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Using existing MPPT Charger instead of DC-DC Charger for two way vehicle charging.

Luk3jay

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Jun 20, 2021
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53
Hi all,
I've recently decided to replace my lead acid aux battery with a LifePo4 pack in my campervan.

My current setup charges the Aux batt from the alternator, if the main batt reaches fully charged level.
And it will charge the Starter batt from the solar if the aux batt reaches charged level.

I really like this and it's a feature I'd like to keep. I've looked into DC-DC chargers but have struggled to find any that charge both ways. some have mppt solar input but the max voltage is only around 25V and my array is higher than that.

I'm considering replacing my Epever charger for an Epever Duoracer which supports charging two separate batteries. one can be configured as LifePo4 with the other configured as Lead Acid.

So That's the solar side done, but I still want to charge the aux battery from the alternator. I considered a b2b charger and then realised I could just feed the alternator via a slight step up converter into the panel input of my solar charger. I would disconnect the starter battery from the solar charger when this happens in order to avoid a feedback loop.

I've attached a very rudimentary diagram to explain my thoughts. There's a slight mistake in it that the relay should be drawn as NC, not NO.

Can anybody see any issues with this? Dare I ask? ? Or any better solutions?

Thanks.
Lifepo and starter 2way.jpg
 
I'm not sure the buck converter won't "confuse" the MPPT algorithm on your SCC.

How about this device?

It's an MPPT SCC with the ability to charge both batteries from solar, or charge the big battery from the alternator.

The site doesn't have much information but if you download the user manual you get all the specs and explanations there.
It seems that they support PV voltage up to 50v, and you have either a 30 amp or 60 amp version.

I Don't have this device, and have never used it, but I remembered coming across it on the web when searching for SCCs.
 
That looks like a good suggestion, thanks. My first thought is that the fans will be a deal breaker. It's only a small van and the electrics are near the bedroom so I certainly wouldn't want any fans. I'll have a look at similar products though.

What do you think will 'confuse' the algorithm with the step up converter?

Thanks.
 
That looks like a good suggestion, thanks. My first thought is that the fans will be a deal breaker. It's only a small van and the electrics are near the bedroom so I certainly wouldn't want any fans. I'll have a look at similar products though.

What do you think will 'confuse' the algorithm with the step up converter?

Thanks.
Well, I tried something similar but with a step-down converter.
My problem was that my new panels exceeded the micro-inverter's rate max voltage.
So I tried connecting a DC-DC step-down to the panels.
The result was that the micro-inverter was never able to lock on the maximum power point. The MPPT tried to increase amperage to find the maximum power point, but it quickly got to the point where the step-down converter's constant-current limit was reached. The step-down did in fact decrease the voltage when that happened, but it seems that the MPPT algorithm just started all over again.

I'm not sure the same will happen with a step-up converter, but surely it will "hide" what is happening on the solar panel to the MPPT algorithm.
As solar panels have a non-linear graph in regards to voltage vs current, the mostly linear action of any DC-DC converter will probably confuse the MPPT SCC.
Thats my understanding anyways...
 
Kinda long, but worth reading (I hope)

I have a one-way configuration, and it has been working great for many years. In my Trailer configuration, input to the EpEver "BN" MMPT is switched to be either the "real" solar panels, or the DC-DC "step-up" converter from the Engine Battery and Alternator. Switching is automatic, so it's connected to the real Solar panels when the Tow Vehicle engine isn't running, and connects the Alternator -> Step-Up -> Trailer Bargman-Connector -> switching Relays -> MPPT path when the Tow Vehicle is running.

I also have a dashboard switch to "enable" the step-up converter in the TV, which I can leave switched off when towing other Trailers, or sitting at a red light with low RPMs.

For the other direction, whenever you have the engine battery running down and need to charge from the MPPT (Solar-connected), I WOULD use a DC->DC charger attached to the "house" electric system with MPPT. It would be pulling power from either the MPPT or the LFP batteries, at LFP voltage - but then charge the "engine start" battery only when the engine isn't running and the batteries are low. Generally, that DC->DC would be running "float mode" into the engine start battery, without regard to the MPPT charging mode selected for the LFP batteries. Only a tiny "loop" would occur, while the engine is running and the MPPT is charging from the boosted alternator. But in that situation, the "pull" from the alternator (at roughly 13.5V) will pretty much already match the DC->DC "float mode" charging voltage, and the reverse current will be tiny or non-existent.

My one-way diagrams are here: https://diysolarforum.com/threads/r...rs-of-mppt-charge-solar-in-the-trailer.20730/ The main limitation in doing this is the fact that output from your MPPT Solar Controller must be "tuned down", in order to avoid overrunning and burning out a smaller Boost Converter. My own "boost" is from engine voltage to 36.0V, rated at 540 watts maximum, and I have my MPPT tuned down to 28A maximum current into my LFP battery string @ 14.4V Voltage (a bit more than 400 watts). 30A would probably be OK, but MPPT inefficiency wiring losses increase the corresponding load at the Boost Converter to about 450 Watts, and I don't want to push it any harder than that.

In short - to prevent MPPT "hunting", I think you must avoid having the Boosted Alternator (constant-voltage) and the genuine Solar String (slightly variable voltage) connected at the same time. My Epever Tracer "BN315" can adjust to either Voltage almost instantly upon switching, but if I had them both connected at the same time (even buck-converting the Solar down to be nearly the same Voltage) it would continue "hunting" indefinitely, because the spike in output at the constant voltage would confuse the microcode algorithm. That algorithm can probably only handle a smooth curve, with ONE peak value.
 
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Thanks for the responses.
It sounds like I need to go back to the drawing board... Or just abandon the idea of charging the starter batt from the solar.
It's just difficult, as I've been saved a few times, by the fact that my solar currently tops the starter battery up.
 
I've looked into DC-DC chargers but have struggled to find any that charge both ways.
IIRC the Ctek and the Renogy 30/50A DC-DC chargers with MPPT send maintenance power to the starter battery. I don't know of any standalones (no SCC) that do it.

My current setup charges the Aux batt from the alternator, if the main batt reaches fully charged level.
And it will charge the Starter batt from the solar if the aux batt reaches charged level....
I've been saved a few times, by the fact that my solar currently tops the starter battery up

If you can live with self-jumpstarting from the aux batts rather than continual maintenance of starter battery then the problem could get easier to solve, I think.

So far I've had to press The Button on my isolator (Battery Doctor VSR) to manually join the batts twice; worked great.
 
Hi all,
I've recently decided to replace my lead acid aux battery with a LifePo4 pack in my campervan.

My current setup charges the Aux batt from the alternator, if the main batt reaches fully charged level.
And it will charge the Starter batt from the solar if the aux batt reaches charged level.

I really like this and it's a feature I'd like to keep. I've looked into DC-DC chargers but have struggled to find any that charge both ways. some have mppt solar input but the max voltage is only around 25V and my array is higher than that.

I'm considering replacing my Epever charger for an Epever Duoracer which supports charging two separate batteries. one can be configured as LifePo4 with the other configured as Lead Acid.

So That's the solar side done, but I still want to charge the aux battery from the alternator. I considered a b2b charger and then realised I could just feed the alternator via a slight step up converter into the panel input of my solar charger. I would disconnect the starter battery from the solar charger when this happens in order to avoid a feedback loop.

I've attached a very rudimentary diagram to explain my thoughts. There's a slight mistake in it that the relay should be drawn as NC, not NO.

Can anybody see any issues with this? Dare I ask? ? Or any better solutions?

Thanks.
View attachment 71745
The basics of energy flow are: it moves from the higher potential to the lower potential (Voltage). If you do not want that to happen (i.e. from an alternator into a solar panel) you must use an isolator bus (two in - two out) or a diode switching device. There are many other options depending on your configuration and needs. An MPPT charger between your LIPO and your lead batteries could also be a solution just make sure the upstream configuration is not overloaded.
 
The basics of energy flow are: it moves from the higher potential to the lower potential (Voltage). If you do not want that to happen (i.e. from an alternator into a solar panel) you must use an isolator bus (two in - two out) or a diode switching device. There are many other options depending on your configuration and needs. An MPPT charger between your LIPO and your lead batteries could also be a solution just make sure the upstream configuration is not overloaded.
Another MPPT charger costs quite a bit, and the "excess voltage" of an LFP battery pack can hardly create much extra current while charging an SLA starter battery. A PWM-based "DC->DC charger" would cost less, and does a perfectly good job.
 
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