Dc to dc charge controller fed with an mppt?

Strangeman24

New Member
Hello all, I’ve been given an unused domestic solar panel that I would like to use on my van, I currently have a renogy 30A DC to DC charger, with in built mppt. But the mppt is limited to 25v and the panel I’ve been given is 37v, could I use an stand alone mppt to reduce the voltage to something the renogy unit could utilise? I understand ther will be an increase in losses by doing this but would it work? Are there any pitfalls I’m over looking? Would a PWM be a better choice for the money? Appreciate any fed back.
 

rin67630

Solar Addict
A MPPT limited to 25V? That is unusual.
You should use a plain mppt SCC, that will charge your battery. (Which one?)
 

Diysolar123

Solar Addict
that is probably a renogy PWM charge controller...useless...just go get an MPPT SCC.
(ok, well, not really useless but unless you are seriously cash strapped spend the $50 and get an MPPT hehe)
 

rin67630

Solar Addict
that is probably a renogy PWM charge controller...useless...just go get an MPPT SCC.
(ok, well, not really useless but unless you are seriously cash strapped spend the $50 and get an MPPT hehe)
A PWM charge controller performs well with "18V" panels and 12V batteries. They perform also well with 36V panels and 24V batteries.
Using them with a 36V panel and a 12V battery is a waste of energy.
 

12VoltInstalls

Solar Enthusiast
I would get a separate MPPT controller for the panel, 25v is really low, might be good for a small ground deploy setup though.
^^^ This is good advice.
Don’t mess with your DC-DC for your solar. Sure it could be made to work but a smaller epever mppt will do that panel AND that your battery well AND you’ll get the most out of the panel if you stay parked for days on end. And the price ain’t bad on epever for the amount of positive feedback on them.
 

rin67630

Solar Addict
It is easy after the first one.
Really? Keeping that waterproof and weather resistant too? I would not want to tamper with them, that might work for the YouTube video, but how does it sustain the harsh weather conditions with extreme temperature/humidity variations?

12 volts = highest potential for diy!
What do you mean with that statement?
12V class panels are good for PWM, 36V class panels good for MPPT with a 12v battery and PWM with a 24V battery.
All that is doable yourself.
Of course > 48V DC is not something for rookies and your insurance may be voided if you try to tamper with it.
Depending oh where you live, you might get a different legal figure.
 

12VoltInstalls

Solar Enthusiast
I’m sorta a 12V proponent. However, some circumstances supplant 12V for practicality and others make 24 or 48V not as practical.

So I’m not thinking either above opinions are the end-all. FWIW my panels are currently wired 24V and I mppt that to 12V batteries. Panels are typically providing 36-40V in full sun. The 24V is a compromise but has more useable benefits than negatives. The 12V bank is also a compromise but for smaller-scale solar offers some advantages and simplicity. But saying 12V is best for diy needs to be qualified- not a useable blanket statement imho.
 

rin67630

Solar Addict
I’m sorta a 12V proponent. However, some circumstances supplant 12V for practicality and others make 24 or 48V not as practical.

So I’m not thinking either above opinions are the end-all. FWIW my panels are currently wired 24V and I mppt that to 12V batteries. Panels are typically providing 36-40V in full sun. The 24V is a compromise but has more useable benefits than negatives. The 12V bank is also a compromise but for smaller-scale solar offers some advantages and simplicity. But saying 12V is best for diy needs to be qualified- not a useable blanket statement imho.
24V Voc panels? or 24V Vmp panels. According to my experience, the voltage span of 18Vmp panels to the 14.4V of a full charged FLA battery is too tiny to run MPPT with a good stability.
 

12VoltInstalls

Solar Enthusiast
24V Voc panels? or 24V Vmp panels. According to my experience, the voltage span of 18Vmp panels to the 14.4V of a full charged FLA battery is too tiny to run MPPT with a good stability.
2S2P 12V panels.
They seem to ceiling at 40V from the panels under full sun. And I even keep them near vertical for self cleaning in summer and easy snow shedding in winter.
Early and late in the day I’m getting at least as good as I was getting w/ pwm sometimes but once the sun is up 40V input to the mppt seems pretty good to me!
 

rin67630

Solar Addict
It's not really.

The MPPT input on my Ctek D250S was limited to 25v I think, thus only 36 cell panels could be used.
Are you sure it was a real MPPT SCC? not a PWMsold as being MPPT?
18Vmp is too low to give a good MPPT span for a 12V FLA battery.
2S2P 12V panels.
They seem to ceiling at 40V from the panels under full sun. And I even keep them near vertical for self cleaning in summer and easy snow shedding in winter.
Early and late in the day I’m getting at least as good as I was getting w/ pwm sometimes but once the sun is up 40V input to the mppt seems pretty good to me!
There is not practical difference between 2x 18Vmp panels in series and one 36Vmp panel alone.
They definitely run better on MPPT almost all the time.
Were they wired parallel on a 12V FLA battery, the picture were different, then PWM is practically as performant especially early and late and under heavy clouds.
 

sunshine

Solar Enthusiast
Keeping that waterproof and weather resistant too?
Silastic sealant over the modification and the leads are zipped tied to the frame. The work is underneath the panel and not exposed directly to the worse of the weather. Often other rips to the underside are given a coating of silastic at the same time since they are older panels.
The thickness of the glass and more solid construction I reckon my older modified panels will outlast the new panels that made them redundant...just loosing watts as time goes on!

''12 volts = highest potential for diy!''
What do you mean with that statement?
12v stuff is readily available, most already have some and it is a lot cheaper.
The only verified data comparing MPPT with PWM only shows a 30% advantage to MPPT....so I add another free panel to make up the difference...and that 30% is only over a very short time frame.

Resistance is obviously greater but only a minor cost soon recuperated.
 

rin67630

Solar Addict
''12 volts = highest potential for diy!''
...
12v stuff is readily available, most already have some and it is a lot cheaper.
I just have misunderstood your meaning for "potential", which means also tension, voltage...

Regarding 30% advantage, i have made a LOT of measures, including the quiescent consumption of the devices.
The 30% is theoretical, and only valid if you have enough voltage difference, which is not the case for 18Vmp panels.

MPPT controllers need much more power for themselves, under cloudy conditions you will just reach the break-even to offset the night own consumption around 11:00.
A 18Vmp panel just delivers its maximum at 15V, which is just above the 14.4V absorption voltage of a FLA battery.
That is also the very reason why 18Vmp has got a quasi standard. Its just optimal for 12V FLA batteries with PWM.
And 36Vmp is optimal for 24V FLA with PWM.
 
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