DCDC and solar charger query

Submitted: Thursday, Mar 14, 2019 at 21:03
ThreadID: 137960 Views:7559 Replies:9 FollowUps:8
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For some time in my vehicle I have a DCDC charger which charges my auxiliary battery from the alternator. I have recently installed solar, which is connected via a mppt solar controller.
Now for the million $ question. When driving, how do I stop the solar controller (victron blue) from detecting the DCDC charge and going into float mode, and the DCDC from doing the same and also going into float mode? The last thing I would want is to do a big drive between camping locations, only to arrive to an undercharged battery, instead of a topped up battery.

Surely I am not the only person who keeps their solar connected 24/7? How have others managed this?

Thank you
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Reply By: Athol W1 - Thursday, Mar 14, 2019 at 21:12

Thursday, Mar 14, 2019 at 21:12

Unfortunately this is a problem faced by many systems where there are 2 charging mediums used at the same time, the one putting out the greatest voltage will 'shut off' the other.

Short of using a single unit that will accept charging current from both, or more than one, source the only way would be to fit a relay to the Solar panels that will 'switch off' the solar panels when the vehicle ignition is on.

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Follow Up By: 4x4PlayNT - Thursday, Mar 14, 2019 at 21:36

Thursday, Mar 14, 2019 at 21:36
I opted to keep the DCDC charger and mppt chargers as separate items, as if I ever had issues with my DCDC unit I could still get by with the mppt.
Having the 2 connected as they are, would the aux battery still receive charge? Both the mppt and DCDC are 20amp units. So I guess it wouldn't matter what is charging as long as it was one or the other instead of none?
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Follow Up By: Keith B2 - Thursday, Mar 14, 2019 at 21:54

Thursday, Mar 14, 2019 at 21:54
I think the one with the highest charge voltage will win the day.
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Reply By: Peter_n_Margaret - Thursday, Mar 14, 2019 at 22:05

Thursday, Mar 14, 2019 at 22:05
If the charge voltages of the 2 devises are even similar, they will play happily together.
I use solar at a boost voltage of about 14.8V and direct charging from the alternator at about 14.4V and they will both contribute charge together - up to 30A from the solar and up to 70A from the alternator. As the batteries come up in charge, the alternator input will gradually drop away.
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Follow Up By: Allan B (Sunshine Coast) - Thursday, Mar 14, 2019 at 22:44

Thursday, Mar 14, 2019 at 22:44
I would go along with Peter on this. There should be no problem.
Both the DC-DC charger and the solar MPPT controller can simultaneously contribute.

What they 'see' is the battery terminal voltage which governs their behaviour and contribution. If the battery SOC is low then its terminal voltage is low and either charge source will contribute in accordance with their charging algorithm. If either of them are capable of raising the battery terminal voltage sufficiently it will be because the battery is becoming charged and the contributions will 'back-off' accordingly. They should not interact unsuitably although their ampere contributions are likely to be unequal.

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Follow Up By: Member - shane r1 - Friday, Mar 15, 2019 at 09:19

Friday, Mar 15, 2019 at 09:19
G’day Peter
That looks like you’re charging via alternator. That’s how my Mazda is set up too, (VSR)and works well
The OP has a Dc/Dc charger so could behave differently.
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Follow Up By: Peter_n_Margaret - Friday, Mar 15, 2019 at 10:05

Friday, Mar 15, 2019 at 10:05
Yes I do, but t should make no difference except the DC-DC charge voltage would be higher and the charge rate would be initially lower, but it would not drop away like the direct alternator does.
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Reply By: RMD - Thursday, Mar 14, 2019 at 23:01

Thursday, Mar 14, 2019 at 23:01

Have you actually parked the vehicle, had the solar running through it's switchmode MPPT unit to batteries AND had the engine running through it's DCDC charger and measure/seen what the charge input is really doing? That way you will know if they conflict.

Some MPPT chargers run MPPT when in BULK, bit switch to PWM when in absorbtion or float.
The alternator to DCDC will do same all the time but drop through it's charge regime.
Unless you try it you won't know if they fight each other. If each one is momentarily stopping it's charge to read the system voltage for it's micro processor to decide what to do next instant, then they may conflict as the voltage fluctuations are being varied from each source.

Not hard to have an opening relay to turn off solar input while travelling as Athol mentioned.

I have used solar through PWM AND direct alternator charge at the same time but when there is switching waveforms and pulses of charge from two sources to the same point it is worth checking before that long drive.
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Follow Up By: 4x4PlayNT - Thursday, Mar 14, 2019 at 23:06

Thursday, Mar 14, 2019 at 23:06
In full sun and vehicle running with a load on the battery, the battery is getting the full 14v. Im not sure how many amps though.
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Reply By: Gbc.. - Friday, Mar 15, 2019 at 06:54

Friday, Mar 15, 2019 at 06:54
What brand dcdc is it? If it combines dc sources and has mppt like most do, just plug the panels straight into that and keep the other mppt for if you have an emergency. You might be over thinking it.
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Reply By: eaglefree - Friday, Mar 15, 2019 at 08:45

Friday, Mar 15, 2019 at 08:45
While on this topic what if you have solar but no dc-dc charger? I have straight from alternator to aux battery through 2x 100amp fuses and solonoid to protect the tug battery.
Altetnator is 110amp. Solar will be 250w (not purchased yet).
AnswerID: 624427

Follow Up By: Gbc.. - Friday, Mar 15, 2019 at 11:15

Friday, Mar 15, 2019 at 11:15
Just run the panels through a solar regulator into the batteries.
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Reply By: David I1 - Friday, Mar 15, 2019 at 09:35

Friday, Mar 15, 2019 at 09:35
I think you will find with the Redarc dc to dc charger that there is a relay to be purchased that switches the Solar connection off when the ignition is on. So when driving the solar panel is doing nothing but when the car is switched off the solar panel will charge.
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Follow Up By: Allan B (Sunshine Coast) - Friday, Mar 15, 2019 at 10:32

Friday, Mar 15, 2019 at 10:32
David, it depends on which model of Redarc charger.
The earlier Redarc chargers had only a single input connection so required a changeover relay as you described.
Later models with the "D" suffix have usual inputs for alternator and solar so require no relay. Both inputs contribute simultaneously.

EDIT: This circuit configuration requires that the solar (without controller) is connected via the relay to the INPUT of the dc-dc charger. It is not appropriate to place a relay in the output of an MPPT controller as the battery should be connected before the solar panel is connected.
I suppose that it would be possible to place a relay in the solar-to-controller (input) circuit to simply 'switch off' the solar contribution leaving the output always connected to the battery but I don't see good reason for this.

I also note that Redarc seem to have removed all references to using a relay in this way. They probably would prefer you to purchase dual-input chargers.
Here is the circuit that they once published:

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Reply By: HKB Electronics - Friday, Mar 15, 2019 at 13:58

Friday, Mar 15, 2019 at 13:58
It will be a matter of suck it and see, some controllers will play happily together others won't. Generally when batteries are low and can accept the entire output of both chargers the chargers will be in current limit mode and both will supply their maximum outputs. It is when the current flow drops below the maximum of both chargers that problems may start, one charger will try to hold its voltage and the other will push it higher causing it to back off more, current drops, the charger that backed off then tries to increase it again and so the cycle repeats.

At this point of time the batteries will most likely be in a fairly high state of charge anyway unless the chargers have very high output capacity or there is sever interaction you might be better off just leaving both working if they are each putting out a fair charge rate, ie if one is putting out 20A and the other is cycling between 5A-15A then your still getting extra into the battery so let them play. If however both a badly interacting with each other and your only getting say 5A out of each but one on its own will give 20A then your better off turning one charger off.

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Reply By: Member - Phil 'n Jill (WA) - Friday, Mar 15, 2019 at 19:33

Friday, Mar 15, 2019 at 19:33
And just when I thought I had all aspects of charging my van batteries sorted out - along comes another curly one.

Yesterday I was feeling quite comfortable having worked out how to go about using a genset to 'top up' the batteries if the situation arose - and now we have a new case to consider - 'possible CONFLICT between charging systems - and as it hasn't been mentioned by any of my installers as the units were added, I am hoping it won't be an issue - but as mentioned earlier we will just have to 'suck it and see'.

My systems now consist of the original CTEK MPS25 which is purely a 240 volt charger - I see no issue there - particularly as I can switch that off quite easily.

The next unit installed not long back and not tested yet is a Jaycar 'Powertech' 4 stage 40A DC - to DC unit to charge purely from the alternator through the Anderson plug and only whilst the engine is operating.

The latest came with the addition of solar panels - an Enerdrive ePower PWM 30A unit.

Each of these is wired separately down to the batteries.

It may be 2 or 3 weeks before I can test it all, but I will most assuredly advise if there are any dramas.

In the interim, if 4x4PlayNT has an issue or anyone else with a similar arrangement I would appreciate any feedback thanks.

Good luck with it all everyone, life was so much simpler back in the dark ages - a tilley lamp and old army tent weighing a ton - we didn't know what we were missing. Ignorance is bliss...

Cheers - Phil

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Reply By: Member - Bigfish - Sunday, Mar 17, 2019 at 18:28

Sunday, Mar 17, 2019 at 18:28
I have 200watts of solar on roof of car and dcdc charger in boot that feeds 2 x 130amp gel fullriver batteries. Solar via a MPPT reg. in 3 years I have never had an issue and batteries always charged up. They happily co-exist being on all the time (naturally dcdc is only on when ignition is on).
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