Need sanity check on dual input DC/DC + MPPT charger upgrade with existing CBE DS300

[herrakonna]

I am upgrading my CBE DS300 based system with a Renogy dual input DC/DC + MPPT charger controller and replacing my CBE CB510 AC charger with a 20A smart lithium charger.

I have studied the DS300 manual closely and have done a fair bit of googling and reading about integrating a DC/DC (B2B) charger to DS300 based systems, and think I have gleaned a sufficient understanding of how the DS300 works and what must be done to integrate the new components, but am still unsure about the ultimate correctness of my planned modifications of and connections to the DS300.

My goal is to remove entirely all charging related behavior from the DS300, both charging the leisure battery from the alternator or AC charger as well as trickle charging the starting battery from the AC charger. However, I want to retain as much of the existing stock behavior of the DS300, e.g. relating to step retraction, 3-way fridge control, display of starter battery voltage in the panel, etc.

My understanding is that if resistor R37 is removed, the parallel battery relay will be disabled and the starter battery and leisure battery bank will never be connected in parallel. In addition, if the AC charger signal "S" connection is never connected, the DS300 will never attempt to trickle charge the starter battery.

In order to ensure that the new charger controller will be able to accurately monitor the voltage of the starter battery without being affected by the DS300 in any way, I intend to install a 50A DC blocking diode, which should block all current from the DS300 towards the charger controller while still allowing the DS300 to sense and display the voltage of the starter battery.

In order to ensure that the DS300 does not attempt to charge the leisure battery from the alternator input, I will install an inline 1A blade fuse. If the parallel battery relay truly is reliably disabled, there should never be any significant current being pulled from the starter battery / alternator by the DS300 so the fuse should always remain entact.

The ignition signal connection will be left connected as it is now, so that the DS300 knows when the engine is running.

With the above modifications and connections, the DS300 functions affected by when the engine is started/running, e.g. controlling the step retraction, fridge switchover, etc. should continue to function as before.

Here is a diagram of my planned installation:

Am I misunderstanding or missing something, such that the above configuration will not achieve my goals? Any suggestions how better to meet my goals?

Thanks.

 

[AdrianChen]

Might be different on my van which has a ds470 but if my main panel is on and the leisure batts are above 13.5v (Solar) the bloody thing charges the cab battery even without EHU so no S signal (and no ehu symbol on panel) Equivalent to R37 is snipped and the relay does not charge hab from cab when the motor is running...... Do you need both ANL's?

 

[herrakonna]

Might be different on my van which has a ds470 but if my main panel is on and the leisure batts are above 13.5v (Solar) the bloody thing charges the cab battery even without EHU so no S signal (and no ehu symbol on panel) Equivalent to R37 is snipped and the relay does not charge hab from cab when the motor is running......

I guess the "S" signal isn't what triggers the trickle charging of the starter battery, just a high enough voltage indicating the leisure bank is charged

I am assuming the planned power diode inline between the starter battery bank connection and the DS300 should block any undesired trickle charging input to the starter battery from the DS300

If I've understood you correctly, with R37 removed, the DS300 does not try to charge the leisure battery from the starter battery / alternator, so at least disabling the parallel battery relay works as documented, right?

Do you need both ANL's?

If you mean one at each end of the starter battery cable, probably not, but I have them, so it can't hurt

 

[AdrianChen]

All I could see the S signal doing was turning on the panel EHU indicator light! Would love to find a way to stop that charge....

Diode should work I guess, but the DCDC charger should achieve the same.

Removing R37 or equivalent on my DS470 has definitely stopped the cab charging the leisure batteries so they are not paralleled

I did leave B1 connected to the start battery so the panel continues to show the voltage, I suppose loosing that connection would stop the DS charging the start battery.

Some folk on here did some clever stuff with relays to get round the paralleling, but I am too simple, and just did the snip!

As for the 2 fuses, I guess it is good in terms of belt and braces, and there are two potential sources.....

 

[herrakonna]

All I could see the S signal doing was turning on the panel EHU indicator light! Would love to find a way to stop that charge....

Diode should work I guess, but the DCDC charger should achieve the same.

Removing R37 or equivalent on my DS470 has definitely stopped the cab charging the leisure batteries so they are not paralleled

I did leave B1 connected to the start battery so the panel continues to show the voltage, I suppose loosing that connection would stop the DS charging the start battery.

Some folk on here did some clever stuff with relays to get round the paralleling, but I am too simple, and just did the snip!

As for the 2 fuses, I guess it is good in terms of belt and braces, and there are two potential sources.....

Thanks for the info. Sounds like my plan will do what I want. Fortunately the new dual input charger controller will provide trickle charging of the starter battery similarly to the DS300 so no loss of functionality there.

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[autorouter]

I'm not convinced about the blocking diode idea. I think the voltage drop across the diode will mean that the starter battery voltage it reports is not accurate. I would think it would be at least 0.5V out, and possibly a volt or more, because the diode will be operating in its very low current region, where the effective resistance is higher.

I think that just the small fuse would be enough to assure you that any current flow is very low. You could of course try it out, and short the diode to see what happens. It should improve the accuracy of the starter battery voltage without blowing the small fuse.

 

[herrakonna]

I'm not convinced about the blocking diode idea. I think the voltage drop across the diode will mean that the starter battery voltage it reports is not accurate. I would think it would be at least 0.5V out, and possibly a volt or more, because the diode will be operating in its very low current region, where the effective resistance is higher.

I think that just the small fuse would be enough to assure you that any current flow is very low. You could of course try it out, and short the diode to see what happens. It should improve the accuracy of the starter battery voltage without blowing the small fuse.

Thanks. I'll try it first with just a small fuse and see how it goes. I can live without any connection, I just may need to rewire the trigger input on the step to be direct to the vehicle AUX+ rather than though the DS300, so it retracts when the engine is running.

I don't every use the AC and 12VDC fridge operation, only propane, and anyway will be swapping the 3-way fridge for a 12V compressor version in the not-too-distant future, so I can live without the DS300 having any starter battery connection.