course forget to switch it off and worse case would be as above.


2. Is there a tap in, on the alternator for example, that would only give a 12v feed once the engine is running?


3. I could connect the 12v feed to only one engine and always start the other first. There is a battery combiner between the 2 start batteries.


Any other solutions?? or am I overthinking it??
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Old 23-04-2024, 10:36   #2
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Re: DC TO DC Charger Question

Quote:
Originally Posted by Tupaia View Post
I have a LifePO house bank (400AH) and 2 x small (24AH) AGM start batteries. (2 engines)



With my old AGM house bank (600AH) no start batteries, I would run my engines when anchoring that would also maintain good voltage on my windlass. While the Lithiums can keep up, the drain seems unnecessary as I would have my engines running anyway.


So I have installed a 40A DC to DC charger.


The DC - DC needs a 12volt feed to initiate charging. The manual says connect this through the Ign switch of the engine. BUT my thoughts are this, if the Lithium bank is low then by using the Ign switch as a feed this would instantly apply a 40A load to the start battery circuit, thus reducing its ability to start the engine.


1. Should I add a manual switch into the ign circuit feed to the DC-DC and only switch it on after the engine is running? I could of course forget to switch it off and worse case would be as above.


2. Is there a tap in, on the alternator for example, that would only give a 12v feed once the engine is running?


3. I could connect the 12v feed to only one engine and always start the other first. There is a battery combiner between the 2 start batteries.


Any other solutions?? or am I overthinking it??
Common issue with the ignition switch and DC2DC, in a bad combo you can even completely drain your starter.
First why not do it right, modify the alternator to external regulation and charge the lifepo4 as well as start from it and get rid of AGM. You added complexity and points of failure plus limited the alternator in output. If it’s not a newer smart temp protected one the modification is cheap and easy.

If you wanna keep the lead starter you can use an argofet instead DC2DC and the regulated alternator makes sure the starter is charged, the AGM work as surge protector too.

3rd option I lately posted the smart alternator sensing relay from an Australian company that detects when an alternator is running and switching on the DC2Dcvia ignition cable. Much easier to install and the relay only switches on charging when the engine and then the alternator is running. Eg on newer Volvo the engine starts without the alternator and then the D+ sends power to the stator coil of alternator so a magnetic field is inducted and the alternator starts to produce power. If the engine is off, so is the alternator and the DC2DC is shut off immediately too and not drained down by the DC2DC.
That’s the relay:
https://perthpro.com.au/products/eas...n-80x-30x-30mm
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Old 23-04-2024, 13:33   #3
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Re: DC TO DC Charger Question

Quote:
Originally Posted by Tupaia View Post
I have a LifePO house bank (400AH) and 2 x small (24AH) AGM start batteries. (2 engines)



With my old AGM house bank (600AH) no start batteries, I would run my engines when anchoring that would also maintain good voltage on my windlass. While the Lithiums can keep up, the drain seems unnecessary as I would have my engines running anyway.


So I have installed a 40A DC to DC charger.


The DC - DC needs a 12volt feed to initiate charging. The manual says connect this through the Ign switch of the engine. BUT my thoughts are this, if the Lithium bank is low then by using the Ign switch as a feed this would instantly apply a 40A load to the start battery circuit, thus reducing its ability to start the engine.


1. Should I add a manual switch into the ign circuit feed to the DC-DC and only switch it on after the engine is running? I could of course forget to switch it off and worse case would be as above.


2. Is there a tap in, on the alternator for example, that would only give a 12v feed once the engine is running?


3. I could connect the 12v feed to only one engine and always start the other first. There is a battery combiner between the 2 start batteries.


Any other solutions?? or am I overthinking it??
I have a 40 amp dc/dc myself with the power wire from the ignition and no you won't deplete your start battery the instant you turn the key on . The dc/ dc requires the voltage to be above the set point before it will start charging . Which means the alternator must be putting out power for the dc dc to charge the house bank . Read your installation manual . If you don't have it let me know what unit you have I have most manuals . Always easier to explain if people have their own materials to reference.
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